West Nile Virus in Horses: Symptoms, Treatment and Prevention

Last Updated on March 18, 2022 by Allison Price

West Nile virus (WNV), a mosquito-borne illness, originated in Africa and was first discovered in the United States in 1999. More than 25,000 WNV cases in horses have been recorded in the United States since then.

The disease has been confirmed in the United States, Canada, Mexico and other Central and South American countries.

How WNV is transmitted

WNV is considered a “host” in wild birds because they carry high levels of virus in their blood. This virus can be transmitted to mosquitoes by eating the birds. After infecting birds, mosquitoes can transmit the virus to horses by biting them.

Horse-to-horse and horse-to–human transmission of the disease are not possible.

Horses can be protected from WNV by getting an annual vaccine.

Limit your horse’s exposure to mosquito season.

  • To repel mosquitoes, use Farnam Mosquito Halt
  • Avoid turning out during peak mosquito activity (generally dusk and dawn).
  • Fans in barns and stalls can be used to stop mosquitoes from landing on horses.
  • To prevent the development of mosquito larvae, empty and clean water troughs once every three to four days
  • Take out vegetation, leaves, and piles of brush. Mosquitoes love these places in the heat of the summer.
  • Make sure gutters are clean

Patterns of Activity

WNV is transmitted by mosquitoes. The disease is more prevalent in areas where there is a high mosquito population. Infection rates for birds and mosquitoes peak around August and fall.

Although you may think that WNV, a mosquito-borne virus, would be more prevalent in hot and humid areas, it is not easy to predict which areas will have the highest number of cases.

“Interestingly, we only see very small areas of WNV activity in the U.S. every year and these are not always in hot climates with year round mosquito activity,” says Martha Mallicote DVM, clinical assistant professor at University of Florida College of Veterinary Medicine, Gainesville, Florida.

Mallicote adds that WNV incidences vary from year to year. Mallicote says that this is more likely due to weather patterns and infected vectors’ local patterns than an overall reflection of vaccine use.

WNV Signs

Horses are susceptible to WNV after a relatively short incubation period. Unvaccinated horses may show symptoms of the disease after being bitten by an infected mosquito for five to fifteen days. Neurologic signs are common with WNV because it can cause inflammation or irritation of the brain, spinal cord, and lining.

One or more of these clinical signs may be seen in horses with WNV infection:

  • stumbling/falling
  • Lack of coordination
  • Limb weakness
  • Partial paralysis
  • Muscle twitching
  • drooping lower lip
  • Inability/difficulty swallowing
  • Acting “colicky”.

If you have any of these symptoms, immediately contact your veterinarian. It is important to get prompt treatment.


A blood test can be used to confirm the presence of WNV in horses.

A blood test called an IgM ELISA is used to diagnose the condition. This antibody increases quickly after being exposed to the virus but then falls back quite quickly after it has been removed. Mallicote explains that this test does not increase due to vaccination.

A thorough neurologic exam is necessary to establish a prognosis.

WNV is not curable. Treatment focuses on managing inflammation and other clinical signs. Horses with WNV cannot transmit or be “carriers” so quarantine is not an option.

West Nile Virus in Horses

The severity of the horse’s neurologic signs will determine the amount of support care required. Mallicote says that we would at the minimum give anti-inflammatory medication and make sure the horse is drinking/eating enough to sustain themselves.”

She notes that horses who are unable to eat or drink by themselves would need intravenous fluids and nutritional supplementation. We would support horses who are so severely injured that they can’t stand or use a sling to help them.

Horses that are unable to stand due to recumbency (or other conditions) have a poor prognosis.

According to the American Association of Equine Practitioners, the WNV mortality rate is around 33%.

This is lower than the mortality rates of Eastern Equine Encephalomyelitis, which has a mortality rate between 75 and 95%, as well as Western Equine Encephalomyelitis, which can kill 20 to 50% of horses. However, WNV can be very serious and require intensive veterinary care. Even horses that survive the disease may experience neurologic deficits such as gait or behavior abnormalities.

How to Protect Your Horse

To protect your horse against WNV and other insect-borne diseases, overall wellness and regular vaccinations are essential. Proper exercise, nutrition tailored to your horse’s size, age and activity, as well as proper nutrition, are all important for keeping him healthy.

Mallicote says that it is important to ensure that horses are healthy and get good preventative care. This includes routine vet exams.

WNV is considered a “core” vaccine in the United States, so all horses should be vaccinated annually. Core vaccines include:

  • Eastern Equine Encephalomyelitis
  • Rabies
  • Tetanus
  • West Nile Virus

Your veterinarian may recommend additional vaccines depending on the specific needs of your horse. Discuss your options with your veterinarian and create a plan for strategic vaccinations that you will need to maintain each year in order to protect your horse from mosquito season.

Mallicote notes that it wasn’t clear when the WNV vaccine first came out that one annual booster would be enough to provide a full year of protection. Therefore, it was suggested to boost more frequently. “At the moment, there are enough data to support annual boosters being sufficient for coverage even in areas where year-round mosquito population is high.”

To provide full protection, the WNV vaccines are required to be administered three to six weeks apart. Your veterinarian will advise you on the best time to begin vaccination if you have a foal, young horse or other animal.

What if you are buying a new horse?

Mallicote says that we have seen horses with WNV and EEE after they were purchased new. If you are not sure about a horse’s vaccination history, it is important to immediately give him a primary series of two doses of vaccine against ALL encephalitis viruses. Also, ensure he is vaccinated against the other core diseases.

“If the horse is living in very cold, mosquito-free conditions and it’s midwinter, these primary vaccines might be delayed for shorttime. She emphasizes that the simple answer is to just vaccinate. “I have seen too many cases of fatal viral encephalitis, including all types, to not be serious about the importance of vaccination.”

Mosquito Control Is Essential

Protecting your horse against WNV is not only important with annual vaccinations, but also requires mosquito control.

The female mosquitoes bite because they need blood to grow eggs. These mosquitoes lay their eggs around or in water, making standing water and damp areas ideal breeding grounds.

Mallicote warns that even the smallest container can become a breeding ground if it is in the right environment and climate. People talk about tires and buckets but forget about outside plants with saucers underneath the pots and shallow containers around the barnyard.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!