Chapter 22 Diseases associated with viruses and Chlamydia – II
VIRAL DISEASES CHARACTERIZED BY RESPIRATORY SIGNS 1307
VIRAL DISEASES CHARACTERIZED BY NERVOUS SIGNS 1368
VIRAL DISEASES CHARACTERIZED BY SKIN LESIONS 1418
DISEASES ASSOCIATED WITH CHLAMIDIAE 1433
Viral diseases characterized by respiratory signs
Viral respiratory tract disease is considered by veterinarians in the United States to be second only to colic among medical diseases in importance to the health and welfare of horses.1 The situation is likely similar in most developed countries and especially those in which equine influenza is endemic. Episodes of upper respiratory tract disease characterized by fever, nasal discharge and cough are common in horses, especially young animals and horses housed in groups in stables and barns. An estimated 17% of equine operations in the United States have one or more horses affected by upper respiratory disease each year, and 1.5% of horses develop the disease every 3 months.2 Upper respiratory disease is most common in spring and least common in winter.2 Strangles was an uncommon cause of disease, occurring in only three horses per 1000 per 3 months.2 Viral respiratory disease is approximately three times more common in horses less than 5 years of age.2
With the exception of Streptococcus equi and possibly Mycoplasma spp., the known causes of infectious upper respiratory disease of horses are viral and include: equine herpesvirus types 1, 2 and 4, equine influenza virus, equine rhinitis virus types A and B (ERAV and ERBV), equine adenovirus, equine viral arteritis, and equine parainfluenza 3 virus. Equine hendra virus and African horse sickness cause signs of severe respiratory disease. Both strangles and equine viral arteritis can be mild and lack outstanding clinical signs, thus closely resembling disease associated with other viral causes of upper respiratory tract disease. Therefore, differentiation among diseases associated with these agents based on clinical signs and epidemiological characteristics is difficult and definitive diagnosis is only achieved through serological or microbiological examination of blood or nasal discharge.
Isolation and identification of a causative organism from nasopharyngeal swabs or airway washings of acutely affected horses provides a definitive diagnosis, although on occasion more than one potential pathogen may be isolated. Demonstration of seroconversion or a three- to four-fold increase in titer from serum samples collected during the acute and convalescent (usually 3 weeks after onset of clinical signs) phases of disease is persuasive evidence of infection. Immunofluorescence, enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) tests may provide rapid diagnosis through detection of viral particles in nasal swabs and tissue specimens. The ability to determine the cause of an outbreak of upper respiratory disease in horses is enhanced by the use of multiple diagnostic tests and obtaining samples from more than one horse in an outbreak.3 However, definitive diagnosis of the cause of nasal discharge, cough, and fever is often not achieved.4
All the agents known to cause upper respiratory disease in horses are relatively sensitive to environmental influences, and spread of the agent is dependent on transmission from infected horses, either directly or on fomites. Introduction of an infected horse into a susceptible population of horses may result in an explosive outbreak of upper respiratory tract disease. Such events are common on stud farms and in racing stables, where relatively closed bands of horses are maintained for much of the year. The movement of horses over long distances may facilitate the introduction of pathogens to which the local population of horses is naive.
The opposite situation occurs when young horses are introduced into larger bands of mixed aged animals, such as happens in racing stables or barns of pleasure horses. The younger, possibly naive, horse is then exposed to endemic pathogens to which the resident horses have developed resistance.
Young horses are at particular risk of developing infectious disease of the upper respiratory tract. The diseases are usually a problem only in yearlings and 2-year-olds; young foals acquire a passive immunity from the dam and adults have acquired a permanent immunity through exposure or vaccination. In a horse population it is the average age and the mix of ages which largely determine its herd resistance, and when 30–40% of that population has not previously been exposed to infection then major outbreaks are likely. All of the diseases are transmitted by droplet infection, and over long distances so that limitation of their spread is possible only by rigid isolation and intensive sanitary precautions, and even the best protected studs are likely to be invaded from time to time.
There are two, and possibly three, strains of equine rhinitis virus, (ERAV, ERBV) that infect horses. ERV is more closely related to foot-and-mouth disease than to other picornaviruses.5 Almost all the information available on ERV infection of horses is for ERAV. The virus has been well characterized.6-8 Serologic evidence of infection with ERAV is present in 50–100% of horses.9,10 The virus is present in nasal discharges, feces and urine of a large proportion (17%) of clinically normal horses. The importance of urine shedding of the virus in transmission of infection is unclear, although the inhalation of aerosols of infected urine might transmit the virus.4 ERAV might be an important but under recognized cause of acute upper respiratory disease in horses.9 The disease thought to be associated with ERAV is characterized by an incubation period of 3–8 days, fever, pharyngitis, pharyngeal lymphadenitis, and a copious nasal discharge which is serous early and becomes mucopurulent later. Viremia is a consistent feature of the early stages of the disease. A cough persists for 2–3 weeks. The uncomplicated disease is mild and self-limiting. Among a group of susceptible horses, there is rapid spread of infection and disease.11 Studies in England have not identified the virus as an important cause of inflammatory airway disease in race horses.12 Virus neutralizing antibody develops within 7–14 days of infection and persists for long periods. Immunity after natural infection is said to be solid and long-lasting. Diagnosis is based on serological testing and tissue culture of the virus, which is environmentally resistant. There is no commercial vaccine available. Planned exposure of young horses to infection has been recommended, but should be reconsidered in light of current knowledge of the prolonged shedding of the virus in urine and feces.9 The virus appears to have minimal zoonotic potential.10,13
Equine rhinitis virus B has been characterized and approximately 24% of horses in Australia and 86% of horses in Austria have serum neutralizing antibodies to the virus.10,14 The role of ERBV in causing disease in horses is unknown.
Upper respiratory tract disease associated with equine parainfluenza-3 (PI-3) is characterized by a mild self-limiting disease which is not clinically distinguishable from the others in the group.15 The epidemiology and economic importance of disease associated with this agent is unknown.
Two antigenic types of equine adenovirus, EAdV-1 and EAdV-2, are recognized that cause respiratory disease in foals and adult horses and diarrhea in foals, respectively.16,17 Infection with EAdV is worldwide, based on seroepidemiological studies using virus neutralization and complement fixation tests, and affects up to 70% of horses. Horses less than 1 year of age are most likely to not have serological evidence of exposure, and are therefore presumably at risk of infection and disease. EAdV usually causes a mild respiratory disease with fever, coughing, nasal discharge and conjunctivitis, although its association with fatal pneumonia in thoroughbred foals is reported. Foals usually acquire the infection from their dams, which secrete the environmentally stable virus in nasal discharge, urine and feces. In Arab foals with inherited combined immunodeficiency adenovirus infection is usually fatal. The virus is not associated with inflammatory airway disease in race horses in England,12 but has been associated with small outbreak of upper respiratory tract disease.17
Diagnosis can be made on cell smears taken from conjunctiva or nasal mucosa that reveal characteristic adenoviral intranuclear inclusion bodies Serological methods include serum neutralization, hemagglutination inhibition, complement fixation, or precipitating antibody tests. The serum neutralization test is most accurate, but the hemagglutination inhibition test is most suitable for a screening test No specific control measures are indicated for normal foals.
A reovirus, or a series of serotypes, cause mild upper respiratory tract disease of horses.18 Infection with these agents appears to be of little clinical or economic importance.
Equine coital exanthema is a venereal disease manifested by papular, then pustular, and finally ulcerative lesions of the vaginal mucosa, which is generally reddened. The ulcers may be as large as 2 cm in diameter and 0.5 cm deep and are surrounded by a zone of hyperemia. In severe cases the lesions extend onto the vulva and the perineal skin to surround the anus. In the male, similar lesions are found on the penis and prepuce. Many mild cases are unobserved because there is no systemic disease and affected horses eat well and behave normally. The effect on fertility is equivocal although there may be a loss of libido during the active stage of the disease in stallions.19 Transmission is usually venereal from affected or clinically normal carrier animals in which the infection is thought to be latent in sciatic ganglion.19 The incubation period is 2–10 days and the course up to complete healing of ulcers is about 14 days. Diagnosis can be achieved by use of virus isolation or demonstration of viral DNA in skin lesions.20 Secondary bacterial infection may lead to suppurative discharge and a longer course. In some outbreaks lesions occur on the skin of the lips, around the nostrils, and on the conjunctiva. They may also be present on the muzzle of the foal. Ulcerative lesions of the pharyngeal mucosa also occur in infections with EHV-2 and with EAdV. Ulcerative lesions of the oral mucosa are of great importance because of the necessity to diagnose vesicular stomatitis early. Control can be achieved by use of artificial insemination.10
1 Traub-Dagatz JL, et al. J Am Vet Med Assoc. 1991;198:1745.
2 Gross DK, et al. Proceedings Am Assoc Equine Pract. 2000;46:274.
3 Mumford EL, et al. J Am Vet Med Assoc. 1998;213:385.
4 Burrell MH, et al. Vet Rec. 1994;134:219.
5 Li F, et al. Proc Natl Acad Sci USA. 1996;93:990.
6 Hartley CA, et al. J Gen Virol. 2001;82:1725.
7 Stevenson RA, et al. J Gen Virol. 2003;84:1607.
8 Varrasso A, et al. J Virol. 2001;75:10550.
9 Studdert MJ. Virus infections of vertebrates: virus infections of Equines 6. Amsterdam: Elsevier, 1996;213.
10 Seki Y, et al. J Vet Med Sci. 2004;66:1503.
11 Li F, et al. J Clin Microbiol. 1997;35:937.
12 Newton JR, et al. Prev Vet Med. 2003;60:107.
13 Kriegshauser G, et al. Vet Microbiol. 2005;106:293.
14 Huang J, et al. J Gen Virol. 2001;82:2641.
15 Ditchfield J, et al. Can J Comp Med Vet Sci. 1965;29:18.
16 Studdert MJ. Virus infections of vertebrates: virus infections of Equines 6. Amsterdam: Elsevier, 1996;67.
17 http://www.ivis.org/special_books/Lekeux/studdert/chapter_frm.asp?LA=1. Accessed February 14th 2005.
18 Studdert MJ. Virus infections of vertebrates: virus infections of Equines 6. Amsterdam: Elsevier, 1996;97.
Five herpesviruses have been associated with various diseases of horses and foals (equine herpesvirus 1–5, EHV-1 to 5). EHV-1, EHV-3 and EHV-4 are alphaherpesviruses, whereas EHV-2 and EHV-5 are slow-growing gammaherpesviruses. Common names are ‘equine abortion virus’ for EHV-1, ‘cytomegalovirus’ for EHV-2, ‘equine coital exanthema virus’ for EHV-3, and ‘rhinopneumonitis virus’ for EHV-4 (although this term is sometimes used, confusingly, for EHV-1). EHV-1 and EHV-4 show extensive antigenic cross-reactivity and were previously considered subtypes of the same virus (EHV-1), but restriction endonuclease fingerprinting has demonstrated them to be different viruses. EHV-1 is closely related to asinine (donkey) herpesvirus 3, which is suggested to be its progenitor.1 Related herpesviruses (asinine herpesvirus 1–5) infect and some cause disease in donkeys.1-3 The recently identified asinine herpesviruses ASV-4 and ASV-5 cause a fatal interstitial pneumonia in donkeys.3
The disease syndromes attributed to equine herpesvirus infection, which are discussed in the following pages, and the viruses associated with them are:
• Upper respiratory tract disease of adult horses, weanlings and older foals is caused principally by EHV-4, although disease attributable to EHV-1 occurs. EHV-2 causes respiratory disease, including pneumonia, of foals, and rarely upper respiratory disease of adults
• Abortion is almost always associated with EHV-1, although rare sporadic cases are associated with EHV-4
• Perinatal disease of foals, including birth of sick and weak foals and development of viral septicemia within 48 hours of birth, is associated with EHV-1
• Myeloencephalopathy is associated with EHV-1 and rarely EHV-4
• Genital disease is an unusual manifestation of EHV-1 infection
Disease associated with EHV-5 has not been identified, although it is suspected to have a role in interstitial pneumonia of adult horses. The diseases associated with EHV-1–4 are discussed below.
Etiology EHV-4, an alphaherpesvirus
Epidemiology Transmission between horses and by mediate contagion. Lifelong latency of infection with putative periodic reactivation of virus shedding. Respiratory disease occurs as sporadic disease and as outbreaks. Younger animals more commonly affected by disease. Immunity following vaccination or infection is apparently short lived
Clinical signs Upper respiratory disease, rarely abortion or myelencephalopathy
Clinical pathology Seroconversion or increase in titer detected by ELISA able to differentiate EHV-1 from EHV-4
Diagnostic confirmation Virus isolation from, or polymerase chain reaction test on, blood, nasopharyngeal swabs or tissue. Seroconversion or increase in titer detected by ELISA able to differentiate EHV-1 from EHV-4
Treatment There is no specific treatment
Control Vaccination (of minimal efficacy). Quarantine. Hygiene
Upper respiratory disease of foals and adults is associated with equine herpesvirus 4 (EHV-4), an alphaherpesvirus. The DNA sequence of EHV-4 has been determined.1 There appear to be strains of EHV-4 that vary in virulence, based on severity of clinical disease, but at present it is not possible to differentiate between strains of low and high virulence by laboratory methods.2
Infection with EHV-4 is endemic in horse populations worldwide. The serologic surveys of prevalence of serologic evidence of infection are of limited value as earlier studies used techniques that were unable to differentiate between antibodies to EHV-1 and EHV-4. The few recent serologic surveys using ELISA tests capable of differentiating between antibodies to EHV-1 and EHV-4 demonstrate that almost all horses and foals >60 days of age have evidence of infection by EHV-4.3-6 Young foals can be seropositive as a result of transfer of immunoglobulins from seropositive dams, making determination of the time of first infection, and active seroconversion, difficult. Furthermore, serologic tests are also unable to differentiate between responses to natural infection and to vaccination.
EHV-4 can be isolated from both clinically normal foals and those with signs of upper respiratory disease with similar frequency.7 Shedding of virus is more likely in foals with nasal discharge.7 There is a marked seasonal distribution to the pattern of shedding, with the most frequent detection of shedding being in early autumn (March).7
Upper respiratory tract disease attributable to EHV-4 is very common and probably affects almost all horses during the first 2 years of life.8 EHV-4 rarely causes abortion in mares, septicemia in newborn foals, or myelencephalopathy in adult horses.9
EHV-4 is highly infectious, and transmission probably occurs by the inhalation of infected droplets or by the ingestion of material contaminated by nasal discharges. Foals infected with EHV-4 have prolonged and profuse shedding of virus in nasal secretions. Mediate infection may occur, the virus surviving for 14–45 days outside the animal.
Infections always arise from other horses, both by direct contact and via fomites. Horses and foals are infectious during the active stage of disease and, because horses become latently infected, presumably during subsequent periods of viral reactivation and shedding. The duration of latency is unknown but is assumed to be lifelong.10 EHV-4 establishes latency in the trigeminal ganglion, which is the origin of the maxillary branch of the trigeminal (5th cranial) nerve that provides sensory innervation to the nasal mucosae.11,12 It is assumed that reactivation of the virus and subsequent virus shedding poses a risk to in-contact, susceptible animals, but this has not been definitively demonstrated in field situations. If this were the case, then clinically normal animals harbor latent virus that during periods of reactivation can infect susceptible animals. If true this feature of the disease has obvious importance in the prevention, control, and management of outbreaks of disease.
Immunity resulting from natural infection of the respiratory tract is of short duration despite the persistence of serum virus-neutralizing (VN) antibodies.10 If similar to EHV-1, immunity to EHV-4 is likely associated with cytotoxic T-cell responses because of the importance of cell-associated virus in dissemination of infection throughout the horse. Because of the short duration of immunity an animal can become clinically affected a number of times during its life, although subsequent disease tends to be milder. Foals born to mares with serum antibodies to the virus acquire a protective passive immunity that persists for up to 180 days, provided that they ingest sufficient high quality colostrum. Unfortunately, VN antibodies are not necessarily an indication of resistance to infection.
