Prevalence of infection and occurrence of disease

Disease complexes associated with this virus have been recognized in most cattle-raising countries of Europe, Asia, North America, Africa, and in Australia and New Zealand. The respiratory form of clinical disease is most common in feedlot cattle, and cattle on diary and beef farms without a routine vaccination program.

Seroprevalence surveys have found that 10–50%, or even higher, of cattle are serologically positive to the virus depending on vaccination practices in individual herds, and the frequency of contact between infected and non-infected animals.³ The percentage can also vary between dairy and beef cattle in the same geographical area. The seroprevalence in a country or area also changes over time. A serological survey in Belgium found that infection is endemic in the cattle population.⁹ In Great Britain in the 1960s, serological surveys indicated that less than 10% of cattle were positive. In the mid- to late-1970s, the incidence of infection increased markedly, and by 1986 35% of the animals and 48% of the herds were positive. In beef herds in northern Australia, up to 96% of bulls and 52% of cows are seropositive, most of which may be venereal because respiratory disease is uncommon in cattle on extensive range conditions. The genital carrier state is important in the maintenance of venereal IBR virus and in the occurrence of sporadic infectious pustular vulvovaginitis (IPV) and infectious pustular balanoposthitis (IPB) in these herds. Outbreaks of IPV and IBP were not uncommon in Britain in the 1960s and 1970s, and then occurred only rarely after the introduction of stricter control measures in artificial insemination centers.¹⁰ In 1997, an outbreak of IPV and IBP occurred in a dairy herd with 70–80% of the cows manifesting signs of both the respiratory and genital form of BHV infection.¹⁰ Both oculonasal and vaginal isolates were typed as type 2b, which is the same as the Oxford isolate commonly associated with both IPV/IBP and typical low/medium virulence outbreaks of IBR.¹⁰

The bovine herpesvirus-1 has not been reported in recent years in a number of countries such as Norway, Sweden, finland, Estonia, Iceland, Denmark, Bulgaria, and Moldova.¹¹ Since 1994, Norway has been free of BHV-1 and certain defined additional guarantees for cattle to be imported are in place to protect the disease status.¹² IBR/IBV is a reportable disease in Norway. Some countries in Europe are in the process of eradicating the virus from the cattle population. In beef herds in Yucutan, Mexico, where vaccination is not practised, seropositivity was associated with large and older herds.¹³

Wildlife

Bovine herpesvirus infections exist in wild ruminants.¹⁴ Infections may be endemic in white-tailed deer in certain parts of Canada, and it is suggested a mild form of the disease occurs in these animals. About 29% of both wild and farmed red deer in Britain are seropositive to the closely related herpesvirus of Cervidae 1. Mule deer are susceptible to infection, the disease has occurred naturally in a goat, and antibodies to the virus have been found in pronghorn antelope in western Canada, and in Tanzania in game animals and cattle. According on serological surveys, the virus is widespread in African wildlife, particularly the buffalo, which may be a reservoir of infection among the wildlife population. The virus has been recovered from the wildebeest in Africa, which suggests further that wildlife may serve as reservoirs. Antibodies to the alpha-herpesviruses were found in reindeer (28.5%), roe deer (3.0%) and in red deer (0.5%) in Norway.¹⁵ In Saskatchewan, Canada, 52% of Woodland Caribou were seropositive to the bovine herpesvirus-1.¹⁶

Morbidity and case fatality

The uncomplicated form of the respiratory disease in cattle is not highly fatal, most losses being due mainly to secondary bacterial bronchopneumonia. The morbidity and case fatality rates in dairy cattle are about 8% and 3%, respectively, while in feedlot cattle the morbidity rate is usually 20–30% in unvaccinated cattle and may rarely reach 100%. The case fatality rate in feedlot cattle is invariably associated with secondary bacterial tracheitis and bronchopneumonia and may reach 10%, but is usually no more than 1%. Morbidity and mortality are higher in feedlot cattle than in dairy herds because of the frequent introduction of susceptible animals into an enzootic situation. The case fatality rate in the systemic form of the infection in newborn calves is almost 100%.

Methods of transmission

The main sources of infection are the nasal exudate and coughed-up droplets, genital secretions, semen, and fetal fluids and tissues. Aerosol infection is the method of spread of the respiratory disease. Experimentally, the BHV-1 can be shed from calves into the environment and transported by air over a distance of at least 3.85 m to sentinel calves housed in a separate building.

Venereal transmission is the method of spread of the genital diseases. The virus may survive for up to 1 year in semen frozen at –196°C (–321°F).

Introduction of animals into a group often precedes an outbreak of the disease. However, it can arise simultaneously in a number of dairy farms in an area and spread from these to adjacent farms until the entire area is affected. The same pattern of occurrence simultaneously in a number of foci is seen in feedlots, and from these foci infection spreads to other pens in the lot. An outbreak usually reaches its peak in the 2nd or 3rd week and ends by the 4th–6th week.

To determine whether or not an infection in a herd will spread from a small inoculum, the number of secondary cases resulting from one infectious animal must be estimated. The ratio of secondary cases to originally infected animals is called the reproduction ratio and is commonly denoted by R_0. When R₀ <1, the infection will not spread and the animal population is effectively protected from the infection. When the infection is established, but R₀ is reduced to less than one, the infection will disappear provided that some additional, commonly satisfied assumptions are met. When R₀ >1, the infection can spread. Therefore, a strategy for controlling an infectious agent is effective when, and only when, the ratio for that strategy is less than one. The value of R₀ for a given host population is determined by a number of different factors including: the transmissibility of the infection; the period over which an infected host is infectious; the population density of hosts; and where appropriate, the density of vectors and the capacity of the vectors to transmit the infection.

Risk factors

Immune mechanisms

Immunity to the virus is complex and consists of relationships between local and systemic antibody, and cell-mediated immunity. Following natural infection or vaccination with the modified live virus (MLV) vaccines, both cell-mediated and humoral components of the immune system are activated. The level of humoral immunity has been used as an indicator of previous infection and an indirect measure of resistance to clinical disease. However, the level of serum neutralizing (SN) antibody is not a reliable indicator of resistance to clinical respiratory disease. Animals with low levels of antibody may be immune because of cell-mediated immunity. The level of cell-mediated immunity can be evaluated using the delayed-type hypersensitivity test. Experimentally, the virus-neutralizing (VN) titers are lower in calves inoculated with both the IBR and parainfluenza-3 (PI-3) viruses than in calves infected with a single virus. This suggests that mixed viral infections may result in greater immunosuppression, although infectious virus synthesis may be suppressed by interference.

Following intranasal infection or the use of a MLV IBR virus vaccine intranasally, local secretory antibody and interferon are produced. The interferon appears in 3 days and persists for 10 days. The presence of the interferon does not protect calves against experimental challenge 3 days after vaccination. However, the presence of even low levels of antibody in the serum or nasal secretion, which appears by day 7 following vaccination, provides varying degrees of resistance to clinical disease for 9 months.

Colostral immunity

Calves acquire colostral antibodies from dams with humoral antibody. The duration of the colostral immunity varies from 1 to 6 months of age dependent on the initial level acquired by the calf. Maternal antibody in the calf may interfere with the successful vaccination of calves before 6 months of age.

Economic importance

BHV-1 infection can cause major economic consequences in a dairy or beef cattle breeding herd, or in a beef feedlot. Losses are incurred due to epidemics of abortion, infertility due to IPV and IPB in bulls, loss of production and deaths from the respiratory form of the disease in all ages of cattle, deaths from the highly fatal systemic form of the disease in newborn calves, and the cost of treatment when secondary bacterial infections of the respiratory tract occur.

Respiratory disease

The BHV-1 virus infects the nasal cavities and upper respiratory tract, resulting in rhinitis, laryngitis, and tracheitis. The pharyngeal tonsil is readily infected by the virus and may be an important lymphoid tissue for early anti-viral responses.³⁴ There is extensive loss of cilia in the trachea leaving the tracheal epithelium covered by microvilli. Intratracheal administration of the virus results in almost complete denudation of tracheal columnar cells, which presumably has an adverse effect on the defense mechanisms of the respiratory tract. Spread from the nasal cavities to the ocular tissues probably occurs by way of the lacrimal ducts and causes conjunctivitis with edema and swelling of the conjunctiva, multifocal plaque formation on the conjunctivae, peripheral corneal edema, and deep vascularization. The virus can also enhance the prevalence and severity of IBR in calves. In neonatal calves, potentially fatal infection, associated with the continued presence of viral antigen and active inflammation, contrasts with repair and clearance of viral antigen in weanling calves.³⁴ Experimentally, the endobronchial inoculation of calves with the BHV-1 causes an interstitial pneumonia.³⁵ The viral antigen can be detected in the desquamated cells and macrophages of bronchoalveolar fluid.

