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

Thursday, August 21, 2014


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 — then infect horses, humans and other mammals. Last year, the United States reported 395 West Nile virus cases in horses.2 Texas and Oklahoma topped the charts with 69 and 41 cases, respectively.2

The number of reported WNV cases was 1,121 in 2006, and the decline is said by health experts to reflect both vaccination and naturally acquired immunity.3,4

“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,” said Kevin G. Hankins, DVM, 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 (EEE) and Western equine encephalomyelitis (WEE).

Researchers recently tested horses' response to six West Nile virus vaccination regimens and found some substantial differences in their immune responses.5 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.

“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 said.

The researchers believe the reduced immune response of horses that 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.3

“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 said. “That makes vaccination a cheap insurance policy.”

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

    Destroying any mosquito breeding habitats by filling in low areas that collect water and adding an aerator to ponds
    Cleaning and emptying any water-holding container, such as water buckets, water troughs and plastic containers, on a weekly basis6
    Apply insect repellents or bring horses inside during the peak mosquito feeding hours between dusk and dawn

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.7 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%.3

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 U.S. Geological Survey. West Nile Virus Maps, 2013 - Veterinary - USA. Available at: http://diseasemaps.usgs.gov/2013/wnv_us_veterinary.html. Accessed June 12, 2014.

3 American Association of Equine Practitioners. Core Vaccination Guidelines. Available at: http://www.aaep.org/-i-165.html. Accessed June 14, 2014.

4 U.S. Geological Survey. West Nile Virus Maps, 2006 - Veterinary - USA. Available at: http://diseasemaps.usgs.gov/2006/wnv_us_veterinary.html. Accessed June 12, 2014.

5 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.

6 Florida Division of Animal Industry. West Nile Virus. Available at: http://www.freshfromflorida.com/ai/main/wnv_main.shtml. Accessed June 12, 2014.

7  Pennsylvania West Nile Virus Control Program. What Horse Owners Should Know About West Nile Virus. Available at: http://www.westnile.state.pa.us/animals/horses.htm. Accessed June 12, 2014.