Foals are infected by EHV-4, presumably from the dam or other mares in the band of mares and foals, early in life and excrete large quantities of virus in nasal secretions.8,13 Horses are infected repeatedly throughout life, with episodes of disease being less frequent and milder with increasing age. EHV-4 is isolated more frequently from younger than from older horses,8 suggesting an age-associated decrease in susceptibility to disease.
Disease associated with EHV-4 is apparently of considerable economic importance because of the loss of training time and opportunities to perform during convalescence and quarantine. Although the upper respiratory disease is a mild inflammation of the respiratory tract of horses, characterized by coughing and nasal discharge, the importance of the disease is the large numbers of animals affected in an outbreak. Fatalities in uncomplicated cases of rhinopneumonitis are rare.
The pathogenesis of EHV-4 infection and disease is assumed to be similar to that of EHV-1, with the exception that the virus does not commonly cause abortion, neonatal septicemia, or myeloencephalopathy.14 The virus is inhaled and binds to epithelium of the upper respiratory tract, enters epithelial cells and reproduces. The infection then spreads throughout the respiratory tract, including trachea and bronchioles, and to lymphoid tissues associated with the respiratory tract. There is a viremia, though this may be of short duration. There is cell death and development of intranuclear inclusion bodies in the respiratory tract and associated lymphoid tissues. The EHV-4 virus then becomes latent as evidenced by isolation of virus from lymph nodes associated with the respiratory tract,15,16 and detection of viral genome in trigeminal ganglia, although this has not been a consistent finding.17 The factors causing viral recrudescence from these latent sites have not been determined. It should be noted that definitive evidence of viral recrudescence of EHV-4 as a cause of outbreaks of disease is lacking, and experimental induction of recrudescence is achieved only by administration of large doses of corticosteroids.16
The classical respiratory tract form of the disease (rhinopneumonitis) is virtually indistinguishable on the basis of clinical signs from the other respiratory tract diseases of horses. There is an incubation period of 2–20 days. Fever, conjunctivitis, coughing and mild inflammation of the upper respiratory tract are the cardinal manifestation of the disease, but inapparent infection is common. The temperature varies from 39 to 40.5°C (102.5 to 105.5°F). There is enlargement, but not abscessation, of the submandibular lymph nodes, especially in foals and yearlings. These signs are more likely to occur in young horses or when horses are assembled in sale barns. Edema of the limbs and diarrhea occur rarely. The length of the illness is usually 2–5 days, although the nasal discharge and cough may persist for 1–3 weeks. Secondary bacterial invasion, usually Streptococcus equi subsp. zooepidemicus, may exacerbate the clinically inapparent viral pneumonia. Young foals can develop primary viral pneumonia.
EHV-4 only rarely causes abortion or neurologic disease.9
Results of hematological and serum biochemical examinations are neither specific nor diagnostic. In adult horses with rhinopneumonitis there may be a pronounced leukopenia, due largely to depression of neutrophils.
Serological tests are of critical importance in diagnosis and control of equine herpesvirus infections. Serum antibody levels to EHV-1/4 may be determined by ELISA,18 virus neutralization (VN),14 or complement fixation (CF) tests. The CF and VN tests are not able to differentiate between seroconversion associated with EHV-1 and EHV-4, whereas an ELISA using recombinant antigens specific for EHV-1 and EHV-4 is able to differentiate infection by each of these types of equineherpesvirus.19 Many, if not all, adult horses have serum antibodies to EHV-4 as a result of previous infection or vaccination. Thus the demonstration of antibodies is not in itself sufficient to confirm a diagnosis of the disease. Complement-fixing antibody appears on the 10th–12th day after experimental infection but persists for only a few months. Demonstration of a three- to four-fold increase in the serum concentration of specific complement-fixing antibodies in acute and convalescent serum samples provides persuasive evidence of recent infection, albeit by either EHV-1 or EHV-4. Complement-fixing antibodies persist for only a short time (several months) while VN antibodies persist for over a year, and testing for them is therefore a more reliable means of determining that previous infection with the virus has occurred. Until recently, serological differentiation of antibodies to EHV-1 and EHV-4 was not possible. However, highly specific ELISA tests based on the variable region of the C terminus of glycoprotein G, at least one of which is commercially available, have been developed that can differentiate between antibodies to EHV-1 and EHV-4 in horse serum.19-22 The ELISA is reported to be more sensitive, easier to perform, more rapid and more reproducible than the virus neutralization test. Importantly, the ELISA test is able to differentiate between infections associated with EHV-1 and EHV-4.19
Identification of the virus in nasal swabs, or blood buffy coat by culture or a PCR test provides confirmation of infection.23 The use of seminested or multiplex PCR provides rapid identification of EHV-4 viral genome in pharyngeal swabs.23 The test is at least as sensitive as viral isolation in identifying presence of virus. However, the use of rapid and innovative diagnostic techniques based on enzyme-linked immunosorbent assay (ELISA), polymerase chain reaction (PCR), immunohistochemical staining with peroxidase, or nucleic acid hybridization probes is often restricted to specialized reference laboratories. Therefore the method of choice for diagnosis of rhinopneumonitis by diagnostic virology laboratories handling many routine samples continues to be the traditional methodology of cell culture isolation followed by sero-identification of the isolated viruses.21 The virus can be isolated in tissue culture, chick embryos and hamsters, from either nasal washings or aborted fetuses.
Samples of nasopharyngeal exudate for virus isolation are best obtained from horses during the very early, febrile stages of the respiratory disease, and are collected via the nares by swabbing the nasopharyngeal area with a 5 × 5 cm gauze sponge attached to the end of a 50 cm length of flexible, stainless steel wire encased in latex rubber tubing. A guarded uterine swab devise can also be used. After collection, the swab should be removed from the wire and transported promptly to the virology laboratory in 3 mL of cold (not frozen) fluid transport medium (serum-free MEM [minimal essential medium] with antibiotics). Virus infectivity can be prolonged by the addition of bovine serum albumin or gelatine to 0.1% (w/v).21
Fatalities are extremely rare in the respiratory forms of EHV-4 infection.
The upper respiratory diseases of horses are listed in Table 16.4. There is no specific treatment although antibiotics are often administered to horses with respiratory tract disease to prevent or treat secondary bacterial infection. There is, however, no evidence that antibiotic treatment shortens the duration of the disease or prevents complications.
Principles of a control program include:
• Enhancing the immunity of individual horses by vaccination
• Minimizing the risk of introducing EHV-4 infection to the farm or stable
• Hygiene to prevent spread of virus on fomites such as clothes and tack
• Rapid isolation of any horse with disease that could be attributable to EHV-4.
Vaccines for protection against rhinopneumonitis contain both inactivated EHV-1 and EHV-4 virus,24 presumably because both viruses cause respiratory disease in horses. None of the currently available vaccines consistently prevent infection of vaccinated horses or provide complete protection against disease associated with EHV-4 although a combined EHV-1/EHV-4 inactivated virus vaccine attenuated the clinical signs of disease in experimentally infected foals.25,26 The development of modified live virus vaccines administered intranasally holds promise for effective control of both EHV-1 and EHV-4 in foals and adults.24
1 Telford EAR, et al. J Gen Virol. 1998;79:1197.
2 Tearle JP, et al. Res Vet Sci. 2003;75:83.
3 Crabb BS, Studdert MJ. Adv Virus Res. 1995;45:153.
4 Gilkerson JR, et al. Vet Microbiol. 1999;68:27.
5 Gilkerson JR, et al. Aust Equine Vet. 1999;17:76.
6 Dunowska M, et al. New Zealand Vet J. 2002;50:132.
7 Gilkerson JR, et al. Vet Microbiol. 1994;39:275.
8 Matsumura T, et al. J Vet Med Sci. 1992;54:208.
9 van Maanen C, et al. Vet Quart. 2000;22:88.
10 Crabb BS, Studdert MJ. In: Studdert MJ (ed) Virus infections of vertebrates: virus infections of Equines 6, 1996; p. 11.
11 Rizvi SM, et al. J Gen Virol. 1997;78:1115.
12 Borchers K, et al. J Gen Virol. 1997;78:1109.
13 Gilkerson JR, et al. Vet Microbiol. 1994;39:275.
14 Kydd JH, et al. Equine Vet J. 1994;26:466.
15 Edington N, et al. Equine Vet J. 1994;26:140.
16 Browning GG, et al. Vet Rec. 1988;123:518.
17 Carvalho R, et al. Arch Virol. 2000;145:1773.
18 McCartan CG, et al. Vet Rec. 1995;136:7.
19 Hartley CA, et al. Am J Vet Res. 2005;66:921.
20 van Maanen C, et al. Vet Microbiol. 2000;71:37.
21 http://www.oie.int/eng/normes/mmanual/A_00085.htm. Accessed February 15th 2005
22 Crabb BS, et al. Arch Virol. 1995;140:245.
23 Varrasso A, et al. Aust Vet J. 2001;79:563.
24 Patel JR, Heldens J. Vet J. 2005;170:14.
Etiology Equid herpesvirus-1 (EHV-1) causes respiratory disease of adults, abortion, neonatal septicemia, and myeloencephalopathy
Epidemiology Transmission between horses and by mediate contagion. Lifelong latency of infection with periodic reactivation of virus shedding. Respiratory disease, abortion and myeloencephalopathy occur prominently as outbreaks, but can affect sole animals
Clinical signs Upper respiratory disease, abortion, neonatal septicemia, and neurologic disease with incontinence, ataxia and recumbency
Clinical pathology Seroconversion or increase in titer using an ELISA able to differentiate between EHV-1 and EHV-4
Diagnostic confirmation Virus isolation from, or polymerase chain reaction test on, blood, nasopharyngeal swabs or tissue. Seroconversion or increase in titer
Treatment There is no specific treatment although acyclovir, an antiviral agent, has been administered. Symptomatic treatment of neurologic signs in horses with myeloencephalopathy
Control Management including quarantine, maintaining mares in small bands, and education of staff about importance of control measures. Vaccination for prevention of abortion. Quarantine. Hygiene
EHV-1 is an alphaherpesviruses, a DNA virus.1 EHV-1 and EHV-4 show extensive antigenic cross-reactivity and were previously considered subtypes of the same virus (EHV-1), but restriction endonuclease fingerprinting has demonstrated both the propinquity and difference of the two virus species. There are two recognized strains of EHV-1, although this designation does not appear to have any association with virulence. Currently, it is not possible to differentiate EHV-1 strains of varying virulence based on in vitro characteristic.2 Virulence is associated with presence of a functional gp2 protein, which is apparently responsible for viral egress from infected cells,3 and glycoprotein D and cell surface glycosaminoglycans that are needed for efficient entry of EHV-1 into cells.4
The most important clinical syndromes associated with EHV-1 infection are abortion, neonatal septicemia, and myeloencephalopathy. Upper respiratory tract disease of adult horses, weanlings and older foals is caused principally by EHV-4, although outbreaks attributable to EHV-1 occur. Genital disease is an unusual manifestation of EHV-1 infection. Infection with EHV-1 causes retinitis and fatal disease in camelids.5,6 EHV-1 also causes disease in wild equids including zebras.7
Infection with EHV-1 is endemic in horse populations worldwide and many adult horses have serologic evidence of infection.8,9 Serologic surveys, which provide an index of the extent of infection in the sampled population, performed before 1995 were hindered by the lack of an assay able to differentiate immune responses to EHV-1 from those to EHV-4. Furthermore, the advent of vaccines eliciting serum antibodies against EHV-1/4, and the inability of diagnostic tests to differentiate between antibodies induced by vaccination or natural infection, complicates assessment of the prevalence of serum antibodies to EHV-1/4. Seroprevalence of EHV-1 specific antibodies is 9–28% in adult Thoroughbred horses, 26% of Thoroughbred brood mares, 11% of Thoroughbred foals, and 46–68 % of 1 and 2-year-old Thoroughbred race horses in Australia.8-11 Sixty-one percent of 82 normal horses and horses with upper respiratory tract disease had antibodies to EHV-1 in New Zealand.12
Upper respiratory tract disease associated with EHV-1 infection has been suggested to occur as outbreaks13 although this is not well documented. Signs of infectious upper respiratory disease affected 20% of Thoroughbred race horses at one race track in Canada over a 3-year period, and seroconversion to EHV-1 occurred in 5–18% of these horses whereas the vast majority of horses seroconverted to influenza.14 However, all horses that seroconverted to EHV-1 also either seroconverted to influenza virus or had been recently vaccinated with a vaccine containing EHV-1. These results suggest that the stress of influenza disease may have triggered reactivation of latent EHV-1 infection in some horses,14 suggesting that EHV-1 did not have a primary role in the outbreak of respiratory disease. Similarly, in England, EHV-1 was not associated with clinical respiratory disease in Thoroughbred race horses.15 EHV-1 was isolated from foals with purulent nasal discharge and respiratory disease concurrent with neurologic disease among the dams in Australia.16
Abortion due to EHV-1 occurs as both sporadic cases and as epizootics (abortion storms). Approximately 3% of abortions in mares are attributable to EHV-1 infection, although the actual incidence probably varies widely among years and geographical regions.17 Outbreaks of EHV-1 abortion and birth of nonviable foals occurs sporadically on farms with sometimes catastrophic losses. Loss of foals through abortion or birth of nonviable foals can be as high as 28% of pregnant mares on the farm.18 Initial cases can, in the absence of appropriate control measures, rapidly spread the infection.18 Vaccination with killed EHV-1 vaccine during late gestation does not reliably prevent the disease18 although conventional wisdom is to ensure that mares are well vaccinated (see ‘Control’ below). EHV-4 rarely causes abortion in mares. Disease of neonates associated with EHV-1 occurs both sporadically and as outbreaks in which up to 25% of foals may be affected.19 Foals infected in utero usually die soon after birth, while those infected in the period after birth may have milder disease and a lower mortality rate (6%).19 One-third of viremic foals may not seroconvert, based on the complement fixation test.19
Myeloencephalopathy occurs as sporadic cases but more often presents as an epizootic within a stable or barn or within a localized area.16,19-22 Morbidity rates in exposed horses range from 1 to 90% with mortality rates of 0.5–40%.16,19-22 Pregnant or nursing mares are suggested to be at greater risk of this disease,8 but outbreaks occur on premises, such as riding schools or race tracks, where there are no foals or pregnant mares.20-22
EHV-1 is highly infectious, and transmission probably occurs by the inhalation of infected droplets or by the ingestion of material contaminated by nasal discharges or aborted fetuses. The virus gains access to the body after binding to respiratory mucosal epithelium Other routes of infection are not recognized.
The virus is efficiently transmitted to in-contact animals and rapid spread of infection results from close contact of an infected animal with susceptible horses. Infection can be spread over short distances in the absence of physical contact or fomite transmission. This likely occurs by airborne spread of virus in droplets of aerosolized nasal secretions.
Infections always arise from other horses, either by direct contact or via fomites. Mediate infection from virus on fomites such as tack, veterinary equipment, vehicles, and housing occurs because the virus survives for 14–45 days outside the animal. The source of the virus is always on of the following:23
• a horse or foal with active infection
• a fetus, fetal membranes, or reproductive tract secretions of a mare immediately after abortion or birth of a weak foal
• virus shed by horses in which latent infection has reactivated.
Horses and foals are infectious during the active stage of disease and, because horses become latently infected, during subsequent periods of viral reactivation and shedding. There is good circumstantial evidence, such as the occurrence of abortion, neonatal disease, or myeloencephalopathy in closed herds, to support a role for latency and reactivation in the genesis of the disease. The duration of latency is unknown but is assumed to be lifelong.1 Latent EHV-1 virus is detectable in the trigeminal ganglion and CD5/CD8 lymphocytes.24-26 Reactivation of the virus might not result in clinical signs in the host animal but there is shedding of virus in nasal secretions. Consequently, clinically normal animals harbor latent virus that can infect susceptible animals during periods of reactivation. This feature of the disease has obvious importance in the prevention, control, and management of outbreaks of disease.
