July through October coincides with peak mosquito activity in some locations, which can place your horse at the highest risk of contracting West Nile virus (WNV) during this time of year. 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.

A viral disease, WNV is transmitted by mosquitoes—which feed on infected birds—to horses, humans, and other mammals. Last year, the United States reported 395 West Nile virus cases in horses; Texas and Oklahoma topped the charts with 69 and 41 cases, respectively. The number of reported WNV veterinary cases fell from 1,121 in 2006 to 157 in 2010, and the decline is said by health experts to reflect both vaccination and naturally acquired immunity.

“It is a good sign that the number of cases has declined over the last decade,” said Kevin G. Hankins, DVM, MBA, senior veterinarian for Zoetis Equine Veterinary Operations. “However, recent news reports of both human and equine cases indicate this disease is still a risk—especially during this time of year.

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. While all of the vaccinated horses demonstrated an initial immune response, by Day 28, the antibody response of the horses vaccinated with a monovalent vaccine (one that protects against one specific disease) was four times higher than those vaccinated with a combination vaccines indicated for WNV.

If your horse has not been vaccinated or is overdue for vaccination, it’s not too late to help protect it against this life-threatening disease with a booster. This added protection can help your horse stay healthy.

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.

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

In conjunction with vaccination, use good techniques for managing mosquitoes and avoiding peak mosquito time. This includes:

  • Destroying any mosquito breeding habitats by removing all potential sources of stagnant water;

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

  • Applying insect repellents or bring horses inside during the peak mosquito feeding hours between dusk and dawn.

Remember, WNV does not always lead to signs of illness. In horses that do become clinically ill, the virus infects the central nervous system and might cause signs such as loss of appetite and depression. Other clinical signs can include fever, weakness or paralysis of hind limbs, impaired vision, ataxia (incoordination), aimless wandering, walking in circles, hyperexcitability, or coma. Horse owners should contact a veterinarian immediately if they notice signs 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%.

By providing proper vaccination and helping to manage mosquito populations, horse owners can do their part to help prevent WNV infections.