Encephalitis

The mechanism by which the brain is infected is presumed to be spread of the virus from the nasal mucosa via the trigeminal peripheral nerve to the trigeminal ganglion, resulting in a non-suppurative encephalitis. However, a viremia has been suspected. Severe encephalitis can be produced experimentally in colostrum-deprived calves with neurovirulent type BHV-1.3.²² Experimental infection with BHV-1.1 produces respiratory disease and a mild encephalitis.²² Intranasal inoculation of young calves and adult cows with BHV-1 can result in non-fatal trigeminal ganglionitis and encephalitis, which may be an important mechanism for latent infection. A rabbit model has been used to study the neuropathogenesis of BHV-5 infection.³⁶

Abortion

Systemic invasion by the virus is followed by localization of the virus in several different tissues. The virus may be transported by peripheral leukocytes to the placenta and transferred to the fetus to cause abortion. The fetus is highly susceptible to the virus, which causes a peracute infection that is usually fatal. Infection in the last trimester of gestation may result in mummification, abortion, stillbirth, or weak calves with the usual lesions of IBR as well as the lesions of the stomachs and intestines that have been produced by experimental administration of the virulent virus to newborn calves.

The systemic form of the infection in newborn calves is characterized by severe inflammation and necrosis of the respiratory and alimentary tracts, including the pharynx, esophagus, lungs, larynx, lymph nodes, liver, and nephritis and encephalitis. There is severe laryngeal edema and respiratory distress which results in difficulty in swallowing and aspiration pneumonia. A severe, highly fatal syndrome characterized by diffuse erosion and ulceration of the upper alimentary tract, including the oral cavity, has occurred in beef feedlot cattle.

Latency

The BHV-1 virus can become latent following a primary infection with a field isolate or vaccination with an attenuated strain.^3,37,38 The virus may remain latent indefinitely and recrudescence, reactivation, and shedding of the virus can occur following the use of large doses of corticosteroids which mimic the effects of stress.³⁹ Transportation of cattle with latent infection can reactivate the virus, resulting in re-excretion of the virus and a rise in neutralizing antibodies. Attenuated vaccine strains can remain in a latent stage and vaccination does not provide protection against the establishment of latent infection with a wild strain.⁴⁰ Vaccination also does not inhibit re-excretion of a wild strain that was in the latent form at the time of vaccination. The vaccine virus and the field isolates can be excreted after live virus vaccination and subsequent field isolate challenge. Colostral antibodies in calves do not prevent initial virus replication, and latency can persist after the decline in colostral immunity and the calves are seronegative.⁴¹

The location of latency of the virus in the body varies; the virus remains localized near the site of its first multiplication and during recrudescence will be re-excreted by the tissue primarily infected. The BHV-1 can be isolated from the trigeminal ganglion of clinically normal cattle during the latent period, and trigeminal ganglionitis can be observed during recrudescence.

Latent infection with virulent BHV-1 virus may occur in the trigeminal ganglion of calves previously vaccinated with the MLV vaccine.^3,38 The virulent virus may spread along the trigeminal peripheral nerve despite the presence of humoral antibodies in vaccinated calves. Recrudescence of the virus from the trigeminal ganglion and spread along the peripheral nerves by intra-axonal flow to the nasal mucosa can occur in calves treated with corticosteroids and, presumably, occurs following stress. The virus has been isolated from the trigeminal ganglia of 10% of clinically normal cattle at slaughter, 40% of which had SN antibody to the virus.

The practical aspect of latency is that all cattle from endemic herds must be considered as potential sources of BHV-1 virus and capable of spreading infection to previously unexposed animals. Some latent carriers do not possess detectable antibodies. The only method of identification is by treatment with dexamethasone to initiate recrudescence and detection of the virus from nasal secretions, or the PCR examination of the trigeminal ganglion at necropsy.

A combined serological and clinical surveillance of 20 dairy herds over three consecutive years revealed wide variations in the circulation of the virus. In some herds there was no identification of active infection, while in others one or two cycles of infection occurred in calves and yearlings, often without any clinical evidence of disease. Reactivation and shedding of the virus can occur in known carrier bulls at the time of mating, which may explain the higher incidence of titers in bulls than cows in some beef herds. Breeding bulls in an artificial insemination center which were vaccinated with a MLV vaccine were shedding the vaccine virus in the semen, and the virus could be recovered from preputial washings⁴² 2–3 months after the last immunization. However, the frequency of recurrent infections and the amount of virus excreted are reduced after vaccination.

The presence of passively acquired antibodies in calves does not prevent virus replication and establishment of latent infection.⁴³ It is also possible to experimentally produce BHV-1 seronegative passively immunized calves which do not have antibody response after infection but develop a cell-mediated immune response after infection detected by a specific interferon gamma assay. The failure to easily detect such animals presents an epidemiological threat for the control of BHV-1 infections. Marker glycoprotein E-negative vaccines can also establish latency not only in naïve but also in passively immunized neonatal calves after a single intranasal inoculation.⁴⁴ This indicates that gE-negative vaccines, when used in calves with passive antibodies can result in seronegative vaccine virus carriers.⁴⁵

The experimental intrapreputial infection of young bulls with BHV-1.2, caused acute balanoposthitis, latent infection, and detection of viral DNA in regional neural (sacral nerve ganglia, pelvic sympathetic plexus) and non-neural tissues (lymph nodes) 50 days after experimental reactivation.⁴⁶ Following experimental infection in calves the BHV-5 also can result in latent infection of surviving animals.⁵

Parturition may also be a stimulus for reactivation and shedding of a thermosensitive vaccine strain of the virus in vaccinated animals.²² Reactivation and shedding of the virus has also been observed in cattle that recovered from the respiratory form of the disease and 5 months later were experimentally infected with Dictyocaulus viviparus. The placenta may harbor the virus in a latent stage for up to 90 days without transmitting the virus to the fetus. Recrudescence may be differentiated from primary infection and re-exposure by the intranasal route based on the distribution of antiviral antibody activity among serum IgM, IgG₁, and IgG₂ isotypes.

Predisposition to pneumonia

The role of the virus in affecting the lung clearance mechanism of cattle in the pathogenesis of pneumonic pasteurellosis has been reviewed and is presented in the section on shipping fever pneumonia in cattle. Experimental aerosol exposure of calves with the BHV-1 virus impairs the function of alveolar macrophages, which allows Mannheimia haemolytica to persist and proliferate in the lung and produce the characteristic lesion. In vitro studies indicate that the BHV-1 virus can interfere with the function of effector cells, such as macrophages, neutrophils, and lymphocytes. Aerosol exposure of calves to BHV-1 can affect the composition of alveolar phospholipids, which can alter the function of lung surfactant and compromise pulmonary defense mechanisms.⁴⁷ The BHV-1 can cause alteration in the glycoconjugate composition of bovine nasal epithelial surfaces, which may promote Mann. haemolytica proliferation in the early stages of pneumonic pasteurellosis.⁴⁸ The virus also causes varying degrees of obstructive lung disease, resulting in increased resistance to breathing, retention of carbon dioxide, and increased resting lung volume. Excessive airway constriction and impairment of bronchial relaxation occurs, which may compromise lung defense mechanisms and allow development of secondary bacterial pneumonia. A severe fatal BHV-1 pneumonia can occur.

Experimentally, active BHV-1 infection function affects bovine peripheral blood neutrophils, enhances the binding of Mannheimia haemolytica leukotoxin to bronchoalveolar leukocytes, and increases their killing.⁴⁷ The virus increases the number of bronchoalveolar leukoctyes, resulting in many more leukotoxin-responsive cells being present in the lung.