Abortion storms are usually attributable to an index case that is:
• A latently infected mare that sheds virus from the respiratory tract, but does not abort
• A mare that aborts an infected conceptus
• A mare that sheds virus from the respiratory tract, and then aborts.1
Mares usually, but not always, abort from EHV-1 infection only once in their lifetime.1 A likely scenario in abortion storms is the reactivation of latent virus in a resident horse with subsequent shedding of virus in nasal secretions or, if the mare aborts, fetal tissues and uterine fluids. Contamination of the environment or horse-to-horse contact spreads infection to susceptible cohorts (primary transmission). The infected cohorts then further spread the virus to other horses in that band of mares (secondary transmission), which then spread infection among other bands of mares and foals, paddocks or fields of horses, or farms (tertiary transmission).18,23
Out breaks of myeloencephalopathy likely occur through similar mechanisms. Most outbreaks are associated with an index case or introduction of a horse with signs of infectious respiratory disease, with subsequent development of new cases in horses that have either direct or indirect (aerosol or fomite) contact with the index case.16,19,21 It has recently been recognized that horses with clinical signs of myeloencephalopathy can spread the disease, contrary to previous supposition.22 This has important implications for handling and care of affected horses, especially those severely affected horses that may be referred for intensive or specialized care.
Studies on Thoroughbred stud farms in Australia have demonstrated the temporal sequence of events that contribute to spread of EHV-1 infection in that region9 and these studies likely have relevance to other regions of the globe. There is a cyclical pattern in which horses are infected at a young age and the source of infection is, depending on the age of the foal, either its dam or other foals. Foals are infected by EHV-1 and shedding virus in nasal secretions as young as 11 days of age,27 often without development of clinical signs but usually associated with mucopurulent nasal discharge.28,29 Peak incidences of cases of respiratory disease associated with EHV-1 are late during the foaling season before weaning, and again after weaning when foals from several groups are housed together. The source of infection in foals before weaning is mares and, as the number of foals in the herd increases over the course of the foaling season, other foals.9 Weanlings spread the disease among their herd during the period shortly after weaning when foals from more than one group are mixed. The incidence density of new cases among weanlings can be as high as 13 new cases per 1000 foal weeks.30 The disease associated with these outbreaks is mild and without long term consequences to the foal or weanling. However, the presence of foals excreting large quantities of EHV-1 has the potential to increase the risk of viral abortion in late term mares in contact with these foals.9 Furthermore, presence of respiratory disease associated with EHV-1 and shedding of virus by foals is associated with development of myeloencephalopathy in mares.16
Immunity against respiratory disease and resulting from natural infection of the respiratory tract is of short duration despite the persistence of serum virus-neutralizing (VN) antibodies.1 The cell-associated nature of the viremia and lack of expression of viral antigens on the surface of infected cells contributes to the poor efficacy of humoral immunity. Immunity to EHV-1 is mediated by cytotoxic T cells, which explains the limited efficacy of inactivated virus vaccines that have minimal effect in stimulating cytotoxic T cells despite being capable of inducing a humoral immune response.31 The presence of EHV-1 cytotoxic T cell precursors correlates well with protection from experimental infection,32 and some of the EHV-1 antigens (‘early proteins’) responsible for this resistance have been identified.33 Because of the short duration of immunity an animal may become clinically affected by respiratory disease a number of times during its life, although subsequent disease tends to be milder. Mares usually only abort from EHV-1 infection once in their lifetime and there are no reports of horses developing myeloencephalopathy more than onc.
Lack of antibodies to EHV-1 was identified as a risk factor in an outbreak of EHV-1 myeloencephalopathy in a herd of mares with foals at foot.16 Mares with strong antibody responses to EHV-1 did not develop disease.
Disease associated with EHV-1 is of considerable economic importance because of the loss of training time and opportunities to perform during convalescence and quarantine, the loss of pregnancies during abortion storms, and deaths due to myeloencephalopathy and infection of neonates.
The three organ systems involved in clinical disease associated with EHV-1 infection are the respiratory tract, uterus and placenta, and central nervous system. The common final pathway for injury in each of these body systems is damage to vascular endothelium with subsequent necrosis, thrombosis, and ischemia.
Following EHV-1 exposure to the upper respiratory tract, virus can be detected in the soft palate and main stem bronchus within 12 hours, and at all levels of the respiratory tract by 24 hours.34 In the respiratory tract there is an initial phase after infection in which there is rapid proliferation of the virus in the nasal, pharyngeal and tonsillar mucosae, with subsequent penetration and infection of local blood vessels.34 This is followed by a systemic, viremic phase in which the virus is closely associated with blood lymphocytes, from which it can be isolated. Infection induces increased production of interferon gamma by T-lymphocytes.35 Absence of viral antigens on the surface of EHV-1 infected peripheral blood mononuclear cells explains their ability to avoid complement-mediated lysis.36 This activity, combined with the immunosuppression that accompanies EHV-1 infection, allows dissemination of the infection to the reproductive tract and central nervous system. Immunosuppression is mediated by production in EHV-1 infected cells of an ‘early protein’ that interferes with peptide translocation by the transporter associated with antigen processing.37 Immunosuppression is evident as reduced in vitro proliferation of peripheral blood monocytes and down regulation of expression of major histocompatability complex class I molecules on the surface of infected cells.38,39 It is from this point that the invasion of lungs, placenta, fetus and nervous tissue occur. Movement of infected mononuclear cells into target tissues is associated with expression of adhesion molecules by endothelium in the gravid uterus and in leukocytes.40
Viral infection of endothelium results in death of endothelial cells, inflammation, activation of clotting factors and formation of blood clots in small vessels. This thrombotic disease causes ischemia of neighboring tissues with subsequent necrosis and loss of function. Another theory is that deposition of antigen–antibody complexes in small vessels results in an Arthus reaction with subsequent ischemia, necrosis and loss of function. However, recent demonstration that mares with no antibody titer to EHV-1 were at increased risk of developing myeloencephalopathy does not support a role for type III hypersensitivity in this disease.16 Regardless of the underlying mechanism, clinical signs are a result of vasculitis and necrosis of tissue in the central nervous system and reproductive tract. This is in contrast to neurologic disease associated with herpesvirus in other species, in which the nervous system disease is a direct result of infection of neural tissues.
Abortion is caused by damage to the placenta, endometrium or fetus. Placental lesions include vasculitis, focal thrombosis and infarction of the microcotyledons of the pregnant uterus.41 The fetus is infected and there are diagnostic lesions present in many aborted foals, including massive destruction of lymphocytes in the spleen and the thymus. In those abortions in which there is no lesion or evidence of virus infection in the foal, there may be extensive damage to the endometrium due to an endothelial lesion and its attendant vasculitis, thrombosis and secondary ischemia.42,43
Foals that are infected in utero but survive to full term may be stillborn or weak and die soon after birth with pulmonary, hepatic, and cardiac lesions. EHV-1 infection in foals not infected before or at birth is usually a self-limiting, mild infection of the upper respiratory tract with an accompanying leukopenia and a transitory immune suppression, although uveitis and occasionally death occur in a small number of foals.19 Virus can be isolated from the nasal mucus and the buffy coat of the blood for some time after clinical signs have disappeared.44
The pathogenesis of myeloencephalopathy in horses contrasts with herpesvirus encephalitis of other species in which there is viral infection of neuronal tissue.1,16 The myeloencephalopathy in horses is, as discussed above, the result of vasculitis, thrombosis and subsequent ischemia of neural tissue. Impairment of blood flow results in hypoxia and dysfunction or death of adjacent neural tissue.
EHV-1 infection manifests as several forms of disease on a farm such that nervous system involvement can occur in an outbreak in which abortion and respiratory disease also feature19 although more commonly one form of the disease (myeloencephalopathy or abortion) occur alone or with mild respiratory disease. Foals, stallions and mares can be affected with one or other form of the disease, although the disease is most commonly seen in adult horses. Onset of neurologic signs is usually, but not invariably, preceded by cases of respiratory disease, fever, limb edema, or abortion.
The classical respiratory tract form of the disease (rhinopneumonitis) is virtually indistinguishable on the basis of clinical signs from the other upper respiratory tract diseases of horses and is identical to that associated with EHV-4.
Outbreaks of abortion might not be preceded by clinically apparent respiratory disease.18 The incidence of abortion is highest in the last third of pregnancy, particularly in the 8 to 10-month period but can occur as early as the 5th month. Abortion occurs without premonitory signs and the placenta is usually not retained. There is frequently no mammary development. Affected mares sometimes prolapse the uterus. Some foals are stillborn whereas others are weak and die soon after birth.
Abortion storms are often long lasting, with a period of 17–22 days separating the index case from cases caused by secondary transmission of the virus,18,45 suggesting an incubation period of 2–3 weeks. Experimental infections induce abortion 15 to 65 days after intranasal inoculation of the virus.46 While most abortions then occur within one month of the first secondary cases, abortions on a farm can continue for many months.18,45
In-utero EHV-1 infection causes abortion or the birth of infected foals, some of which are normal at birth, but become weak and die within 3–7 days of birth with signs of respiratory distress and septicemia. A less severe form of the disease, characterized by pyrexia, nasal discharge and chorioretinits, occurs in slightly older foals that are apparently infected after birth.19,47 Affected foals that survive sometimes do not have serum antibodies to EHV-1.19 Death may be associated with secondary bacterial infection with Escherichia coli or Actinobacillus equuli, although EHV-1 infection alone is sufficient to cause death.
The disease initially occurs in an index case, which might or might not have had signs of infectious respiratory disease alone or with signs of neurologic disease.16,19-21 Signs of neurologic disease develop in other horses approximately 2 weeks after disease in the index case. Disease then develops in a number of horses over a short period of time (3–10 days).16,20,21
Fever, without signs of respiratory disease, often precedes signs of neurologic disease by 24–72 hours.21 The onset of neurologic signs is usually rapid, with the signs stabilizing within 1–2 days. Signs are variable but usually referable to spinal white matter involvement. Affected horses have variable degrees of ataxia and paresis manifest as stumbling, toe dragging, pivoting and circumduction that is most severe in the hind limbs. Signs are usually symmetrical. There is often hypotonia of the tail and anus.
Fecal and urinary incontinence are common and affected horses often dribble urine, have urine scalding of the skin of the perineum and legs, and require manual evacuation of the rectum. The severity of signs can progress to hemiplegia or paraplegia manifesting as recumbency and inability to rise. Less commonly, cranial nerve deficits, such as lingual or pharyngeal paresis, head tilt, nystagmus or strabismus, are present. Affected horses are usually alert and maintain their appetite.
Severity of neurologic disease varies among horses within an outbreak, and the prognosis is related to the severity of disease. In general, horses that become recumbent have a poor prognosis for both short-term and long-term survival despite intensive nursing care.16,19-21 However, less severely affected horses have a good prognosis for survival, with case fatality rates are low as 2–3% in some outbreaks.21 Horses with mild signs of neurologic disease often recover completely and return to their previous level of performance,20,21 although some have persistent neurologic deficits after one year.
Results of hematological and serum biochemical examinations are neither specific nor diagnostic. EHV-1 infection of adult horses results in leucopenia that is attributable to both neutropenia and T-cell lymphopenia, with B-cell lymphocytosis occurring during the recovery period.48 EHV-1 septicemia of foals is characterized by profound leukopenia, neutropenia with a left shift and lymphopenia.
Cerebrospinal fluid (CSF) of horses with EHV-1 encephalomyelopathy is characteristically xanthochromic and has an increased total protein concentration (>1 g/L) with a normal white cell count.49 The interpretation of EHV-1 antibody in CSF is uncertain, although normal horses are not expected to have detectable antibodies to EHV-1 in the CSF.49
Serological tests are of critical importance in diagnosis and control of equine herpesvirus infections. Many horses have serum antibodies to EHV-1 and EHV-4 as a result of previous infection or vaccination. Thus the demonstration of antibodies is not in itself sufficient to confirm a diagnosis of the disease. Complement-fixing antibody appears on the 10th–12th day after experimental infection but persists for only a limited period. Demonstration of a three- to four-fold increase in the serum concentration of specific complement-fixing antibodies in acute and convalescent serum samples provides persuasive evidence of recent infection. Complement-fixing antibodies persist for only a short time (several months) while VN antibodies persist for over a year, and testing for them is therefore a more reliable means of determining that previous infection with the virus has occurred. Until recently, serological differentiation of antibodies to EHV-1 and EHV-4 was not possible. However, highly specific ELISA tests based on differences between EHV-1 and EHV-2 in the variable region of the C terminus of glycoprotein G, at least one of which is commercially available, have been developed that can differentiate between antibodies to EHV-1 and EHV-4 in horse serum.50-53 The ELISA is reported to be more sensitive, easier to perform, more rapid and more reproducible than the virus neutralization test. Importantly, the ELISA test is able to differentiate between infections associated with EHV-1 and EHV-4.53
Identification of the virus in nasal swabs, or blood buffy coat by culture or a PCR test provides confirmation of infection.54 The use of seminested or multiplex PCR provides rapid identification of EHV-1 viral genome in pharyngeal swabs.54 The test is at least as sensitive as viral isolation in identifying presence of virus. However, the use of rapid and innovative diagnostic techniques based on enzyme-linked immunosorbent assay (ELISA), polymerase chain reaction (PCR), immunohistochemical staining with peroxidase, or nucleic acid hybridization probes is often restricted to specialized reference laboratories. Therefore the method of choice for diagnosis of EHV-1-associated disease by diagnostic virology laboratories handling many routine samples continues to be the traditional methodology of cell culture isolation followed by sero-identification of the isolated viruses.51 The virus can be isolated in tissue culture, chick embryos and hamsters, from either nasal washings or aborted fetuses.
Samples of nasopharyngeal exudate for virus isolation are best obtained from horses during the very early, febrile stages of the respiratory disease, and are collected via the nares by swabbing the nasopharyngeal area with a 5 × 5 cm gauze sponge attached to the end of a 50 cm length of flexible, stainless steel wire encased in latex rubber tubing. A guarded uterine swab devise can also be used. After collection, the swab should be removed from the wire and transported promptly to the virology laboratory in 3 mL of cold (not frozen) fluid transport medium (serum-free MEM [minimal essential medium] with antibiotics). Virus infectivity can be prolonged by the addition of bovine serum albumin or gelatine to 0.1% (w/v).51
Fatalities are extremely rare in the respiratory forms of EHV-1 infection. Macroscopic findings in aborted fetuses include petechial and ecchymotic hemorrhages, especially beneath the respiratory mucosae. The most consistent finding is an excess of clear yellow fluid in the pleural and peritoneal cavities. Focal hepatic necrosis and slight icterus may also be present. In some aborted fetuses the cut surface of the spleen reveals unusually prominent lymphoid follicles, which are swollen due to necrosis and edema. Acidophilic intranuclear inclusion bodies may be evident histologically in a variety of cell types, including the bronchiolar and alveolar epithelium, hepatocytes and dendritic cells of the lymphoid tissues. Although the microscopic pathology is unimpressive, examination of the placenta via immunohistochemical techniques can be a useful aid in the diagnosis of EHV-1 and EHV-4 Induced abortions.55,56 In foals that are alive at birth but die soon afterwards there is usually massive pulmonary congestion and edema, with collapse of the lung and hyaline membrane development in those that survive longer.
In the nervous or paralytic form of the disease there is an acute disseminated myeloencephalopathy. Hemorrhages may be visible grossly but often there are no macroscopic changes. Disseminated vasculitis occurs in the experimental disease57 and the malacic lesions present in the nervous tissue are the result of leakage from these damaged vessels. The virus can be isolated from the brain and the isolation is facilitated by use of an indirect peroxidase stain58 to establish the location of the virus. The virus infects endothelial cells within the central nervous system (CNS) but has also been demonstrated within neurons and astrocytes and has been linked to chorioretinitis in a foal.59 In rare cases the virus may cause lesions in other tissues, such as the intestinal mucosa and spleen60 or pharynx.61
The laboratory examination of aborted fetuses should include a search for virus by tissue culture and immunohistochemical or PCR techniques, as well as a histological examination of the lung and liver for the presence of inclusion bodies. A direct fluorescent antibody test has also been used.62 A serological examination of the foal may provide useful information in those cases where attempts at isolation are negative but seroconversion has occurred. However, a recent study found that fetal serology was an unreliable means of diagnosing EHV-1 abortion and that IHC was slightly more sensitive than virus isolation.63
• Virology – chilled lung, liver, spleen, thymus and thoracic fluid of aborted fetuses or neonates. Spinal cord or brain of horses with nervous disease (VI, PCR, FAT, serology)
• Histology – fixed lung, liver, spleen, thymus, trachea from fetuses or neonates
• Fixed brain and spinal cord from several sites, as well as Bouin’s-fixed eye should be examined in adults with nervous disease (LM, IHC).
