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 demonstr