Be Aware of West Nile Virus in Horses

West Nile Virus (WNV) is a mosquito-borne virus that can be transmitted to horses. While we haven't seen an equine case of WNV in Pennsylvania yet as of June 2022, there have been mosquito populations confirmed positive for WNV in multiple counties across the state. This means that there are currently mosquitoes in PA carrying the virus. As horse owners, we should be aware of what WNV is, how it can impact our horses, and what we can do to protect them.

A horse becomes infected with WNV when it is bitten by a mosquito carrying the virus. Therefore, the likelihood of contracting WNV is higher during seasons when mosquitoes are alive and active (mostly spring, summer, and fall). West Nile Virus affects the horse's central nervous system. When a horse contracts WNV, most of the signs are neurological in nature. They include incoordination, muscle twitching, weakness, and paralysis. There is a 35% mortality rate for horses showing clinical signs of WNV. When diagnosing WNV, a blood test is required to confirm a positive diagnosis so always contact a veterinarian if illness is suspected.

What can I do to protect my horse from West Nile Virus?

There are multiple ways you can protect your horse. First, you should have it vaccinated for WNV annually. This is the recommendation of the American Association of Equine Practitioners (AAEP). Vaccines are 93-95% effective and horses that become infected are often not vaccinated. Work with your veterinarian to make sure your horse receives the vaccine in springtime before mosquito season has begun. Your vet may also recommend more frequent vaccinations if your horse is at higher risk of contracting WNV. A horse might be at higher risk if they are juvenile (under 5 years of age) or geriatric (over 15 years of age). Horses kept in a geographic area where mosquito populations are high may also benefit from additional vaccinations.

Limiting your horse's exposure to mosquitoes can also help reduce its risk of contracting WNV. Mosquitos breed in standing, stagnant water, so try to reduce these areas around your barn. Refresh water buckets and troughs frequently. Dump out standing water in items not being used such as old tires or troughs around the farm. Try to limit your horse's access to areas where mosquitoes are prevalent. They are often found in high concentrations in wooded areas with water nearby. They are also more prevalent during early morning and evening, so keep horses stabled or away from high-risk areas during these times if possible. Applying permethrin-based fly sprays can also help reduce bites when applied appropriately and frequently.

For more information on WNV visit:

• West Nile Encephalitis in Horses - Penn State Extension
• West Nile Virus - American Association of Equine Practitioners
• Mosquitoes and Mosquito-Borne Diseases - Penn State Extension