Respiratory disease may be associated with a variety of agents (Table 16.4).
Abortion can be associated with leptospirosis, Salmonella abortusequi, placentitis associated with Streptococcus zooepidemicus or Escherichia coli, associated with mare reproductive loss syndrome, or congenital abnormalities, among other causes. When other pregnant mares are at risk abortion in a late-term mare should always be considered to be due to EHV-1 until proved otherwise.
Neurologic diseases with clinical presentations similar to that associated with EHV-1 include rabies, equine protozoal myeloencephalitis, neuritis of the cauda equina (equine polyneuritis), trauma, acute spinal cord compression (cervical stenotic myelopathy) and equine degenerative myelopathy. Fever is rare in other neurologic diseases of horses and any horse with neurologic disease and fever or a history of fever within the previous week should be considered to have EHV-1 myeloecephalopathy. Outbreaks of posterior paresis or ataxia, especially in horses without fever, should prompt consideration of ingestion of intoxicants such as Astragalus spp., Swainsonia spp., or sorghum. Rye grass staggers can produce similar signs of ataxia.
Neonatal septicemia may be associated with E. coli, Streptococci spp. and other bacteria, especially in foals with failure of transfer of maternal immunoglobulins.
Because of the highly contagious nature of EHV-1 infections, horses with respiratory disease, abortion or neurologic disease, especially if these occur as an outbreak, should be isolated until the cause of the disease is identified.
There is no specific treatment for the diseases associated with equine herpesvirus infection although acyclovir was used to treat horses in recent outbreaks of myeloencephalopathy. The drug is effective against EHV-1 in vitro and pharmacokinetic studies suggest that administration of 10 mg/kg orally every 4–6 hours (five times daily) or 10 mg/kg IV every 8 hours results in acceptable concentrations of drug in the blood.64 However, other studies have not demonstrated adequate concentrations of the drug in blood after oral administration of 20 mg/kg.77 The efficacy of this drug in treatment of EHV-1 myeloencephalopathy has not been determined.
Antibiotics are often administered to horses with respiratory tract disease to prevent or treat any secondary bacterial infection. There is, however, no evidence that antibiotic treatment shortens the duration of the disease or prevents complications.
Horses with EHV-1 myeloencephalopathy require intense supportive care.65 Administration of corticosteroids to these horses is controversial,19,65 but many clinicians administer dexamethasone sodium phosphate (0.05–0.25 mg/kg IM every 12–24 h) or prednisolone (1–2 mg/kg orally or parenterally every 24 h) for 3–5 days. Administration of corticosteroids may be contraindicated because of the presence of replicating virus in affected horses. Nursing care to prevent urine scalding, pressure sores and pneumonia is important in horses with myeloencephalopathy. Recumbent or severely ataxic horses should be supported to stand if at all possible. While a rope tied to the tail and slung over an overhead beam may be used to assist the horse to stand, a sling may be necessary to support more severely affected horses.
Neonatal foals with septicemia should be treated aggressively with antibiotics and supportive care, including enteral or parenteral nutrition and fluid administration (see ‘Principles of providing care to the critically ill neonate’). Treatment with acyclovir has been reported.47 Failure of transfer of passive immunity should be rectified with oral or intravenous administration of colostrum or plasma, respectively.
Recommendations for programs to prevent introduction of infection and to control disease outbreaks are available from several sources.12,66
The principles are:12
• enhanced immunity, currently attempted by vaccination
• subdivision and maintenance of the farm population in groups of horses to minimize spread of the infection
• minimize risk of introduction of infection by new horses
• minimize risk of reactivation of latent infection in resident horses
• develop plans for implementation of these routine control measures, and for actions in the event of an abortion
• educate management and staff as to the importance of strict adherence to these procedures.
The relative importance of each of the above measures has not been determined, but implementation of control measures, including allocation of mares to small bands based on anticipated foaling date, quarantine of new introductions, and vaccination of pregnant mares has reduced the incidence of EHV-1 abortion in central Kentucky. The most striking association has been an apparent reduction in the incidence of abortion storms. It must be emphasized that vaccination does not replace any of the other management procedures in control of this disease and that abortions have occurred among vaccinated mares on farms on which the other management procedures have been ignored.
Vaccination against respiratory disease and abortion associated with EHV-1 is widely practised despite lack of clear cut evidence that vaccination reduces the incidence or severity of either of these diseases. Information regarding field efficacy of equine herpesvius vaccines is lacking, and that derived from experimental challenge models is often contradictory or incomplete.67 Give these caveats, the following recommendations are made based on generally accepted practices.
None of the currently available vaccines consistently prevent infection of vaccinated horses or provide complete protection against disease associated with EHV-1. The principal objective of vaccination has been to protect mares against abortion associated with EHV-1, although vaccines intended to prevent rhinopneumonitis and containing both EHV-1 and EHV-4 are available. Additionally, vaccination of mares is intended to reduce transmission of EHV-1 to foals in an attempt to interrupt the cyclical nature of infection on stud farms.9,68 Vaccines consisting of a modified live EHV-1, inactivated EHV-1, or a mixture of inactivated EHV-1 and EHV-4 are available for intramuscular or intranasal administration to horses.69 Both inactivated and modified live EHV-1 vaccines elicit virus-neutralization and complement fixation antibody responses in horses,1 although high antibody titers are not necessarily related to resistance to infection.69
Resistance to infection might be more closely related to cytotoxic T-cell responses.33 Widespread use of a combined EHV-1 and EHV-4 killed virus vaccine in Australia has not reduced serologic evidence of infection in foals on farms where mares are vaccinated,68 although the vaccine was effective in preventing disease induced by experimental infection. Complicating assessment of vaccine efficacy is the variable response to vaccination by some mares and foals, with certain animals having minimal responses to vaccination which in other horses elicits a strong immune response.70 Efforts are underway to develop modified live vaccines that can be administered intranasally.67 Intranasal administration of one such EHV-1 vaccine induced protection against experimentally induced EHV-1 (and EHV-4) respiratory disease and abortion in mares, and prevented infection of foals even when administered in the presence of maternally-derived antibodies.71-73 An alternative approach is the development of subunit vaccines using the envelope glycoprotein D which has been shown to elicit protective immunity in laboratory animal models of EHV-1 disease and administration of which induces virus neutralizing antibody and glycoprotein D-specific ELISA antibodies in horses.74 However, at the time of writing these products are not commercially available nor has their efficacy in field situations been demonstrated.
Despite the incomplete protection afforded by vaccines, vaccination against EHV-1 is an important part of most equine herd health programs in the vaccination of pregnant and non-pregnant mares, foals, and adult horses. The intent of vaccination of mares is to prevent abortion associated with EHV-1. One inactivated virus vaccine is reported to decrease the incidence of abortion by 65%, although others have not been able to replicate this success and there are reports of abortion storms on farms of well vaccinated mares1,18,75 An inactivated virus vaccine containing EHV-1 and EHV-4 prevented abortion in five of six mares exposed experimentally to EHV-1, whereas all six non-vaccinated mares aborted.46 Mares are vaccinated with the inactivated vaccine during the 5th, 7th, and 9th months of gestation. Additional vaccinations at breeding and 1 month before foaling are recommended by some authorities.
Foals are an important source of infection (see ‘Infection cycling’ above) and control of infection in foals is considered critical to control of infection on a farm.9 Consequently, attention has been paid to the responses of foals to vaccination at various ages, given the risk of passive immunity interfering with vaccination and the early age at which foals are infected by EHV-1.9 Current recommendations vary with some authorities recommending vaccination of foals after 5 months of age, to avoid the interfering effect of passive immunity on response to vaccination.76 However, vaccination of foals at this age likely misses the period of time when foals are first infected by EHV-1 from their dam or other mares in the band.9 One recommendation is that foals be vaccinated in their 3rd month, with revaccination 1 month and 6 months later. Modified live virus vaccine is given to foals at 3–4 months of age, non-pregnant mares and other horses as two doses administered 3 months apart, followed by revaccination every 9 months. Because of the short duration of immunity following vaccination, frequent vaccination, perhaps at intervals as short as 3 months, of horses at high risk is recommended. However, the efficacy of such a program is uncertain.
The efficacy of vaccination in preventing myeloencephalopathy appears to be minimal and there is concern that well vaccinated horses might be at increased risk of the disease, although this has not been conclusively demonstrated and there is evidence to the contrary.16
Maintenance of small groups of horses of similar age and reproductive status is recommended to minimize the chances of spread of infection.12 Pregnant mares, after weaning of foals, should be maintained in a herd that does not have access to foals, weanlings, nonpregnant mares or other equids (donkeys). Similarly, weaned foals should be separated from horses of other ages in recognition of the high rate of infection and viral shedding in weanlings.9 Failure to adhere to these procedures can result in rapid spread of infection and abortions among at risk mares.18 Pregnant mares should be combined into small groups (∼10) early in pregnancy based on their anticipated foaling dates.12,66 Multiparous mares should not be mixed with mares that are pregnant for the first time.
Management practices should be introduced that minimize the opportunities for viral spread. Ideally, pregnant mares are handled using facilities separate to those used to handle mares with foals or weanlings. If common facilities must be used, pregnant mares should be handled first, after thorough cleaning of the facility, followed by mares with foals and finally weanlings and other horses.
The only sources of virus are recrudescence of latent infection and introduction by newly arrived horses shedding virus. All horses must be considered as potentially shedding EHV-1 on arrival at a farm and should be isolated from resident horses. Introduction of new horses to the small groups of pregnant mares should be avoided if at all possible, or if absolutely necessary preceded by a 21 day isolation period.23,66 If at all possible, avoid mingling resident and non-resident mares even after quarantine of non-resident animals.
The factors inciting reactivation of latent infection and viral shedding are unknown. However, stressful events, such as transportation or other disease, have the potential to cause reactivation of latent infection. For this reason pregnant mares should not be shipped within 8 weeks of expected foaling and all efforts, including vaccination, should be made to prevent other infectious diseases.66
The principles underlying control of abortions due to EHV-1 include:12,66
Every abortion in a late term mare should be considered to be associated with EHV-1 until proven other wise. Therefore, rapid and early diagnosis of the abortion in important to instituting control measures. Means of diagnosing EHV-1 abortion are detailed above. In regions with large numbers of breeding mares, all abortions in mares should be investigated by detailed post mortem examination of the fetus and serologic examination of the mare.
Diligent and concerted efforts must be made to prevent dissemination of infection from the initial focus. Infected fetal tissues and fluids, and contaminated materials such as bedding, should be placed in impervious containers and either transported to a laboratory for examination or destroyed by incineration. Samples for laboratory examination should be handled in a manner to prevent spread of infection. Facilities and equipment that might have been contaminated should be disinfected by thorough cleaning followed by application of a phenolic or iodophor disinfectant.23
The mare should be isolated until results of laboratory examination are negative for EHV-1 or until the second estrus, at which time it is unlikely that there is shedding of virus from the reproductive tract.66 Other mares in the same band as the mare that aborted should be considered exposed and at risk of abortion. These mares should be held in strict isolation until the results of laboratory examination are negative for EHV-1, or until they foal or abort. Other recommendations for horse movement include:
• When an abortion occurs on the stud, no mares should be allowed to enter or leave it until the possibility of EHV-1 infection is excluded. However, maiden and barren mares, i.e. mares that have foaled normally at home but that are not in foal, coming from home studs where no signs of the disease are occurring, may be admitted because they are considered to be not infected
• If EHV-1 infection is identified on the stud, all pregnant mares due to foal that season (i.e. late pregnant mares) should remain at the stud until they have foaled. The incubation period for EHV-1 abortion ranges between 9 and 121 days
• All non-pregnant animals and mares that have foaled should remain at the stud for 30 days after the last abortion.
The main problem that arises in this program is in deciding what to do with mares that come into contact with the respiratory disease but not the abortion disease. This may occur very early in pregnancy and prolonged isolation would be onerous. The decision usually depends on the owner’s risk aversion and the availability of facilities to maintain long-term isolation.
Outbreaks of EHV-1 induced neurologic disease often occur in riding schools and similar situations where there is constant movement of horses on and off the property. As such it is exceedingly difficult to institute control measures that prevent introduction of the disease and that are compatible with the use of the horses. Having said that, the principles outlined above for preventing introduction of infection on to breeding farms also apply for prevention of myeloencephalopathy at riding stables.
Outbreaks of neurological disease attributable to EHV-1 should be handled as follows:12
• Affected horses should be isolated as there is strong evidence that these horses are infectious22
• The diagnosis should be confirmed by virus isolation, PCR, or histological examination of tissues from affected horses that die or are euthanized
• There should be no movement of horses on or off the premises for at least 21 days after the last case has occurred12
• Movement among bands of horses on the farm should be avoided
• Bands of horses should be monitored for clinical, serological or virological evidence of infection
• Animals should leave or move between bands only when there is no evidence of continued active infection in their group
• Vaccination in the face of an outbreak of EHV-1 myeloencephalopathy is not recommended
• Prophylactic use of acyclovir has been reported although the efficacy of this practice is unknown.64
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3 von Einem J, et al. J Virol. 2004;78:3003.
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5 Bildfell R, et al. J Zoo Wildl Manage. 1996;27:409.
6 House JA, et al. J Vet Diagn Invest. 1991;3:137.
7 Blunden AS, et al. J Comp Pathol. 1998;119:485.
8 Crabb BS, Studdert MJ. Adv Virus Res. 1995;45:153.
9 Gilkerson JR, et al. Vet Microbiol. 1999;68:27.
10 Gilkerson JR, et al. Aust Equine Vet. 1999;17:76.
11 Gilkerson JR, et al. Aust Vet J. 2003;81:283.
12 Dunowska M, et al. New Zealand Vet J. 2002;50:132.
13 Matsumura T, et al. J Vet Med Sci. 1992;54:208.
14 Morley PS, et al. J Am Vet Med Assoc. 2000;216:535.
15 Newton JR, et al. Prev Vet Med. 2003;60:107.
16 Studdert MJ, et al. Vet Rec. 2003;153:417.
17 Hong CB, et al. J Vet Diag Invest. 1993;5:560.
18 Barrandeguy ME, et al. Equine Vet Educ. 2002;14:132.
19 McCartan CG, et al. Vet Rec. 1995;136:7.
20 van Maanen C, et al. Equine Vet J. 2001;33:191.
21 Friday PA, et al. J Vet Int Med. 2000;14:197.
22 Reed SM, Toribio RE. Vet Clin Nth Am Equine Pract. 2004;20:631.
23 Allen GP. Equine Vet Educ. 2002;14:136.
24 Chesters PM, et al. J Virol. 1997;71:3437.
25 Rizvi SM, et al. J Gen Virol. 1997;78:1115.
26 Borchers K, et al. J Gen Virol. 1997;78:1109.
27 Foote CE, et al. Equine Vet J. 2004;36:341.
28 Gilkerson JR, et al. Aust Vet J. 1998;76:677.
29 Gilkerson JR, et al. Aust Equine Vet. 1997;15:128.
30 Gilkerson JR, et al. Aust Vet J. 2000;78:277.
31 Minke JM, et al. Vet Res. 2004;35:425.
32 O’Neill T, et al. Vet Immunol Immunopathol. 1999;70:43.
33 Soboll G, et al. J Gen Virol. 2003;84:2625.
34 Kydd JH, et al. Equine Vet J. 1994;26:466.
35 Breathnach CC, et al. Vet Immunol Immunopathol. 2005;103:207.
36 van der Meulen KM, et al. J Gen Virol. 2003;84:93.