Reproductive failure

The intrauterine inoculation of the BHV-1 into cattle results in an acute necrotizing endometritis in the uterine body and caudal portions of the uterine horns but minimal lesions in the anterior parts of the horns. Experimental inoculation of the virus into heifers on the day after estrus and insemination can result in lesions of the ovaries consisting of focal necrosis and cellular infiltration. Commercially available vaccinal strains of the BHV-1 virus can produce similar lesions. The ovarian lesions have marked effects on luteal function, and plasma progesterone values in the first estrus after inoculation are markedly lower than those in subsequent normal cycles. Whether the BHV-1 virus causes reproductive failure as a result of necrosis of the corpus luteum or embryonic infection remains to be determined. Recently hatched bovine embryos can be infected with any of several strains of BHV-1 and such infection in vitro is embryocidal. Experimentally induced infection during early pregnancy (7–28 days) will cause oophoritis and, in some cases, embryonic mortality. The effects of the virus on the genital tract and on reproductive performance in cattle have been reviewed.¹

Bovine mastitis

The BHV-1 and BHV-4 have been associated with mastitis in cattle.^7,8 Both viruses, and including the foot-and-mouth disease virus, and the PI-3 virus have been isolated from milk. The BHV-4 has been isolated from cows with clinical mastitis which also developed antibodies against the virus at the time of the mastitis and no bacteria were isolated from the milk samples.⁷ Bovine umbilical cord endothelial cells were used to culture the virus. Experimental inoculation of the ductus papillaris of the teat has resulted in replication of the virus and subclinical mastitis after BHV-4 infection.⁸ Simultaneous intramammary and intranasal inoculation of lactating cows with BHV-4 did not induce clinical but subclinical mastitis.⁴⁹ It is unlikely that BHV-4 is a major mastitis pathogen.

Rhinitis, tracheitis and conjunctivitis (red nose)

After experimental infection there is an incubation period of 3–7 days, but in infected feedlots the disease occurs 10–20 days after the introduction of susceptible cattle.

There is considerable variation in the severity of clinical signs following natural infection, dependent on the strain of the virus, the age susceptibility, and environmental factors. In North America, where the disease is endemic, the clinical disease is usually mild in dairy cattle and in range beef cattle. A severe form of the disease can occur in feedlots where crowding and commingling from several sources occur. A severe form of upper respiratory tract disease and encephalitis have been reported in neonatal beef calves.⁵⁰

There is sudden onset of anorexia, loud coughing, fever (up to 42°C, 108°F), severe hyperemia of the nasal mucosa, with numerous clusters of grayish foci of necrosis on the mucous membranes of the nasal septum visible just inside the external nares, a serous discharge from the eyes and nose, increased salivation, and sometimes a slight hyperexcitability. A marked fall in milk yield may be the earliest indication in diary cattle. The respirations are increased in rate and are shallow, but only an increase in the loudness of breath sounds are audible on auscultation of the lungs unless secondary pneumonia is present. A severe primary viral, or secondary bacterial, tracheitis may cause inspiratory dyspnea with abnormal tracheal breath sounds transmitted to the lungs. Respiratory distress is evident on exercise. A short, explosive cough is characteristic of some outbreaks but not in others. Sudden death within 24 hours after first signs appear can result from extensive obstructive bronchiolitis.

In dairy cattle, many animals in a herd become affected within a few days. The disease is usually mild, characterized by inappetence, coughing, profuse bilateral serous nasal discharge, excessive salivation, nasal lesions, moderate fever, moderate drop in milk production, and recovery in a few days. Several animals may have the corneal form of the disease with obvious corneal edema, conjunctivitis, and profuse ocular discharge. The affected animals as a group do not return to full production for 10–14 days. The outbreak of respiratory disease will be followed by abortions in several days up to 90 days after the index case occurred.

In feedlot cattle the illness is often more prolonged, the febrile period is longer, the nasal discharge becomes more profuse and purulent, and the convalescent period is longer. Some deaths may occur in the acute febrile period, but most fatalities are due to a secondary bronchopneumonia and occur after a prolonged illness of up to 4 months in which severe dyspnea, complete anorexia, and final recumbency are obvious signs. Some recovered animals may have a persistent snoring respiration and a grossly thickened, roughened nasal mucosa accompanied by nasal discharge.

Ocular form of IBR

Conjunctivitis is a common finding in typical ‘red nose’, but outbreaks of conjunctivitis may occur as the major clinical finding. One or both eyes may be affected, which is easily misdiagnosed as infectious keratoconjunctivitis (pinkeye) associated with Moraxella bovis. However, the IBR lesions are confined to the conjunctiva and there are no lesions of the cornea except diffuse edema. The conjunctiva is reddened and edematous, and there is a profuse, primarily serous, ocular discharge. Calves less than 6 months of age may develop encephalitis, which is marked by incoordination, excitement alternating with depression, and a high mortality rate. Salivation, bellowing, convulsions and blindness are also recorded.¹

Systemic disease in newborn calves

In newborn calves under 10 days of age, the systemic form of the disease is severe and highly fatal. Sudden anorexia, fever, excessive salivation, and rhinitis, often accompanied by unilateral or bilateral conjunctivitis, are common. The oral mucous membranes are usually hyperemic, erosions of the soft palate covered by tenacious mucus are common, and an acute pharyngitis covered by tenacious mucopurulent exudate is characteristic. The larynx is usually edematous and respiratory distress is common. Bronchopneumonia is common, and loud breath sounds, crackles and wheezes associated with consolidation are present. Outbreaks of the disease commonly occur in highly susceptible herds where the herd immunity has declined, the dams are not vaccinated, and there is minimal, if any, specific colostral immunity. Diarrhea and dehydration, referred to as the alimentary form of BHV-1 infection, occur in some affected calves. The cause of the diarrhea is uncertain but it may be related to the ruminal lesions.

Abortion

Abortion is a common sequel and occurs some weeks after the clinical illness or parenteral vaccination of non-immune pregnant cows with the MLV vaccine of bovine tissue culture origin. Abortion may occur up to 90 days following vaccination if the virus becomes latent in the placenta and infects the fetus much later than usual. This raises the possibility that vaccination even with safe vaccines may appear to be the cause of abortion if natural infection preceded vaccination. It is most common in cows that are 6–8 months pregnant. Retention of the placenta often follows, but residual infertility is unimportant. However, endometritis, poor conception and short estrus can occur after insemination with infected semen. The infectious bovine rhinotracheitis virus has been isolated from semen 12 months after storage.

Infectious pustular vulvovaginitis is characterized by frequent urination, elevation of the tail, and a mild vaginal discharge. The vulva is swollen, and small papules, then erosions and ulcers, are present on the mucosal surface. Mucosal ulcers may coalesce and sloughing of brown necrotic tissue may occur. Recovery usually occurs in 10–14 days unless there are complications.

Balanoposthitis is characterized by similar lesions of the glans penis and preputial mucosa.

Virus isolation or detection

Isolation of the virus from nasal swabs using tissue culture, combined with a four-fold rise in antibody titers, between acute and convalescent phase sera are desirable for a positive diagnosis of the disease.¹ When using nasal swabs, cotton and polyester swabs are recommended rather than calcium alginate swabs, which are viricidal within 2 hours. The virus can be detected in nasal swabs by the use of an ELISA, direct and indirect immunofluorescence techniques, immunoperoxidase, and by electron microscopic examination which may reveal herpes-like viral particles. The sensitivity of the direct immunofluorescence techniques is comparable to the cell culture technique. The ELISA is highly sensitive. A combination of the indirect immunofluorescence test and virus isolation from both ocular and nasal swabs of several animals will increase the recovery rate.¹

The PCR assay is as sensitive as virus isolation and is a practical alternative for the rapid detection of the virus.¹ The results are available in 1 day, compared to virus isolation which requires 7 days. Virus could be detected in nasal swabs for up to 14 days after experimental infection of cattle, and the assay can also detect the virus in bovine fetal serum and semen samples. The PCR assay can be used for detection of virus in semen and is considered equivalent to that of standard virus isolation and dot blot hybridization.⁵¹ The PCR assay with Southern blot hybridization is considered to be highly sensitive and can detect the virus in semen before they develop any detectable antibody.⁵² The PCR assay can also detect five times as many positive semen samples as the virus isolation on egg yolk-extended semen.⁵³

Using restriction endonuclease analysis of viral DNA it is now possible to distinguish field isolates of the virus from vaccine strains, which may be useful in the investigation of vaccine-induced epidemics of the disease.

