Western Equine Encephalitis in Horses: Signs, Risk, and Prevention

Western equine encephalitis, often shortened to WEE, is a viral neurologic disease of horses. The virus is spread by infected mosquitoes and can inflame the brain and spinal cord. That inflammation can lead to fever, behavior changes, incoordination, weakness, seizures, and death.

WEE is part of the equine encephalitis group that also includes eastern equine encephalitis and Venezuelan equine encephalitis. In North America, WEE has historically been associated with the western United States and western Canada, although the virus has also been identified in Mexico and parts of South America. Horses do not usually spread the virus directly to each other in the barn.

The good news is that WEE appears to be reported much less commonly in recent years than it was in the past. Vaccination is thought to be a major reason. Even so, any horse with sudden neurologic signs should be treated as an emergency because WEE can look similar to other serious conditions, including rabies, West Nile virus, equine herpesvirus neurologic disease, trauma, and toxicities.

WEE is caused by Western equine encephalitis virus, an alphavirus. In nature, the virus is maintained mainly in a cycle between mosquitoes and wildlife reservoirs, especially birds. In western North America, Culex tarsalis is considered an important mosquito vector.

A horse becomes infected after being bitten by an infected mosquito. Horses are considered dead-end hosts, which means they usually do not develop enough virus in the bloodstream to keep the transmission cycle going. In practical terms, that means one sick horse is not typically the source of infection for another horse in the barn.

Risk tends to rise during mosquito season and in environments that support mosquito breeding, such as standing water, irrigated pasture, wet areas, and warm weather. Unvaccinated horses are at the greatest risk of clinical disease. Even vaccinated horses with neurologic signs still need prompt evaluation, because no vaccine is 100% protective and several other serious neurologic diseases can look similar.

Your vet will start with a full history and neurologic exam. Timing matters. A horse with fever and neurologic signs during mosquito season, especially in a region where arboviral disease is possible, raises concern for encephalitis. Still, there are no pathognomonic signs for WEE, so diagnosis always involves ruling out other causes of neurologic disease.

Initial testing often includes bloodwork and, depending on the case, referral laboratory testing for arboviral disease. Diagnostic confirmation commonly relies on antibody testing such as IgM capture ELISA or other specialized testing selected by your vet and diagnostic lab. Cornell's equine diagnostic resources list arboviral panels using EDTA whole blood and serum for encephalitis-related testing, and referral testing may include broader encephalomyelitis panels.

Because neurologic horses can be unsafe and because some look-alike diseases have public health or regulatory importance, your vet may recommend isolation precautions, referral, and reporting steps. Suspected equine encephalitis cases may need to be reported through animal health channels depending on the diagnosis and state requirements.

Prevention centers on vaccination plus mosquito control. In the United States, the AAEP considers EEE/WEE vaccination a core vaccine for all horses residing in or traveling within the country. Adult horses previously vaccinated should be revaccinated before mosquito season in the spring. Unvaccinated adults usually need a 2-dose primary series given 3 to 6 weeks apart, then revaccination before the next vector season and annually after that. Foals typically begin a primary series at 4 to 6 months, with a second dose 4 to 6 weeks later and a third dose at 10 to 12 months. Previously vaccinated pregnant mares are generally boosted 4 to 6 weeks before foaling.

Mosquito control matters because vaccines lower risk but do not remove it completely. Work with your vet on a practical barn plan: remove standing water, clean troughs and buckets often, improve drainage, reduce manure and wet organic debris, use fans in stalls when appropriate, and consider equine-safe repellents or fly sheets based on your horse's needs.

If one horse on the property develops fever or neurologic signs, call your vet promptly and review the vaccine status of all horses on the farm. Your vet can help you decide whether boosters, movement limits, or additional mosquito-control steps make sense for your specific region and season.