West Nile Virus: Is Your Horse Ready for Peak Mosquito Season?

Learn how you can help protect your horses.

From AQHA Corporate Partner Zoetis

In the United States, July through October coincides with peak mosquito activity, which places your horse at the highest risk of contracting West Nile virus (WNV) during this time of year.1 However, with the right vaccine and preventive measures, it’s not too late for horse owners to help protect their horses against this life-threatening disease.

West Nile virus is transmitted by mosquitoes, which feed on infected birds and then infect horses, humans and other mammals. In 2014, the U. S. Centers for Disease Control and Prevention reported 19,623 West Nile virus cases; case numbers include mosquitoes, birds, humans and horses.2 California topped the charts with 7,008 cases, followed by Texas with 2,484 cases. 2

“It is a good sign that the overall number of cases has declined over the last decade. However, there continue to be an alarming number of human and equine cases every year, especially in the late summer and fall,” says Dr. Kevin G. Hankins, senior veterinarian, Equine Veterinary Operations, Zoetis.

Vaccination remains the most effective way to help protect horses against West Nile and other mosquito-borne diseases, such as eastern equine encephalomyelitis and western equine encephalomyelitis.

Researchers recently tested horses' response to six West Nile virus vaccination regimens and found some substantial differences in their immune responses.3 While all of the vaccinated horses demonstrated an initial immune response, by Day 28, the response of the horses vaccinated with WEST NILE-INNOVATOR® was four times higher than those vaccinated with the one-dose, big combination WNV-containing vaccines.

Learn more about West Nile virus and other common horse health concerns with AQHA's Common Horse Health Issues report.

“We thought that WEST NILE-INNOVATOR would produce a higher immune response than the large one-dose combination West Nile vaccines but did not think it would be nearly four times higher,” Dr. Hankins says.

The researchers believe the reduced immune response of horses who received the large one-dose combination West Nile vaccines could be caused by:

    • Antigen interference
    • Antigen load (exposure to substances that trigger an immune response)
    • Or other unknown factors

Regardless of the cause, Dr. Hankins suggests that veterinarians should consider the possible consequences of a lower WNV antibody response with West Nile combination vaccines when developing and implementing vaccine programs for horse owners.

West Nile is considered a core vaccination requirement, along with vaccinations for EEE, WEE, tetanus and rabies, according to the American Association of Equine Practitioners guidelines.4

“We have a disease that is here to stay and an effective vaccine but no reliably effective treatment in the case of infection,” Dr. Hankins says. “That makes vaccination a cheap insurance policy.”

Get AQHA's Common Horse Health Issues report and make sure you are well informed about West Nile virus and other equine health concerns.

Along with vaccination, use other techniques for managing mosquitoes, including:

    • Cleaning and emptying any water-holding container, such as water buckets, water troughs and plastic containers, on a weekly basis5

Remember that WNV does not always produce signs of illness. In horses that do become clinically ill, the virus infects the central nervous system and can cause symptoms such as loss of appetite and depression. Other clinical signs may include fever, weakness or paralysis of hind limbs, impaired vision, ataxia, aimless wandering, walking in circles, hyperexcitability or coma.6 Horse owners should contact a veterinarian immediately if they notice signs or symptoms of WNV infection in their horses, especially if they are exhibiting neurological signs. The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33 percent.4

By providing proper vaccination and helping to manage mosquito populations, it’s not too late for horse owners to do their part to help prevent WNV infections.

Ask your veterinarian or retailer for WEST NILE-INNOVATOR today.

1 Reed SM, Bayly WM. Equine Internal Medicine, 3rd Ed. 2010; 630.
2 Center for Disease Control. Number of West Nile Positive Mosquitos by State. Available at: Accessed May 6, 2015.
3 Cortese V, Hankins K, Holland R, Syvrud K. Serologic Responses of West Nile Virus Seronegative Mature Horses to West Nile Virus Vaccines. J Equine Vet Sci 2013;33(12):1101-1105.
4 American Association of Equine Practitioners. Core Vaccination Guidelines. Available at: Accessed May 6, 2015.
5 Florida Division of Animal Industry. West Nile Virus. Available at: Accessed May 6, 2015.
6 Pennsylvania West Nile Virus Control Program. What Horse Owners Should Know About West Nile Virus. Available at: Accessed May 6, 2015.
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