A PCR assay is used to screen large numbers of milk samples for the presence of BHV-4.⁵⁴

Serology

Several serological tests are available for the detection of antibody and a rise in titer between the acute and convalescent phases of the infection.

The primary immune response to BHV-1 experimental inoculation of cattle is characterized by the formation of IgM and IgG antibodies, primarily IgG₁, by postinoculation day 7. Secondary immune responses are characterized primarily by the formation of IgG₂ antibody. A secondary immune response resulting from abortion induced by intra-amniotic virus inoculation is characterized by a substantial increase in IgM antibody. A secondary BHV-1 exposure by the intranasal route does not result in secondary IgM antibody formation.

The VN test has been widely used and is the standard by which other techniques have been evaluated.⁵⁵ The ELISA is a specific, sensitive, and practical test for detection of BHV-1 antibodies and has advantages over the SN test. The IgM– ELISA test is useful for the diagnosis of recent infection with BHV-1 in calves.⁵⁶ A micro-ELISA test is being used for the control program of BHV-1 infection in Switzerland. The test is simple, rapid, and convenient compared to the SN test, which requires cell culture facilities and is time-consuming.

The detection of latent BHV-1 infection in cattle is important in control programs and in international trade activities. Therefore, tests to detect specific antibodies in serum must be highly sensitive in order to detect low levels of BHV-1-specific antibodies.⁵⁷ This emphasizes the need for international standardization of tests to detect BHV-1-specific antibodies in cattle. In a comparison of European laboratories to evaluate a panel of test sera, including negative, weak and strong positive samples as well as international reference sera, VN tests and ELISAs demonstrate high specificity.⁵⁷ The quality of most laboratories was adequate. The VN test and the gB-specific ELISAs were most sensitive for the detection of antibodies in serum; the indirect ELISAs are the tests of choice for assaying milk samples. Most of the ELISAs demonstrate 100% specificity. Discrepancies occur with the low amounts of specific antibody. An indirect ELISA using undiluted test serum demonstrated 100% sensitivity. A commercial anti-BHV-1 blocking ELISA test kit is available to differentiate between cattle immunized with a marker BHV-1 vaccine and naturally infected cattle, but the sensitivity is only 74%.⁵⁸

An immunofluorescence assay using monoclonal antibodies can discriminate between the four ruminant alphaherpesviruses related to the BHV-1.⁴

They include the bovine herpesvirus-5, caprine herpesvirus-1 (CpHV-1), cervine herpesvirus-1 (CvHV-1), and cervine herspesvirus-2 (CvHV-2). Buffalo herpesvirus-1 and elk herpesvirus are also closely related to BHV-1.⁴

Four serological tests have been evaluated to detect serum colostral antibodies to BHV-1 in calves.⁵⁹ A blocking ELISA demonstrated superior sensitivity in the detection of antibodies in calves up to 9–11 months of age compared to calves up to 7 months of age.⁵⁹

The serological tests used for the BHV-1 eradication programs in The Netherlands have been compared.⁶⁰ The combination of the gB-ELISA (for screening), and the Danish test – a blocking and an indirect ELISA (for conformation), provides a sensitivity of >99.0 % and a specificity of >99.9%.

Bulk tank milk testing for BHV-1 antibodies may be useful in eradication and monitoring programs because it offers the possibility of rapid and inexpensive screening.^61,62 The correlation between the bulk milk test and the within-herd prevalence of seropositive animals can be as high as 0.86.⁶¹ If BHV-1 is detected in the bulk milk, there is a high probability that more than one animal in a herd is infected and that the infection has spread.⁶³ The BHV-1 blocking ELISA is in use on bulk milk samples as part of the Danish surveillance system for BHV-1 infection in dairy herds.⁶⁴ The test can detect seropositive herds, with prevalence proportions as low as one seropositive cow out of 70 cows.

Specific antibody against BHV-1 may be detectable in fetal fluids and increases the rate of diagnosis of abortion.

Samples for confirmation of diagnosis

Antimicrobial therapy

Broad-spectrum antimicrobials are indicated if secondary bacterial tracheitis and pneumonia are present. Affected cattle should be identified, isolated, and monitored frequently for evidence of secondary bacterial disease accompanied by anorexia and toxemia, and treated accordingly. The tracheitis is particularly difficult to treat; antimicrobials daily for several days are necessary and often slaughter for salvage is the most economical course.

Natural exposure or vaccination

Vaccination programs in herds

Beef breeding herds. Beef calves should be vaccinated 2–3 weeks before weaning as part of a preweaning preconditioning program. Calves vaccinated with the parenteral MLV BHV-1 vaccine before colostral BHV-1 antibody titers reach low levels do not develop an immediate, active serological response, as indicated by serological titers, but are sensitized to the virus. Revaccination at a later date, when maternal antibodies have decreased to undetectable levels, results in a marked serological response. Heifer and bull replacements are vaccinated at least 2 weeks before breeding. When outbreaks of the respiratory disease occur in unvaccinated beef herds, all cattle in the herd may be vaccinated with the intranasal vaccine. Whether or nor beef herds should be vaccinated annually following the initial vaccination is uncertain. There are field reports of outbreaks of abortion due to the virus in beef cattle that were vaccinated 3 years previously, which suggests that revaccination of breeding females every 2 years may be indicated. Since both natural infection and vaccination results in latent infection it may be that the persistence of the virus, combined with natural exposure, may result in persistence of antibody. The duration of protective immunity following vaccination is uncertain, but usually lasts 1 year. Antibodies last for at least 5.5 years in heifers following experimental infection and complete isolation during that time.

The MLV BHV-1 vaccine given intranasally or parenterally can enhance the prevalence of infectious bovine keratoconjunctivitis in beef calves vaccinated between 4 and 10 months of age, when the risk for the ocular disease is highest. The explanation for the pathogenetic mechanism is uncertain.

Feedlot cattle. Feedlot cattle should be vaccinated at least 10 days before being placed in the lot, especially one in which the disease may be enzootic. If this is not done a high incidence of the respiratory form of the disease may occur in recent arrivals. If vaccination before arrival is not possible, the next best procedure is to vaccinate the cattle on arrival and place them in an isolation starting pen for 7–10 days during which time immunity will develop.

Prevention of pneumonia. A field trial of pre-shipment vaccination of cattle with a combination BHV-1 and PI-3 vaccine administered intranasally 3 weeks before shipment from western Canada to Ontario did not have a significant effect on treatment rates of the animals for pneumonia after their arrival in the feedlot. The vaccination trial was designed to examine the hypothesis that vaccination with a BHV-1 virus vaccine prior to the stress of shipment would decrease the incidence of bovine respiratory disease, most of which is pneumonic pasteurellosis. Experimentally, vaccination of cattle with the BHV-1 virus vaccine prior to inoculation with an aerosol of BHV-1 virus, followed later by an aerosol of Mann. haemolytica, provides protection against the bacterial pneumonia. There was no significant difference between vaccinated and non-vaccinated animals in terms of liveweight gains and incidence rate of subclinical disease. A commercially available MLV BHV-1 intranasal vaccine has been used in veal calves for this purpose but the lack of controls makes it difficult to interpret the results.

