West Nile Virus in Horses: Prevent the Preventable

By: SallyAnne L. DeNotta, Ph.D., DVM, DACVIM
Contributions and Editing By: Ashley Pfeifer, DVM – March 2025 

When it comes to horse health, there are rarely any guarantees. Even horses with impeccable management can fall victim to illness or injury at any time. Thankfully, we can prevent some serious diseases, such as West Nile Virus (WNV), easily and effectively through proper vaccination. 

The American Association of Equine Practitioners has included WNV on its list of core vaccines, meaning it’s recommended for all horses regardless of where they live or whether they travel. Unfortunately, in 2023 there were still over 300 equine cases of WNV in the United States. Most, if not all, of these cases could have been prevented with proper vaccination. Two key measures you can take to help protect your horse against WNV include adopting management strategies to reduce mosquito exposure and vaccinating against the disease.  

Background on WNV 

West Nile virus is a member of the Flaviviridae family in the genus Flavivirus. It was first introduced into the U.S. in 1999 and spread rapidly across all 48 contiguous states in the years following, resulting in thousands of equine infections. Various wild bird species can carry high levels of virus and serve as reservoirs for it. Mosquitoes transmit the virus by feeding on the blood of infected birds and then on susceptible species such as horses and humans. 

Upon transmission to a susceptible horse, the virus crosses the blood-brain barrier and damages the brain and spinal cord. Infected horses that develop illness typically show neurologic signs such as ataxia (limb incoordination, especially of the hind limbs); face, neck, or trunk muscle twitching; and fever. Many horses also exhibit behavioral changes such as hyperreactivity or depression. In severe cases, horses might progress to recumbency (inability to rise), paralysis, coma, and death.  

Veterinarians diagnose WNV in horses based on physical examination findings and the detection of specific types of antibodies to WNV in the blood. The virus itself can also occasionally be found in blood, serum, and cerebrospinal fluid using a PCR test. While there is no specific treatment for WNV, it often involves intense supportive care. Infected horses with clinical signs often benefit from anti-inflammatory medication plus intravenous and/or oral fluids. Many horses recover partially or fully. Horses that become recumbent carry a poor prognosis and are often euthanized. 

Vaccination Is Key 

An equine vaccine for WNV became available in 2001 and was fully licensed by the USDA in 2003. Widespread vaccination efforts resulted in a dramatic drop in infection rates across the country. Both the killed and recombinant forms of the WNV vaccine promote excellent immunity when administered according to recommendations. For both vaccine types, veterinarians recommend administering an initial two-dose series, four to six weeks apart. The second booster vaccine is crucial for protective immunization, and horses that only receive the first vaccine are not considered fully protected.  

Following the initial vaccine series, your veterinarian should administer regular boosters at least annually. While each of the available WNV vaccines have been shown to provide protection for 12 months, in the states where mosquitoes remain active throughout the year, many vets recommend horses receive boosters every six months to ensure strong protective immunity. Foals from vaccinated mares should receive an initial three-vaccine series starting at 4-6 months of age, while foals from unvaccinated mares might need to receive their initial vaccines earlier to be fully protected. It is important to give vaccinations at least a month prior to mosquito season to allow for protective immunity to develop before infection risk. Consult your veterinarian for recommended vaccination schedules in your region. 

Reduce the Source 

Mosquito control represents another crucial component for reducing WNV risk to your horse and yourself. Eliminating mosquito breeding grounds by removing sources of standing water can reduce populations on farms dramatically. Clean and refill water troughs regularly. Fly sprays containing pyrethrin are effective mosquito repellents and should be applied regularly.  Remove bird nests from barns. Also protect yourself from WNV by wearing protective clothing and mosquito repellent. It is important to note that WNV is not passed from horse to human or human to horse.  

With so many unforeseeable equine health risks, it’s crucial that we take action to prevent the avoidable diseases. Now is the time to consult your veterinarian and make a plan to prevent WNV from affecting you and your horses.