37 Ambagala APN, et al. J Gen Virol. 2004;85:349.
38 Rappocciolo G, et al. J Gen Virol. 2003;84:293.
39 Hannant D, et al. Viral Immunol. 1999;12:313.
40 Smith DJ, et al. Equine Vet J. 2001;33:138.
41 Smith KC. Vet J. 1997;153:253.
42 Smith KC, et al. Equine Vet J. 1992;24:256.
43 Smith KE, et al. Equine Vet J. 2004;36:79.
44 Gibson JS, et al. Arch Virol. 1992;123:351.
45 van Maanen C, et al. Vet Quart. 2000;22:83.
46 Heldens JGM, et al. Vaccine. 2001;19:4307.
47 Murray MJ, et al. J Vet Int Med. 1998;12:36.
48 McCulloch J, et al. Vet Microbiol. 1993;37:147.
49 Donaldson MT, Sweeney CR. J Am Vet Med Assoc. 1998;213:671.
50 van Maanen C, et al. Vet Microbiol. 2000;71:37.
51 http://www.oie.int/eng/normes/mmanual/A_00085.htm. Accessed February 15th 2005.
52 Crabb BS, et al. Arch Virol. 1995;140:245.
53 Hartley CA, et al. Am J Vet Res. 2005;66:921.
54 Varrasso A, et al. Aust Vet J. 2001;79:563.
55 Szeredi L, et al. J Comp Path. 2003;129:147.
56 Gerst S, et al. 2003;35:430.
57 Sharma PC, et al. Equine Vet J. 1992;24:20.
58 Whitwell KE, Blunden AS. Equine Vet J. 1992;24:13.
59 Slater JD, et al. Vet Rec. 1992;131:237.
60 Carman S, et al. J Vet Diag Invest. 1993;5:261.
61 Del Piero F, et al. Vet Pathol. 2000;37:672.
62 Gunn HM. Irish Vet J. 1991;44:37.
63 Szeredi L, et al. Acta Vet Hung. 2003;51:153.
64 Wilkins PA. Proceedings Am Coll Vet Int Med. 2004;22:170.
65 Wilson WD. Vet Clin North Am Equine Pract. 1997;13:53.
66 Anonymous. Aust Equine Vet. 2002:20.
67 Patel JR, Heldens J. Vet J. 2005;170:14.
68 Foote CE, et al. Aust Vet J. 2003;81:283.
69 Minke JA, et al. Vet Res. 2004;35:425.
70 Foote CE, et al. Vet Microbiol. 2002;88:13.
71 Patel JR, et al. Equine Vet J. 2004;36:447.
72 Patel JR, et al. Vet Microbiol. 2003;92:1.
73 Patel JR, et al. Vet Microbiol. 2003;91:23.
74 Foote CE, et al. Vet Immunol Immunopathol. 2005;105:47.
75 Burki F, et al. Vet Q. 1990;12:80.
76 Wilson W, Rossdale PD. Proc 8th Int Conf Equine Infect Dis 1999; p. 428.
EHV-2 is a slow-growing gammaherpesviruses.1 The viral genome has been elucidated.2 There is considerable antigenic, genetic and biologic heterogeneity among EHV-2 isolates which might reflect variations in pathogenicity.3
Almost all adult horses and foals over 2 months of age have serologic evidence of infection by EHV-2.3-6 EHV-2 was cultured from bronchial or other lymph nodes in 97% of horses sampled at an abattoir and from blood of 31–76% of live adult horses (clinically normal or with respiratory disease) in Europe and New Zealand.3,7,8 Virus was isolated from blood of 76 of 77 normal foals examined in the United States6 and in all of 16 foals from a stud with endemic respiratory disease in New Zealand.5 Viral genome was detected in blood of ∼70% of live, adult horses in Europe and in trigeminal ganglion of 6 of 12 seropositive ponies.8,9 Thirty-four virus isolates were obtained from peripheral blood leukocytes of 139 horses in Poland.10 Viral DNA was detected in blood samples of all foals examined in Europe.11 These data demonstrate the ubiquitous nature but uncertain clinical significance of EHV-2 infection of horses worldwide.
The frequency with which disease attributable to EHV-2 occurs in horses is uncertain. In one report, 10 of 16 foals naturally infected with EHV-2 developed respiratory disease, and two died.5 EHV-2 is isolated from tracheal aspirates of foals with respiratory disease much more often than it is isolated from tracheal aspirates of foals without respiratory disease.6 Isolation of EHV-2 is statistically associated with upper respiratory disease in horses,8 and experimental infection causes conjunctivitis, lymphadenopathy, and coughing.12 The virus is isolated more frequently from eyes of horses with keratoconjunctivitis than from eyes of normal horses, suggesting a role for EHV-2 in this disease.13 The observation that EHV-2 can be isolated more frequently from the blood of horses with upper respiratory disease, ataxia or abortion might indicate a causative role for the virus, or that intercurrent disease induces reactivation of EHV-2 infection.3
EHV-2 is highly infectious, and transmission probably occurs by the inhalation of infected droplets or by the ingestion of material contaminated by nasal discharges. Virus is secreted in nasal and ocular discharge of recently infected horses, and in nasal secretions and tracheal fluid of foals with respiratory disease associated with natural infection.5,6,12 If survival ex vivo is similar to that of the other equine herpesviruses, mediate infection may occur for 14–45 days, although this has not been specifically investigated for EHV-2.
Infections always arise from other horses, both by direct contact and by fomites. Horses and foals are infectious during the active stage of disease and, because horses become latently infected, during subsequent periods of viral reactivation and shedding. The duration of latency is unknown but is assumed to be lifelong.1 Latent EHV-2 virus is detectable in the trigeminal ganglion, lymph nodes, and peripheral blood monocytes of clinically normal horses.3,7-9 Reactivation after experimental infection can be triggered by administration of dexamethasone.12 Factors influencing reactivation of the virus and the importance of reactivated infections in dissemination of infection and induction of disease is unknown, but is likely important as for other equid herpesviruses.
It appears that foals are infected at a young age and that infection is persistent into adulthood.5,6,11 EHV-2 can be isolated by 25 days of age from foals that were seronegative at birth and there is a progressive increase in virus neutralization from 1 to 5 months of age.6 Viral shedding is persistent until development of high antibody titers at 6–9 months of age.5
The pathogenesis of EHV-2 has not been defined. However, it is apparent that infection occurs at a young age, as early as several weeks of age, and becomes latent in lymph nodes, especially those draining the respiratory tract, peripheral blood mononuclear cells, and nervous tissue including the trigeminal ganglion. As with EHV-1, there is evidence that EHV-2 causes immunosuppression14 and it is speculated that this might play a role the development of other infections, including pneumonia associated with Rhodococcus equi or other bacteria.5
EHV-2 or equine cytomegalovirus causes a long-term infection in foals, some of which develop clinical signs of purulent nasal discharge, fever and lymphadenopathy in a syndrome which lasts about 1 week. Affected animals rarely die although deaths of foals from pneumonia have been recorded.5,6,8 EHV-2 was isolated from 20 of 30 foals with signs of lower respiratory disease including fever, abnormal lung sounds and radiographic abnormalities.6 Bacteria were isolated from tracheal aspirates of 29 of these foals.
EHV-2 is statistically associated with keratoconjunctivitis in horses of varying ages.13
Results of hematological and serum biochemical examinations are neither specific nor diagnostic. A blocking ELISA has been developed that detects serum antibody specific for EHV-2, and is useful to detecting new infection in foals.15 Identification of the virus in nasal swabs, blood buffy coat or fetal tissue by culture or a PCR test provides confirmation of infection.16
Necropsy findings for diseases associated with EHV-2 infection are poorly reported. Two foals that died of respiratory disease associated with EHV-2 infection had pneumonia, chronic pharyngitis and lymphoid depletion in the thymus and spleen.5
Differential diagnosis includes other causes of respiratory diseases in horses (Table 16.4).
1 Crabb BS, Studdert MJ. In: Studdert MJ (ed) Virus infections of vertebrates: virus infections of Equines 6, 1996; p. 11.
2 Telford EA, et al. J Mol Biol. 1995;249:520.
3 Borchers K, et al. Arch Virol. 1997;142:917.
4 Browning GF, Studdert MJ. Vet Bull. 1988;58:775.
5 Edington N, et al. Equine Vet J. 1994;26:140.
6 Fu ZF, et al. NZ Vet J. 1986;34:152.
7 Murray MJ, et al. Equine Vet J. 1996;28:432.
8 Nordengrahn A, et al. Vet Res. 2002;33:251.
9 Nordengrahn A, et al. J Vet Diag Invest. 2001;13:389.
10 Dunowska M, et al. New Zealand Vet J. 2002;50:132.
11 Borchers K, et al. Virus Res. 1998;55:101.
12 Kershaw O, et al. Virus Res. 2001;80:93.
13 Rizvi SM, et al. J Gen Virol. 1997;78:1115.
14 Dunowska M, et al. Res Vet Sci. 2001;71:111.
Equine coital exanthema is a venereal disease associated with infection by equine herpesvirus 3 and manifested by papular, then pustular, and finally ulcerative lesions of the vaginal mucosa, which is generally reddened. The ulcers may be as large as 2 cm in diameter and 0.5 cm deep and are surrounded by a zone of hyperemia. In severe cases the lesions extend onto the vulva and the perineal skin to surround the anus. In the male, similar lesions are found on the penis and prepuce. Many mild cases are unobserved because there is no systemic disease and affected horses eat well and behave normally. The effect on fertility is equivocal although there may be a loss of libido during the active stage of the disease in stallions.1 Transmission is usually venereal from affected or clinically normal carrier animals in which the infection is thought to be latent in sciatic ganglion.1 The incubation period is 2–10 days and the course up to complete healing of ulcers is about 14 days. Diagnosis can be achieved by use of virus isolation or demonstration of viral DNA in skin lesions.2 Secondary bacterial infection may lead to suppurative discharge and a longer course. In some outbreaks lesions occur on the skin of the lips, around the nostrils, and on the conjunctiva. They may also be present on the muzzle of the foal. Ulcerative lesions of the pharyngeal mucosa also occur in infections with EHV-2 and with equine adenovirus. Ulcerative lesions of the oral mucosa are of great importance because of the necessity to diagnose vesicular stomatitis early. Control can be achieved by use of artificial insemination.
Etiology Equine arteritis virus
Epidemiology Outbreaks of disease due to lateral transmission by infected body fluids. Venereal transmission by persistently infected, clinically normal, stallions with subsequent lateral spread among mares
Clinical signs Abortion. Upper respiratory disease with systemic signs including edema and respiratory distress
Clinical pathology Serology. No characteristic changes in hemogram or serum biochemistry
Diagnostic confirmation Virus isolation from blood, sperm-rich fraction of semen, nasopharyngeal swabs or tissue. Seroconversion or increase in complement fixation titer
• The systemic disease – viral respiratory disease
Treatment There is no specific treatment
Control Vaccination, especially of stallions and seronegative mares to be inseminated by seropositive stallions, and to control outbreaks at racetracks. Quarantine. Hygiene
Viral arteritis of horses, donkeys and mules is associated with an arterivirus, formerly classified as a non-arthropod borne togavirus but now recognized as a member of the coronavirus-like superfamily.1 The virus is single-stranded RNA. There is considerable genomic variation among isolates with EAV of North American and European origin clustering in geographically distinct viral clades.2,3 Although different isolates of EAV vary in virulence, consistent variations in virulence among clades have not been demonstrated.2 Novel phenotypic variants of EAV can emerge during persistent infections in stallions and this might be an important feature in the development of disease in exposed mares and foals.3-5 A strain recently isolated from donkeys and mules in South Africa causes only mild disease in horses.6 The virus resists freezing but not heat.
Serologic evidence of infection by equine arteritis virus (EAV) is found in horse populations in North and South America, Europe, Africa, Asia, and Australia.7 Recent disease outbreaks have been identified in North America, Britain, Spain, Italy, France, Poland, The Netherlands, South Africa, and Germany.7 It is probable that the disease is now present in most countries with substantial populations of horses. International shipment of horses and frozen semen contributes to the spread of the EAV.8
The proportion of seropositive horses varies considerably among populations, with there being marked differences among breeds. Overall, 2% of horses in the United States are seropositive to EAV (serum neutralization titer >1:4) with 8.4% of horse operations having seropositive horses.9 Twenty-five percent of operations whose principal activity was breeding had at least one unvaccinated seropositive horse, whereas 4% of racing operations had a least one unvaccinated seropositive horse.9 The prevalence of titers to EAV is higher in mares and in horses used for breeding.9 The frequency with which horses in the United States have serum titers >1:4 varies with breed with 24% of Standardbreds, 4.5% of Thoroughbreds, 3.6 % of Warmbloods and 0.6 % of Quarter horses being seropositive.9 Approximately 19% of Warmblood horses imported into the United States have antibodies to EAV, with horses from Germany and the Netherlands having the highest prevalence (21 and 25% respectively).10 Between 55–93% of Warmblood and Lipizzan breeds in Austria have serologic evidence of exposure to EAV.11 Disease in Great Britain and North America has been associated with importation of infected stallions or semen.8,10 Horses of all age groups are susceptible. The disease spreads rapidly in a group of susceptible horses, and although the course of clinical disease is short, an outbreak in a group of horses may persist for a number of weeks. Naturally acquired infections in newborn foals can occur as an outbreak and cause severe disease.12
1 Horizontal transmission of virus by predominantly nasal fluid, but also by urine, feces, lacrimal fluid and vaginal discharge of infected horses
2 Venereal transmission from stallions to susceptible (seronegative) mares.
Horizontal transmission through infected nasal discharge and body fluid is effective and is the means of disease spread in outbreaks in racing stables, and among mares and foals at breeding farms. Virus is found in the respiratory secretion for 7–14 days and in other tissues for 28 days.11 Close contact between horses is probably required for transmission of the virus – it has been reported to spread after contact of horses across a fence.8 Duration of viability of the virus in the environment has not been reported, but the potential for spread of infection on fomites includes clothing and tack should be considered when dealing with an outbreak.
Venereal transmission: Stallions are infected by horizontal transmission of the virus, subsequently excrete the virus in semen and infect susceptible mares at the time of mating. Clinically normal stallions are also capable of transmitting the virus horizontally to other stallions in a breeding operation, demonstrating the potential for horizontal spread of infection from stallions in the absence of clinical disease or sexual contact.13 Between 30 and 60% of infected stallions excrete the virus in semen for weeks to months.14 Some stallions excrete virus for years, and lifelong infection and virus excretion can occur.11 Prolonged infection of stallions is associated with mutation of the virus and secretion by the stallion of viral strains that vary over time. However, disease resulting from transmission of infection from a stallion to a mare, and subsequent spread of infection to other horses, is associated with a single viral strain.5 In other words, stallions can excrete a variety of strains of the virus during their life, but outbreaks of disease are associated with a single viral strain; multiple viral strains are not detected during outbreaks of disease.5
Prolonged excretion of the virus in semen is likely important in the maintenance of the virus in populations of horses. Introduction of a persistently infected stallion into a naive population or insemination of seronegative mares with semen from an infected stallion have been implicated as the cause of outbreaks of viral arteritis.8 The carrier stallion infects mares at mating, the mares develop disease and shed virus in nasal and other body fluids and infect in-contact susceptible horses and foals by horizontal transmission.
Vaccination or recovery from natural infection results in the development of a strong serum antibody virus neutralizing response which is believed to be important in clearance of the virus and resistance to infection.15 Naive, pregnant mares infected by horizontal transmission may abort or, less commonly, give birth to infected foals that subsequently die.
Foals of immune mares are resistant to infection, and viral neutralization antibodies are present in mare’s colostrum and foal’s serum after sucking, with persistence of the antibodies to the age of 2–6 months in the foals.16 Persistence of passive immunity in foals has important implications for resistance to infection and for timing of administration of modified live vaccines.
