West Nile Virus

The majority of infected horses do not display clinical signs and WNV encephalitis occurs in only a small proportion of infected horses. The incubation period for equine WNV encephalitis following mosquito transmission ranges from three to 15 days. Horses that do become ill may show mild to severe disease.

Typical signs include ataxia (incoordination, stumbling, limb weakness) that might appear suddenly or develop gradually and worsen. Other disease signs include:

• sleepiness
• dullness
• listlessness
• facial paralysis (droopy eyelids, lower lip), and
• an inability to rise.

Some horses may develop a mild fever, blindness, muscle trembling and seizures. Treatment is supportive and signs may resolve or progress to terminal recumbency. The mortality rate in some outbreaks is approximately one in three clinically affected unvaccinated horses.

Horses vaccinated for WNV and foals of positive-testing mares are likely to give a positive blood test for the virus and veterinarians should consider the possibility of other neurological diseases in such circumstances. Differential diagnoses in horses include other arboviral encephalitis (e.g. Eastern, Western or Venezuelan equine encephalomyelitis, Japanese encephalitis), equine protozoal myeloencephalitis (Sarcocystis neurona), equine herpervirus-1, borna disease and rabies.