Exotic Diseases

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Equine viral encephalomyelitis

Species Infected

• affects mainly horses, donkeys, mules
• man susceptible - disease varies from influenza-like to encephalitis in the young/old
• a wide range of mammal, bird and other animal species can be infected subclinically
• - 3 viruses
  • - Eastern equine encephalomyelitis (EEE) virus
  • - Western equine encephalomyelitis (WEE) virus
  • - Venezuelan equine encephalomyelitis (VEE) virus

Clinical Signs

• incubation period 1-3 weeks
• short fever followed by anorexia, depression
• development of nervous signs
• - hypersensitivity
• - transient periods of excitement/ restlessness
• involuntary muscle movements
• - face
• - shoulder
• - penile erection
• blunder as if blind
• paralysis develops
• - increasing incoordination
• - terminally, complete paralysis
• death 2-4 days in majority (90%) of cases
• for WEE, a milder disease, up to 30% mortality

Lesions

• no gross lesions
• histopathology: diffuse encephalomyelitis

Pathogenesis

• transient low titre viraemia occurs with WEE, EEE
• persistent high titre viraemia exists with VEE
• infection does not produce signs except fever unless virus penetrates brain
• clinical signs correlate with brain lesions - e.g. early derangements are associated with infection of grey matter of cerebral cortex, thalamus and hypothalamus

Aetiology

• Alphavirus (Togaviridae)
• 3 viruses are
• - Eastern EE virus
• - Western EE virus
• - Venezuelan EE virus
• are related antigenically to one another
• are arboviruses that proliferate in vector
• virus can be found in most tissues of vector
• VEE virus has 2 subtypes
• - sylvatic - nonpathogenic for Equidae does cause disease in man
• - epidemic - pathogenic for Equidae and man

Epidemiology

• 2 cycles
  • - endemic cycle - continuously maintained and acts as reservoir for epidemic disease between summers
    • - for EEE and WEE viruses, the endemic cycle involves mosquitoes and wild birds - - for VEE virus, wild rodents as well as birds and mosquitoes are involved
  • - epidemic cycle - when virus moves into the equine and human population - low transient viraemias seen in EEE and WEE means little horse to horse spread; secretions are infective but have only been shown experimentally to transmit disease
    • - high persistent viraemia found in VEE means that horses can act as amplifiers of the disease

Differential Diagnoses

1. bacterial encephalitis
2. Australian viral encephalidites
3. botulism
4. lead poisoning
5. hepatic encephalopathies
6. rabies
7. EHVI myeloencephalopathy
8. Indigofera toxocity