Seven serotypes1-7 of equine encephalosis virus (EEV) infect equids of southern Africa including Kenya, Botswana and South Africa.1 The virus is an insect-borne orbivirus that is transmitted by a variety of Culicoides spp.2 and is closely related to bluetongue and epizootic hemorrhagic disease viruses. It has characteristics in cell culture similar to African Horse Sickness.5 The virus replicates in midges although the rate differs depending on species of midge and strain of the virus. The genetic and phenotypic stability of strains of the virus are unknown and there exists the potential for emergence, or recognition, of new strains. Variations in pathogenicity are not recognized, perhaps because of incomplete knowledge of this importance of this virus, but might exist. There is independent persistence of virus serotypes in a maintenance cycle based on observation of increased rates of seasonal seroconversion to a specific serotype with ongoing low level of infection by other serotypes.1 Horses, donkeys, and zebra in southern Africa frequently have antibodies to a group epitope of the virus indicating widespread infection of these Equidae. Seventy seven percent of 1144 horses, 57% of 518 horses, 49 % of 4875 donkeys and up to 88% of zebra in South Africa have antibody to EEV.1-4 Elephant seldom have antibodies to EEV.4
The virus was originally isolated from a horse with signs of neurologic disease, hence the name. However, the disease associated with infection by EEV is poorly documented and, given the high prevalence of infection, EEV might be falsely incriminated in some situations. Most infections are subclinical based on the high seroprevalence rate and lack of reports of outbreaks of the disease. Clinical signs commonly attributed to EEV infection include fever, lassitude, edema of the lips, acute neurologic disease and enteritis. Abortion has been associated with infection by EEV. Disease associated with EEV has not been recorded in donkeys or zebra.2 Characteristic abnormalities in serum biochemistry or hematology are not reported. Antibodies to the virus are detected by serum neutralization assays (which are serotype specific) and ELISA, which is not serotype specific. A group specific, indirect sandwich ELISA detects EEV antigen and does not cross react with African Horse Sickness virus, Bluetongue virus, or epizootic hemorrhagic disease virus.6 Necropsy examination reveals cerebral edema, localized enteritis, degeneration of cardiac myofibers and myocardial fibrosis but whether all these abnormalities are attributable to EEV is unclear.5 Definitive diagnosis is difficult, if not impossible, at the current time because of the high prevalence of seropositive animals and the ill-defined clinical and necropsy characteristics of the disease. There are no recognized treatment, control or preventive measures. There is no vaccine.
1 Howell PG, et al. Ond J Vet Res. 2002;69:79.
2 Paweska JT, Venter GJ. Med Vet Entomol. 2004;18:398.
3 Venter GJ, et al. Equine Infect Dis III. Newmarket, England: R & W Publications, 1999.
4 Barnard BJH. Ond J Vet Res. 1997;64:95.
5 Laegrid WW. Virus infections of Equines. Amsterdam: Elsevier, 1996;125.
Etiology Venezuelan encephalitis virus (types 1AB, 1C, and, to a lesser extent, 1E)
Epidemiology Disease limited to the Americas. Arthropod, usually mosquito, borne virus. Mammals, including horses, are accidental hosts. Horse are amplifying hosts and can spread VEE. Care fatality rate 5–70%. VEE occurs as epidemics
Clinical findings Fever, muscle fasciculation, severe depression, head pressing, incoordination, recumbency, opisthotonos and paddling, and death
Lesions Non-supparative encephalomyelitis
Diagnostic confirmation Virus isolation and identification. Identification of viral antigen by indirect immunofluorescence. Serological confirmation of exposure, preferably demonstrating an increase in hemagglutination inhibition, virus neutralization, or complement fixation titer
Treatment No specific treatment. Supportive care
Control Vaccination with formalin-inactivated or modified live virus. Insect control
Venezuelan equine encephalomyelitis is associated with an arthropod borne alphavirus (family Togaviridae) Venezuelan equine encephalomyelitis virus (VEE). The VEE complex has one virus, VEE, with six antigenically related subtypes: I, VEE; II, Everglades; III, Mucambo; IV, Pixuna; V, Cabassou; and VI, AG80-663. Within subtype I are at least five variants. Epidemic (pathogenic) VEE in horses is associated with variants IAB (originally identified as distinct variants, A and B are now considered the same variant) and IC; all other subtypes of I (D–F), and other variants of VEE virus, are usually non-pathogenic for horses and are found in sylvatic or enzootic, non-equine cycles. The epidemic variants are exotic to the United States. Outbreaks occurred in Mexico in 1993 and 1996, and in Venezuela and Columbia in autumn 1995.1,2 The Columbian outbreak affected 90 000 people and killed an estimated 4000 horses.1 The strain involved in the Columbian outbreak was IC, while that involved in the Mexican outbreaks was a variant of the usually non-pathogenic IE.2 The outbreak in Mexico was associated with a variant of VEE that did not cause viremia in horses, although it was capable of causing neurologic disease in this species, and it might have been this attribute that abbreviated the course of the epidemic.3 There is evidence of continuing enzootic circulation of VEE IE in southern Mexico.4
The viruses is extremely fragile and disappear from infected tissues within a few hours of death.
The encephalitis viruses cause disease in horses, humans, pigs, and various birds including ratites and domestic pheasants.5
Pathogenic or epidemic VEE is found in northern South America, Central America, Mexico and, rarely, in the southern United States.5 Non-pathogenic, or endemic, strains of VEE are found in South and Central America and in parts of the southern United States.
VEE exists as both non-pathogenic and pathogenic strains. Non-pathogenic VEE viruses persist in sylvatic cycles in northern South America, Central America and parts of the southern United States, and are important because they are the source of the epizootic strains of the virus that emerge at infrequent intervals.2,6-8 The enzootic strains also confound the diagnosis of VEE because of the extensive serological cross-reactivity among endemic and epidemic VEE viruses. However, recent advances in diagnostic techniques may have solved this diagnostic problem. The non-pathogenic viruses are maintained in rodents associated with swamps, and transmitted by mosquitoes of the genus Culex, and perhaps other hematophagous insects. Humans, horses, cattle, pigs, dogs, and ratites are accidental hosts of the virus. Epidemics of VEE occur irregularly, the latest being in northern Columbia in 1995, and Mexico in 1993 and 1996.1,2 The source of virus during outbreaks is infected horses. Horses develop a profound viremia and are amplifying hosts that aid in the spread of the epizootic; other domestic species, including cattle, pigs, and goats are not considered to be amplifiers of the virus. During epizootics, all species of mosquitoes that feed on horses, including Aedes, Psorophora and Deinocerites species, are thought to be capable of spreading the infection. Epizootics end as the population of susceptible horses decreases below a critical level, either by death or vaccination. The reservoir of the virus between outbreaks, which may be up to 19 years, was unknown until it was demonstrated that epidemic VEE type IAB virus arises by mutation of endemic strains (types ID–F and II–vi), or that type IE (enzootic) mutates into an epizootic form serologically very similar to IE. This mutation of the endemic virus into the epidemic form has occurred on at least three occasions associated with epidemics of VEE.2,6-8 It is likely that pathogenic strains of VEE will continue to emerge in areas where the non-pathogenic strains of the virus are endemic.8
Recovered horses are resistant to infection for at least 2 years, and vaccination confers immunity of variable duration (see under ‘Control’). Housing and exposure to mosquitoes are important risk factors for EEE, and presumably VEE.
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 is usually 40–80% and may be as high as 90% with VEE.
The susceptibility of humans to the causative virus gives the disease great public health importance. Humans can become infected with sylvatic and epizootic VEE subtypes.1 A recent outbreak of VEE in Columbia caused 75 000 human cases, 300 fatalities and killed approximately 4000 horses.1 Human infections generally follow equine infections by approximately 2 weeks. The infection in humans is usually a mild, influenza-like illness in which recovery occurs spontaneously. When clinical encephalitis does occur it is usually in very young, or older, people. Occurrence of the disease in humans can be limited by the use of a vaccine in horses, thus limiting the occurrence of the disease in horses in the area. There is a strong relationship between the mosquito population and the incidence of the disease in horses and in humans.10 The occurrence of the disease in humans may be predicted by an unusually high activity of virus in mosquitoes. There are usually, but not always, widespread mortalities in horses before the disease occurs in humans.10 VEE infections have occurred among laboratory workers as a result of aerosol infections from laboratory accidents and from handling of infected laboratory animals. The TC83 live attenuated VEE virus vaccine may be teratogenic in humans.11
Inapparent infection is the mildest form of the disease and may be characterized by only a transient fever. A more severe form of the disease is manifested by tachycardia, depression, anorexia, occasional diarrhea, and fever.
Viremia persists throughout the course of the disease in VEE and the blood provides a source of infection for biting insects. Transplacental transmission of the VEE virus can occur in pregnant mares infected near term.12 The virus is present in saliva and nasal discharge, and this material can be used to transmit the disease experimentally by intranasal instillation.
Penetration of the virus into the brain does not occur in all cases and the infection does not produce signs, other than fever, unless involvement of the central nervous system occurs. The lesions produced in nervous tissue are typical of a viral infection and are localized particularly in the gray matter of the cerebral cortex, thalamus and hypothalamus, with minor involvement of the medulla and spinal cord. It is this distribution of lesions that is responsible for the characteristic signs of mental derangement, followed at a later stage by paralysis. The early apparent blindness and failure to eat or drink appear to be cortical in origin. True blindness and pharyngeal paralysis occur only in the late stages.
The diseases associated with the different viruses are clinically indistinguishable. The incubation period for VEE is 1–6 days. Uncomplicated disease usually lasts about 1 week. In the initial viremic stage there is fever, which may be accompanied by anorexia and depression, but the reaction is usually so mild that it goes unobserved. In the experimental disease, the temperature may reach 41°C (106°F) persisting for only 24–48 hours, with nervous signs appearing at the peak of the fever. Animals that have shown nervous signs for more than 24 hours may then have a temperature within the normal range.
Early nervous signs include hypersensitivity to sound and touch, and in some cases transient periods of excitement and restlessness, with apparent blindness. Affected horses may walk blindly into objects or walk in circles. Involuntary muscle movements occur, especially tremor of shoulder and facial muscles and erection of the penis. A stage of severe mental depression follows. Affected horses stand with the head hung low; they appear to be asleep and may have a half-chewed mouthful of feed hanging from the lips. At this stage the horse may eat and drink if food is placed in its mouth. The pupillary light reflex is still present. The animal can be aroused, but soon relapses into a state of somnolence.
A stage of paralysis follows. There is inability to hold up the head, and it is often rested on a solid support. The lower lip is pendulous and the tongue may hand out. Unnatural postures are adopted, the horse often standing with the weight balanced on the forelegs or with the legs crossed. Head-pressing or leaning back on a halter are often seen. On walking, there is obvious incoordination, particularly in the hindlegs, and circling is common. Defecation and urination are suppressed and the horse is unable to swallow. Complete paralysis is the terminal stage. The horse goes down, is unable to rise and usually dies within 2–4 days from the first signs of illness. A proportion of affected horses do not develop paralysis and survive, but have persistent neurological deficits.
In the experimental infection of horses with the endemic strain of the VEE virus, a fever and mild leukopenia occurs.13 Following infection with the epidemic strain of the virus, a high fever and severe leukopenia are common, and a high level of neutralizing antibodies develop about 5–6 days after infection. Clinical findings include profound depression, accompanied by flaccidity of lips, partially closed eyelids, and drooped ears; some horses chew continuously and froth at the mouth. In the terminal stages, there is recumbency and nystagmus.
There are no characteristic hematological or biochemical abnormalities. The absence of biochemical indication of liver disease (hyperbilirubinemia, increased activity in serum of liver-specific enzymes such as sorbitol dehydrogenase and gamma glutamyl transferase, absence of hyperammonemia) rules out hepatic encephalopathy.
Diagnostic confirmation is achieved by one or more of several means:14
• Isolation of virus from an affected animal
• Detection of viral antigen or nucleic acid in an animal with appropriate clinical signs
• Seroconversion or an increase in serum titer of sick or recovered animal.
Virus isolation provides definitive proof of infection. However, viremia may have resolved by the time nervous signs have developed, and it may be advantageous to sample febrile animals instead of animals showing more advanced signs of the disease. Virus can be cultured in intracranially inoculated suckling mice, weanling mice, guinea pigs, cell culture, newly hatched chicks, or embryonated eggs.9 Virus isolates can be identified by complement fixation, hemagglutination inhibition, virus neutralization, PCR, IFA, and antigen capture ELISA.9,14 A recently developed indirect fluorescent test using monoclonal antibodies enables the differentiation of endemic from epidemic strains of VEE.15 Interpretation of the results of serological tests of horses in an area where endemic, non-pathogenic VEE virus exists is difficult because of the cross-reaction between endemic and epidemic strains of the virus. Therefore, in areas where there is endemic, non-pathogenic VEE, demonstration of the presence of antibodies should not be considered persuasive evidence of the presence of the disease.
Acute and convalescent sera taken 10–14 days apart for the presence of neutralizing, hemagglutination-inhibiting, or complement-fixing antibodies in the serum of affected or in-contact horses, is of value in detecting the presence of the virus in the group or in the area. A four-fold increase in complement-fixing antibodies is considered positive.
Demonstration of viral nucleic acid in tissue, blood or insects by PCR test may be a useful indicator of the presence of the virus. There may be sufficient viral antigen to be detected by ELISA in clinical material, and this may provide a useful test in the early stages of an epidemic.14
The brain meninges may appear congested, but there are generally no gross changes. Histological examination of the brain reveals perivascular accumulations of leukocytes and damage to neurons.16 The gray matter of the forebrain and midbrain are the most severely affected areas. In some cases of VEE, liquefactive necrosis and hemorrhage are visible in the cerebral cortex.17 Cell culture and transmission experiments utilizing brain tissue as an inoculum are the traditional means of confirming a diagnosis, and require that the brain be removed within an hour of death. Transmission is by intracerebral inoculation of brain tissue into sucking mice or duck embryo tissue culture. Fluorescent antibody tests have been developed to detect VEE virus6 and EEE virus in brain tissue.6
• One half of midsagittally-sectioned brain and liver and spleen should be submitted for fluorescent antibody and PCR testing, virus isolation and bioassay
• One half of midsagittally-sectioned brain, fixed in formalin, should be submitted for light microscopic examination.
Note the zoonotic potential of these organisms when handling the carcass and submitting specimens.