The chief impact of the disease on breeding farms is the loss of foals through abortion and the cost of quarantine and control measures. The systemic illness may be severe, but the mortality rate is low. During outbreaks at race tracks the economic impact is a result of lost opportunities for training and racing sick or convalescing horses, and the effect of quarantine and control measures. Additional costs are incurred by the inconvenience and cost of vaccinating mares to be bred to stallions infected with the virus and import regulations controlling movement of horses and semen including the inability to export mares, fillies and non-carrier stallions that are seropositive (perhaps as a result of vaccination), and the limited opportunities for export of semen from infected stallions or export of the stallions themselves.
The pathogenesis of disease associated with horizontal transmission of EAV has been elucidated.17 After inhalation of the virus it binds to the respiratory epithelium and infects alveolar macrophages and is detectable in bronchial lymph nodes by 48 hours after infection. Three days after infection the virus is detectable in circulating monocytes with subsequent systemic distribution of infection. The virus localized in vascular endothelium and medial myocytes by days 6–9 and there is significant damage to blood vessels by day 10. The virus infects renal tubular epithelium and can persist there for up to 2 weeks. Medial necrosis of blood vessels might cause anoxia of associated tissues. Virus is not detectable in any tissue by 28 days after infection, with the exception of accessory sex glands in intact male horses.
Abortion is caused by a severe necrotizing myometritis and presumed consequent reduction in fetal blood flow. The are usually no lesions in the fetus, although the fetus is sometimes infected with the virus.
Infection by EAV is usually clinically inapparent, especially after venereal infection of mares. Abortion is not necessarily associated with clinical disease in the mare. Systemic disease is usually mild to moderate and self-limiting with recovery in 5–9 days in the vast majority of horses.
Systemic disease is characterized by an incubation period of 1–6 days followed by the appearance of fever (39–41°C, 102–106°F). A serous nasal discharge which may become purulent and be accompanied in some horses by congestion and petechiation of the nasal mucosa, urticaria, conjunctivitis, excessive lacrimation developing to purulent discharge, keratitis, palpebral edema, and blepharospasm. Opacity of the aqueous humor and petechiation of the conjunctiva may also occur. Signs of pulmonary disease, such as respiratory distress and coughing are attributable to pulmonary edema and congestion, but are uncommon. The appetite is reduced or absent and, in severe cases, there may be abdominal pain, diarrhea and jaundice. Edema of the limbs is common and more marked in stabled horses than those at pasture. In stallions, edema of the ventral abdominal wall may extend to involve the prepuce and scrotum. Depression is usual and varies in degree with the severity of the syndrome. The disease is acute and severe, and deaths may occur without secondary bacterial invasion. In these cases dehydration, muscle weakness and prostration develop quickly. It must be emphasized that the disease may be much milder than that described above.
Clinical disease in neonatal foals is characterized by fever, profound depression, weakness, limb and facial edema, and respiratory distress. Severely affected foals usually die. Foals can be affected at birth, or be born apparently normal and develop disease 1–19 days after birth.12
Abortion occurs within a few days of the onset of clinical illness, although it is not usually associated with clinically apparent disease. Abortions may occur in 10–60% of at-risk mares during an outbreak and during the 3rd–10th months of gestation.11 Abortion occurs 12–30 days after exposure. The abortion is not foreshadowed by premonitory signs, and the placenta is not retained.
Hematological examination of adults and foals during the acute phase of the systemic disease is characterized by leukopenia and thrombocytopenia.12
Serological confirmation of infection is achieved using complement fixation, serum neutralization and ELISA tests.7 Seroconversion occurs within 1 week of infection, and demonstration of a rising antibody titer, based on acute and convalescent serum samples, or seroconversion is considered evidence of recent infection. False-positive results for the virus neutralization test have occurred using OIE prescribed rabbit kidney (RK-13) indicator cells when testing serum from horses vaccinated with an EHV-1/4 tissue cultured derived vaccine.18 The false-positive results are likely a result of vaccine-induced anticellular antibody response against the RK-13 cells.
Virus isolation from blood, body fluids, fetal or placental tissue is readily achieved during the acute phase of the disease. Appropriate samples for virus isolation include nasopharyngeal or conjunctival swabs and anticoagulated whole blood (heparin, EDTA or citrate are suitable anticoagulants).11 Virus is continuously excreted in the semen of infected stallions and is readily isolated from the sperm-rich fraction of the semen. A nested PCR can detect the presence of virus in naturally infected semen at concentrations as low as 2.5 plaque forming units per mL with a specificity of 97% and a sensitivity of 100%, and may be useful for the rapid diagnosis of EVA shedding stallions.19
Antemortem diagnosis of EAV disease can be achieved by examination of skin samples using monoclonal antibody immunoperoxidase histochemistry. Examination of skin samples obtained by biopsy reveals edema and vasculitis and presence of intracytoplasmic EAV antigen.20
Gross lesions include edema of the eyelids and petechiation of the upper respiratory tract, and the serosae of the abdominal and thoracic viscera. There is an abundant serofibrinous pleural and peritoneal effusion with generalized edema of the lungs, mediastinum, and abdominal mesenteries. A hemorrhagic enterocolitis, as well as hemorrhage and infarction in the spleen may be noted. Characteristic histological changes are found in the small arteries and include fibrinoid necrosis of the tunica media and karyorrhexis of the infiltrating leukocytes. Fluorescent antibody or immunohistochemical staining demonstrates viral antigen within the endothelial cells of these blood vessels. An immunoperoxidase method has also revealed viral antigen within endothelial cells and macrophages of an aborting mare and her fetus and within skin biopsies of animals exhibiting a maculopapular rash.20 Serological tests performed on samples collected at necropsy can also be used to confirm that exposure to the virus has occurred.
The virus can be isolated from the lung and spleen of aborted fetuses, but no consistent, specific lesions are present. Necrotizing arteritis, similar to that in the mare, may be detectable.21
• Virology – chilled lung, spleen and thymus (VI, PCR, FAT)
• Serology – heart-blood serum or fetal thoracic fluid (VN, ELISA, CF)
• Histology – fixed lung, spleen, adrenal, jejunum, colon and heart (LM, IHC).
Definitive diagnosis is based on isolation of EAV from affected cases, or the demonstration of seroconversion or an increase in serum antibody titer.
The systemic disease must be differentiated from that associated with EHV-1 or EHV-4 infection, equine influenza, strangles (see Table 16.4), infection with Getah virus in Japan, equine infectious anemia, African horse sickness, and purpura hemorrhagica.
Abortion should be differentiated from that associated with EHV-1, Salmonella abortusequi, leptospirosis, mare reproductive loss syndrome, and congenital malformations.
Similar disease in neonates can be associated with EHV-1, immaturity or premature birth, and bacterial septicemia.
There is no specific treatment for equine viral arteritis. Most horses recover without specific care. Severely affected foals require intensive care.
Control of EAV infection is based on the strong immunity induced by natural infection or vaccination with a modified live virus, and an understanding of the role of carrier stallions in the disease. The following practices are suggested:9
1. Isolate all new arrivals (and returning horses) to farm or ranch for 3 to 5 weeks
2. If possible, segregate pregnant mares from other horses
3. Blood test all breeding stallions for EAV antibodies
4. Check semen of any unvaccinated, antibody-positive stallions for EAV to identify carriers before breeding
5. Once tested negative for EAV antibodies, vaccinate all breeding stallions annually
6. Physically isolate any EAV carrier stallions
7. Restrict breeding EAV carrier stallions to vaccinated mares or mares which test positive for naturally acquired antibodies to the virus
8. Vaccinate mares against EVA at least 3 weeks prior to breeding to a known carrier stallion
9. Isolate mares vaccinated for the first time against EVA for 3 weeks following breeding to an EAV carrier stallion
10. In breeds or areas with high rates of EAV infection, vaccinate all intact males between 6 to 12 months of age.
Testing of mares and stallions permits identification of serologically negative, and therefore at-risk, animals. Seronegative mares should not be mated with infected stallions nor inseminated with fresh or frozen semen from infected stallions because of the risk of transmission of infection to the mare.8 Seropositive mares, or mares that have been vaccinated at least 3 weeks, can safely be bred to stallions that have serological evidence of infection. Seropositive mares should be separated from seronegative mares for at least 3 weeks after mating to a seropositive stallion. Seropositive stallions that have not been vaccinated should have their semen cultured to determine if they are excreting the virus. Stallions excreting virus in their semen should be kept isolated from susceptible horses but can be bred to seropositive mares, as described above. Because the virus survives cooling and freezing, similar principles should be applied to the use of artificial insemination in horses. One control program requires that all stallions be vaccinated with a modified live virus vaccine 28 days before the beginning of each breeding season.11
Vaccination with a modified live virus vaccine induces strong immunity, although revaccination is necessary to insure continuing immunity.7,11 The vaccine protects mares exposed to stallions shedding the virus in semen and has been used to control outbreaks of the respiratory form of the disease at racetracks.22 The modified live virus vaccine is regarded as safe,11 although there is mild fever and leukopenia, and evidence that the vaccine virus replicates in the vaccinates.23 A killed virus vaccine has limited availability and its efficacy in the field has not been reported.21 Antibodies induced by the vaccine cannot be differentiated from those resulting from natural infection, a situation that may be problematic when import restrictions require the horse to be seronegative, presumably as proof of lack of exposure to virulent EAV.
Vaccination of foals from immune mares results in good protection provided that the timing of vaccination is delayed until maternal antibodies to EAV are no longer present in the foal.
1 Boon JA, et al. J Virol. 1991;65:2910.
2 Larsen LE, et al. Vet Microbiol. 2001;80:339.
3 Balasuriya EBR, et al. J Gen Virol. 2004;85:379.
4 Hedges JF, et al. J Virol. 1999;73:3672.
5 Balasuriya UBR, et al. J Gen Virol. 1999;80:1949.
6 Paweska JT, et al. Onderstepoort Vet J. 1996;63:189.
7 De Vries AAF, et al. In: Studdert MJ (ed) Viral infections of vertebrates: viral infections of Equines 6, 1996; p. 171.
8 Balasuriya UBR, et al. J Am Vet Med Assoc. 1998;213:1586.
9 NAHMS. Equine Viral Arteritis (EVA) and the US Horse Industry. USDA:APHIS:VS, CEAH. National Animal Health Monitoring System. Fort Collins, CO 2000; #N3150400.
10 Hullinger PJ, et al. J Am Vet Med Assoc. 2001;219:946.
11 Timoney PJ, McCollum WH. Vet Clin North Am Equine Pract. 1993;9:295.
12 Del Piero F, et al. Equine Vet J. 1997;29:178.
13 Guthrie AJ, et al. Equine Vet J. 2003;35:596.
14 Timoney PJ, et al. J Reprod Fertil (Suppl). 1987;35:95.
15 Castillo-Olivares J, et al. J Gen Virol. 2003;84:2745.
16 Hullinger PJ, et al. J Am Vet Med Assoc. 1998;213:839.
17 Del Piero F. Vet Pathol. 2000;37:287.
18 Newton JR, et al. Vaccine. 2004;22:4117.
19 Gilbert SA, et al. J Clin Microbiol. 1997;35:2181.
20 Del Piero F. Vet Pathol. 2000;37:486.
21 Glaser AL, et al. Vet Q. 1996;18:95.
Etiology Influenza virus H3N8 (previously A/equine 2) of two lineages and numerous, evolving, strains. Historically, H7N7 virus
Epidemiology Short incubation period and highly contagious nature of the virus result in explosive outbreaks of disease. Viral shedding by subclinically affected horses is important for introduction of infection to populations. Prolonged carrier state is not recognized
Clinical signs Upper respiratory disease complicated by pneumonia. Abortion is not a feature of the disease
Clinical pathology None characteristic
Lesions Rhinitis, pneumonitis. Rarely causes death
Diagnostic confirmation Demonstration of virus in nasopharyngeal swab either by culture, ELISA, PCR, or membrane bound immunoassay
Treatment Supportive care. There is no specific treatment
Control Vaccination, often every 4 months, quarantine and hygiene
Equine influenza is associated with infection by equine influenza A/H7N7 (previously referred to as equine influenza A1) or equine influenza A/H3N8 (previously referred to as equine influenza A2) virus, members of the influenzavirus A genus of the family Orthomyxoviridae. Of the two serologically distinct subtypes, all reported outbreaks in the past two decades have been associated with strains of EIV-A/H3N8. There are no reports of disease associated with EIV-A/H7N7 in the past 25 years and reports of seroconversion might be related to use of vaccines containing EIV-A/H7N7 antigen.1 There are no reports of other influenza viruses, such as the H1N1 avian virus, causing disease in horses, although the avian-like influenza A/I/Jilin89 (H3N8) caused severe disease and high mortality among horses in China in 1989.2 Equine influenza H3N8 virus can infect dogs and cause serious disease and death.3
Currently, two major lineages of EIV-H3N8 circulate in horse populations – a Eurasian lineage and an American lineage. The terms Eurasian and American do not denote geographical isolation of these lineages, and viruses of American lineage circulate in Europe and North America, although to date the only virus of Eurasian lineage isolated in North America was the Saskatoon isolate (1/90).4 Within the American lineage there is further division of the virus into three distinct lineages (South American, Florida and Kentucky) with the predominant lineage varying from year to year with evidence of cycling.4,5 Cycling is the predominance of one lineage one year, and the other lineage in the subsequent year, with reversion to the initial lineage in the third year, and so on.5 This is believed to be a mechanism for viral perpetuation, allowing avoidance of eradication of the virus by changing the immunological target from year to year without viral evolution.5
The predominating virus lineage or strain varies from year to year and from region to region. The important point is that there is continual change in the viral strain in some populations of horses and that constant monitoring of viral strains is vital for appropriate composition of vaccines and for molecular epidemiology. For instance, the majority of viruses from Europe (France, Italy and the UK) and North America characterized antigenically and/or genetically between January 2003 and April 2004 were of the American lineage. Based on hemagglutination inhibition (HI) testing most viruses isolated in Europe, represented by A/equine/Newmarket/5/03, were antigenically closely related to the vaccine strains, A/equine/Newmarket/1/93 (American lineage), A/equine/Kentucky/97 and A/equine/Kentucky/98 (American lineage). Viruses recently isolated in South Africa and the USA were distinguishable from these three vaccine strains and were more closely related to A/equine/South Africa/4/03.6 The South African strain was identical to an American lineage strain originally isolated in Wisconsin in 2003 (A/Equine/Wisconsin/1/03).7,8 The HA1 sequences of 2003 American-like viruses fall within a single phylogenetic sub-group referred to as the ‘Florida’ lineage.5 In 2003 and 2004 a few viruses, isolated in Benelux, were of the ‘European’ lineage,6 and were antigenically similar to some viruses circulating in Europe during 2002, represented by A/equine/Lincoln/1/02, and were distinguishable from the prototype vaccine strain A/equine/Newmarket/2/93.6 The European lineage viruses circulating in 2002 were heterogeneous in their antigenic and genetic characteristics. Information about EIV strains changes constantly and is available at the equiflunet website.6
The existence of lineages and strains of virus is important in the epidemiology of the disease because the antigenic differences among strains can be sufficient to prevent cross-protection provided by natural infection or vaccination. Cross-protection refers to the ability of one antigen (virus strain) to produce immunity in the horse against infection with another type of antigen (virus strain). Infection or challenge with the same type of antigen is referred to as homologous challenge, whereas that with a different antigenic type is referred to as heterologous challenge.9 Strains of influenza virus circulate between and among populations of horses, with more than one strain of virus circulating at any one time in some horse populations,10 although individual disease outbreaks are associated with a single viral strain. Many, but not all, of these virus strains are constantly evolving and evolution of the viruses is necessary for perpetuation of cycles of infection through the emergence, or reemergence by cycling, of heterologous strains.5,11 Evolutionary stasis, the continued circulation of older strains of virus, occurs and has importance for vaccine composition.11 However, emergence of new strains is common and of great importance for control of the disease. Evolution of strains of equine H3N8 virus occurs through antigenic drift. Antigenic drift, the accumulation of point mutations in the gene coding for the major surface protein hemagglutinin, occurs continuously in virus circulating in horse populations. Antigenic drift occurs most rapidly in hemagglutinin protein but also occurs in M and NS genes.12 Antigenic drift, by producing heterologous viral strains, contributes to the continuing susceptibility of horses to infection and the reduced efficacy of some vaccines.