Dairy cattle. The necessity of vaccinating dairy cattle will depend on the prevalence of the disease in the area and in the herd, and the movement of cattle in and out of the herd. A closed herd may remain free of BHV-1 infection indefinitely and vaccination may not be indicated. But to avoid unpredictable abortion storms due to the virus in dairy herds, heifer replacements should be vaccinated for the disease 2–3 weeks before breeding. Vaccination of a large dairy herd with a persistent BHV-1 infection has been successful in controlling the respiratory form of the disease. The intranasal vaccine has been used extensively in newborn calves in problem herds, but its efficacy at such an age is unknown. The parenteral vaccination of beef calves under 3 days of age with a MLV BHV-1 and PI-3 vaccine caused high mortality.²⁶ If the systemic form of the disease poses a threat to a potential calf crop, the pregnant cows could be vaccinated with the intranasal vaccine in late pregnancy; this will increase the level of colostral antibody available to the newborn calf and will provide newborn calves with protection against the highly fatal systemic form of the disease.¹⁷

Bulls intended for use in artificial insemination centers present a special problem of disease control because the virus in semen can have severe consequences on reproductive performance. Bulls that are seropositive to the virus must be considered as carriers and potential shedders of the virus, and should not be allowed entry to these centers. Not all bulls that are seronegative can necessarily be considered free of the virus, and regular attempts at the isolation of the virus must be made from preputial washing and semen. Bulls that become infected while at the centers should be kept isolated, and culled and replaced with clean bulls. Bulls from herds that routinely vaccinate against BHV-1 should not be vaccinated with conventional vaccines if destined for an artificial insemination center. Cattle destined for export should not be vaccinated in case importing countries prohibit the introduction of seropositive cattle. This will not guarantee that such animals will not become positive from natural infection. The use of marker vaccines has some potential in breeding bulls intended for artificial insemination units and for export.

Eradication

Eradication of the BHV-1 virus from a single herd or the cattle population in a country can be considered as an alternative to vaccination. One of the prerequisites for the exportation of pedigree cattle is that the animals are serologically negative, which requires that the herd be free of infection with BHV-1. With careful planning and health management, it is possible to establish a seronegative herd. In a preliminary study in one closed beef herd in which there was latent BHV-1 infection, the calves were raised in isolation separate from the cows, following weaning. All of the maternally derived BHV-1 titers in the calves decayed to zero at weaning time and remained seronegative while raised in isolation. Serologically positive animals are removed or culled, and only seronegative animals introduced into the herd.

In 1983, Switzerland began a national program for the eradication of IBR.⁸³ The disease was made notifiable and the use of vaccination was prohibited. The program is based on:

By 1987, the breeding stock was virtually free of IBR.⁸⁴ The virus was also eradicated from a beef feedlot farm in Switzerland that raised 750 calves over a period of 12–13 months for market at 500 kg BW. Seropositive animals were identified serologically, kept separate from seronegative animals, and monitored serologically every 3 months. Eradication was complete within 9 months.⁸⁴

In other countries, control is being attempted by segregation and elimination of seropositive animals and reduction of animal movement to prevent spread. This approach is not feasible in countries with extensive cattle populations and where management practices result in movement of cattle from one region to another.

Eradication using marker vaccines. Some countries are beginning an immunization program with the marker vaccines, which will protect the cattle against disease but still allow differentiation between vaccinated animals and those that have been naturally infected and are potential carriers of the latent virus. These infected animals could be eliminated over a period of time. Successful eradication depends not only on the efficacy of the vaccine but also on the quality of the tests. False-positive test results can lead to unnecessary culling of cattle, an increase of costs, and reduced co-operation of farmers in the eradication program.

In The Netherlands, a compulsory eradication program for BHV-1 began in 1998. The program required that farms either vaccinate all cattle twice yearly or be approved for a certified BHV-1 free status – or SPF (specific pathogen free). To become a certified BHV-1 free herd, cattle have to be sampled individually and all seropositive animals culled as soon as their status is known. The BHV-1 free herd status is monitored by monthly bulk milk samples.⁸⁵ The spread of BHV-1 between herds can be prevented using a surveillance system of sampling herds annually, both individual milk samples and blood samples.⁸⁶

Herds with BHV-2 infected (seropositive) animals are required to vaccinate with a glycoprotein E (gE)-negative BHV-1 vaccine.⁸⁷ The vaccine may be either an inactivated or live vaccine both based on a spontaneous BHV-1 mutant without the complete gE gene.⁷⁸ These so-called marker vaccines or ‘diva’ (differentiating infected from vaccinated animals) vaccines allow the identification of cattle infected with the wild-type BHV-1 within a vaccinated population using a gE-ELISA or a commercially available gE-blocking ELISA (Idexx) which both specifically detect gE antibodies. The eradication program is based on the presumption that all BHV-1 wild-type strains express gE and induce antibodies which can be measured with a gE-blocking ELISA.

The success of the marker vaccines depends on their capability to reduce transmission of the virus in the field. To prevent major outbreaks on a farm, the transmission ratio R₀ between animals must be below 1. If the Ro between animals is below 1 on all farms eradication of BHV-1 would be certain. However, even when Ro between animals is not below 1, eradication can still occur when the Ro between herds is below 1, because the reduced transmission within a farm, makes farms less susceptible and less infectious. For BHV-1 it is possible that vaccination with a live gE-negative vaccine would reduce Ro in a herd below 1.

The efficacy of a live E-negative BHV-1 vaccine to reduce the transmission of BHV-1 in cattle was evaluated in a randomized, double blind, placebo-controlled field trial in 84 dairy herds in the Netherlands.⁸⁸ The incidence of BHV-1 infections during 17 months was monitored by detecting antibodies against glycoprotein E. The transmission ratio R₀ in the placebo-treated herds was estimated at 2.5 and 1.2 in the vaccinated herds. Thus the use of the live gE-negative BHV-1 vaccine reduces the incidence and transmission of infection in the field.⁸⁸

Before the eradication program began, a large field trial to evaluate the efficacy of the live gE-negative BHV-1 vaccine found that some placebo herds, which were not vaccinated. were seronegative for gE-negative BHV-1, but seropositive for BHV-1 by glycoprotein B (gB) blocking ELISA.⁸⁷ The source of the gB seropositive occurrence was undetermined. Antibodies against BHV-5 may be differentiated from those of BHV-1 in a BHV-1 glycoprotein E blocking ELISA.⁸⁹ The gE-negative BHV-1 vaccine strain is not re-excreted after corticosteroid treatments, and not transmitted to susceptible cattle.⁹⁰

An epidemiological and economic simulation model to evaluate the spread and control of BHV-1 in the Netherlands indicates that compulsory vaccination would be necessary to reach a BHV-1 free status.⁹¹ Simulation modeling of BHV-1 control programs at the national level with special attention to sensitivity analysis has been described.⁹²

Side effects of BHV-1 marker vaccine in the Netherlands

Between May 1, 1998 and February 22, 1999 it was compulsory for Dutch cattle farmers to take measures to control BHV-1. Cattle on farms not already certified as BHV-1 free had to be vaccinated twice yearly with a gE-negative BHV-1 marker vaccine. In February 1999, the Dutch Animal Service advised all veterinary practices to postpone vaccination against BHV-1 using the marker vaccine.⁹³ Severe disease had occurred in herds vaccinated with the marker vaccine. Using monoclonal antibodies, bovine virus diarrhea virus (BVDV) type 2 was found in the vaccine batch. Batches of vaccine which contained the BVDV type induced clinical signs of BVDV when inoculated into susceptible animals.⁹⁴ The first clinical signs of illness occurred 6 days after vaccination. Morbidity was up to 70%; on some farms none. During the first week feed intake and milk production decreased. In the second week, some animals become severely ill with fever, nasal discharge, and diarrhea. By the third week the number of affected animals increased rapidly and some died. Necropsy findings included erosions and ulcers of the digestive tract. Contamination of the marker vaccine with the BVDV was a consequence of infection of fetal calf serum used in vaccine manufacture. Vaccination of calves with a BHV-1 marker vaccine containing BVDV type 1 did not induce clinical disease.⁹⁵

By the end of 1999, 6997 cattle farmers had lodged complaints related to the vaccine.⁹⁶ During the compulsory vaccination period, 13% of the herd vaccinations resulted in clinical disease and complaints.

