EQUUS ‘Farm Calls’ episode 24: West Nile virus in horses

More than 250,000 horses have been reported to have West Nile virus (WNV) since it was first identified in the United States in 1999.

West Nile virus (WNV) is a vector-borne virus that can cause fever and neurologic disease in horses, humans and other mammals. It is usually spread by mosquitoes.

“Since 1999, when it was first identified in the United States, more than 250,000 cases have been reported in horses,” said Sarah Colmer, VMD, DACVIM. She is currently a fellow in Large Animal Neurology under Dr. Amy Johnson at the University of Pennsylvania’s New Bolton Center in Kennett Square, Pennsylvania. She recently completed her residency there to become board certified in Large Animal Internal Medicine.

No one knows how many cases of West Nile virus have gone undiagnosed in this country.

“This is certainly something to be on the lookout for!” she noted.

Varying signs

Clinical signs of West Nile virus vary, noted Colmer. “Some horses are exposed to West Nile virus and have no clinical signs,” she said. “If it ‘takes hold,’ it will be about two weeks before clinical signs become apparent.”

She said some horses will have neurologic signs, and others might have signs of colic. Those signs can vary between horses.

“Fever is the first sign, but it might not be noticed,” said Colmer. “Then there can be a variety of neurologic signs. Mostly they are asymmetric (one side but not the other), but not always. You might see ataxia (unsteadiness) or balance weakness. Or the horse might become unable to rise.”

Colmer said some horses get muscle fasciculations (tremors), a drooping lip or even twitching of the face muscles.

Some horses will have a change in attitude from dull and quiet to aggressive.

“If you think a horse is neurologic, alert your vet right away,” she warned owners. “If a horse is unsteady on its feet, keep people and other animals safe.”


Colmer said prevention is important. “Watch for changes in weather patterns that make a better environment for mosquitoes,” she said. “Minimizing standing water where mosquitoes can breed is an easy management strategy.”

She noted that there are multiple vaccine options to prevent West Nile virus in horses. “They are part of the ‘core vaccines’ that the AAEP recommends where West Nile virus exists,” she added. “Your veterinarian can outline when the best time to vaccinate and give boosters should be in your area.”

More WNV information

In the podcast, Colmer went through the diagnostic procedures that veterinarians might use. She also discussed prognosis, treatment and supportive care

She reminded horse owners that some horses can get West Nile virus and get better, then can relapse.

Colmer pointed out one positive note: “If you are dealing with an encephalitides in horses, you hope it is West Nile virus. It’s the least deadly of that group.”

This episode of the EQUUS “Farm Calls” podcast is brought to you by Farnam—Your Partner in Horse Care. Visit farnam.com to learn more.




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