Influenza virus is an RNA virus that has eight segments to its genome which encodes ten proteins. The hemagglutinin and neuraminidase proteins are used for antigenic characterization of virus strains. Mutations in these genes or poor fidelity RNA copying results in changes in amino acid composition of viral proteins that may be detected by serological tests (see ‘Clinical Pathology’) and that might have, as discussed above, important consequences for infectivity and pathogenicity of the virus. Antigenic shift is an event in which there is a dramatic alteration in the viral genome occurring by reassortment of viral genes during co-infection of a cell by two different types of virus (for example infection of a pig by both avian and human influenza viruses). Antigenic shift, which has not been documented for influenza viruses infecting horses, has the potential to produce new viruses with markedly different host infectivity and pathogenicity to either parent virus.
Worldwide, the only large horse populations in which influenza virus infection does not occur are in Australia and New Zealand. Widespread use of aircraft to move horses between countries in short periods has increased the spread of equine influenza viruses, as exemplified by the 2003 outbreak of equine influenza in horses in South Africa associated with a virus from North America, and an earlier outbreak in Hong Kong. In both cases virus was introduced by imported horses.8,13,14
Epidemics of equine influenza have occurred in Europe or North America in 1956 (H7N7), 1963 (H3N8), 1969, 1979, and 1989, although this does not represent a comprehensive listing of large scale outbreaks.15,16 Epidemics affecting >1 million horses occurred in China in 1989 (associated with the novel H3N8 Jilin virus) and 1993/1994 (associated with a conventional H3N8 virus closely related to 1991 European isolates).16 Epidemics in Europe and North America have been associated with introduction of a novel virus (for example the 1963 appearance of H3N8 virus in Miami) or antigenic drift of existing viruses and resultant inefficacy of extant vaccines.15
Localized outbreaks of disease in stables or race courses occur almost annually in countries in which the disease is endemic, likely related to the movement of horses into the training and racing populations, with subsequent introduction of virus and development of disease in at risk horses (see ‘Animal Risk Factors’). Disease associated with equine influenza virus usually occurs as outbreaks associated with the introduction of virus into a population of susceptible horses. Virus may be introduced by clinically affected horses or, more commonly, by horses that are not noted to be clinically ill. Vaccinated horses may become infected and shed influenza virus while not becoming ill, especially if vaccinated with heterologous strains, and this is likely a common method of introduction of virus into susceptible populations.
Outbreaks of influenza virus infection may cause clinical disease in nearly all (98%) horses in a susceptible population, although in populations of horses of mixed age and with varying serum titers to equine influenza the morbidity rate may be much lower (16–28%).17 The incidence of disease in one race track population was approximately 130 cases per 1000 horses at risk per month18 although this rate likely varies widely among outbreaks. The mortality rate is usually very low (< 1%) with most deaths associated with secondary bacterial infections. However, a recent outbreak of disease in China associated with a novel H3N8 (Jilin virus) strain was associated with a morbidity rate of 80% and mortality rate of 20–35%.2
The disease in populations of vaccinated or previously exposed horses is associated with a lower morbidity and mortality, and slower spread, due to the milder disease induced by influenza virus infection of immune or partially immune horses. In an outbreak among vaccinated racehorses in Hong Kong, 75% of horses had serological evidence of infection, 37% had clinical signs of infection, and 0.2% died.14 Horses imported from Australia and New Zealand, where the disease does not occur, had a morbidity rate of 52%, while horses from the northern hemisphere had a morbidity rate of 20%, likely reflecting the effect of previous exposure to influenza virus or repeated vaccination.14
The profile of an epidemic can vary from explosive with a large proportion of a small group of susceptible horses housed in close proximity, such as a small band, developing clinical disease within 24–48 hours, to much more prolonged outbreaks lasting several weeks in larger groups of horses of varying susceptibility housed in multiple barns. During larger outbreaks among horses of varying susceptibility there is a characteristic three phase pattern.18 The first stage is associated with the first cases of disease and slow spread over 10–14 days. This stage is followed by one of rapid spread of the disease to horses clustered in stalls around horses affected during the first phase of the outbreak. The third phase is characterized by declining numbers of cases.18
Equine influenza virus is relatively susceptible to environmental conditions, and during an outbreak infection must originate from an infected horse although the proximate source of virus can be contaminated equipment or other fomites. Transmission of equine influenza virus occurs by direct contact, inhalation of aerosols of infected material, and on fomites. Survival of the virus on clothing and surfaces, including vehicles used to transport horses shedding the virus, can result in transmission of infection in the absence of horse to horse contact.8 Fomite transfer on veterinary clothing, equipment or vehicles was likely responsible for the spread of infection from quarantined horses in both South African outbreaks.8,19 However, in most instances, horses are infected by other horses that are in close proximity or have physical contact, for instance exercise ponies (horses or ponies used to accompany race horses from the stable to the track in preparation for racing or training gallops) or stable mates.17 Aerosol spread occurs over distances of 35 meters, possibly further, and is enhanced by the frequent coughing characteristic of the disease. Equine influenza virus in aerosols survives longer (24–36 hours) than human or porcine strains (15 hours).
Clinically affected horses excrete more virus than do horses in which the infection is inapparent. The duration of infectivity of clinically affected horses is 3–8 days and with the short incubation period of 2–3 days combine to produce the potential for a very rapid new infection rate and a characteristic explosive outbreak.
All age groups of horses, including newborn foals, are susceptible. The greatest risk appears to be between the ages of 2 and 6 months,20 serum levels of passively acquired antibodies being lost by foals at 2 months of age.21 A recent survey of >8000 horses in the United States revealed that only 20.2% of horses aged 6 to 17 months had a detectable influenza antibody titer (HI), as compared to 89.0% of horses aged 20 years or more.22 The percentage of horses that had a high equine influenza antibody titer increased as the horse’s age increased such that 45–51% of horses older than 5 years had high titers. This observation is consistent with most cases of the disease occurring in 2-year-old or younger horses, probably because older horses are immune through either natural exposure or vaccination. Thoroughbred race horses ≤2 years were 5–8 times more likely to develop influenza than were horses ≥5 years of age in a well characterized series of outbreaks.17 Seronegativity to a H3N8 virus (Saskatoon/90) was associated with a 13–38 fold increase in likelihood of developing influenza, independent of the effect of age.17 It is probable that outbreaks occur as a result of a natural accumulation of young animals which have not been previously exposed, the co-mingling of these susceptible animals with older infected ones at race and show meetings, and to the significant level of antigenic ‘drift’. This capacity of the virus to change slightly and continuously in antigenic composition leads to the frequent appearance of new strains which are likely to breach existing natural and induced immunological barriers.
Outbreaks can occur at any time of year, and their timing probably depends on husbandry and management practices, such as yearling sales, transport of horses for racing and sale, and movement of show and breeding animals. These events often provide the combination of a population of susceptible animals housed in crowded, poorly ventilated barns that facilitate transmission of the virus.23Immunity depends on the means of exposure (vaccination or natural infection), the strain of the virus, and the time since exposure. After infection, protective immunity to homologous strains of the virus is present and persists for 1 year, possibly up to 2 years. Field studies of disease outbreaks indicated that the concentration of antibodies in serum that provide some resistance to disease might be less than that suggested from experimental studies.17 Protective immunity induced by natural infection is characterized by production of IgA in nasal secretions and IgGa and IgGb in serum, whereas administration of an inactivated, alum-adjuvanted commercial vaccine induces only a serum IgG(T) response that is not protective against challenge.24 Immunity after vaccination lasts for a much shorter period of time, 3–4 months, and is specific for the subtypes, and their strains, of virus included in the vaccine. Immunity following infection or vaccination is less protective against infection by a heterologous strain.23 Similarly, vaccination exposure to a heterologous virus may induce only a poor anamnestic immune response.23 These observations are consistent with the concurrent circulation of multiple viral strains influenza virus in horse populations, and the cycling of virus strains causing disease in consecutive years.5
Housing of large numbers of horses in close contact, or in enclosed environments such as large barns or stables, provides optimum conditions for facilitating contact and aerosol spread of the virus. Shed barns, which characteristically have poorer ventilation and greater stocking density than pole barns, are associated with a 4-fold increase in risk of influenza.17
Presence of small numbers of horses with access to large numbers of at risk horses might impact the course of an epidemic. Track ponies, which have close contact with large numbers of horses on a daily basis, are important in spread of influenza in racing barns.17
Influenza causes minimal loss through death of horses, but it causes much inconvenience in racing stables because it occurs in explosive outbreaks and affected horses have to break training. Such outbreaks have the capacity to close down the racing industry in a country for a period of months.14 An additional cost is incurred because of restrictions on international movement of horses and associated quarantine periods.
Equine influenza A viruses can infect humans although such infections are unusual and subclinical.25
The disease is principally one of inflammation of the upper respiratory tract, although pulmonary lesions are common in adult horses and the disease can cause severe, fatal pneumonia in foals.7,26,27 The virus is inhaled, attaches to respiratory epithelial cells with it hemagglutinin spikes, fuses with the cell, and is released into the cytoplasm where it replicates. New virions are released from the cell surface and infect other cells or are expelled into the environment. Initial viral infection and replication occurs mainly in the nasopharyngeal mucosa, but by 3–7 days after infection, virus can be recovered from cells throughout the respiratory tract. Infection of the respiratory mucosa results in death of epithelial cells, inflammation, edema, and loss of the protective mucociliary clearance. Death of cells is a result of influenza virus-induced apoptosis of respiratory epithelial cells and local and systemic increases in interferon and interleukin-6.28,29 Proliferation by opportunistic bacteria, commonly Streptococcus zooepidemicus, occurs because of the disruption of normal clearance mechanisms, and can exacerbate the inflammation and cause bronchopneumonia. Viremia, if it occurs, is mild and brief, although it may be related to some of the systemic signs of the disease. Some speculate that myocarditis, myositis and encephalitis occur occasionally in response to influenza virus infection, but definitive proof is lacking.23 Influenza virus has not been isolated from tissues other than those of the respiratory tract.23 Enteritis was reported in horses in the 1989 Chinese outbreak (Jilin/89), but is not reported for disease associated with conventional virus strains.
Outbreaks of equine influenza are characterized by a sudden onset and rapid spread of disease. Typically, in a large group of susceptible horses the incidence of the disease peaks about 1 week after the first case is noticed and new cases do not develop after 21–28 days.14,17,30 The disease may have an attenuated clinical course in a population of vaccinated or previously exposed horses. The milder disease in immune animals may be clinically indistinguishable from upper respiratory diseases associated with other common agents such as EHV-4, equine rhinitis virus and arteritis virus.
Clinically, the disease starts with a fever (38.5–41°C, 101–106°F) after an incubation period of 24–72 hours. Horses may be depressed, refuse feed, and reluctant to move. The dominant sign is cough, which is dry and hacking in the beginning and moist later, and which commences soon after the temperature rise and lasts for 1–3 weeks. It is easily stimulated by manual compression of the upper trachea. During the early stages of the disease, nasal discharge is not a prominent sign and, if it occurs, is watery. There is no marked swelling of the submaxillary lymph nodes but they may be painful on palpation in the early stages of the disease, especially in younger horses. Limb edema or swelling is unusual in horses with influenza. Abnormal lung sounds, characterized by crackles, wheezes, and increased intensity of normal breath sounds may be apparent in both uncomplicated disease and in horses with secondary bacterial pneumonia. Ultrasonographic examination of lungs of horses with influenza, even clinically mild disease, reveals pulmonary consolidation, fluid bronchograms, and peripheral irregularities.27,31 Tracheal aspirates are neutrophilic, yield heavy growth of S. zooepidemicus, and are consistent with bronchitis and pneumonia.31 Horses, unwisely, forced to exercise have reduced endurance. Horses that are protected against environmental stress pursue an uncomplicated course with most horses have complete recovery in 7–14 days, although a mild cough can persist for weeks.
The above is a description of the classical disease. However, in outbreaks there is a range of disease severity.17 Mucopurulent nasal discharge is observed in 75–90% of horses, cough in approximately 60%, fever in 20–50%, inappetance in 20–30%, and signs of depression in 20–40%.17 Undoubtedly, the proportion of horses showing each of these signs will vary from outbreak to outbreak depending on the age and susceptibility of horses in the population, among other factors.
Complications and a more severe disease occurs in a small number of horses. Horses that are worked, transported, or exposed to adverse climatic conditions can experience a worsening of the cough, and severe bronchitis, pneumonia and edema of the legs may develop. Complications are usually associated with secondary bacterial infection, usually Strep. zooepidemicus, that results in a mucopurulent nasal discharge, persistent fever, and markedly abnormal lung sounds. Icterus, encephalitic signs, incoordination and myoglobinuria are reported as rare complications.23 Electrocardiographic abnormalities have been reported in horses with influenza, and were attributed to myocarditis. However, there is no objective evidence of myocarditis secondary to influenza infection of horses nor is there a clear association between influenza infection and electrocardiographic abnormalities.
A more severe form of the disease, associated with an antigenically distinct strain of equine influenza 2, is reported from China. The mortality rate is 35%, and death is due to pneumonia and enteritis.2
A severe form of the disease is also reported in young foals. Foals develop fever, severe respiratory distress and acute interstitial pneumonia that is commonly fatal.10,26,32 The disease is not invariably associated with failure of transfer of passive immunity.
There are no characteristic changes on hematologic or serum biochemical examination of horses clinically affected by equine influenza virus infection.
Confirmation of the diagnosis of infection by equine influenza virus is achieved through virus isolation, indirect demonstration of virus in nasopharyngeal swabs, and/or serology.33
Measurement of antibody concentrations against the viral hemagglutinin antigen is important in determining susceptibility to infection, vaccine efficacy, and exposure – factors important in implementing control measures (see below). Documentation of seroconversion, a three- to four-fold increase in hemagglutination inhibition (HI) antibody titer, or a doubling in antibody titer measured by the single radial hemolysis test, in paired sera collection 14–21 days apart provides retrospective confirmation of the diagnosis. The single radial hemolysis test is more reproducible than the hemagglutination inhibition test and is the preferred test for determining concentrations of antibody against the hemagglutinin antigen.34 For the single radial hemolysis test virus is coupled to red blood cells that are then included in agarose. Wells are punched in the agar plate filled with test sera. Influenza antibodies then cause lysis of red cells, with the diameter of the zone of hemolysis proportional to the concentration of the strain specific antibody in the serum.25 Antibodies against the non-structural protein (NS1) are detectable in horses after natural infection, but not after vaccination with an inactivated virus thereby permitting differentiation of immunologic responses to vaccination and infection.35
However, because rapid identification of the cause of the outbreak is important when instituting control measures, timely demonstration of virus in nasopharyngeal swabs is desirable. Rapid demonstration of viral shedding can be accomplished by use of tests for rapid diagnosis.
Directigen Flu A test (Becton Dickinson) is a rapid test designed for use with humans, that identifies influenza viral nucleoprotein by a membrane-bound enzyme immunoassay. It has been validated for use in horses and is effective because of the conserved nature of the target antigen across influenza A strains.36 Results are available in as little as 15 minutes. The test had sensitivity of 83%, specificity of 78%, positive predictive value of 70%, and negative predictive value of 88% compared to virus culture. Sensitivity was 54%, but specificity and positive predictive value were 100% when compared with serological diagnosis. The low sensitivity compared to serology was ascribed to inadequate collection of nasopharyngeal swabs, or collection of samples when horses were not excreting virus.36 The high specificity and positive predictive value of the test mean that a positive result confirms the diagnosis of influenza infection. The relatively low specificity means that samples should be collected from a number of horses in various stages of the disease. Nasopharyngeal swabs should be collected by inserting a cotton gauze swab approximately 30 cm (12 inches) into the nostril or, preferably, nasopharynx of an adult horse and leaving it in place for 60 seconds.25 The swab should then be transferred to specialized transport media and shipped to the laboratory.33
Other rapid diagnostic tests include the Flu OIA (Biostar) assay for influenza A and B viral antigen. The test cross reacts with equine herpesvirus 2 and is therefore not useful for diagnosis of upper respiratory disease of horses.6 The test is more sensitive than the Directigen test, possibly because it detects nonviable viral material present in exfoliated epithelial cells.37 Other tests include the ZstatFlu (ZymeTx), QuickVue Influenza (Quidel) and NOW Flu (Binax) assays, none of which have been validated for use in horses at the time of writing.6
Use of rapid tests is not a substitute for viral isolation, which is important for typing of the isolate and subsequent epidemiological studies and vaccinal applications. Isolation of the virus provides a definitive diagnosis.