Clinically, the disease has very great similarity to the other viral encephalomyelitides, from which it can often be discriminated by the geographical location of the horse, and to the hepatic encephalopathies and a number of other diseases (see below and in Table 22.1).
• West Nile virus encephalomyelitis
• Borna disease – occurs in Europe
• Japanese encephalitis – occurs in Asia
• Various other viral infections that are geographically restricted
• Hepatic encephalopathy, such as that associated with poisoning by Crotalaria, Senecio, and Amsinckia spp.; acute serum hepatitis or hepatopathy
• Botulism causes weakness evident as muscle fasciculation, recumbency and dysphagia, but does not cause cerebral signs (irritation, behavioral abnormalities)
• Yellow star thistle poisoning (Centauria solstitialis) and poisoning by fumonisins can produce similar clinical signs to that of the encephalitides, with the exception of fever.
There is no definitive or specific treatment. Supportive treatment may be given with the intention to prevent self-inflicted injury, and maintain hydration and nutritional status.
Control of VEE of horses is based on:18-20
• Accurate clinical and laboratory diagnosis of the disease in horses
• Use of sentinel animals to monitor the presence of the virus in the region
• Quarantine of infected horses to stop movement of virus donors
Vaccination of horses is important not only because it minimizes the risk of disease in vaccinated horses but also because it prevents viremia, subsequent infection of feeding mosquitoes, and propagation spread of VEE.
One of the most important aspects of the control of VEE is the vaccination of the horse population to minimize the number of horses that are viremic and serve as amplifying hosts. A tissue culture-attenuated virus vaccine TC83, is available for immunization of horses against VEE. The vaccine is considered to be safe and efficacious21; concerns about reversion of the virus to virulence appear unfounded,9 but should be reconsidered in light of the observation that the source of epidemic VEE virus is mutation of endemic strains.2,6,8
A highly effective immunity is produced within a few days following vaccination, and serum neutralizing antibodies persist for 20–30 months.1 The vaccine causes a mild fever, leukopenia and a viremia and, because of conflicting reports about its capacity to cause abortion, should not be used in pregnant mares.1 Antibodies to the heterologous alphaviruses, WEE and EEE, existing at the time of TC83 vaccination, may suppress the VEE antibody response to the vaccine.
However, the response to the vaccine is adequate to provide protection against VEE, and the interference is not considered significant.22 There is inconclusive evidence that WEE and EEE antibodies protect horses against infection with virulent VEE virus, or conversely that VEE antibodies protect against infection with WEE and EEE viruses.23 Simultaneous vaccination using formalin-inactivated EEE, WEE, and VEE (the TC83 strain of VEE) is effective and recommended in areas where all three viruses may be present.
Housing of horses indoors at night, especially in flyproofed stables, and the use of insect repellents might restrain the spread of the virus.
Widespread spraying of insecticides to reduce the population of the vector insects has been used in the control of VEE in humans, along with vaccination of horses.24 Complete eradication of the virus appears to be impossible because of the enzootic nature of the ecology of the virus: epidemic VEE arising by chance mutation of endemic strains of VEE, make elimination of the virus impossible with methods currently available.
1 Rivas F, et al. J Infect Dis. 1997;175:828.
2 Smith JF, Oberste MS. Factors in the emergence of arbovirus diseases. Amsterdam: Elsevier, 1997;235.
3 Gonzalez-Salazar D, et al. Emerg Infect Dis. 2003;9:16.
4 Estrada-Franco JG, et al. Emerg Infect Dis. 2004;10:2113.
5 Elvinger F, et al. J Am Vet Med Assoc. 1994;205:1014.
6 Weaver S. Factors in the emergence of arbovirus diseases. Amsterdam: Elsevier, 1997;241.
7 Powers AM, et al. J Virol. 1997;71:6697.
8 Rico-Hesse R, et al. Proc Natl Acad Sci USA. 1995;92:5278.
9 Walton TE. J Am Vet Med Assoc. 1992;200:1385.
10 Grady GF, et al. Am J Epidemiol. 1978;107:170.
11 Casamassima AC, et al. Teratology. 1987;36:287.
12 Justines G, et al. Am J Trop Med Hyg. 1980;29:653.
13 Walton TE, et al. J Infect Dis. 1973;128:271.
14 Calisher CH, Walton TE. Studdert MJ, editor. Virus infections of vertebrates: virus infections of Equines 6. Amsterdam: Elsevier. 1996:141.
15 Roehrig JT, Bolin RA. J Clin Microbiol. 1997;35:1887.
16 Erickson GA, Mare CJ. Am J Vet Res. 1975;36:167.
17 Monath TP, et al. Am J Vet Res. 1981;42:1418.
18 Calisher CH, et al. J Am Vet Med Assoc. 1983;183:438.
19 Wong FC, Lillie LE. Am J Publ Health. 1976;67(Suppl 1):15.
20 Omohundro RE. J Am Vet Med Assoc. 1971;161:1516.
21 Baker EF, et al. Am J Vet Res. 1978;39:1627.
22 Ferguson JA, et al. Am J Vet Res. 1978;39:371.
Japanese encephalitis is associated with the Japanese encephalitis flavivirus (JEV), a member of the Flaviviridae family.1 Antigenically related viruses include Murray valley encephalitis virus, Kunjin virus, and West Nile virus. There appear to be a number of variants of JEV, with there being genetic and antigenic differences between endemic and non-endemic strains, and isolates of different years.1,2 The virus cycles between avian and mammalian amplifying hosts and the mosquitoes.1 The pig is the principal mammalian amplifying host among domestic animals. Horses and humans become infected but likely play only a minor role in the spread of the virus. The disease is listed by the OIE.
The disease occurs throughout the Orient and South-East Asia and has extended into Papua New Guinea, the Torres strait, and northern Australia. Outbreaks of disease occurred in the Torres strait in 1995 and disease in humans has occurred rarely in northern Australia.3 Outbreaks of disease have not occurred in Australia, despite large populations of wild pigs, wading birds, and mosquitoes probably because the mosquitoes prefer to feed on marsupials, which are poor hosts for JEV.3
Horse deaths are now uncommon in Japan, due to vaccination of most horses, but 15–70% of race horses have antibodies to JEV that are not induced by vaccination.4 Seroepidemiological surveys of cattle in Japan reveal that about 68% of animals are positive.5 Disease in horses and humans occurs in China.1 The prevalence of the disease is related to the population of pigs, the main amplifying host, the mosquito vector, and susceptible human and equine hosts.1 Factors affecting the number of mosquitoes include availability of suitable habitat, such as rice field in which survival of mosquito larvae is enhanced by application of nitrogenous fertilizers and the presence of phytoplankton which provide food and shelter for the larvae.6,7
The clinical manifestations of the disease in horses vary widely in severity. Mild cases show fever up to 39.5°C (103°F), anorexia, sluggish movements, and sometimes jaundice for 2–3 days only. More severe cases show pronounced lethargy, mild fever, and somnolence. Jaundice and petechiation of the nasal mucosa are usual. There is dysphagia, incoordination, staggering, and falling. Transient signs include neck rigidity, radial and labial paralysis, and blindness. In the most severe cases there is high fever (40.5–41.5°C, 105–107°F), hyperexcitability, profuse sweating, and muscular tremor. Violent, uncontrollable activity may occur for a short period. This severe type of the disease is uncommon, representing only about 5% of the total cases, but is more likely to terminate fatally. In most cases complete recovery follows an illness lasting from 4 to 9 days. A variety of tests are available to detect antibodies to JEV.8 A latex agglutination test provides accurate detection of antibodies in the field.9 However, definitive diagnosis of Japanese viral encephalitis should not be based exclusively on serology because infection with antigenically related viruses including Murray Valley encephalitis virus, Kunjin virus, and West Nile virus can cause false-positive (from the perspective of JEV) results.8
The disease in cattle, sheep and goats is usually clinically inapparent and of little overall significance. Widespread losses, however, have been reported in swine, particularly in Japan. The disease occurs as a non-suppurative encephalitis in pigs under 6 months of age. Sows abort or produce dead pigs at term.10 There are no characteristic gross changes. As is typical of most viral encephalitides, microscopic changes include a nonsuppurative encephalomyelitis, focal gliosis, neuronal necrosis, and neuronophagia. Isolation of this flavivirus is difficult and bioassay techniques are comparatively slow. As a result, detection via PCR is likely to be increasingly utilized. Immunohistochemistry can be used to demonstrate this virus in formalin-fixed, paraffin-embedded sections
• Virology – mL chilled CSF fluid, chilled brain (split along midline) (ISO, BIOASSAY, PCR)
• Histology – fixed samples of other half of brain, lung, spleen, liver, heart (LM, IHC).
Note the zoonotic potential of this organism when handling carcass and submitting specimens.
There is no specific treatment for the disease. Control is by vaccination. Formalinized vaccines afford excellent protection in pigs and horses.4
1 Getah virus infection. In: Calisher CH, Walton TE, Studdert MJ, editors. Virus infections of vertebrates: virus infections of Equines 6. Amsterdam: Elsevier; 1996:141.
2 Yamada A, et al. Vet Pathol. 2004;42:62.
3 van den Hurk AF, et al. Med Vet Entomol. 2003;17:403.
4 Konishi E, et al. Vaccine. 2004;22:1097.
5 Sakai T, Horimoto M. Prev Vet Med. 1989;7:39.
6 Sunish IP, Reuben R. Med Vet Entomol. 2001;15:381.
7 Sunish IP, Reuben R. Med Vet Entomol. 2001;16:1.
8 Anon. OIE Manual of diagnostic tests and vaccines for terrestrial animals. http://www.oie.int/eng/normes/mmanual/A_00092.htm. Accessed February 26 2005.
Etiology West Nile virus, a flavivirus
Epidemiology Maintained in a bird–mosquito cycle. Mammals are incidentally infected. Enzootic in Africa and, recently, North America. Epizootics in the Mediterranean littoral and southern Europe. Affects a wide variety of species including horses, humans, sheep, camelids, and dogs
Clinical signs Weakness, incoordination, altered mentation, muscle fasciculations, recumbency
Clinical pathology MAC-ELISA for diagnosis
Lesions Polioencephalomyelitis
Diagnostic confirmation MAC-ELISA, clinical signs, lesions
Encephalitis in horses, humans, and other species is associated with West Nile virus, an arthropod borne flavivirus in the Japanese encephalitis virus group. Other viruses in the group include Japanese encephalitis virus (Japan and South-East Asia), St Louis encephalitis virus (United States), Kunjin virus (now considered a subtype of West Nile virus, Australia), Murray Valley encephalitis virus (Australia), and Rocio virus (Brazil).1 The virus was first isolated in 1937 from a human with fever in Uganda. There are at least two lineages of the virus, with one lineage (lineage 1) isolated from animals in central and North Africa, Europe, Israel, and North America whereas the other (lineage 2) is enzootic in central and southern Africa.2 The recent outbreak in North America was associated with a virus identical to that isolated from diseased geese in Israel.3 Viruses of both lineages can circulate at the same time in the same geographic region.1 Virus of either lineage can cause disease, although that of lineage 1 appears to be associated with more severe disease in horses and other species. There are anecdotal reports of Kunjin virus being associated with isolated cases of encephalomyelitis in horses in Australia.
The West Nile virus causes disease in humans, horses, birds (including geese, raptors, and corvids), sheep, alpacas, and dogs.2,4-6
West Nile encephalitis virus is enzootic to Africa and sporadic outbreaks of the disease occurred in the 1960s in Africa, the middle East and southern Europe.7 Recently, outbreaks affecting horses and other animals have occurred in southern France, Tuscany, Israel and other parts of southern Europe.8-11 The virus was introduced into New York city in North America in 1999 and subsequently spread widely across the continent, including Canada, Mexico and the Caribbean, in the next several years, and reaching the west coast by 2004.12 The virus caused widespread deaths of wild birds and disease and death in humans, horses and other species in North America during this period.13
Introduction of the infection to North America was associated with an epizootic of disease that over several years moved across the continent. During the initial years of the epizootic there were large numbers of cases in horses (15 000) and humans (4000) and death of at least 16 500 birds.14 As the front of the epizootic moved across the country, the infection became enzootic and the number of cases in horses in these regions decreased markedly over those in the first year.
The virus is maintained by a cycling between amplifying hosts, usually birds, and insect vectors. Large mammals, including horses and humans, are incidentally infected and are not important in propagation of the virus. Amplifying hosts are those in which the viremia is of a sufficient magnitude and duration (1–5 days) to provide the opportunity to infect feeding mosquitoes. Mammals, and in particular horses, are in general not amplifying hosts because of the low level of viremia.15 The virus is spread by the feeding of ornithophilic mosquitoes, usually of the genus Culex. Infected mosquitoes carry the virus in salivary glands and infect avian hosts during feeding. The virus then multiplies in the avian host causing a viremia that may last for up to 5 days. Mosquitoes feeding on the avian host during the viremic phase are then infected by the virus. This pattern of infection of amplifying hosts and mosquitoes is repeated such that the infection cycles in these populations. Increases in mosquito number, such as occur at the end of the summer, and enhanced viral replication in mosquitoes at higher ambient temperatures, increase the likelihood that avian hosts, or incidental hosts, will become infected. This results in an increase in the incidence of disease in late summer and early autumn.16-19
The principal avian host and vector species vary markedly between geographic regions. In North America the house sparrow (Passer domesticus) is the principal amplifying host and Culex pipens is the principal vector. Culex pipens, and other mosquito vectors, feed almost exclusively on passerine and columbiform birds early in the season, but later in the summer in temperate regions switch to feeding on mammalian hosts.20 This change in feeding behavior is associated with increased frequency of infection and disease in mammals, including horses and humans, in the late summer.
