On Sept. 25, 2023, the Washington State Department of Health confirmed the presence of West Nile virus in two horses from Grant and Benton counties. The horses, which were not vaccinated or were under-vaccinated for the disease, are both recovering.

The Washington Animal Disease Diagnostic Laboratory (WSDA) in Pullman, operated by Washington State University, reported the positive test results to WSDA’s State Veterinarian’s Office.

The disease is fatal to horses in about a third of the cases where clinical signs are apparent. However, most horses do not become ill and show no symptoms. Horses that do become ill can display loss of coordination, loss of appetite, confusion, fever, stiffness, and muscle weakness, particularly in their hindquarters.

Besides vaccination, horse owners can take action to limit horse exposure to mosquitoes. For example, reduce or eliminate sources of stagnant or standing water, stable horses during peak mosquito periods (dawn and dusk), use equine-approved mosquito repellants, place fans inside barns or stalls to maintain air movement, and avoid using incandescent bulbs inside stables at night.

Washington State Veterinarian Dr. Amber Itle recommends spring as the best time to vaccinate horses against West Nile virus or obtain an annual booster shot.