Equine West Nile Virus in Washington State

Published on July 18, 2017

There have been two recent cases of West Nile virus in horses in Washington State:

Dr. Liana Wiegel and the PVH equine department would like to remind horse owners that vaccination for West Nile is recommended once yearly, prior to the onset of mosquito season. (In areas where WNV is more prevalent, such as the southeastern U.S., twice yearly vaccination is recommended.)

West Nile virus causes serious disease in horses, humans and other species.

In horses, clinical signs vary from neurologic disease to subtle signs. Horses that develop neurologic disease show hind limb weakness, trouble eating and swallowing, changes in personality, and tremors, and may progress to the point that the horse is unable to stand.

The onset of symptoms is frequently sudden, and symptoms progress over a course of seven to 10 days.

Other diseases that have similar clinical signs include rabies, equine protozoal myeloencephalitis (EPM) and equine herpesvirus type 1 (EHV-1), among others.  

Most horses that develop the neurologic signs of WNV are horses that were unvaccinated. Vaccinated horses tend to develop less severe clinical signs and may only develop a fever, decreased appetite, and depression when exposed to WNV.

If you have any questions about your horse's vaccinations or WNV in horses, please call Pilchuck at 360.568.3111.