The virus cycles between the avian host and insect vectors year round in tropical regions. However, in temperate regions in which mosquitoes do not survive during the winter the mechanism by which the virus survives over winter is unknown.13
Transmission is only by the bite of infected insect vectors. There is no evidence of horizontal spread of infection among horses. The disease can be spread in humans by transfusion of blood or transplantation of organs obtained from an infected person.21
The disease occurs in parts of the world as epidemics apparently associated with sporadic introduction of the virus into non-endemic regions, such as the Mediterranean littoral. Introduction of the virus to these regions occurs infrequently enough that horses have no active immunity and are susceptible to infection and disease. Horses immune through either natural infection or vaccination are resistant to the disease. The effect of immunity was evidenced in North America by the marked decrease in morbidity and mortality among horses after the epizootic waned and the disease became enzootic. The decrease in morbidity was attributed to both natural and vaccinal immunity. Interestingly, although the number of cases in horses decreased rapidly there was not a similar decrease in the number of human cases, perhaps because of the lack of a vaccine for use in humans.13
Horses of all ages appear to be equally susceptible to infection. Disease is report in horses aged from 5 months to >20 years. There does not appear to be any predilection based on breed or sex.16
The incidence of the disease during an epizootic can be as high as 74 cases per 1000 horses at risk.17 The case fatality rate among horses treated in the field is 22–38%16,17,22 whereas it is 30–43% of horses in referral centers.19,23
Infection of humans by West Nile virus can result in fatal encephalitis, although less severe disease or inapparent infection is more common. The virus has zoonotic potential and tissues from potentially infected animals and virus cultures should be handled in containment level 3 facilities, particularly material from potentially infected birds.24
Horses are infected by the bite of infected mosquitoes. Feeding by as few as 7 infected mosquitoes is sufficient to cause infection in seronegative horses.15 Viremia, which persists for less than 2 days, occurs 2 to 5 days after feeding by infected mosquitoes.15 West Nile encephalitis occurs in only a small proportion of infected horses. The virus localizes in cells in the central nervous system where it induces a severe polioencephalomyelitis with the most severe lesions being in the spinal cord.25 Lesions are often evident in the ventral horn of the spinal cord, which is consistent with clinical signs of weakness.
The incubation period after natural infection is estimated to be 8–15 days. Fever occurs early in the disease but is uncommon at the time that signs of neurologic disease become evident.16,17 Affected horses are often somnolent, listless, or depressed although hyperexcitability has been reported. The signs of neurologic disease, including muscle fasciculation, weakness, and incoordination, develop within a period of hours and may progress over several days. Muscle fasciculations are common in the head and neck, but can occur in any muscle group. Weakness is most pronounced in limb and neck muscles and severely affected horses are recumbent with flaccid paralysis. Signs of neurologic disease are usually, but not reliably, bilaterally symmetrical. Altered mentation, blindness, and cranial nerve abnormalities, if they occur, usually become evident after signs of spinal cord disease are apparent.
Weakness with or without ataxia is present in almost all affected horses, whereas altered mentation is detected in approximately 66% of horses.19 Cranial nerve abnormalities are evident in approximately 40% of horses, whereas apparent blindness or lack of menace reflex occurs in 3–7% of horses.17,19
Median recovery time for horses treated in the field is 7 days, with a range of 1–21 days.17
The prognosis depends on the severity of clinical signs. Horses that become recumbent and unable to rise are approximately 50 times more likely to die than are horses that remain able to stand while affected by the disease.22 Most horses that survive the initial disease do not have signs of neurologic dysfunction 6 months later.19
Disease associated with West Nile virus is documented in small numbers of other species, including squirrels, chipmunks, bats, dogs, cats, reindeer, sheep, alpacas, alligator and a harbor seal during intense periods of local viral activity.24 The disease in camelids is characterized by acute recumbency and altered mentation.2,5
Affected horses are often mildly lymphopenic, and hyperbilirubinemic (likely due to anorexia), and occasionally azotemic.19 These changes are not diagnostic of West Nile encephalitis.
Cerebrospinal fluid is abnormal in approximately 70% of horses with signs of neurologic disease. Abnormalities include mononuclear pleocytosis and elevated total protein concentration.26
Antibody can be identified in equine serum by IgM capture enzyme-linked immunosorbent assay (IgM capture ELISA, MAC-ELISA), hemagglutination inhibition (HI), IgG ELISA or plaque reduction neutralization (PRN).23 Equine WN-specific IgM antibodies are usually first detec table 7–10 days after infection and persist for 1–2 months.15 Because the incubation period of the disease after infection by bite of infected mosquitoes is at least 8 days, West Nile-specific IgM is usually present at the time of development of clinical signs of the disease. The MAC-ELISA is therefore a useful test in the diagnosis of the disease.19
WNV neutralizing antibodies are detectable in equine serum by 2 weeks post-infection and can persist for more than 1 year. In some serological assays, antibody cross-reactions with related flaviviruses (St Louis encephalitis virus or Japanese encephalitis virus), can be encountered.23 The PRN test is the most specific among WN serological tests and all affected horses have titers ≥1:100 4–6 weeks after recovering from the disease, and 90% of horses maintain this titer 5–7 months after recovery.27
Detection by MAC-ELISA of West Nile specific IgM in serum at dilutions greater than 1:400, in the presence of appropriate clinical signs, is considered diagnostic of WNV.28 Similarly, a four-fold increase in PRN titer in serum collected during the acute and convalescent stages of the disease, in the absence of vaccination and in the presence of appropriate clinical signs is considered diagnostic.
The virus can be grown in cell culture and viral nucleic acid can be demonstrated in tissues of infected animals by RT-PCR.29 Note that infected horses have much lower concentrations of virus than do infected birds, and failure to demonstrate viral antigen in infected horses is not uncommon, especially if less sensitive techniques, such as immunohistochemistry, are used.24,25
Gross lesions are infrequently seen. When present they consist of multifocal areas of congestion and hemorrhage within the medulla oblongata, midbrain, and spinal cord. Histopathologic changes include a nonsuppurative poliomeningoencephalomyelitis with multifocal glial nodules and neuronophagia. The inflammatory changes and viral distribution are concentrated in the rhombencephalon and spinal cord, with comparatively little damage to the cerebrum. One immuohistochemical study of naturally infected horses concluded that examination of the spinal cord is required to accurately identify WNV infection.25 Another report, in which RT-PCR was employed, concluded that high quality samples of medulla were sufficient to detect the presence of the virus.30 Postmortem confirmation of the diagnosis through virus isolation is possible but the sensitivity is generally inferior to molecular biology-based techniques. RT-PCR is generally superior to IHC. The processing of tissue from multiple CNS sites is recommended in order to increase the chances of finding a virus-rich focus. High concentrations of WNV are not found in non-CNS tissues of infected equids, in contrast to the distribution of the virus in many other species.
• Virology – minimum sample is one half of sagittally sectioned hindbrain (must include medulla). Ideally a segment of thoracolumbar spinal cord as well. Submit samples chilled (VI, RT-PCR)
• Histology – same samples, fixed in formalin (LM, IHC, RT-PCR).
Note the zoonotic potential of this disease when collecting and submitting specimens. Some authorities recommend using containment level 3 precautions when handling potentially infected tissues, such as that from birds.
Differential diagnoses for West Nile encephalitis include (Table 22.1):
There is no specific treatment for West Nile encephalitis although administration of interferon or hyperimmune globulin has been advocated. Affected horses are often administered non-steroidal anti-inflammatory drugs such as flunixin meglumine, dimethyl sulfoxide, or corticosteroids in an attempt to reduce inflammation in neural tissue.19 Administration of corticosteroids minimally but statistically significantly increases the likelihood of survival22 but this practice is controversial. Treatment is based on supportive care and prevention of complications of neurologic disease and includes assistance to stand, including use of a sling support, administration of antimicrobials, and maintenance of hydration and nutrition.
Control of disease associated with West Nile virus is achieved by vaccination and minimization of exposure. Elimination of the virus is not practical given that it cycles through avian and insect vectors and that the horse is incidentally infected.
Vaccination is believed to be effective in preventing development of disease, and has been demonstrated to reduce the likelihood of death in horses with West Nile encephalitis by approximately 2–3 times.17,22 Efficacy of vaccination in preventing disease in horses has not been formally demonstrated although there is general agreement that vaccination is an important aspect to control of the disease.13 There is no evidence that administration of the inactivated virus vaccine increases the risk of fetal loss in mares.31 Vaccination prevents viremia in most horses following exposure to WNV-infected mosquitoes.32
Both inactivated virus vaccine and a live canarypox-vectored recombinant vaccine are available in North America. The inactivated virus vaccine should be administered in two doses at an interval of 3–6 weeks in early summer in the first instance, and then again once to twice yearly before the season of peak disease incidence. Foals from unvaccinated mares should be administered the vaccine beginning at 2–3 months of age, and foals of vaccinated mares should be administered the vaccine beginning at 7–8 months of age.
Administration of the recommended two doses of inactivated virus vaccine fails to induce an adequate plaque reduction titer in approximately 14% of horses 4–6 weeks after vaccination, and in 30% of horses 5–7 months after vaccination.27 This effect was especially evident in horses >10 years of age.27 These results indicate that some horses will not develop protective immunity against WNV despite administration of vaccine in the recommended dose and interval.
Minimization of exposure of horses to the virus includes reducing the population density of mosquitoes and protecting horses from being bitten. Reducing the population of mosquitoes includes widespread spraying with insecticides and elimination of mosquito breeding sites. Widespread spraying in cities is employed when the disease is a risk for humans but is not practical for controlling mosquitoes in rural areas. Environmental concerns make this approach to control unacceptable in many regions.
Removal of larval habitat by draining standing water is recommended for control of WNV, although the efficacy of this approach has not been demonstrated. Standing water includes not just dams and ponds but also poorly maintained outdoor swimming pools, bird baths, discarded vehicle tires, and other receptacles that could hold water. Use of larvicidal compounds in standing water is recommended by some authorities.
Minimizing the frequency with which horses are bitten by mosquitoes has the potential to reduce the risk of contracting the disease. However specific recommendations are not available. Housing during periods of peak mosquito activity, especially at dawn and dusk, might reduce the risk of disease.
1 Solomon T. New Engl J Med. 2004;351:4.
2 Kutzler MA, et al. J Am Vet Med Assoc. 2004;225:921.
3 Banet-Noach C, et al. Virus Genes. 2003;26:135.
4 Tyler JW, et al. J Vet Int Med. 2003;17:242.
5 Dunkel B, et al. J Vet Int Med. 2004;18:365.
6 Buckweitz S, et al. J Vet Diag Invest. 2003;15:324.
7 Burt FJ, et al. Emerg Infect Dis. 2002;8:820.
8 Steinman A, et al. Vet Rec. 2002;151:47.
9 Autorino GL, et al. Emerg Infect Dis. 2002;8:1372.
10 Murgue B. Emerg Infect Dis. 2001;7:692.
11 Cantile C, et al. Equine Vet J. 2000;32:31.
12 Petersen LR, Hayes EB. New Eng J Med. 2004;351:2257.
13 Castillo-Olivares J, Wood J. Vet Res. 2004;35:467.
14 Anon. Morb Mort Weekly Report. 2003;52:1160.
15 Bunning ML, et al. Emerg Infect Dis. 2002;8:380.
16 Ward MP, et al. J Am Vet Med Assoc. 2004;225:84.
17 Schuler LA, et al. J Am Vet Med Assoc. 2004;225:1084.
18 Steinman A, et al. Vet Rec. 2002;151:47.
19 Porter MB, et al. J Am Vet Med Assoc. 2003;222:1241.
20 Bernard KA, Kramer LD. J Immunol. 2001;14:319.
21 Pealer LN, et al. New Eng J Med. 2003;349:1236.
22 Salazar P, et al. J Am Vet Med Assoc. 2004;225:267.
23 Weese JS, et al. Can Vet J. 2003;44:469.
24 http://www.oie.int/eng/normes/mmanual/A_00133.htm. Accessed March 11 2005.
25 Cantile C, et al. Vet Pathol. 2001;38:414.
26 Wamsley HL, et al. J Am Vet Med Assoc. 2002;221:1303.
27 Davidson AH, et al. J Am Vet Med Assoc. 2005;226:240.
28 Tardei G, et al. J Clin Micro. 2000;38:2232.
29 Johnson DJ, et al. J Vet Diag Invest. 2003;15:488.
30 Kleiboeker SB, et al. J Vet Diag Invest. 2004;16:2.
Borna disease is an infectious encephalomyelitis of horses and sheep first recorded in Germany. It is associated with a negative sense, single-stranded RNA virus classified as Bornavirus within the order Mononegavirales.1 The disease and the virus are indistinguishable from near Eastern equine encephalomyelitis. Borna disease is now recognized as a subacute meningoencephalitis in horses, cattle, sheep, rabbits, and cats in Germany, Sweden and Switzerland.2-4 There are reports of encephalitis with Borna disease virus genome detected in lesions by PCR in a horse and a cow in Japan.5,6 Encephalitis associated with BDV was detected in young ostriches in Israel.8
Borna disease virus (BDV) is suspected of causing disease in humans, including lymphocytic meningoencephalitis, but BDV infection is not associated with an increased prevalence of psychiatric disorders.8,9 Others suggest that the presence of circulating BDV immune complexes (BDV antigen and specific antibodies) is associated with severe mood disorders in humans.10 The role, if any, of BDV in human neurologic or psychiatric disease has not been established with any certainty.
Detection of BDV genome by PCR analysis suggests that, while the spontaneous disease in horses and sheep occurs predominantly if not exclusively in Europe, clinically unapparent BDV infection is widespread in a number of species including horses, cattle, sheep, cats, and foxes.11,12 However, concern has been raised that some of these reports might be based on flawed laboratory results as a consequence of contamination of PCR assays.11 Antibodies to BDV in serum or cerebrospinal fluid have been detected in horses in the eastern United States, Japan, Iran, Turkey, France, and China,11,13-17 and in healthy sheep and dairy cattle in Japan.18,19 In areas in which the disease is not endemic between 3% (US) and 42% (Iran) of horses have either antibodies or BDV nucleic acid, detected by PCR, in blood or serum. Similarly, approximately 12–20% of horses have serological evidence of exposure to BDV in areas of Europe where the disease is endemic. Antibodies to BDV and nucleic acid have been detected in humans in North America, Europe, and Japan.20,21 Closed flocks of sheep and herds of horses have evidence of persistent infection of some animals, based on serological testing.22,23 It is worth noting that animals infected with the virus and clinically ill may have undetectable to very low antibody titers.22
The method of transmission of infection between animals is unknown, but it is thought to be horizontal by inhalation or ingestion. Seropositive, clinically normal horses and sheep can excrete virus in conjunctival fluid, nasal secretions and saliva,22,24 suggesting that they might be important in the transmission of infection. Removal of all seropositive and BDV RNA positive sheep from a closed flock did not prevent seroconversion of other animals in the flock the following year.22 The possibility of vertical transmission is raised by the finding of BDV RNA in the brain of a fetal foal of a mare that died of Borna disease.25
There is a seasonal distribution to the prevalence of the disease, with most cases in horses occurring in spring and early summer.21 The virus has not been isolated from arthropods, including hematophagous insects.