Material for viral culture should be collected as early as possible during the illness and inoculated into the transport medium quickly. The transport medium should contain phosphate buffered saline, glycerol, and antibiotics, among other constituents.25
Virus can be detected rapidly in clinical specimens by a reverse transcription PCR (RT-PCR) test for nucleoprotein gene, hemaglutinin gene of H3N8 viruses and hemaglutinin gene of H7N7 virus.33 RT-PCR and viral isolation are more sensitive than use of Directigen assay in detection of virus.38
Necropsy material is rarely available and the lesions in these fatalities are usually complicated by other pathogens. Histologically, a necrotizing bronchiolitis accompanies widespread pulmonary edema. Foals dying of acute respiratory distress associated with influenza infection have severe diffuse interstitial pneumonia which is characterized histologically by necrotizing bronchitis and bronchiolitis and multifocal interstitial pneumonia.10
Currently, there is no specific treatment of influenza virus infection of horses.
Amantadine is used in humans for prophylaxis and treatment of influenza infection in high-risk populations, and it has been investigated for use in horses.39 Amantadine administered intravenously caused transient neurologic abnormalities in experimental horses.39 Rimantidine (30 mg/kg PO q 12 hour) administered 12 hours before experimental inoculation of horses with equine influenza KY/91 mitigated signs of disease but did not eliminate viral shedding.40 However, the safety and efficacy of amantadine and rimantidine in horses with naturally occurring disease have not been demonstrated at this time. Until these issues are resolved, and because the infection has such a low case fatality rate, the use of these drugs in horses cannot be recommended.
Antibiotic treatment of uncomplicated cases is probably not warranted but horses that develop prolonged fever (greater than 5 days), signs of pneumonia, or a profuse mucopurulent nasal discharge should be treated with broad-spectrum antibiotics, such as potentiated sulfonamides (15–30 mg/kg, PO, IM, or IV, every 12 h), ceftiofur (2.2 mg/kg, IM, every 12 h), or procaine penicillin (20000 IU per kg, IM, every 12 h) with or without gentamicin (6.6 mg/kg, IM, every 24 h). The usual cause of secondary bacterial infection is S. zooepidemicus, which is susceptible to penicillin.
Supportive treatment includes rest, provision of a dust-free environment and, on occasion, administration of non-steroidal anti-inflammatory drugs (NSAIDs). However, NSAIDs should be used judiciously, as their analgesic properties may mask signs of complications, such as pleuritis. Corticosteroids are contraindicated in the treatment of this disease. Cough suppressants are also contraindicated, as coughing is a normal protective mechanism that aids in the clearance of material from the airway. Mucolytics can be administered but their efficacy is unknown. Clenbuterol administration does not alter the course of the disease and is not recommended.41
The fundamental aims of a control program are to:
• Increase the immunity of both individual animals and the population to infection
• Reduce the opportunities for spread of infection between horses
• Prevent the introduction of novel strains of the virus into a population.
These aims are achieved by vaccination, hygiene and quarantine.
Immunity to influenza through administration of inactivated vaccines can be assessed by measurement of serum antibody concentrations against hemagglutinin, using the single radial hemolysis test, whereas immunity gained through natural infection is independent of serum antibody concentration.34 However, serum antibody concentration is currently used as an indicator of susceptibility of individual horses to infection, and as a guide in the development and application of vaccination protocols, including monitoring of need for vaccination in individual horses. Serum antibody concentrations to hemagglutinin measured by single radial hemolysis are specific for the strain of virus and are strongly predictive for resistance to disease associated with that virus in both experimental and field challenge.34,42,43 Failure of a commercial inactivated virus multivalent vaccine to induce detectable increases in antibody concentration in Thoroughbred race horses was associated with lack of protection against natural infection by a heterologous influenza virus.44 It is important to reiterate that resistance to disease after vaccination or natural infection is greatest for homologous virus and minimal for challenge by heterologous virus.45 Thus, horses with antibody concentrations protective to disease associated with homologous virus are susceptible to disease associated with heterologous virus.34,42,43
Vaccination against equine influenza is now in general use in countries where the disease occurs, and use of efficacious vaccines is effective in limiting the severity of clinical illness and morbidity during an outbreak14,46 Vaccine efficacy is limited by the short duration of immunity induced by vaccination, the presence in horse populations of multiple viral strains and of antigenic drift in these strains, and the poor immunity induced by vaccines (and natural infection) to challenge by heterologous virus.34,46 Furthermore, the immune responses induced by administration of inactivated virus or subunit vaccines, which are primarily an increase in serum IgG(T) antibody titer, differ markedly from the immune responses to natural infection, which are production of IgA in nasal secretions and IgGa and IgGb in serum.24
Multiple factors are important in determining the efficacy of a vaccine in protecting against disease. Factors include efficacy of the vaccine in stimulating an immune response, viral strains included in the vaccine amount of antigen in a dose of vaccine adjuvant, and timing and frequency of administration of the vaccine.
A complete listing of current commercially available vaccines, the viral stains or antigens included, and the adjuvant used is available at the equiflunet website.47 This site should be consulted for up to date information on equine influenza vaccines as this is a rapidly developing field. For instance, during the period January 2003 to April 2004, H3N8 viruses of the American lineage caused widespread outbreaks in Europe, with well-vaccinated horses frequently affected. Furthermore, viruses responsible for recent outbreaks in South Africa (2003) and circulating in North America were antigenically distinguishable from the currently recommended vaccine strains. On the other hand, viruses of the European lineage were associated with limited, more localized, outbreaks in Europe and were antigenically and genetically similar to some viruses circulating during 2002. Based on these observations, it is recommended that vaccines are updated to contain an A/equine/South Africa/4/03 (H3N8)-like virus (American lineage) and an A/equineNewmarket/2/93 (H3N8)-like virus (European lineage). A/eq/Suffolk/89 and A/eq/Borlänge/91, currently used vaccine strains, continue to be acceptable.
There are several forms of vaccine available including those that contain one of inactivated virus, modified live virus, hemagglutinin and neuraminidase subunits, or hemagglutinin proteins.47 Furthermore, the type of adjuvant varies among vaccines with carbomer, aluminum hydroxide, carbopol, saponin, immune-stimulating complex (ISCOM), and various proprietary products (Meta-stim, PureVax) being used.47 A comparison of each vaccine is beyond the scope of this text, but several principles should be considered when selecting a vaccine: the vaccine should induce a measurable immune response and demonstrable protection against disease (natural or experimental), it should contain pertinent viral strains, it should be safe, and it should be practical (i.e. readily administered).
Most vaccines are comprised of inactivated or subunits of virus combined with an adjuvant. Inclusion of an adjuvant is important in maximizing the immune response to vaccination. The important factor in vaccine composition is the inclusion of adequate amounts of antigen of pertinent strains of virus. H7N7 virus is no longer a cause of clinical disease and it should not be included in contemporary vaccines.34 Inclusion of antigen from both American and Eurasian lineages of H3N8 virus is essential, and vaccine composition should be regularly updated to reflect those viruses currently circulating in the horse population.34,46,47 The vaccine must include an adequate amount of antigen, measured by the single radial diffusion assay, preferably, the single radial hemolysis assay, as there is a clear relationship between dose of antigen and magnitude and duration of antibody response.46 There is increasing concern, and some evidence,45 that inclusion of multiple antigens in vaccines (for example, tetanus toxoid, equine herpesvirus, encephalomyelitis virus) reduces the efficacy of influenza vaccines. While this concern has yet to be proved conclusively, it should be borne in mind when formulating vaccine programs for horses at high risk of influenza.
A modified live virus vaccine is available in North America and has proven to be effective in experimental studies in preventing disease against heterologous virus challenge (both American and Eurasian lineages).48 Furthermore, the duration of protection is at least 6 months after completion of a course of vaccination. Vaccinated ponies had had significantly lower clinical scores, smaller increases in rectal temperature, and shed less virus over fewer days than did the unvaccinated controls in response to challenge 6 months after vaccination.49 After challenge at 12 months, vaccinates had rectal temperatures, duration and concentration of virus shed significantly reduced to that in unvaccinated animals. The results of this study showed that 6 months after a single dose of vaccine the duration and severity of clinical signs were markedly reduced amongst vaccinated animals exposed to a severe live-virus challenge. Appropriate use of this vaccine should lead to a marked reduction in the frequency, severity and duration of outbreaks of equine influenza in North America.49
A live recombinant canary pox vector vaccine has been introduced in Europe.46 The vaccine uses the viral vector to introduce influenza hemagglutinin genes into host cells. The recombinant virus expresses the hemagglutinin gene of both A/eq/Newmarket/2/93 (European lineage) and A/eq/Kentucky/94 (American lineage).50 The canary pox infection of the host cell is abortive, with no virus produced, but influenza viral gene is expressed and presented through MHC class 1 by the host cell, with subsequent induction of an immune response.34 In preliminary reports it provides protection against infection by homologous virus, or by A/eq/Newmarket/05/03, the cause of the 2003 outbreak of influenza in Britain, 2 weeks after vaccination.50 The vaccine was used to aid in the control of the 2003 influenza outbreak in South Africa.
Other novel vaccine strategies include use of DNA or vector vaccines. Although effective in inducing a protective response, technological issues currently limit the widespread use of DNA vaccines.51,52
The objective of a vaccination program is to insure that horses have maximal immunity at the times of greatest risk of exposure to influenza virus. Therefore, young horses should be adequately vaccinated before being introduced into larger populations of horses. Older horses should receive frequent booster vaccinations before, and during, the racing or show season. Mares should be revaccinated before being shipped to breeding farms. It is important in any control program that all horses in a herd be vaccinated so that the population immunity to infection is maximal.
Timing of vaccination of foals depends on the immune status of the mare, and consequent acquisition of passive immunity by the foal. The presence of even small amounts of maternally derived antibody interferes with the immune response of foals to vaccination.53,54 Furthermore, vaccination of foals while they continue to have passive immunity can result in impaired responses to subsequent vaccinations.53,54 The practical significance of this latter observation is unknown but because of its potential importance should be considered when developing vaccination protocols for foals. Therefore, vaccination of foals born to mares vaccinated more than once yearly should be delayed until the foals are at least 24 weeks of age when the immunity resulting from the vaccination is much better; this might leave some foals unprotected because passively acquired immunity is shortlived and some foals of recently vaccinated dams are seronegative by 4 weeks of age.21 Foals of unvaccinated mares can be vaccinated at less than 1 month of age. Vaccinations are carried out at 6 to 12-week intervals for at least two injections. Subsequently, booster injections are given at least once a year, although more frequent vaccination confers a greater immunity.
Yearlings and young horses are at increased risk of disease, and careful attention to their vaccination status is important in reducing the incidence of disease in this group. Yearlings and 2 year olds in racing stables in Great Britain typically have antibody concentrations against influenza prior to vaccination on arrival at the stable that are not protective.55 Vaccination increases antibody titer such that approximately three quarters of yearlings and 2 year olds have protective titers.55 For yearlings entering training the important predictors of antibody titer prior to vaccination on arrival at the stable were the time since a previous vaccination, total number of previous vaccinations, and the age at first vaccination.55 This study demonstrates the need for appropriate vaccination of young horses before they enter larger populations of horses, both to protect the young horse from disease and also to confer herd immunity on the population that they are entering.
Vaccination of race and show horses and other horses at increased risk of exposure should be frequent. Booster vaccines should be timed to maximize immunity at the time of greatest exposure, such as introduction to a new stable or at the beginning of the show season. For maximal protection subsequent booster injections should be administered at intervals of 6, or even 4, months. Measurement of antibody concentrations by single radial hemolysis can be useful in determining the need for booster vaccination. Previously, vaccination during the racing season was disliked by trainers because of transitory swellings at injection sites and an infrequent mild systemic reaction, however administration of contemporary vaccines is rarely associated with these adverse effects. In general, vaccination appears to have no adverse effect on performance.
Various schedules have been proposed for influenza vaccinations of horses, with different regulatory bodies having specific recommendations.6 The FEI requires all horses competing in FEI competition to provide evidence of sufficient vaccination against equine influenza. This involves regular six monthly booster vaccinations following a primary vaccination course. All horses and ponies for which an FEI Passport or a National Passport approved by the FEI has been issued must have the vaccination section completed and endorsed by a veterinarian, stating that it has received two injections for primary vaccination against equine influenza, given between 1 and 3 months apart. In addition, a booster vaccination must be administered within each succeeding 6 months (± 21 days) following the second vaccination of the primary course. None of these injections must have been given within the preceding 7 days including the day of the competition or of entry into the competition stables.
The UK Jockey Club has strict vaccination requirements which must be complied with to enter horses in their competitions or onto their premises. The program includes a 1st equine influenza vaccination to be followed by a 2 nd vaccination 21 to 92 days later, with a 3rd vaccination 150 to 215 days from the 2 nd vaccination.6 Thereafter vaccinations should be annually, with the last permissible day being the same date as the previous year’s vaccination. Horses may not race until the 8th day after the day of vaccination.
A schedule proposed for control of influenza in a large area includes the following rules:
• Mandatory vaccination for all horses entering racing premises
• Horses not to race in the 10-day period following vaccination
• Horses coming from international locations must be vaccinated before departure
• All horse events, including shows, sales and gymkhanas, should apply the same restrictions.
The recommended vaccination program using inactivated or subunit vaccine is:
• Mares should be vaccinated during the final 4–6 weeks of gestation to ensure adequate passage of passive immunity to the foal
• Vaccination of foals at 6 months of age
• Two vaccinations initially at 21 days, and not more than 92 days apart
• A booster vaccination 5–7 months later
• Annual boosters or, in the face of increased infection pressure or when the risk of infection is high, boosters should be at 6-month or even 4-month intervals
• When vaccination schedules break down and a horse goes longer than 12 months without a booster, recommence with a two-vaccination schedule
• Yearlings and 2-year-olds may require an additional vaccination between the second vaccination of the primary series and the booster at 6 months.53
Hygienic precautions can be of value in limiting the spread of the disease. Vehicles used for the transport of horses are thought to play a large part in transmission and should be thoroughly disinfected between shipments.19
Quarantine is imperative to prevent introduction of virus by animals in the incubation period of the disease or subclinically infected horses. The most common introduction of infection, especially internationally, is through importation of subclinically infected horses.23 Also, because vaccinated animals can be infected and be shedding virus but not have signs of infection, isolation of introduced animals is an essential precaution, especially when an outbreak is in progress. The period of isolation should be at least 21 days. The degree of isolation required cannot be specified because of lack of basic information, but it is suggested that droplet infection can occur over a distance of 32 m and that maximum security with regard to clothing, utensils and personnel must be practised. Additional measures are a requirement for recent (4–8 weeks) vaccination, measurement of antibody concentrations in imported horses to ensure that they have protective immunity, and testing of imported horses on arrival for viral shedding.34 The later can be accomplished through use of rapid diagnostic tests (see above).
Control measures during an outbreak are intended to eliminate sources of infection, reduce transmission of virus, enhance the resistance of at-risk horses, and decrease the number of horses at risk.23 Infected horses (identified by stall-side or rapid laboratory tests) and clinically affected horses should be removed from the group and isolated for 3–4 weeks. Ventilation of shed rows and barns should be optimal to minimize aerosol spread of the virus. No horses should be introduced or allowed to leave until the outbreak is over, probably about 4 weeks after the first case is identified. Movement of horses between barns or paddocks should be avoided. Training and racing should be suspended. The opportunity for fomite transfer on clothing, tack, feed utensils or vehicles should be minimized by strict hygiene. Vaccination of clinically normal horses in the face of an outbreak can enhance the immunity of at-risk horses, and is probably safe.
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