West Nile Virus in Horses

By: Mary Newell Sanders, DVM

TVMA Member
Bellville, TX

Published July 2014

Photo by Tracy Fant Colvin, DVM

What is West Nile Virus?

West Nile Virus is a viral disease that is transmitted by mosquitos. Mosquitos first become exposed to the virus when they feed on infected birds. Once the mosquito is infected, it can then transmit the virus to people and other animals when it bites them. It first appeared in the United States in 1999 on the East Coast and has since spread rapidly to most other states. (1) It can be transmitted to both humans and animals and cause brain and/or spinal cord infection. Horses are one of the species prone to severe infection. Though people cannot get West Nile Virus from an infected horse, it is important to be able to recognize the signs and know how to prevent this serious and sometimes fatal disease in your horse.

Symptoms of West Nile and Other Mosquito Diseases

West Nile infects the central nervous system and causes symptoms of encephalitis (brain inflammation). Clinical signs develop rather rapidly and include loss of appetite, depression, fever, weakness or paralysis, muscle twitching, loss of vision, incoordination, head pressing, circling or aimless wandering, seizures, hyperexcitability or coma in horses. There are a number of other mosquito-borne viral diseases, such as Eastern, Western and Venezuelan encephalitis, that display clinical signs that are very similar to West Nile Virus. Equine Protozoal Myelitis (EPM) can also mimic West Nile Virus. A thorough physical exam should be done on any horse displaying neurologic signs. A blood test is necessary to definitively diagnose West Nile Virus infection.

Treatment & Prevention for West Nile

There is no specific treatment for this dangerous virus. Supportive care, such as intravenous fluids and anti-inflammatories, should be provided until the horse has fully recovered. In most cases, improvement is noted within days. Those horses that are severely affected and are unable to stand or have seizures are unlikely to respond to treatment and should be humanely euthanized.

There is a vaccine for West Nile. Current recommendations are an initial vaccination followed by a booster three weeks later and annually thereafter. Horses should also be vaccinated annually against the equine encephalitis viruses, which are also transmitted by mosquitoes. Discuss with your Texas veterinarian the best vaccination schedule for your horse. Vaccination is not, however, a guarantee of protection against infection. Reducing your horse’s exposure to mosquitos is one of the best ways to prevent infection by West Nile Virus in Texas. This involves eliminating mosquito breeding sites. Mosquitos will breed in any puddle of water that stands stagnant longer than four days, so dispose of any unwanted water containers on the property. Do not forget discarded tires. If you have to have containers that will hold water, drill holes in the bottoms so they can drain. Turn over plastic pools and wheelbarrows. Do not let stagnant water collect in birdbaths or gutters. Empty and refill buckets and water troughs every few days. Aerate or stock tanks with fish, and keep pools clean and chlorinated.

This disease is easily preventable with proper vaccination and environmental management. Vaccinating all horses as recommended by your veterinarian and keeping your property clean, dry and free of standing water will go a long way in minimizing your horse’s risk of contracting this deadly virus.


1. “The Outbreak of West Nile Virus Infection in the New York City Area in 1999.” The New England Journal of Medicine. http://www.nejm.org/doi/full/10.1056/NEJM200106143442401


Dr. Mary Newell Sanders practices at Marek Veterinary Clinics in Bellville and Sealy, Texas.

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