The morbidity in Borna disease is not high, approximately 0.006–0.23% of horses affected per year in endemic areas of Germany,21 but most affected animals die.
The pathogenesis of the disease involves infection of cells of the central nervous system. It is assumed that the virus gains entry to the central nervous system through trigeminal and olfactory nerves, with subsequent dissemination of infection throughout the brain.27,28 Viral transcription and replication occurs within the cell nucleus.28 Viral replication does not appear to result in damage to the infected neuron. However, infected cells express viral antigens on their surface which then initiate a cell-mediated immune response by the host that then destroys infected cells – immunosuppression prevents development of the disease.29 The inflammatory response is largely composed of CD3+ lymphocytes.30 The disease is subacute; infection and the development of lesions may take weeks to months. Clinically inapparent infection appears to be common in a number of species, including horses.
In field outbreaks the incubation period is about 4 weeks, and possibly up to 6 months.28
Clinical signs of the disease in horses include:
• Hyperesthesia.27
Lethargy, somnolence and flaccid paralysis are seen in the terminal stages, and death occurs 1–3 weeks after the first appearance of clinical signs. Infection without detectable clinical signs is thought to be common on infected premises. The frequency with which BDV is detected in horses with gait deficits is greater than in clinically normal horses, suggesting a role for the virus in inducing subtle disease.31,32
The disease in cattle has a similar presentation as that in horses, with affected animals having reduced appetite, ataxia, paresis, and compulsive circling.2,3 The disease ends in the death of the animal after a 1–6-week course.2
Hematology and routine serum biochemistry are typically normal, with the exception of fasting induced hyperbilirubinemia in anorexic horses. Clinicopathological identification of exposed animals is achieved with the complement fixation, ELISA, western blot, or indirect immunofluorescent tests.33
At necropsy there are no gross findings, but histologically there is a lymphocytic and plasmacytic meningoencephalitis, affecting chiefly the brain-stem, and a lesser degree of myelitis. The highest concentration of virus is in the hippocampus and thalamus.27 The diagnostic microscopic finding is the presence of intranuclear inclusion bodies within neurons, especially in the hippocampus and olfactory bulbs. The virus can be grown on tissue culture and demonstrated within tissues by immunofluorescence and immunoperoxidase techniques.28 BDV can also be detected in formalin-fixed, paraffin-embedded brain tissues using a nested PCR.34
Specific control measures cannot be recommended because of the lack of knowledge of means of transmission of the virus. The role of inapparently infected horses in transmission of the disease is unknown, and there is no widespread program for testing for such horses. An attenuated virus vaccine was produced by continued passage of the virus through rabbits and used in the former East Germany until 1992.21 However, its use was discontinued because of questionable efficacy.
1 Torre JC. J Virol. 1994;68:7669.
2 Bode L, et al. Vet Rec. 1994;135:283.
3 Caplazi P, et al. J Comp Path. 1994;111:65.
4 Lundgren AL, Ludwig H. Acta Vet Scand. 1993;34:101.
5 Taniyama H, et al. Vet Rec. 2001;148:480.
6 Okamoto M, et al. Vet Rec. 2002;150:16.
7 Ashash E, et al. Avian Dis. 1996;40:240.
8 Kubo K, et al. Clin Diag Lab Immunol. 1997;4:189.
9 Richt JA, et al. J Neurovirol. 1997;3:174.
10 Bode L, et al. Mol Psych. 2001;4:481.
11 Kao M, et al. Vet Rec. 1993;132:241.
11 Staeheli P, et al. J Gen Virol. 2000;81:2123.
12 Dauphin G, et al. 2001;82:2199.
13 Nakamura Y, et al. Vaccine. 1995;13:1076.
14 Bahmani MK, et al. Virus Res. 1996;45:1.
15 Galabru J, et al. Vet Rec. 2000;147:721.
16 Hagiwara K, et al. Vet Microbiol. 2001;80:383.
17 Yilmaz H, et al. Arch Virol. 2002;147:429.
18 Hagiwara K, et al. Med Microbiol Immunol. 1996;185:145.
19 Hagiwara K, et al. Clin Diag Lab Immunol. 1997;4:339.
20 Takahashi H, et al. J Med Virol. 1997;52:330.
21 Durrwald R, Ludwig H. J Vet Med B. 1997;44:147.
22 Vahlenkamp TW, et al. J Virol. 2002;76:9735.
23 Inoue Y, et al. J Vet Med Sci. 2002;64:445.
24 Richt JA, et al. Med Microbiol Immunol. 1993;182:293.
25 Hagiwara K, et al. Vet Microbiol. 2000;72:207.
27 Bilzer T, et al. Tierarztl Prax. 1996;24:567.
28 Becht H, Richt J. Borna disease. In: Studdert MJ, editor. Virus infections of vertebrates: virus infec tions of Equines 6. Amsterdam: Elsevier; 1996:235.
29 Morimoto K, et al. Proc Natl Acad Sci USA. 1996;93:13345.
30 Caplazi P, Ehrensperger F. Vet Immunol Immuno pathol. 1998;61:203.
31 Berg AL, et al. Arch Virol. 1999;144:547.
32 Hagiwara K, et al. Vet Microbiol. 2002;84:367.
There are a number of Bunyaviridae that cause disease in horses in the western hemisphere.1 The viruses are maintained in a mosquito-vertebrate host-mosquito or midge-vertebrate host-midge cycles with horses being infected and developing signs of neurologic disease occasionally.
The California serogroup of viruses are mosquito-transmitted viruses of the family Bunyaviridae which can cause acute encephalitis in horses.1,2 There are 12 serotypes isolated in Africa, Europe, Asia, and North and South America. Snowshoe hare and Jamestown Canyon are two of these serotypes that have been isolated in Canada and California, and that have the potential to cause disease in humans. The snowshoe hare virus is the most widely occurring arbovirus in Canada and is maintained in an amplification cycle involving small mammals, such as snowshoe hares, and mosquitoes, primarily of the genus Aedes.2 In one reported case, the horse recovered completely within 1 week and there was seroconversion to the snowshoe hare serotype of the California serogroup of viruses.3 Approximately 15% of horses in California have antibodies to Jamestown Canyon virus in horses in southern California.4 The virus has been isolated from vesicular lesions in a horse.5
The Cache Valley virus has been isolated from a clinically normal horse and the high seroprevalence of specific antibody suggests enzootic transmission.6
The Main Drain virus has been isolated from a horse with severe encephalitis in California.1 Clinical findings included incoordination, ataxia, stiffness of the neck, head-pressing, inability to swallow, fever, and tachycardia. The virus is transmitted by rabbits and rodents and by its natural vector, Culicoides varipennis.
The Powassan virus, a flavivirus, occurs in Ontario and eastern United States, and produces a non-suppurative, focal necrotizing meningoencephalitis in horses.7 Approximately 13% of horses sampled in Ontario in 1983 were serologically positive to the virus. Experimental intracerebral inoculation of the Powassan virus into horses resulted in a neurological syndrome within 8 days.8 Clinical findings include a ‘tucked-up’ abdomen, tremors of the head and neck, slobbering and chewing movements resulting in foamy saliva, stiff gait, staggering, and recumbency. Pathologically, there is a non-suppurative encephalomyelitis, neuronal necrosis, and focal parenchymal necrosis. The virus has not been isolated from the brain.
Nigerian equine encephalitis, a disease with low morbidity but high mortality, is characterized by fever, generalized muscle spasms, ataxia, and lateral recumbency of 3–5 days duration. The virus has not been identified.9
Nipah virus is a cause of encephalitis in humans and pigs in south-east Asia.10 The virus is a member of the henipavirus family, which includes Hendra virus, which is transmitted from frugiverous bats (Pteropus sp.) to pigs, among which it spreads horizontally to other pigs and humans. Horses can be exposed and develop antibodies to the virus, and there is one anecdotal report of dilated meningeal vessels in a horse from which Nipah virus was isolated.11
Salem virus has been isolated from horses, but does not appear to cause disease in this species.12
1 Emmons RW, et al. J Am Vet Med Assoc. 1983;183:555.
2 Lynch JA, et al. J Am Vet Med Assoc. 1985;186:389.
3 Heath SE, et al. Can Vet J. 1989;30:669.
4 Nelson DM, et al. Comp Immunol Microbiol Infect Dis. 2004;27:209.
5 Sahu SP, et al. J Vet Diag Invest. 2000;12:80.
6 McLean RG, et al. Am J Vet Res. 1987;48:1039.
7 Keane DP, Little PB. Aust Vet J. 1987;28:497.
8 Little PB, et al. Vet Pathol. 1985;22:500.
9 Adeyefa CA, et al. Vet Rec. 1996;138:323.
10 Nor MNM Disease Information. 12;1999:20.
11 Hooper PT, Williamson MM. Vet Clin North Am Equine Pract. 2000;16:597.
Getah virus and Ross river virus are both alphaviruses within the Semliki Forest complex of togaviruses. These are small enveloped viruses with a single-stranded, positive sense RNA genome. Getah virus causes disease in horses and pigs, whereas Ross River virus causes disease in humans and, arguably, horses.
The geographic range of the viruses is distinctive, with Getah virus reported from Japan, Hong Kong, south-east Asia, Korea, and India, and Ross River virus found in most areas of continental Australia, Tasmania, West Papua and Papua New Guinea, New Caledonia, Fiji, Samoa, and the Cook Islands.1 Reports from the 1960s document antibodies to Getah virus in animals in Australia, but the presence of this virus in Australia has not been confirmed using modern techniques that are able to differentiate antibodies to Getah virus from those of the related Ross River virus and other viruses in this complex. There are no reports of disease caused by Getah virus in Australia. There is considerable sequence homology between Getah and Ross River virus genomes.2 There is geographic genetic variability among isolates of Ross River virus, and temporal, but not geographic, variability among isolates of Getah virus from South-east Asia and Japan.3,4
Both viruses are arthropod-borne, and infection is through the bite of an infected mosquito. The virus is maintained in the mosquito-vertebrate-mosquito host cycle typical of arboviruses. The definitive, amplifying vertebrate host for Getah virus is unknown although a number of vertebrates including horses, cattle and pigs can be infected by the virus. Horses and pigs become viremic and presumably can infect mosquitoes, although this does not appear to have been confirmed experimentally. The life cycle of Getah virus has not been explicated. The virus is assumed to be maintained in a mosquito– pig–mosquito cycle in those areas in which there is mosquito activity year round.5 Persistence of the virus in areas in which mosquito activity is seasonal has not been explained, and whether transovarial or transtadial transmission occurs within the mosquito population is not reported.5 The vertebrate hosts of Ross River virus include a large number of eutherian, marsupial, and monotreme mammals and birds.1 Macropod species, including kangaroos and wallabies, are assumed to be the most important amplifying hosts, although this is debated.1
There is suspicion that during outbreaks of disease Getah virus is spread by horse-to-horse contact, based on the rapidity of spread among horses, the short duration of the outbreak, and the lack of mosquito activity at the time that some horses developed the disease.6,7 However, experimental evidence suggests that this route of spread is likely of limited importance in propagation of epidemics because of the low concentration of virus in nasal and oral secretions of infected horses, and the large inoculum required to cause disease in horses by the intranasal route.8
The prevalence of serological evidence of infection of horses by Getah virus in Japan ranges from 8 to 93%, depending on the region of the country in which the samples were collected and the disease history of the band or stable of horses.6,9 Seroprevalance was 17% in India and 25% in Hong Kong.8,10 These results confirm the widespread incidence of subclinical infection of horses by Getah virus in endemic areas.
There is a similar high incidence of Ross River virus infection of horses in endemic regions of Australia. Prevalence of seropositive horses in Queensland, an area in which year-round mosquito activity was likely, is approximately 80%, whereas that of horses around the Gippsland lakes in southern Australia, a region with seasonal mosquito activity, is 50%.11 These high rates of infection, in the absence of similarly high rates of clinical disease, suggest that the virus is minimally pathogenic in horses, and increases the likelihood that seroconversion or virus isolation from horses with clinical abnormalities is a chance event and not causally related.
The disease syndrome caused by infection by Getah virus is better defined than that of Ross River virus, but infection by either virus appears to cause disease in horses that has a number of clinical features in common.6,7,11,12 Disease associated with Getah virus infection is characterized by pyrexia, edema of the limbs, and an abnormal gait, often described as ‘stiffness’.6,7 Eruptions of the skin, urticaria, and submandibular lymphadenopathy are reported in some horses with the disease in Japan, but not in India.6,7 The clinical disease persists for 7–10 days. Abortion is not a feature of the disease and foals born of mares that have had the disease during gestation are normal.13 Subclinical infection is very common.
Hematological abnormalities induced by Getah virus infection in horses include lymphopenia.6 Increases in serum activity of muscle derived enzymes, such as creatine kinase, are not characteristic of the disease. Affected horses can have mild to moderate hyperbilirubinemia secondary to inappetance.6
The disease associated with Ross River virus infection of horses is typified by pyrexia, lameness including ‘stiffness’, swollen joints, inappetance, reluctance to move, and mild colic.11,12 The duration of disease caused by Ross River virus in horses is uncertain, and some veterinarians consider that the disease can persist for weeks to months, or recur in horses. There is some skepticism regarding the pathogenicity of Ross River virus in horses because, at least in part, the disease syndrome associated with Ross River virus infection of horses is not well characterized. Descriptions of the disease are based on a very small number of horses in which there was demonstrated viremia concurrent with development of clinical signs of disease, or larger number of horses with serum antibodies to the virus. Horses infected experimentally with Ross River virus have minimal clinical signs of disease.14 There are insufficient reports of disease to determine if characteristic or diagnostic abnormalities in serum biochemistry or hematology occur in affected horses.
Diagnosis of infection by Ross River virus is achieved by virus isolation from serum or heparinized blood samples collected during the acute phase of the disease, or detection in serum of antibodies to the virus.12 Detection of IgM antibodies to Ross River virus is indicative of recent infection whereas detection of IgG antibodies is indicative of more distant infection.12 Seroconversion confirms exposure, and presumably infection, by the virus. Isolation of Ross River virus has been achieved from horses with IgM antibody to the virus, but not with IgG antibody, likely because of the temporal pattern of antibody appearance in blood of infected horses.12 In addition to culture of the virus in mice or tissue culture, Ross River virus can be detected in blood and synovial fluid using an RT-PCR.14 It is important to remember that subclinical infection of horses in endemic regions is very common and that this high rate of subclinical infection increases the risk of incorrect attribution of clinical abnormalities to infection by the virus – it is possible that clinical abnormalities in a horse with Ross River viremia or serum antibodies to the virus are not attributable to infection by Ross River virus.
