West Nile Virus in Horses

Trisha Consunji: 916-649-0599 (California Veterinary Medical Association)


(Sacramento, CA) The California Veterinary Medical Association (CVMA) urges horse owners to have their veterinarian vaccinate their horses as soon as possible to inoculate the animals against West Nile virus.

Saying it is not too late to protect horses from the rapidly spreading virus, CVMA past-president Jon Klingborg, DVM emphasized, “The only sure way to protect your horse from West Nile virus is to practice mosquito control and vaccinate.” The initial dose of vaccine must be followed with a second dose three to six weeks later and then with yearly boosters, but it’s not too late to protect horses for at least part of this year’s mosquito season.

The CVMA recommends that owners contact their veterinarians to have their horses vaccinated. It is important to ensure the vaccine has been handled properly and is administered correctly. Horse owners should not attempt to vaccinate their horses themselves, said Dr. Klingborg, in part because of the following reasons:

• Many manufacturers sell vaccines only to veterinarians. Since pet and feed stores may have diverted product, the manufacturer will not guarantee the product’s efficacy under circumstances in which they have no control over shipping and storage.

• Vaccines must be administered by the proper route and in the correct location to be safe and effective.

• Vaccines are not curative. They must be given far enough in advance of exposure to allow the animal to develop immunity.

• Non-symptomatic animals could still develop signs of disease within a few days of vaccination. In these cases, the animals did not get sick from the vaccine, but rather because they did not develop sufficient immunity to the virus prior to exposure.

• Early vaccination and boostering at proper intervals is essential for protection against any disease.

In addition to vaccination by a veterinarian, horse owners should practice mosquito control by eliminating breeding sites like standing water in buckets, wheelbarrows, pastures, and non-aerated ponds. Horse owners can also keep their horses in stables at dawn and dusk (peek mosquito feeding times) and use fans, flysheets, and insecticide misting systems to deter the vectors.

Clinical signs of West Nile virus in horses include stumbling, staggering, wobbly gait, circling, hind limb weakness, inability to stand, paralysis, and death. Only one out of four affected horses develops a fever. West Nile Virus cannot be transmitted to humans from horses, nor can it be transmitted from horse to horse or to other animals by infected horses. It is only transmitted by infected mosquitoes.

For more information, please contact Trisha Consunji (916) 649-0599.


The California Veterinary Medical Association is the largest state veterinary medical association in the United States, with more than 7,000 members. Founded in 1888, its mission is to serve its membership and community through innovative leadership and to improve animal and human health in an ethically and socially responsible manner.

© 2020 California Veterinary Medical Association

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