West Nile Virus (WNV) infection poses a health threat to domestic and free-ranging horses. As in humans, WNV in horses may result in an inapparent infection, illness, or death from encephalitis (inflammation of the brain). Clinical signs of WNV infection in horses include:

• ataxia (stumbling or incoordination)
• depression or apprehension
• weakness of limbs
• partial paralysis
• muscle twitching
• death
• fever is not often observed

Veterinarians can test horses for WNV and other diseases. About one third of the horses diagnosed with WNV die or are euthanized. In 2002, WNV was diagnosed in almost 15,000 horses; however, may more cases likely went undetected. Although horses can become ill and may die from WNV encephalitis, there is no documentation that they play a role in spreading the disease. Therefore, infected horses do not need to be quarantined or euthanized. However, there are a number of other serious diseases that are transmissible to humans (e.g., rabies virus) that have similar signs. Therefore, precautions to avoid exposure and veterinary diagnostics should be performed.

No treatment, other than supportive care, is available for horses with WNV. However, a fully licensed killed vaccine as well as a recombinant vaccine are now available for use in equids. Vaccination is an excellent means of disease prevention. Consultation with a practicing veterinarian is encouraged to determine if vaccination of domestic horses is indicated and to determine a vaccination schedule. Vaccination of free-ranging horses is, in most cases, not practical due to the need for multiple injections.

Reducing exposure of horses to mosquitoes can also help prevent WNV infection. Sanitation practices, such as removing artificially contained standing water and cleaning water troughs, to reduce mosquito breeding can help protect domestic horses and free-ranging horses in developed areas. Domestic horses can also be protected by housing in screened or indoor stalls when mosquitoes are active and by use of insect repellents. In accordance with NPS policy, all proposed use of pesticides (including insect repellents for employees, horses, USPP, or concessions facilities) must be approved by an NPS integrated pest management specialist prior to use.

Animal and Plant Health Inspection Service (APHIS)

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