There are not reports of postmortem examination of horses with disease confirmed to be caused by Ross River Virus.
Diagnosis of disease caused by Getah virus is achieved by detection of clinical signs consistent with the disease, isolation of the virus from blood of affected horses, and seroconversion to the virus.4 Interpretation of serological data from horses in Japan is hindered by the widespread use of a vaccine against Getah disease that induces detectable antibodies to Getah virus in serum.9
Reports of postmortem examination of horses with disease caused by Getah virus are limited to experimental studies because the disease is typically not fatal. Horses with disease induced by inoculation with pathogenic Getah virus typically have mild changes including atrophy of splenic and lymphoid tissue with destruction of lymphocytes, and perivascular and diffuse infiltration of focal skin lesions by lymphocytes, histiocytes, and eosinophils. Lesions in the central nervous system are equivocal and limited to mild perivascular cuffing in the cerebrum and small hemorrhagic foci in the spinal cord.13
Treatment of affected horse is supportive. Affected horses might benefit from administration of analgesics and antipyretics such as phenylbutazone. Administration of antimicrobials is not indicated in uncomplicated cases.
An inactivated virus vaccine is available in Japan for immunization of horses against disease caused by Getah virus.9 The vaccine, which is combined with that for Japanese encephalitis, is considered effective. For both Getah virus and Ross River virus minimizing the exposure of horses to infected mosquitoes is prudent although the efficacy of this technique in preventing infection is unknown. During outbreaks of disease caused by Getah virus it is prudent to isolate affected horses, given the potential for horse-to-horse spread of the virus.
There is no vaccine to prevent infection or disease of horses by Ross River virus.
Disease of humans caused by Getah virus has not been documented.
Disease associated with Ross River virus infection is common in humans in Australia with an estimated 4800 cases per year, and much larger numbers during epidemics of the disease.15 The horse is believed to be an amplifying host of the virus because experimentally infected horses can infect mosquitoes.16 The disease in humans is characterized by mild pyrexia and constitutional signs initially, with subsequent development of a rash on the skin and oral lesions. Arthritis or arthralgia is common and affects primarily the wrists, knees, ankles and small joints of the extremities. These signs and symptoms can persist for 2–3 months, and the disease can relapse.
1 Russell RC. Annual Rev Entomol. 2002;47:1.
2 Strauss JH, Strauss EG. Microbiol Rev. 1994;58:491.
3 Lindsay MDA, et al. J Virol. 1993;67:3576.
4 Wekesa SN, et al. Vet Microbiol. 2001;83:137.
5 Calisher CH, Walton TE. Studdert MJ, editor. Virus infections of Equines. New York: Elsevier. 1996:157.
6 Brown CM, Timoney PJ. Trop Anim Health Prod. 1998;30:241.
7 Sentsui H, Kono Y. Res Vet Sci. 1980;29:157.
8 Kamada M, et al. J Vet Med Sci. 1991;53:855.
9 Sugiura T, Shimada KJ. Vet Med Sci. 1999;61:877.
10 Shortbridge KF, et al. Vet Rec. 1994;134:527.
11 Azuolas JK. Aust Equine Vet. 1998;16 volume 2.
12 Azuolas JK, et al. Aust Vet J. 2003;81:344.
13 Wada R, et al. Jpn J Vet Sci. 1982;44:411.
14 Studdert MJ, et al. Aust Vet J. 2003;81:76.
15 www.health.gov.au/internet/wcms/publishing/content/health-ross. river virus Accessed January 2 2006.
Etiology Lyssavirus of family Rhabdoviridae
Epidemiology Occurs in all farm animals worldwide except Australia and New Zealand. Major zoonoses. Transmitted by bites of infected animal. Many different wildlife are vectors depending on geographic location; vampire-bats in South America, foxes in Europe and North America, skunks in North America, mongoose in Africa, raccoon in the United States recently
Signs Incubation period varies from 2 weeks to several months. Cattle – Paralytic form: bizarre mental behavior (yawning, bellowing), incoordination, decreased sensation of hindquarters, drooling saliva, recumbency, and death in 4–7 days. Furious form: hypersensitive, belligerent, then paralysis and death as in paralytic form. Sheep – Outbreaks common; sexual excitement, wool pulling, attacking, incoordination, and then paralysis. Horses – Abnormal postures, lameness or weakness, depression, ataxia, pharyngeal paralysis, recumbency, hyperesthesia, biting, loss of anal sphincter tone, death in 4–6 days. Pigs – Excitement, attack, twitching of nose, clonic convulsions, paralysis
Clinical pathology No antemortem test
Lesions Non-suppurative encephalomyelitis
• Cattle: Lead poisoning, lactation tetany, hypovitaminosis-A, listerial meningoencephalitis, polioencephalomalacia, nervous acetonemia
• Horse: Viral encephalomyelitis, herpes viral paralysis, cerebrospinal nematodiasis, equine degenerative myeloencephalopathy, protozoal encephalomyelitis, neuritis of cauda equina, horsetail poisoning, Borna, Japanese encephalitis, botulism
• Sheep: Enterotoxemia, pregnancy toxemia, louping-ill, scrapie
• Pig: Pseudorabies, Teschen’s disease, Glasser’s disease, and other meningitides (E. coli and Streptococcus suis).
Diagnostic confirmation Fluorescent antibody test of brain. Negri bodies histologically
Treatment None. All rabies cases are fatal
Control Prevention of exposure. Vaccination of domestic animals and wildlife. Quarantine and biosecurity to prevent entry of virus into country
Rabies is associated with the rabies virus of the genus Lyssavirus of the family Rhabdoviridae.1 The genus is composed of at least six genotypes.
It was recognized long ago that the strain of virus known as the ‘street’ rabies virus differed in some way from ‘fixed’ strains which had been cultivated for vaccine production (grown in cell culture or passaged through serial generations of laboratory animals). It is now known that t there are several strains of rabies virus, which are adapted to particular host species but remain infective for any warm-blooded mammal.2
Rabies occurs in all warm-blooded animals.1,3 The disease occurs in cattle, sheep, pigs, and horses in most countries, except the insular countries that exclude it by rigid quarantine measures or prohibition of the entry of dogs. However, the genus Lyssavirus can still cause surprises. In 1996 and 1998, two women died in Queensland, Australia, from infections with a newly discovered rabies-related virus (Australian bat lyssavirus).4 In 2002, a man died in Scotland after contracting European bat lyssavirus rabies indicating that after a century of apparent freedom from rabies, the disease is now enzootic in the UK.4
Rabies in cattle is a major economic and public health problem in South America, where vampire bat-transmitted rabies results in cyclic outbreaks. Bovine paralytic rabies is endemic in the tropical regions extending from northern Mexico, to northern Argentina, and on the island of Trinidad.1 An outbreak in cattle in Guyana was associated with a large number of bats that had inhabited a culvert that was not cleaned regularly because of excessive rainfall.
In Europe, sylvatic rabies is a major problem where the red fox is the principal vector. The disease is still spreading from a focal point that developed in Poland in the mid-1930s. It is endemic in Yugoslavia and Turkey, and has spread westward to Germany, Denmark, Belgium, Czechoslovakia, Austria, Switzerland, and France. Spread continues at the rate of about 30–60 km (18–37 miles) per year and the threat to the United Kingdom increases each year. finland had been free of rabies since 1959, but in 1988 sylvatic rabies occurred with the raccoon dog as the vector. Monoclonal antibody indicated the virus was an Arctic-type strain possibly related to the red fox. France reported more than 2000 cases of rabid cattle between 1968 and 1982, and had more cases in sheep than in either dogs or cats during the same time period. In former East Germany, sheep were the second most frequent animal diagnosed with rabies after foxes.
Epidemiological studies of rabies in Lithuania from 1990 to 2000 found that rabies among wildlife comprised 54% with the majority of cases in foxes (27%), followed by raccoon dogs (21%).5 The incidence of rabies in foxes and raccoon dogs increased over the period of study. Also, the number of humans attacked by domestic animals and wild animals has increased.
Rabies occurs in most countries in the African continent, but the reported incidence is surprisingly low for an area with such a high population of wild carnivores. The incidence of rabies, and the range of species involved, is increasing in Africa, and a number of wildlife hosts has been identified, including wild dogs, jackals and mongoose. Because of dislocation of civilian life, rabies in Zimbabwe has increased in prevalence and geographical distribution in recent years.
Rabies is now a very serious zoonotic disease in South Darfur, Sudan.16
Over a 4-year period, of all the domestic animal rabies cases reported, cattle accounted for one-half of the rabies cases in South African domestic animals. The mongoose accounted for 70% of the wild animal cases reported.7 Widespread distribution of the rabies virus occurs when the young mongooses are evicted from their parents’ territory during the winter months, forcing them to scatter over a wide area. This increases the probability of domestic animals coming in contact with rabid animals.
The arctic fox variant of rabies invaded most of Canada south of 60°N and east of the Rocky Mountains in the early 1950s largely by the migration of arctic foxes into the populated areas.8 It died out in most of that range, but persisted for over 40 years in southern Ontario with sporadic incursions into narrow adjacent strips in western Quebec and northern New York. The principal vectors were red foxes (Vulpes vulpes) and, to a lesser extent, striped skunks (Mephitis mephitis). During the period 1957 to 1989, Ontario experienced more animal rabies cases than any almost every North American jurisdiction almost every year, and over 95% of those cases were limited to the southernmost 10% of the province’s land area.
A second major outbreak, involving striped skunks, progressed from North Dakota into the prairie provinces during the late 1950s and 1960s. In the 1990s, the endemic areas in Canada are southern Ontario, which accounts for 85% of the Canadian diagnoses, and the prairie provinces where rabies is endemic in skunks. In western Canada, the main reservoirs of the rabies virus are skunks, bats, and foxes.
In southern Ontario, Canada, the ecogeographic patterns of rabies indicate that townships could be aggregated into 12 rabies units or clusters.9 The units had different behaviors in terms of species composition, persistence, and periodicity. The ratio of rabid skunks to rabid foxes varied between areas which may be due to seasonal factors and urban development. This information is useful for planning rabies control programs.
Information on rabies surveillance in the United States is published annually by the Centers for Disease Control and Prevention.10-14 From 1999 to 2003, more than 90% of cases occurred in wild animals, 6 to 9% in domestic animal species. The disease occurred in humans, raccoons, skunks, bats, foxes, cats, dogs, cattle, sheep and goats, horses and mules, mongoose, rodents and lagomorphs.
Most cases of rabies reported annually in the United States occur among three groups of carnivores: raccoons, skunks, foxes, and among bats.15 However, between 1960 and 2000, a total of 2851 cases of rabies in 17 other carnivore taxa were reported to the Centers for Disease Control and Prevention, Atlanta, Georgia.15 Three species of other carnivores (mongooses, coyotes, and bobcats accounted for 92% of the cases reported among other carnivorous mammals.
The most frequently reported rabid wildlife cases occurred in raccoons, skunks, bats, and foxes.10 The relative contributions of those species continue to change in recent decades because of fluctuations in enzootics of rabies among animals infected with several distinct variants of the rabies virus.
During the past 30 years, rabies in domestic animals has steadily decreased in the United States, whereas annual occurrence in wild animals has increased. Wild animals accounted for 92% of all reported cases of rabies in 1995, a decrease from 1994. Raccoons were the most frequently reported rabid animal (50.3% of all animal cases), followed by skunks (22.5%), bats (10.0%), foxes (6.5%) and other wild animals, including rodents and lagomorphs (2.7%). Domestic animals accounted for nearly 8% of all rabid animals in the United States in 1995. In recent years, the number of rabid cattle was equal to, or greater than, the number of rabid dogs. The incidence of rabies in horses is low compared to wildlife or domestic small animals but some yearly fluctuations occur.
In 1990, for the first time since surveillance began in 1950, the number of cases in raccoons exceeded that in skunks. Raccoon rabies spread from Florida to raccoons in Georgia, Alabama, South Carolina, and North Carolina by natural spread.2 A separate focus in the northeastern States was caused by a translocation of raccoons from Florida, with legal permits to Virginia for restocking of hunting preserves.2 From the index case in a raccoon in 1977, near the Virginia–West Virginia border, over the next 17 years, 20 000 cases of rabies were recorded in raccoons, and several thousand associated cases in domestic dogs and other animals. In Canada, the rabies virus isolates from rabies-positive raccoons from 1982 to 1994 were the same strains found in foxes and skunks in eastern Canada,8 and is different from the ‘Mid-Atlantic’ strain found in raccoons in the eastern United States. Local dynamics of epidemics of rabies in raccoons in the United States can be predicted.16
Historically, in North America, the number of cases of rabies in skunks exceeded that in either raccoons or foxes. Endemic skunk rabies occurs mainly in four geographical regions: southern Ontario and Quebec and upper New York State; the north central United States and the Canadian provinces of Manitoba, Saskatchewan, and Alberta; California; and south central United States. Within these broad areas, the disease persists in enzootic foci and erupts every 6–8 years. Experimental studies suggest that the species specificity of endemic rabies is due to differences in the pathogenicity of variants of rabies virus. Skunk rabies peaks in the spring and early winter, which is probably a reflection of certain life history events within the skunk population.
The prevalence of rabies in bats in the United States is about 6%, and transmission to humans is rare even though sensational journalism has caused many people to consider bats as a serious threat to health. All of the confirmed cases of rabies in bats in Michigan in 1993 were associated with the big brown bat; in New York the prevalence in 1993 was 4.6% and nearly 90% of rabid bats were the common big brown bat. However, the silver-haired bat was associated with two human cases of rabies.
Trends in national surveillance for rabies among bats in the United States from 1993 to 2000 have consistently found a diffuse geographic pattern of rabies in bats throughout the continental United States.17 Although spillover infection of bat variants of rabies among terrestrial animals such as dogs and cats are rare, these variants of rabies virus have been associated with 92% of the indigenously acquired human rabies infections in the United States since 1990.11 Data from 37 states from 1993 to 2000 indicate an increased risk of rabies among certain groups of bat species was consistently found across season and most geographic regions of the US.17 The Brazilian free-tailed, eastern pipistrelle, and the silver-haired bats, when considered as a single group, were more rabid more frequently than were other bat groups.