Following recognition of the outbreaks associated with the marker Vaccine some dairy farmers identified a ‘chronic wasting’ syndrome in their cows which they attributed to the vaccine. field investigations of these cases could not associate them with the vaccine.^97,98 A review of the research into the chronic wasting syndrome of dairy cows found no evidence to associate the syndrome with the vaccine.⁹⁹ Vaccination of pregnant heifers in their third trimester experimentally with a high dose of the marker vaccine did not have any adverse effects.¹⁰⁰ Comparison of performance of dairy herds which were or were not vaccinated with the marker vaccine in 1998 could not discern any differences due the vaccine.⁹⁸

Loss of certification. The probability of and risk factors for the introduction of BHV-1 into SPF Dutch dairy farms has been examined.⁸⁵ A total of 95 SPF dairy farms were monitored for 2 years during which time 14 introductions of infectious diseases occurred on 13 of the 95 farms for a total incidence rate per herd-year at risk was 0.09. Outbreaks were usually associated with allowing cattle to return to their farm, more often grazed cattle at other farms, and less often provided protective clothing to the veterinarian. For a successful eradication program, farms should remain BHV-1 free which can be achieved by a more-closed farming system.¹⁰¹ A more-closed farming system is one which rules out the possibility of direct contact with other cattle from other farms. Also, the farmer requests that professional visitors like veterinarians and AI technicians to wear protective farm clothing when handling cattle. Protective farm clothing are coveralls or overcoats and boots which can be worn over ‘off-farm’ clothing and which the farmer provides to the visitors before handling cattle. A sanitary barrier is a covered area outside the barn in which visitors put on protective farm clothing over their ‘off-farm’ clothes. A sanitary barrier has a ‘dirty’ side, where visitors leave, their ‘off-farm’ boots and a ‘clean’ side, where visitors wear protective clothing and can enter the barn. All of these measures would be economical.⁸⁵

M. ovipneumoniae

M. ovipneumoniae is considered to be important in the disease complex and may be the initiating cause.^1-4 It is commonly isolated in large numbers from the lungs of affected sheep, but can also be isolated from the nasal cavity of some normal sheep and less occasionally from normal lung. Experimental challenge with pure cultures of the organism produces minimal lesions, but aerosol or intrabronchial challenge with homogenates of affected lung that contain the organism produces proliferative interstitial and lymphoid pneumonic lesions indistinguishable from the natural disease.^3,5 M. ovipneumoniae is a facultative pathogen that requires compromised lung defense mechanisms in order to initiate lesions; infection with this organism subsequently predisposes the lung to secondary infection with organisms such as Past. haemolytica.^1,3 There is considerable heterogeneity in M. ovipneumoniae and several different strains may be isolated from a pneumonic lung.⁵ Differences between strains in pathogenicity are not determined. Other mycoplasma, including M. mycoides subsp. mycoides, M. mycoides subsp. capri, M. putrifasciens, and M. argininii may be associated with chronic enzootic pneumonia in tropical zones.⁶

B. parapertussis

B. parapertussis is a common isolate from the nasal cavities and lungs of sheep with chronic enzootic pneumonia and is also believed to have an initiating role in the disease.⁷ It produces a cytotoxin that damages ciliated epithelium in the trachea and experimental challenge of colostrum-deprived lambs produces mild pulmonary lesions similar to those seen early in the natural disease. B. parapertussis also can predispose pneumonic pasteurellosis.⁸

Parainfluenza-3 (PI-3) virus

PI-3 is a cause of a mild undifferentiated pneumonia in sheep, and surveys around the world have shown that it is a widespread infection.^2-4 The disease is clinically mild and marked by the presence of interstitial pneumonia. Antibodies to PI-3 are present in lambs soon after birth, but the half-life is short and lambs are susceptible by the time they are weaned and mixed with other lambs, which is when clinical disease often occurs. In the experimentally produced disease in lambs⁹ there is a slight seromucosal nasal discharge, coughing, increased sensitivity to tracheal compression, and fever of 40–41°C (104–106°F). At necropsy there is obvious hyperemia of the upper respiratory mucosa, including the trachea, the bronchial lymph nodes are enlarged, and there are small foci of catarrhal inflammation of pulmonary parenchyma of the apical and cardiac lobes.⁹ However, challenge of lambs at 2 weeks of age with this virus and Mann. haemolytica, while producing disease, did not result in prolonged disease lasting to slaughter, and it was concluded that these agents, without other factors, were not the cause of enzootic pneumonia.¹⁰ This conclusion is supported by the results of vaccine trials with PI-3 against enzootic pneumonia.¹¹

Bovine respiratory syncytial virus (BRSV)

BRSV has resulted in pneumonia following experimental challenge of sheep and is evidenced clinically by fever and hyperpnea, and pathologically by multifocal pulmonary consolidation and necrosis of epithelial cells. There is little evidence for BRSV as a cause of significant respiratory disease in sheep.^3,4,12

Other agents

Adenovirus¹³ and a type-3 reovirus¹⁴ have been used experimentally to produce pneumonic lesions, and a vaccine has been produced to protect lambs against the adenovirus infection.¹⁵ Similarly, sheep herpesvirus, caprine herpesvirus-1, will produce an interstitial pneumonia in experimentally challenged SPF lambs, but there is no evidence of a causal association with chronic enzootic pneumonia.

Autoantibodies to upper respiratory cilia have been detected in sheep colonized with M. ovipneumoniae and it is suggested that they contribute to the pathogenesis of coughing in this disease.¹⁶

Occurrence

Enzootic pneumonia affects animals up to 12 months but may commence as early as 6 weeks of age. The disease can occur in both lambs at pasture and housed lambs. In many affected flocks, 80% of 4–5-month-old lambs have clinical signs and lesions, and the disease is credited with causing a significant depression in growth rate after weaning in lamb flocks with a high prevalence. This has been confirmed in controlled studies on the effect of the experimentally produced disease on weight gain in housed and pasture-fed lambs.^2-4

Enzootic pneumonia has a seasonal pattern which differs according to locality and management. In Australia and New Zealand, the period of peak prevalence is in the late summer and autumn. In a longitudinal slaughter study of lambs in New Zealand the prevalence of pneumonic lesions was found to increase from early summer to early autumn with an overall prevalence of pneumonia of 42%. There were significant differences in prevalence between different regions of the country.¹⁷ Factors such as co-mingling sheep from different sources and environmental stress can precipitate clinical disease.

Environmental risk factors

In Australia and New Zealand, clinical outbreaks of enzootic pneumonia in lambs aged 5–8 months are often associated with heat stress, frequent yarding after weaning, use of plunge or shower dips, and transport or mustering of sheep in hot dry conditions. Cases commence within 1–3 weeks after transport. In contrast in the United Kingdom and Europe this disease occurs primarily in the late winter and early spring and in the Northern hemisphere the disease is commonly associated with problems in the housing environment. In Ireland, an association has been made between the occurrence of lesions at slaughter and the extent of rain and windchill experienced by the sheep in the 2 months prior to slaughter.¹⁸

Economic importance

Death loss from this disease is minor but economic loss is considerable and includes reduced growth rate, prolonged periods on the farm before reaching slaughter weight, the drug and labor costs associated with treatment, slaughterhouse wastage, and downgrading of carcasses with pleural adhesions and an effect on carcass quality. The situation is similar to that with enzootic pneumonia of pigs.

Occurrence

The earliest reports of the disease were from South Africa and the United States, but it now occurs in all major sheep-producing countries with the exception of Australia, New Zealand, Iceland, and Finland. The disease was present in Iceland but was eradicated in 1965. Maedi virus was introduced into Iceland in 1933 by the importation of infected sheep, and because of breed susceptibility of the local sheep and management practices it developed to a problem of major national significance. In individual flocks the annual mortality was often 15–30% and in these circumstances sheep farming was not economically feasible. Approximately 105000 sheep are believed to have died of the disease and 650000 sheep had to be slaughtered in order to eradicate it from the country.⁴

The international movement of sheep has facilitated the spread of the disease and it is believed to have been introduced into Denmark, Norway, Sweden, and Great Britain since the 1970s through the importation of infected sheep. No other country has experienced the severity of disease that occurred in Iceland.

