ISDA Confirms Multiple Equine West Nile Cases

Friday August 27, 2021

The Idaho State Department of Agriculture (ISDA) Animal Health Laboratory has confirmed multiple cases of West Nile Virus (WNV) in horses. The affected horses are located in Lemhi, Twin Falls, Canyon, Ada, and Blaine counties.  One horse was euthanized due to the severity of the disease, and the other horses are recovering under the supervision of a veterinarian.

The most common signs of WNV in horses are fever and weakness, usually in the hindquarters which sometimes is characterized by a widened stance, stumbling, leaning to one side and toe dragging. Mental conditions such as fearfulness, lip-smacking, chewing movements and fine muscle tremors may be noticed. Paralysis and inability to stand may follow in extreme cases of WNV infection.

Most commonly, WNV is spread to people and horses through the bite of an infected mosquito. The public should continue to take precautions in areas with WNV-carrying mosquitoes and precautions should extend to animals.

The ISDA recommends two essential protections for equines and small camelids: vaccination and physical precautions.

“The WNV vaccination continues to be an essential preventive measure and should be given annually to provide an adequate level of protection from the virus,” said Dr. Scott Leibsle, Idaho State Veterinarian. “Nearly 98 percent of horses that test positive for WNV are non-vaccinated or under-vaccinated. Horse owners should speak with their veterinarians about the WNV vaccination and other annual core vaccinations as well as how to follow an annual booster schedule. Horses that have been vaccinated for WNV in previous years must receive an annual booster; a one-time vaccination is not enough.”

Physical WNV precautions include using fans and repellants to keep mosquitoes away from equines. Additionally, horse owners should remove standing water on or near their property whenever possible.

“There is no vaccine for people,” said Dr. Leslie Tengelsen, State Public Health Veterinarian with the Idaho Department of Health and Welfare. “Although most infections in people are mild, some people can become quite ill. Avoid mosquito bites by taking simple precautions.” Precautions can include using insect repellant according to the label, wearing long-sleeved shirts and long pants from dusk to dawn when mosquitoes are most active, and making sure house screens are in working order.

Additional information about the virus and a map of known affected counties is available from the Idaho Department of Health and Welfare’s website,

For questions regarding control of mosquito populations, please contact your local mosquito abatement district.