Peak mosquito activity in the United States occurs July through October, which places horses at the highest risk of contracting West Nile virus (WNV). Much of the United States has experienced increased rainfall this spring, and the first cases of West Nile have already been documented this year. With the right vaccine and preventive measures, it’s not too late for owners to help protect their horses against this life-threatening disease.

West Nile is transmitted by mosquitoes—which feed on infected birds—to horses, humans, and other mammals. In 2014, the United States recorded 19,623 cases of West Nile disease or infection in mosquitoes, birds, humans, and horses, the Centers for Disease Control and Prevention reported as of January 2015. California topped the charts with 7,008 cases, followed by Texas with 2,484 cases.

Vaccination remains the most effective way to help protect horses against WNV and other mosquito-borne diseases, such as Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE).

Researchers  recently tested horses’ responses to six WNV 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 titer response of the horses vaccinated with a monovalent vaccine (containing only a vaccine against WNV; in this study the team tested West Nile Innovator, produced by Zoetis, which is one of the monovalent vaccines available in the United States) was four times higher than those vaccinated with the one-dose, big combination WNV-containing vaccines