All warm-blooded animals, with the possible exception of opossums, are susceptible, and there is no variation in susceptibility with age; 1-day-old pigs have been affected. Variation in susceptibility between species is noticeable. Foxes, cotton rats, and coyotes are extremely susceptible; cattle, rabbits, and cats are highly susceptible; dogs, sheep, and goats are moderately susceptible; and opossums little if at all.
The Lyssavirus genus belongs to the Rhadoviridae family of the Mononegavirales order and includes unsegmented RNA viruses causing rabies encephalomyelitis. They are well fitted to vectors belonging to the orders Carnivora (flesh-eating mammals including skunks), and Chiroptera (the order which comprises all of the 178 genera in 16 families of bats). Seven genotypes have so far been delineated within the genus. These genotypes are divided into two immunopathologically and genetically distinct phylogroups. Phylogroup I includes two African genotypes: Mokola virus, which has been isolated from shrews and cats, although its reservoir remains unknown, and Lagos bat virus, which has been found mainly in frugivorous bats but also in an insectivorous bat. Phylogroup II has five genotypes: Duvenhage virus (Africa), European bat lyssavirus I (EBLV-1; Europe), EBLV-2 (Europe), Australian bat lyssavirus (Australia), and the classical Rabies virus (RABV, worldwide). Members of the genotypes Duvenhage virus, EBLV-1, and EBLV-2 are exclusively found in insectivorous bats, members of the genotype Australian bat lyssavirus are found in both insectivorous and frugivrorous bats, and member of the genotype RABV are found in carnivorous and American bats (insectivorous, frugivorous, and hematophagus). The fact that lyssaviruses are well established in two ecologically distinct mammal orders may very likely be the consequence of successful host switching.
Analysis of 36 carnivoran and 17 chiropteran lyssavirus representing the main genotypes and variants strongly supports the hypothesis that host switching occurred in the history of the lyssaviruses. In fact, lyssaviruses evolved in chiroptera long before the emergence of carnivoran rabies, very likely following spillovers from bats.18 Using dated isolates, the emergence of carnivoran rabies from chiropteran lyssaviruses is estimated to have occurred 888 to 1459 years ago.
In Europe, bat rabies is associated with two specific virus strains: European bat lyssavirus type 1 and European bat lyssavirus type 2. European bat lyssavirus type 1 isolates have been found in serotine bats in France.19 European bat lyssavirus type 2 virus have now been found in Daubenton’s bats in England and Scotland.20
In North America, variants of rabies virus are maintained in the wild by several terrestrial carnivore species, including raccoons, skunks, and a number of bat species.21 Each antigenically and genetically distinct variant of the virus in mammalian species occurs in geographically discrete areas and is strongly associated with its reservoir species.11 Within each area, a spillover of rabies into other species occurs, especially during epidemics. Temporal and spatial analysis of skunk and raccoon rabies in the eastern United States, indicated that epidemics in raccoons and skunks moved in a similar direction from 1990 to 2000. However, to date there is no evidence that the raccoon rabies virus variant is cycling independently in the skunk population of the eastern United States or that the variant has undergone any genetic adaptations among skunks.21
Within broad geographic regions, rabies infections in terrestrial mammals can be linked to distinct virus variants, identified by panels of monoclonal antibodies or by genetic analysis.2 These analyses have demonstrated substantial differences between isolates from various parts of the world. Most outbreaks of rabies tend to be host species-specific. Each variant is maintained primarily by intraspecific transmission within a dominant reservoir, although spillover infection of other species may occur within the region. Geographic boundaries of the currently recognized reservoirs for rabies in terrestrial mammals have been established. Reservoirs for rabies virus are found worldwide. The virus is maintained at endemic and epidemic levels in a wide variety of Carnivora and Microchiroptera (bats) species. There are also antigenically similar rabies-like viruses, including the Makola virus, the Lagos bat virus, Duvenhage virus, European bat lyssavirus 1 and European bat lyssavirus 2 rabies, which are found principally in small mammals (rodents, insectivores, insectivorous or frugivorous bats). These strains appear to be limited in their geographical distribution to regions in Africa, unlike rabies virus which is distributed worldwide. The rabies-related viruses represent potential public and veterinary threats because of the lack of effective vaccines and the difficulties with diagnosis.
The geographic boundaries of the currently recognized reservoirs for rabies in terrestrial species in North America are as follows:
• Raccoons in the southeastern United States
• Red and arctic foxes in Alaska, resulting in spread across Canada as far east as Ontario, Quebec, and the New England states
• Striped skunks in California, the north central States, and the south central States
• Gray foxes in small reservoirs in Arizona
• Coyotes in south Texas as a result of spread from domestic dogs in a long-standing reservoir at the Texas–Mexico border.
The first reported occurrence of rabies in a human being infected with the raccoon rabies virus was from Virginia in 2002.14
In Ontario, wildlife rabies persists in two predominant species: the red fox and the striped skunk. Molecular epidemiology studies indicate that there is no host specificity, but rather there are very clear and consistent differences in the virus from distinct geographical regions. Such analyses will allow further epidemiological study of the behavior of the virus in different regions.
Overlying the disease in terrestrial mammals are multiple, independent reservoirs for rabies in several species of insectivorous bats. Distinct viral variants can be identified for different bat species, but geographical boundaries cannot be defined for rabies outbreaks in the highly mobile bat species.
Certain antigenic variants exist in nature against which conventional vaccines do not fully protect. In Canadian studies, two major antigenic groups can be distinguished among the rabies virus isolates examined. One group is found in Ontario, Quebec and the Northwest Territories and is represented in the wild by endemic red fox and striped skunk rabies that originated in northern Canada. The second group is found in Manitoba where striped skunk rabies is endemic.
The epidemiology of rabies in Chile and the animal species which serve as rabies reservoirs have been examined.22 None of Chilean isolates segregated with viruses from the terrestrial reservoirs. No non-rabies lyssaviruses were found, and the Chilean samples were not related to viruses of the sylvatic cycle maintained by the common vampire bat. The Brazilian free-tailed bat was identified as the reservoir for the rabies genetic variant most frequently isolated in Chile from 1977 to 1998. The close association of a group of rabies viruses obtained from a domestic dog, Brazilian free-tailed bats, and a red bat with viruses maintained by red bat species in North America implicated species of this genus as the possible reservoirs of this particular genetic variant in Chile.
In Trinidad, bovine rabies is common and is due to the bat type.23
The source of infection is always an infected animal, and the method of spread is almost always by the bite of an infected animal, although contamination of skin wounds by fresh saliva may result in infection. Not all bites from rabid animals result in infection because the virus is not always present in the saliva and may not gain entrance to the wound if the saliva is wiped from the teeth by clothing or the coat of the animal. The virus may appear in the milk of affected animals, but spread by this means is unlikely as infection. The rabies virus is relatively fragile, susceptible to most standard disinfectants, and dies in dried saliva in a few hours.
One of the most important parameters in rabies models is the transmission rate, or the number of susceptible animals infected by a diseased animal per unit of time. In a population of 19 raccoons feeding at a concentrated, common food source available during the summer in rural eastern Ontario, raccoons bite and are bitten an average of 0.99 to 1.28 times per hour, respectively.24
Because of the natural occurrence of rabies in animals in caves inhabited by infected insectivorous bats, inhalation as a route of infection came under suspicion. It is now accepted that interbat spread, and spread from bats to other species is principally by bites, but that infection by inhalation also occurs. That infection can occur by ingestion has been put to use in devising systems of vaccinating wildlife by baiting them with virus-laden baits. This also has implications for epidemiological study generally. For example, attenuated viruses used in baits could be taken by other than the target species, thus creating an unexpected seropositive segment of the animal population. It is also considered likely that outbreaks occurring naturally amongst carnivores may originate by them eating bats that have died of rabies.
Traditionally, the dog, and to a minor extent the cat, have been the main source animals. However, native fauna, including foxes, skunks, wolves, coyotes, vampire, insectivorous and fruit-eating bats, raccoons, mongoose, and squirrels provide the major source of infection in countries where domestic carnivora are well-controlled. In general, foxes are less dangerous than dogs, foxes tending to bite only one or two animals in a group, while dogs will often bite a large proportion of a herd or flock. Raccoons and skunks are major reservoir of rabies in North America.
Bats are the important species in which subclinical carriers occur. Multiplication of the virus without invasion of the nervous system is known to occur in fatty tissues in bats, and may be the basis of the ‘reservoiring’ mechanism in this species. Violent behavior is rare in rabid animals of this species, but has been observed. Bats represent a serious threat of spread of rabies because of their migratory habits. Most spread is within the species, but the threat to humans and animal species by bats cannot be completely disregarded. Although rodents can be infected with the rabies virus they are not thought to play any part in the epidemiology of rabies, either as multipliers or simply as physical carriers of the virus. Many of the viruses they carry are rabies-like rather than classical rabies.
Rabies has occurred in swine herds where the skunk population is high, where farms were settled from rough terrain resulting in considerable interface between wildlife and domestic animals, and in which the management system allows the pigs to run free on the premises. The disease has occurred in pigs reared in a closed feeder barn where access by wildlife was very unlikely.
There is a difference in role between vectors. For example, in Europe it is thought that foxes carry the infection into a new area, but other species disseminate it within an area.25 Foxes are the principal vectors and, as in Canada, cattle are the principal receptors. In western Canada, the main reservoirs of infection are skunks, bats, and foxes. This would have important consequences for control programs based on wildlife surveillance.
Domestic livestock like cattle are rarely a source of infection, although chance transmission to humans may occur if the mouth of a rabid animal is manipulated during treatment or examination. The virus may be present in the saliva for periods up to 5 days before signs are evident.
Spread of the disease is often seasonal, with the highest incidence in the late summer and autumn because of large-scale movements of wild animals at mating time and in pursuit of food. In Canada, the frequency of rabies infection in livestock populations increases in the fall when adolescent foxes mature, begin mating behavior, and travel over large areas.
Because of rapid developments in virological techniques, especially serological screening of animal populations to obtain presumptive diagnoses of the presence of a virus in the population, the question of latent infection and inapparent carriers of rabies has assumed some importance. The presence of rabies antibodies in animals in a supposed rabies-free area is likely to arouse concern. Inapparent carriers do occur in bats and there is some evidence that latent infections can occur in other species.
The prime importance of rabies is its transmissibility to humans, with veterinarians being at special risk. European data indicate that by far the greatest proportion of humans requiring pretreatment for rabies have been exposed to a rabid domestic animal, not a wild one.1,3
Human rabies is extremely rare in countries where canine rabies is controlled by regular vaccination. In the United States, a total of 28 cases of human rabies occurred from 1980 to 1995. Most were due to viral variants associated with bats. Rates of post-exposure prophylaxis in developing countries are about 10 times higher than those in the United States, and rates of human rabies are approximately 100 times higher. According to the WHO, over 30 000 people die each year from rabies and more than 10 million undergo post-exposure treatment, having been bitten by a rapid animal.3 In 1987, according to surveillance conducted by the World Health Organization, dogs were responsible for 91% of all human rabies cases; cats, 2%; other domestic animals, 3%; bats, 2%; foxes, 1%; and all other wild animals, fewer than 1%. The disease is a major occupational hazard for veterinarians who should receive pre-exposure prophylaxis. Because horses will bite each other and their handlers, rabid horses that are aggressive pose a serious threat to humans.
There is a lack of general rabies knowledge among the public.26 The laissez-faire attitude toward rabies by Americans causes instances of rabies exposure to be commonplace. A survey of middle school children in Texas found a lack of basic knowledge about rabies. Only 0.3% of children achieved a minimum score of 75% on a survey of knowledge about rabies.26 Respondents lacked knowledge about the disease is transmitted, and less than one-third were even aware of rabies epidemic in southern Texas despite the fact that a rabies epidemic had been occurring in southern Texas for the previous 13 years. Even more astonishing was the finding that 80% knew of the risk of acquiring rabies from an unvaccinated pet buy 57% claimed to own unvaccinated pets.
Rabies is not of major economic importance in farm animals, although individual herds and flocks may suffer many fatalities. The disease in human has always been considered fatal. Since 1970, there have been reports of five patients said to have survived rabies encephalitis.4 All patients had received some rabies vaccine before the onset of clinical signs but none had had rabies immunoglobulin.
A 15-year-old girl who developed rabies one month after being bitten by a bat survived following intensive medical therapy including induced coma while a native immune response matured, and treatment with ketamine, midazolam, ribarvirin, and amantadine.27 Probable drug-related toxic effects included hemolysis, pancreatitis, acidosis, and hepatotoxicity. The patient was discharged to her home after 76 days, and at nearly 5 months after her initial hospitalization she was alert and communicative, but with choreoathetosis, dysarthria, and an unsteady gait.
Rabies in the Americas. Between 1993 and 2002, the number of human and canine rabies cases in the Americas Region fell by approximately 80%.28 There were 39 human cases in 2002, 63% of them transmitted by dogs. Human rabies transmitted by wildlife, mostly by bats is a risk to inhabitants in many countries in the Region. This sharp reduction is attributable mainly to the control measures implemented by the countries in the Region, such as mass vaccination of dogs and prophylactic treatment of people who have been exposed.
The economic costs of rabies in a country are associated with pet animal vaccinations, animal bite investigations, confinement and quarantine of domestic animals which bite humans or which are suspected of exposure to rabid animals, salaries of animal control officers, laboratory diagnosis, the costs of pre-exposure and post-exposure prophylaxis and treatment and consultation, public education, staff training and clerical costs.
The cost of the ‘point infection control’ program as a response to raccoon rabies introduction in Ontario in 1999 was $363 000 (Cdn) or $500 Cdn/km2. The costs were justified as by containing the spread of raccoon rabies, annual savings to Ontario are estimated at $8 to 12 million.29 The reported associated costs in Ontario before raccoon rabies occurred were estimated at about $6 million annually, excluding pet vaccination costs.
Following the deep introduction of rabies virus by the bite of a rabid animal, initial virus multiplication occurs in striated muscle cells at the site. The neuromuscular spindles then provide an important site of virus entry into the nervous system. Entry into the nervous system may also occur at motor end plates. In the olfactory end organ in the nares, neuroepithelial cells are in direct contact with the body surface and these cells extend without interruption into the olfactory bulb of the brain. Following entry of the virus into nerve findings, there is invasion of the brain by passive movement of the virus within axons, first into the spinal cord then into the brain1,4 The immune response during this phase of the infection is minimal and explains why neutralizing antibody and inflammatory infiltration are usually absent at the time of onset of encephalitic signs. Antibody titers reach substantial levels only in the terminal stages of the disease. Following entry of rabies virus to the central nervous system (CNS), usually in the spinal cord, an ascending wave of neuronal infection and neuronal dysfunction occurs.