Host range

Sheep and goats are the only species known to be susceptible and infection cannot be established by experimental challenge in cattle, deer, pigs, dogs, horses, chickens, mice, and rats.⁶ Rabbits are susceptible, but infection is limited to the acute stage prior to the production of antibody, and chronic infection does not occur as it does in sheep and goats.⁵ A serological survey of wildlife in the United States has shown no evidence of infection in bighorn sheep, elk, white tail deer, or antelope.⁵ All breeds of sheep appear to be susceptible to infection, but there may be differences in breed susceptibility based on breed differences in seroprevalence in flocks with more than one breed of sheep.^6,7 Differences in breed susceptibility are not consistent and it is possible that in any one flock they reflect differences in the susceptibility of family lines within the breed.⁷ Apparent differences in breed susceptibility in surveys could also be a reflection of management differences between flocks and regions.^7,8

Prevalence

The prevalence of infection varies between farms, breeds and countries. In the United States, true prevalence is not known, but a seroprevalence of 26% is recorded in samples voluntarily submitted from sheep in 29 States, and 48% of flocks had one or more seropositive sheep. An earlier study found 1–68% reactors in culled ewes from different States. Infection appears more common in western States.⁹ A random nationwide survey in Canada¹⁰ found 19% of sheep over 1 year of age with antibodies; 63% of flocks were infected, and the mean flock prevalence was 12%; similar prevalence rates are recorded for the province of Ontario.¹¹ The disease does not have significant national occurrence in countries such as the United Kingdom where infection has been introduced relatively recently, but there is concern that the prevalence may be increasing.^12,13 Prevalence rates vary markedly between countries and between flocks in the same country.^4,8,11,14

There is considerable variation in the prevalence of seropositive sheep between flocks. Rates of seropositivity increase with age, and flock seroprevalence is influenced by the average age of the flock. Flock seroprevalence also has been positively associated with the use of foster ewes, allowing lambs older than 1 day to have contact with other lambing ewes, flock size, close contact during confinement for lambing, stocking density on pasture, and the length of time that the flock has been in existence.^10,11 Rates of seropositivity are much higher in flocks that also are infected with pulmonary adenomatosis than in those which are not.¹⁵

Transmission

The disease is spread by the respiratory route, by ingestion of infected milk and by in utero infection. The relative importance of these routes appears to vary with the flock and its management, but lateral transmission is important in all.

Lambs may contract the infection at birth, or shortly following, from contact with infected ewes or from ingestion of infected colostrum and milk. Mononuclear cells in the colostrum and milk of infected ewes are infected with virus and virus from these cells may pass through the intestinal wall to infect the lamb.¹⁶ Lambs born to seropositive ewes have a significantly greater risk for infection than those from seronegative ewes, and lambs born to ewes that have been infected for a long time are at greater risk for infection. The chance of transmission to lambs from infected ewes increases with the period of contact but can occur within the first 10 hours of life.^7,11

Lateral transmission can also occur in older sheep. Lateral transmission appeared an important method of transmission of the disease in Iceland and the Basque area and has been a significant component of the spread of infection of the virus in flocks in the United Kingdom since its initial introduction.^12,13,17 In some flocks, the spread of infection can be rapid and the majority of the flock can seroconvert within a few years of the introduction of infected sheep.^12,18

Infection can be transmitted in utero but the relative importance of this route in the spread of the disease in flocks is debated.^13,19 Virus is also shed in the semen of infected rams that have leukospermia.⁸

The spread of infection is frequently very rapid in flocks that are concurrently infected with the retrovirus causing pulmonary adenomatosis.^12,20 Macrophages are numerous in the lungs of sheep affected with pulmonary adenomatosis, and these cells will be infected with ovine progressive pneumonia virus where there is a dual infection. The copious lung fluid produced by sheep with pulmonary adenomatosis contains maedi–visna virus in dual-infected sheep and is believed to increase the risk for lateral transmission.

Economic importance

The economic importance of the disease rests with losses associated with decreased longevity, mortality with clinical disease, decreased value of cull animals and possible effects of subclinical infection on productivity.

Clinical disease occurs in sheep 2 years old or older, usually in sheep 3 to 4 years of age and is more likely to appear when the infection prevalence exceeds 50%.⁴ The case fatality rate is 100%. Whereas infection is common in many flocks, the occurrence of clinical disease is commonly rare as most infections are subclinical. High mortality rates occurred in infected flocks in Iceland and have been recorded in Texel flocks in The Netherlands and in some flocks in the USA, but this is the exception and in most infected flocks clinical disease is rare. Despite the high serological prevalence in areas of Canada, ovine progressive pneumonia is not a common diagnosis in the region’s diagnostic laboratories.²¹

It is possible that the major economic loss associated with infection with these viruses rests with the effects of subclinical infection on productivity of infected flocks. Subclinical infection of breeding ewes in some flocks has been associated with a reduction in conception rate, as well as lowered birth weights in some flocks and/or reduced growth rates in their lambs.^14,18,21 The reduction in growth rate of the lamb has a significant assocition with changes in the udder of the ewe and probably results from lowered milk intake.¹⁸ This may be reflected in depression of growth rate only in lambs from older parity ewes.¹⁴ In other flocks there has been no evidence of effect on the birth weight or growth rate of lambs born of infected ewes.⁶ Subclinical infection has no effect on mature ewe body weight or greasy fleece weight.⁶

Samples for confirmation of diagnosis

Segregated rearing

This requires a management system of separating the lambs from the ewes at birth, giving them no colostrum, or bovine colostrum, and rearing them on milk replacer quite separately from other sheep. This method is effective in establishing an infection-free flock^31,32 and is of particular value where there is a requirement to retain genetic lines in the eradication procedure. However, it is very labor-intensive and expensive, and there is no cash flow unless the infected sheep are maintained in production pending the establishment of a mature infection-free flock. This can create a considerable potential for reinfection of the artificially reared flock.

Test and cull

This involves the detection and culling of seropositive animals and is the preferred method where lateral transmission is the dominant mode of transmission in the flock.¹⁷ All sheep (and goats) on the farm are serologically tested annually or twice a year, and seropositive animals and their progeny of less than 1 year of age are culled. Ideally they should be slaughtered but this may not be economically feasible, in which case they must be kept separate from the seronegative sheep. The seronegative flock must subsequently be kept isolated from infected sheep, as well as people and equipment in contact with the seropositive animals. Testing is continued semiannually or annually until there are at least two consecutive negative tests. The offspring of older seronegative ewes are kept for replacements.

Future flock introductions with both methods should be from a seronegative flock.

Other control procedures

Control procedures that attempt to limit or delay the spread of infection, and consequently the occurrence of clinical disease within an infected flock, have limited success. Shed lambing and close lambing is thought to be very conducive to spread of the disease and its discontinuance is recommended in infected flocks. In flocks that have a high incidence of clinical disease, the determination of the age of onset of clinical disease and the establishment of a culling policy based on this information can reduce the economic impact of the disease.

In countries where the disease is enzootic, there is often a great deal of movement of animals between farms, especially of rams, and in some management systems of replacement ewes. Flock introductions should be from seronegative flocks where possible. In several countries, breed societies, or other bodies, have established certification programs for flocks free of this infection. In the absence of such programs, severe restriction of inter-farm movement and outdoor housing may limit the spread of the disease.

The fact that some sheep in infected flocks retain freedom from infection in the face of continual exposure to infection suggests that there is a genetic resistance to infection. The identification of these determinants may prove to be the future method for the control of this disease. There is currently no effective vaccine.⁸

Flock biosecurity

Once infection is introduced to a flock it is difficult and expensive to eradicate and all efforts should be directed to prevent its introduction. The specificity and sensitivity of most currently available serological tests are inadequate to determine the infection status of an individual and the results of flock tests of a potential source of replacement sheep should be used coupled with an examination of postmortem records in the potential source flock, if available. Rams and replacement ewes should be acquired from accredited free flocks in countries where these are registered. The sheep should be transported directly from the source farm rather than through a market.

Occurrence

The disease has worldwide distribution and is recorded in most countries that have significant sheep populations, with the exception of Australia and New Zealand.³ Until recently there has been no practical method to detect infected sheep and estimates of the prevalence of jaagsiekte are largely based on clinical or postmortem observations. The prevalence of the disease appears to vary depending upon the breed of sheep and the type of flock management. In most endemically infected flocks annual losses attributable to jaagsiekte are between 2% and 10%, although the tumor is present in a much higher proportion of the flock and infection without lesions is also common.^3-6 Annual mortality can be higher in flocks where the infection has recently been introduced and before the disease becomes endemic. PCR analysis of peripheral blood leukocytes of sheep in infected flocks show significantly higher rates of non-clinical infection.