The primary lesions produced are in the CNS, and spread from the site of infection occurs only by way of the peripheral nerves. This method of spread accounts for the extremely variable incubation period, which varies to a large extent with the site of the bite. Bites on the head usually result in a shorter incubation period than bites on the extremities. The severity and the site of the lesions will govern to a large extent whether the clinical picture is primarily one of irritative or paralytic phenomena. The two extremes of the paralytic or dumb form and the furious form are accompanied by many cases that lie somewhere between the two. Gradually ascending paralysis of the hindquarters may be followed by severe signs of mania, which persist almost until death. Destruction of spinal neurons results in paralysis, but when the virus invades the brain, irritation of higher centers produces manias, excitement, and convulsions. Death is usually due to respiratory paralysis. The clinical signs of salivation, indigestion and pica, paralysis of bladder and anus, and increased libido all suggest involvement of the autonomic nervous system, including endocrine glands. At death, there are viral inclusions and particles in almost all neurons in the brain, spinal cord and ganglia, but none in the supportive cells of the CNS. Electron microscopic examination also shows the presence of the virus in the cornea, which it reaches centrifugally along the peripheral nerves.
Virus reaches the salivary glands and many other organs in the same way, but the highly infective nature of saliva arises from passage of the virus along the olfactory nerve to taste buds and other sensory end organs in the oropharynx, rather than from the salivary glands. Experimentally, infection of non-nervous tissues in skunks and foxes has been reproduced in the adrenal medulla, cornea, and nasal glands.19 The virus may be found in milk, in some organs and in fetuses, but the virus cannot be demonstrated in the blood at any time.
Variations in the major manifestations as mania or paralysis may depend upon the source of the virus. Virus from vampire bats almost always causes the paralytic form. ‘fixed’ virus that has been modified by serial intracerebral passage causes ascending paralysis in contrast to ‘street’ virus, which more commonly causes the furious form. The site of infection and the size of the inoculum may also influence the clinical course. There is also geographical difference in the proportion of animals affected by the furious or paralytic form of the disease. In the Americas most cases are paralytic. In Africa and India most cases in farm animals are the furious form.
The disease is always fatal, but infrequently an experimentally infected animal shows clinical signs of the disease but recovers. There are two recent records of spontaneous recovery in man, and the occurrence of non-fatal rabies in all species has been reviewed. There appears to be no field occurrence in domestic animals of the finding in experimentally infected mice that some strains of virus invade only peripheral nerves and spinal ganglia leaving a number of survivors with permanent nervous disability. The pathogenesis of recovery from rabies is important relative to vaccination and serological testing to determine the incidence and prevalence of the disease. The literature on the animal models used for the study of the pathogenesis and treatment of rabies has been reviewed.1
Among farm animals, cattle are most commonly affected. The incubation period in naturally occurring cases is about 3 weeks, but varies from 2 weeks to several months in most species, although incubation periods of 5 and 6 months have been observed in cattle and dogs. In one large-scale outbreak in sheep, deaths occurred 17–111 days after exposure.
Experimentally, in cattle the average incubation period was 15 days and the average course of the disease was 3.7 days. Unvaccinated cattle had shorter incubation and clinical duration of disease than vaccinated cattle. Major clinical findings included excessive salivation (100%), behavioral change (100%), muzzle tremors (80%), vocalization (bellowing 70%), aggression, hyperesthesia and/or hyperexcitability (70%), and pharyngeal paralysis (60%). The furious form occurred in 70%.
In sheep, experimentally, the average incubation period was 10 days, and the average course of the disease was 3.25 days. Major clinical findings included muzzle and head tremors (80%), aggressiveness, hyperexcitability and hyperesthesia (80%), trismus (60%), salivation (60%), vocalization (60%), and recumbency (40%). The furious form occurred in 80% of sheep.
In the paralytic form, knuckling of the hind fetlocks, sagging and swaying of the hindquarters while walking, often deviation or flaccidity of the tail to one side, are common early signs. Decreased sensation usually accompanies this weakness and is one of the best diagnostic criteria in the detection of rabies. It is most evident over the hindquarters. Tenesmus, with paralysis of the anus, resulting in the sucking in and blowing out of air, usually occurs late in the incoordination stages just before the animal becomes recumbent. This is a characteristic finding but it may be transient or absent. Drooling of saliva is one of the most constant findings. The yawning movements are more accurately described as voiceless attempts to bellow. When paralysis occurs, the animal becomes recumbent and unable to rise. Bulls in this stage often have paralysis of the penis. Death usually occurs 48 hours after recumbency develops and after a total course of 6–7 days. The paralytic form of rabies has been reproduced experimentally by the IM injection of brain tissue from naturally occurring cases of paralytic rabies (‘derriengue’) in cattle in Mexico.
In furious rabies the animal has a tense, alert appearance, is hypersensitive to sounds and movement, and is attracted to noise so that it may look intently or approach as though about to attack. In some cases, it will violently attack other animals or inanimate objects. These attacks are often badly directed and are impeded by the incoordination of gait. Frequently, loud bellowing is usual at this stage. The sound is characteristically hoarse and the actions are exaggerated. Sexual excitement is also common, bulls often attempting to mount inanimate objects. Multiple collections of semen for artificial insemination have been made during very short periods from bulls that later proved to be rabid. With this violent form of the disease the termination is characteristically sudden. Severe signs may be evident for 24–48 hours and the animal then collapses suddenly in a paralyzed state, dying usually within a few hours.
There is no consistent pattern in either the development or the range of signs. Body temperatures are usually normal but may be elevated to 39.5–40.5°C (103–105°F) in the early stages by muscular activity. Appetite varies also. Some animals do not eat or drink, although they may take food into the mouth. There is apparent inability to swallow. Others eat normally until the terminal stages. The course may vary from 1 to 6 days. So wide is the variation in clinical findings that any animal known to be exposed and showing signs of spinal cord or brain involvement should be considered rabid until proved otherwise.
In sheep, rabies often occurs in a number of animals at one time due to the ease with which a number of sheep can be bitten by a dog or fox. Clinically, the picture is similar to that seen in cattle. The minority show sexual excitement, attacking humans or each other, and vigorous wool pulling; sudden falling after violent exertion, muscle tremor, and salivation are characteristic. Excessive bleating does not occur. Most sheep are quiet and anorectic. Goats are commonly aggressive, and continuous bleating is common.
Most recorded cases in horses are lacking in distinctive nervous signs initially, but incline to the paralytic form of the disease. Experimentally, the average incubation period was 12 days and the average duration of disease was 5.5 days.30 Unvaccinated animals had shorter incubation periods and duration of clinical disease. Muzzle tremors were the most frequently observed and most common initial signs. Other clinical findings included pharyngeal paresis (71%), ataxia or paresis (71%), and lethargy or somnolence (71%). The furious form occurred in 43% of cases, some of which began as the dumb form. The paralytic form was not observed.
In naturally occurring cases, the initial clinical findings may include abnormal postures, frequent whinnying, unexplained aggressiveness and kicking, biting, colic, sudden onset of lameness in one limb followed by recumbency the next day, high-stepping gait, ataxia, apparent blindness, and violent head-tossing. Lameness or weakness in one leg may be the first sign observed, but the usual pattern of development starts with lassitude, then passes to sternal recumbency and lateral recumbency, followed by paddling convulsions and terminal paralysis.
In a series of 21 confirmed cases in horses, the clinical findings at the time of initial examination included ataxia and paresis of the hindquarters (43%), lameness (24%), recumbency (14%), pharyngeal paralysis (10%), and colic (10%).31 The major clinical findings observed over the course of hospitalization included recumbency (100%), hyperesthesia (81%), loss of tail and anal sphincter tone (57%), fever ∼38.5°C (52%), and ataxia and paresis of the hindquarters (52%). Mean survival time after the onset of clinical signs was 4.47 days (range, 1–7 days). Clinical findings of the furious form of rabies, such as aggressiveness (biting), compulsive circling, and abnormal vocalization, were evident in only two horses. Supportive therapy, given to nine horses, had no effect on survival time and did not correlate with the detection of Negri bodies at necropsy. Horses developing the furious form show excitement, become vicious, and bite and kick. Their uncontrolled actions are often violent and dangerous and include blind changes, sudden falling and rolling and chewing of foreign material or their own skin. Hyperesthesia and muscular twitching of the hindlimbs followed by crouching and weakness are also recorded in the horse.
Pigs manifest excitement and a tendency to attack, or dullness and incoordination. Affected sows show twitching of the nose, rapid chewing movements, excessive salivation, and clonic convulsions. They may walk backward. Terminally, there is paralysis and death occurs 12–48 hours after the onset of signs. The clinical findings in pigs are extremely variable, and individual cases may present in a variety of ways and only one or two of the classical findings may occur.
No antemortem laboratory examination is of diagnostic value, but tests for lead on blood, urine, and feces may help to eliminate lead poisoning as a possible diagnosis. Virus neutralization tests are available, but the presence of antibodies is not diagnostic. Other available tests are passive hemagglutination, complement fixation, radioimmunoassay, and indirect fluorescent antibody staining. These are used to determine immune status rather than as a diagnostic aid. An ELISA is available for measurement of rabies-specific antibody in the sera of major domestic and wildlife reservoirs in North America.
Confirmation of a diagnosis of rabies depends on careful laboratory examination of fresh brain. The recommended laboratory procedure includes the following three tests and it is recommended that at least two of them be used on all specimens.
• A fluorescent antibody test (FAT) on impression smears from the brain – current recommendations include sampling of the hippocampus, medulla oblongata, cerebellum or gasserian ganglion. However, a recent publication stipulates that the hippocampus and cerebellum are less desirable samples than the thalamus, pons, or medulla for the detection of viral antigen, and that the current sampling recommendations stem from the visibility of Negri bodies, rather than the true distribution of viral antigen.32 An FAT can be completed in approximately 2 hours and is highly accurate when done routinely by experienced personnel. The reliability of FAT confirmed by the mouse inoculation test is over 99%.14 Those specimens that are negative on FAT, and have contact with humans, are inoculated into experimental mice. The incubation period in mice before clinical signs are seen averages 11–12 days (range of 4–18 days), and death occurs in 7–21 days. The mouse brain is harvested as soon as signs appear and is submitted to the same tests described above. Thus a positive result can be obtained as soon as 4–7 days after inoculation. Some mice must be left for the full 21 days because only a negative result at that time can give a complete negative to the test. A tissue culture infection test is now available, which allows demonstration of the virus in stained tissue culture cells within 4 days. This may replace the mouse inoculation test
• A dot ELISA is available for the detection of rabies antigen in animals.33 It is rapid, simple, economical and, in comparison with the FAT, the agreement is 95%.
A histological search for Negri bodies in tissue sections with results available in 48 hours. Because of false-positive diagnoses the technique is in some disrepute. An immunoperoxidase test for rabies can be used on formalin-fixed, paraffin-embedded brain tissues of domestic animals and wild animals when fresh tissues are not available.16,23 In some cases, the brain tissue may be negative for the rabies virus using standard diagnostic techniques but immunohistochemical tests may detect the presence of antigen.20 A reverse transcriptase polymerase chain reaction test has been found of value in detecting rabies infection in decomposed brain samples that were negative by the direct fluorescent antibody test.34
The histopathologic changes of rabies infection include a non-suppurative encephalomyelitis and ganglioneuritis, with neuronal necrosis and the formation of glial nodules. Negri bodies are most commonly found in the Purkinje cells of the cerebellum in ruminants. Spongiform change has also been reported in the brain of a heifer infected with rabies virus.35
• Histology – one half of midsagittally-sectioned brain, cervical spinal cord (including root ganglia), gasserian ganglion, parotid salivary gland (LM, IHC)
• Virology – one half of midsagittally-sectioned brain, cervical spinal cord (FAT, ISO, BIOASSAY).
Note the zoonotic potential of this organism when handling carcass and submitting specimens.
The diagnosis of rabies is one of the most difficult and important duties that a veterinarian is called upon to perform. Since in most cases there is a probability of human exposure, failure to recognize the disease may place human life in jeopardy. It is not even sufficient to say that if rabies occurs in the area one will classify every animal showing nervous signs as rabid, because nervous signs may not be evident for some days after the illness commences. In addition, many animals suffering from other diseases will be left untreated. The best policy is to handle all suspect animals with extreme care but continue to treat them for other diseases if such treatment appears to be indicated. If the animal is rabid, it will die and the diagnosis can then be confirmed by laboratory examination.
Several diseases are characterized by signs of abnormal mental state or paralysis, or a combination of both (see Table 22.1 for the horse; Table 32.3 for cattle). Rabies must be differentiated from the following common diseases affecting the nervous system, according to species:
• Lead poisoning. In acute and subacute lead poisoning in cattle the clinical findings are similar to those of furious and dumb rabies. In acute lead poisoning, the common clinical findings are blindness, convulsions, champing of the jaws with the production of frothy saliva, and twitching of the eyelids and ears. In subacute lead poisoning in cattle there is blindness, stupor, head-pressing, grinding of the teeth, and almost no response to treatment. Rabid cattle are usually not blind, and signs of motor irritation such as convulsions and twitching of the facial muscles usually do not occur. However, there are signs of bizarre mental behavior, such as wild gazing, bellowing, yawning, attacking, and compulsive walking
• Lactation tetany occurs in lactating cattle on lush pasture in the spring during cold wet and windy weather, and is characterized by hyperesthesia, tremors, convulsions, recumbency, and rapid death
• Vitamin A deficiency occurs in groups of young cattle from 6 months to 18 months of age not receiving adequate carotene intake or vitamin A supplementation and is characterized by blindness in the ocular form and episodes of tremors and convulsions
• Polioencephalomalacia in cattle and sheep is characterized by blindness, nystagmus, opisthotonos, and convulsions; bellowing, loss of sensation, and tenesmus do not occur
• Listeriosis in cattle and sheep is manifested by localizing signs of circling and facial nerve paralysis
• Enterotoxemia in sheep is usually confined to lambs on heavy carbohydrate diets
• Pregnancy toxemia is a disease of pregnant ewes and is readily differentiated by the presence of ketonuria
• Louping-ill in sheep is transmitted by insects, has a seasonal occurrence, and a localized geographical distribution.
In pigs, rabies must be differentiated from pseudorabies, Teschen’s disease, and involvement of the brain in several other diseases of the pigs, such as hog and African swine fever, meningitis associated with Streptococcus suis type II, Haemophilus spp., Glasser’s disease, Escherichia coli, septicemia, and erysipelas.