Prevalence varies between countries and there can be areas of high prevalence within countries; in Britain, the Borders and the east coast of Scotland, and East Anglia in England, appear to be foci of infection from which other outbreaks arise.^3,6 The prevalence may be higher than generally recognized and, in a biased sample, histological evidence of jaagsiekte was detected in 25% of cases of pneumonia in sheep submitted to a diagnostic laboratory in Scotland over a 6-year period.⁷ The disease is also a significant cause of mortality in adult sheep in South Africa and Peru, but is a minor disease in the United States and Canada.

The disease occurred in epizootic proportions in Iceland during the same period of time as the maedi-visna epizootic but has been eradicated by a rigorous slaughter policy.

Animal and environmental risk factors

Mature sheep, 2 to 4 years of age are most commonly affected but the disease can occur in younger animals. There are reports of the occurrence of jaagsiekte in goats at very low prevalence rates in India and Greece, and the disease has been experimentally transmitted to goat kids.^8,9 The lesions produced were small and circumscribed, and goats have low susceptibility to infection.⁹

Jaagsiekte has a prolonged clinical course and is uniformly fatal. In some reports there is a greater prevalence of onset of clinical disease in the winter months but in others there is no seasonal variation in clinical onset. Ewes may show a sudden onset of clinical disease in late pregnancy.

The incubation period in natural cases is 1–3 years, but may be as short as 5–12 months after experimental transmission. Clinical disease is rare in sheep younger than 2 years and is most common at 3–4 years of age. Very rarely, cases occur in lambs 3–6 months old and disease can be reproduced in lambs of this age by challenge of very young lambs.³ A genetic or familial susceptibility to the disease is suspected.^3,6

Because of the method of spread, the disease is likely to assume more importance in systems of sheep husbandry where there are significant periods of close contact as, for example, occurs with intensified lamb-rearing systems. Close housing during the winter is a potent predisposing cause and probably accounted for the occurrence of the disease in epizootic form in Iceland. However, the disease occurs commonly in range sheep in other countries. Sheep that have a combined infection with jaagsiekte and the maedi-visna lentivirus have an increased ability to transmit maedi-visna infection,¹⁰ and flocks with the combined infection can suffer high losses from pneumonic disease.¹¹

Transmission

Experimental transmission has been effected by pulmonary or IV injection, or by intratracheal inoculation of infected lung material.³ The incubation period of the experimental disease in young lambs is much shorter than that in mature sheep. The disease has also transmitted by inhalation of infected droplets when sheep are kept in close contact, and it is assumed that the natural mode of transmission is by droplet infection from respiratory secretions, which are copious in sheep with clinical disease. A longitudinal study of the natural transmission showed that infection established readily and rapidly in young lambs and also horizontally in adult sheep, but that the majority of infected sheep did not show clinical disease during their commercial life span.⁵

Samples for confirmation of diagnosis

Distribution

Equine Eastern and Western encephalomyelitis viruses are restricted to the Americas. The two viruses have distinct geographical ranges that may overlap: EEE is restricted to South America and North America generally east of the Mississippi river whereas WEE is found west of the Mississippi river and predominantly in the western United States and Canada, although it also occurs in Florida and South America.

Viral ecology

Humans, horses, cattle, pigs, dogs, and ratites are accidental hosts of the virus. The EEE and WEE viruses are normally maintained in a host–vector relationship by cycling between mosquitoes, and some other hematophagous insects, and the definitive host. However, there are some important differences in the ecology of the different viruses.

Animal risk factors

Recovered horses are resistant to infection for at least 2 years, and vaccination confers immunity of variable duration (see under ‘Control’). Unvaccinated horses are at increased risk of disease – the risk of a vaccinated horse contracting EEE is only 0.14 that of an unvaccinated horse.^8,11 The disease is more severe, and mortality is higher, in unvaccinated horses than in vaccinated horses.¹² The mortality in young foals from non-immune mares, that are infected with WEE, is always high, often as high as 100%.

Housing and exposure to mosquitoes are important risk factors for EEE, and presumably WEE. During an outbreak in 1831, only horses kept at pasture were affected.¹³ The use of insect repellants reduces the odds of a horse being infected with EEE to 0.04 that of an unprotected horse.¹¹ Similarly, keeping horses at pasture near woods increases the risk of disease by almost four times, and the presence of swamp land increases the risk by over two times.¹¹ Horses kept in areas with high precipitation have an increased risk of the disease, presumably because of the density of mosquitoes in these areas.¹⁴

Morbidity and case fatality

Morbidity varies widely depending upon seasonal conditions and the prevalence of insect vectors; cases may occur sporadically or in the form of severe outbreaks affecting 20% or more of a group. The prevalence of infections, as judged by serological examination, is much higher than the clinical morbidity. The case fatality rate differs with the strain of the virus; in infection with the WEE virus it is usually 20–30% and with the EEE it is usually between 40 and 80% and may be as high as 90%.

Zoonotic implications

The susceptibility of humans to the causative virus gives the disease great public health importance. Humans can become infected with the EEE virus and the WEE virus.¹

Pigs

EEE causes an encephalitis and myocarditis of piglets less than 2 weeks of age.³ The disease is characterized by incoordination, seizures, vomition, weight loss, and paddling. Recovered piglets can have retarded growth.³

Samples for postmortem confirmation of diagnosis

Note the zoonotic potential of these organisms when handling the carcass and submitting specimens.

Vaccination

Vaccination of horses is important not only because it minimizes the risk of disease in vaccinated horses but also because of the possibility that for EEE it prevents viremia, subsequent infection of feeding mosquitoes, and propagation of the epidemic.

Formalin-inactivated EEE and WEE virus vaccines are available and are effective although over 50% of horses with EEE had been vaccinated within the previous year.^17,25 This apparent poor protection can be explained by many horses not developing a detectable change in antibody titer after vaccination with a bivalent vaccine and rapid decreases in antibody titer from a peak value achieved 2–4 weeks after vaccination.¹⁷ Vaccines are available as univalent or bivalent preparations and in combination with other antigens (for instance, tetanus toxoid). Horses should be vaccinated well in advance of the anticipated encephalomyelitis season in a given area. Vaccination against both strains of the virus is advisable in areas where the strain has not been identified or where both strains exist. The currently recommended vaccination schedule consists of two doses of the vaccine initially, 10 days apart, followed by annual revaccination using two doses. Annual revaccination is currently recommended because the duration of effective immunity beyond 1 year is not known. It is probable that the initial two-dose vaccination lasts for up to 3–4 years. The emphasis in a vaccination program should be on the young horses.

Colostral antibody can be detected in the blood of foals from vaccinated dams for up to 6–7 months, after which time it declines rapidly. Foals from vaccinated dams should be vaccinated at 6–8 months of age and revaccinated at 1 year of age. Foals from unvaccinated dams may be vaccinated at 2–3 months of age and again at 1 year of age. Colostral antibodies in the foal will prevent the development of autogenous antibodies, and foals vaccinated when less than 6 months should be revaccinated when they are 1-year-old or, in high-risk areas, foals from vaccinated mares should be vaccinated at 3, 4, and 6 months of age.

Experimental DNA vaccines hold promise for the prevention of WEE.²⁶

Protection from insects

Housing of horses indoors at night, especially in flyproofed stables, and the use of insect repellents may restrain the spread of the virus. Use of insect repellents decreases the risk of EEE in horses to 0.04 that of unprotected horses.

Widespread spraying of insecticides to reduce the population of the vector insects has been used in the control of VEE, however, such measures are not practical for preventing sporadic cases of EEE or WEE, and the environmental impact of widespread insecticide use should be considered.

Complete eradication of the virus appears to be impossible because of the enzootic nature of the ecology of the virus. The horse being an accidental host for EEE and WEE virus make elimination of the virus impossible with methods currently available.

Zoonotic aspects of control

Control of the disease in humans in areas where the disease may occur is dependent on insect control, and a monitoring and surveillance early warning system is necessary to decide whether or not to take control measures. In areas where WEE occurs, clinical cases of the disease in unvaccinated horses usually precede the occurrence of the disease in human.²⁷ The establishment of a reporting system whereby practicing veterinarians report all clinical cases of the disease in horses will also assist in predicting potential epidemics of WEE virus infection in the human population. Serological surveys of wildlife may also serve as good indicators of the geographical distribution and seasonality of circulation of these viruses and provide an early warning system prior to the detection of human cases.