In horses, rabies must be differentiated from several diseases of the nervous system (summarized in Table 22).
The most common include: viral encephalomyelitis, herpes virus myeloencephalopathy, cerebrospinal nematodiasis, equine degenerative myeloencephalopathy, equine protozoal myeloencephalitis, neuritis of the cauda equina, horsetail poisoning, Borna, Japanese encephalitis, botulism.
No treatment should be attempted after clinical signs are evident. If the bite is seen, immediately after exposure, irrigation of the wound with 20% soft soap solution or a solution of Zephiran may prevent the establishment of the infection. Immediate and thorough washing of all bite wounds and scratches with soap and water is perhaps the most effective measure for preventing rabies in veterinarians bitten by rabid animals. In experimental animals, simple local wound cleansing has been shown to markedly reduce the likelihood of rabies. Post-exposure vaccination is unlikely to be of value in animals, as death usually occurs before appreciable immunity has had time to develop. Euthanasia of suspect animals must be avoided, particularly if human exposure has occurred, since the development of the disease in the animals is necessary to establish a diagnosis. Antirabies serum may become available for animal treatment at some future date. In some countries, cases of rabies in farm animals are notifiable to the animal health and disease regulatory bodies.
The major goal of rabies control in domestic and wild animals is the reduction or elimination of human rabies. The most rational approach to reducing human rabies is to reduce the prevalence and incidence of disease in animals. In developed countries, this has been accomplished by vaccination of dogs and cats, leaving much rabies in wildlife to be controlled. In countries without wildlife reservoirs, such as the Philippines, it would be economically advantageous to eliminate dog rabies. In Africa, where the incidence of rabies as well as the range of species involved is increasing, there is a need to develop new and economical methods of vaccinating domestic animals.
Pre-exposure immunization for individuals, like veterinarians, who are at high risk to rabies, has been recommended by the World Health Organization, since it reduces risk and provides a more rapid anamnestic response, eliminating the need for human globulin should exposure occur. Rabies pre-exposure vaccination is now mandatory in many veterinary colleges. Despite some mild adverse reactions, immunization against rabies is an important prophylaxis measure well-accepted by veterinary students.
For farm animals, there are two useful control techniques: the prevention of exposure and pre-exposure vaccination.
This can be achieved by controlling access of wildlife species which are likely to come into contact with the farm livestock in particular areas or through vaccination of the wildlife. Foxes accounted for a very large proportion (85% in Europe) of wildlife rabies, and a control program aimed at reducing their population using poison or traps was attempted until the 1970s.36 This method of population reduction failed to control outbreaks or reduce enzootic rabies.
Point infection control. To control the introduction of raccoon rabies in Ontario in 1999, ‘point infection control’ was used to control the epidemic.29 This involves the use of three tactics: population reduction, trap-vaccinate-release, and oral vaccination with baits to control the spread of raccoon rabies. Some raccoons were captured and euthanized which resulted in an 83 to 91% reduction in the raccoon population in an area of 225 km2 around the location of the three original cases of raccoon rabies. Raccoon density in the population reduction zones declined from 5.1 to 7.1 km2 to 0.6 to 1.1 km2 following control. Cats were also captured, vaccinated and released. Raboral V-RG oral rabies vaccine was distributed aerially to vaccinate free ranging raccoons. The point infection control program is considered to be highly successful and will continue to be used to contain isolated cases of raccoon rabies.29
The most successful form of rabies prevention is pre-exposure vaccination. In human medicine, there are no reported cases of rabies deaths in anyone who has had pre-exposure vaccination followed by a booster vaccination if exposed.4
The Centers for Disease Control (CDC) and Prevention has published the recommendations of the Advisory Committee on Immunization Practices (ACIP) for human rabies prevention, which indicate that rabies pre-exposure vaccination should be offered to persons more likely to be exposed to rabies virus than the population of the United States at large.37 The recommendations of the ACIP for pre-exposure prophylaxis and maintenance of a detectable antibody titer differ depending on the estimated degree of risk of exposure to the virus. Four risk categories have established: continuous; frequent; infrequent; and rare. The classification depends on factors such as the occupation of the individual and geography.38
With directed continuing education, common sense, first aid, and the availability of modern biological agents, human rabies is nearly always preventable.1 Rabies pre-exposure vaccination is recommended for anyone at increased risk of exposure to rabies, including veterinarians, veterinary students who work in university veterinary teaching hospitals, laboratory staff working with rabies, vaccine producers, animal and wildlife control personnel, and zoologists.
The standard pre-exposure regimen is three doses of vaccine IM or ID on days 0, 7, and 28 (or 21). A booster dose after 1 year increases and prolongs the antibody response.4 This pre-exposure vaccination permits post-exposure vaccination to consist of two doses of vaccine on days 0, and 3 instead of 5 on days 0, 3, 7, 14, and 28 and avoids the need for postexposure of administration of human rabies immunoglobulin.
A large proportion of at-risk staff members working in veterinary clinics, animal shelters, and wildlife rehabilitation centers in a study area did not receive rabies pre-exposure vaccination according to the recommendations of the ACIP of the CDC.37,38 Cost may be factor because many of these employees are commonly short-term, part-time, or volunteer workers.
Modern post-exposure treatment is highly successful if done adequately. Wound care with passive and active rabies immunization are essential especially after severe exposure. Post-exposure treatment is assumed to neutralize or inactivate virus while it is still in the wounds, before it gains access to the nervous system where it is protected from the immune system. Therefore, treatment after exposure to rabies virus is very urgent, even if the patient was bitten months before. Thorough washing of rabies-infected wounds with soap and water can increase survival by 50%.3 However, this inexpensive, readily available treatment is omitted in most cases.
The World Health organization recommends a multi-site intradermal regimen of 0.1 mL of vaccine at eight sites on day 0, at four sites on day 7, and at one site each on days 28 and 90.39
Passive immunization with human rabies immunoglobulin lowers mortality after severe exposure.
An effective post-exposure protocol for unvaccinated domestic animals exposed to rabies includes immediate vaccination against rabies, a strict isolation period of 90 days, and administration of booster vaccinations during the third and eighth weeks of the isolation period.40 The protocol has been effective in dogs, cats, cattle, and horses.40
A Compendium of Animal Rabies Control is published annually by the National Association of State Public Health Veterinarians, Inc. in the United States and Canada.25 It provides recommendations for immunization procedures in domestic animals, the vaccines licensed and marketed in the United States. Detailed information is provided on pre-exposure vaccination, management of dogs and cats and livestock, post-exposure management, and control methods in wild animals. Such publications should be consulted when necessary. In general, for cattle, sheep, and horses the primary vaccination is given at 3 months of age and boosters given annually. Farm livestock in endemic areas where clinical cases of rabies occur commonly should be vaccinated.
In countries where vampire bats are a major vector for rabies in farm livestock, vaccination of livestock is necessary but in countries such as Argentina, vaccination does not support a cost benefit analysis.41
Almost all rabies vaccines for domestic animals are inactivated.25 Inactivated tissue culture cell vaccines given to cattle result in neutralizing antibodies in 1 month after the primary vaccination. A booster given 1 year later increases the titers, which are detectable 1 year after the booster. A vaccine inactivated with binary-ethylenimine, and containing aluminum hydroxide adjuvant, provides excellent protection for up to 3 years and is very useful for the control of rabies in cattle in Latin America where the vampire bat is the main vector.
Vaccinal antibodies are present in the colostrum of vaccinated cows and it is recommended that, where cattle are vaccinated annually, calves be vaccinated at 4 months of age and again when 10 months of age. Calves from unvaccinated dams can be protected by vaccinating them at 17 days of age. Postvaccinal paralysis does not occur after its use.
A post-exposure vaccination protocol including immediate rabies vaccination, with a minimum of one booster vaccination prior to release from quarantine, and 90 days strict isolation, was 99.7% successful in unvaccinated cattle, horses, sheep, goats, and pigs.
The literature on oral rabies vaccination of wild carnivores in the United States has been reviewed.42 Mass oral vaccination of terrestrial wild animals is a rabies control method that is feasible, effective, and internationally accepted.36,42,43 It is based on the concept of applied herd immunity. The vaccines are efficacious when fed as vaccine-baits. The factors affecting acceptance of baits for delivery of oral rabies vaccine to raccoons have been examined.44
The oral immunization of foxes has resulted in a substantial decrease in the number of rabies cases in Europe. As a result of oral vaccination of the red fox (Vulpes vulpes) against rabies, using hand and aerial distribution of vaccine-laden baits, the rabies virus has almost been completely eradicated from Western and Central Europe.45 The same dramatic decrease occurred in southern Ontario, Canada. In most countries, vaccine baits were distributed twice yearly; during the spring (March to May) and autumn (September to October). Several European countries have become rabies-free: Belgium, Luxembourg, France, Italy, Switzerland, finland and the Netherlands.43 With the European Union consisting of 25 countries from May 2004, all the scientific knowledge is available for establishing efficient and adapted oral vaccination programs aimed at eliminating terrestrial rabies from this area.
Progress has been made in applying oral rabies vaccination to contain and eliminate some strains of terrestrial rabies in North America.42 Notable examples include near elimination of rabies from red foxes in southern Ontario. Containment and elimination of canine rabies coyotes from south Texas, containment and near elimination of raccoon rabies from Ohio, prevention of raccoon rabies spread through the Lake Champlain Valley in New York and across northern Vermont and New Hampshire, and reduced incidence of rabies where other oral rabies vaccination projects targeting raccoons have occurred. As of 2005, both Ontario and New Brunswick were free of raccoon rabies for greater than 10 months and 2 years, respectively, after implementation of ‘point infection control’ strategies29 but continued surveillance is critical to monitor effectiveness.
Raboral V-RG is the only rabies vaccine licensed for use in the United States. It has not produced sufficient levels of population immunity in skunks in the wild at the current dose and V-RG may be less effective in skunks than other species. Skunks are a major contributor to rabies in North America with 38% of cases associated with the raccoon variant of rabies virus involved skunks in 2001.12 This has raised concerns about an independent maintenance cycle for raccoon rabies in skunks. The national rabies management goals of virus containment and elimination of will likely remain elusive until an oral vaccine is licensed that is immunogenic in all terrestrial rabies reservoir species. Skunk rabies virus, which has the broadest geographic distribution of all terrestrial rabies variants in the USA can currently be addressed only through local trap-vaccinate-release or population suppression programs.42
A satisfactory vaccine for oral mass vaccination of skunks has not yet been developed. Many experimental and commercial-produced live-modified or recombinant-based oral rabies virus vaccines have been tested in skunks with contradictory results. An oral modified live rabies virus Vaccine SAD B19, used in striped skunks was innocuous and may be safer and more effective in skunks after oral vaccination than previously considered.46 An attenuated SAG-2 vaccine was efficacious in challenge studies in skunks and raccoons and may satisfy both safety and efficacy concerns for oral rabies vaccination of major North American rabies reservoirs.47
During the 1990s in the United States, oral vaccination programs concentrated upon raccoons, gray foxes, and coyote, with similar success. As a result, raccoon rabies has not spread west of its initial concentration in the eastern states, and grey fox rabies in contained in the west central Texas, and no recent cases of rabies have been reported in coyotes away from the Mexican border for several years. However, vaccination is not a panacea and should be considered as an important adjunct to traditional prevention and control techniques in human and veterinary medicine.
It is notable that no practical vaccination methods have been developed for bats.36 Phylogenetic analyses of viruses from bats and carnivores suggest a historical basis for still existing viral origins due to interactions between these taxa. Thus the possibility for pathogen emergence resulting from transmission by rabid bats with subsequent perpetuation among other animals cannot be discounted easily on any continent. Vampire bats have been vaccinated IM, scarification, oral, or aerosol routes using a vaccina-rabies glycoprotein recombinant vaccine.48 The highest antibody titers occurred in animals vaccinated by the IM and scarification routes. All animals vaccinated by the IM, scarification, and oral routes survived experimental challenge, except 1 of 8 receiving the aeosol vaccination died.
Most consistent progress was achieved and maintained where:
• Large coherent territories covering certain sized areas were treated simultaneously
• All areas of fox habitat were included by using both manual and aerial distribution
• Vaccination zones were progressively expanded towards the infected area
The vaccination will succeed in reducing or eradicating rabies only if a sufficient proportion of the target population can be immunized. Mathematical modeling techniques are now being tested to examine the population biology of rabies in wildlife species such as raccoons and skunks.49 Mass immunization of foxes by aerial distribution of vaccine-baits containing liquid rabies vaccine was highly successful in controlling rabies in both urban and rural areas of Ontario.36,42
In 1989, the Ontario began a 5-year experiment to eliminate terrestrial rabies from a study area in the eastern end of southern Ontario.50 Baits containing oral rabies vaccine were dropped annually in the area at a density of 20 baits/km2 from 1989 to 1995. The experiment was successful in eliminating the arctic fox variant of rabies from the entire area. In the 1980s, an average of 235 rabid foxes per year were reported in the study area. Between 1993 and 2001, no cases were reported. Cases of fox rabies in other species also disappeared. In 1995, the last bovine and companion animal cases were reported and in 1996 the last rabid skunk occurred. Only bat variants of rabies were present until 1999, when the raccoon variant entered from New York.
The most effective method of preventing the entry of rabies into a country free of the disease is the imposition of a quarantine period of 4–6 months on all imported dogs. This system has successfully prevented the entry of the disease into island countries, but has obvious limitation in countries that have land borders. The occurrence of the disease in two dogs in the United Kingdom in 1969–1970 in which the incubation period appeared to last 7–9 months suggests that the more usual period of 6 months may give incomplete protection. Therefore, vaccination on two occasions with an inactivated vaccine while the animal is still in quarantine for 6 months is the current recommendation. To require a longer period of quarantine would encourage evasion of the law by smuggling. The situation in the United Kingdom, and in any country where the disease does not occur, is a vexed one. It is possible to rely chiefly on quarantine and act swiftly to stamp the disease out if it occurs. The shock eradication program would include quarantine of, and vaccination in, a risk area, ring vaccination around it, and destruction of all wildlife. This procedure is likely to be adopted in countries where the risk is small, such as Australia. Where the risk is great, consideration must be given to mass vaccination of wildlife by baits, because wildlife are the cracks in the defense armor. The use of combined vaccines containing rabies vaccine in other vaccines used in dogs would be an effective and panic-free way of increasing the immune status of the pet population.
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