Equine West Nile Virus reported in four states so far this month

Vaccination against the virus is considered a "core" immunization, recommended for every horse in the United States.

Since the beginning of August, the Equine Disease Communication Center (EDCC) has reported six cases of equine West Nile Virus (WNV) spread across four states.  

• A 28-year-old mare in Midland County, MIchigan was euthanized after becoming progressively weaker and eventually unable to rise. She was undervaccinated for WNV, meaning she either did not receive the full course of the vaccine or was overdue for an annual booster. 

• In Yakima County, Washington, two horses at a private boarding facility contracted WNV. One horse, who was unvaccinted, died and the second horse, who was undervaccinated is currently alive and being treated.

A horse with an uncertain vaccination history from Kittitas County, Washington, contracted WNV and was euthanized. According to the EDCC, this horse spent the previous month in Southern Idaho at rodeos.

• A vaccinated 3-year-old gelding in Fresno, County, California developed signs of WNV, including ear, eyelid and muzzle twitching. He is currently alive and being treated.

• A horse with an unknown vaccination status in Jackson County, Oklahoma developed what the EDCC reports as “waxing and waning [signs] of rear limb stiffness with a wide based stance.” His current status is listed as unknown.

WNV virus is spread by birds, primarily crows and jays.

Carried by birds, primarily crows, and spread by mosquitoes, WNV is a flavivirus that affects the central nervous system. Horses are considered dead-end hosts, meaning once they are infected they do not directly infect others.

Most horses bitten by mosquitoes carrying WNV will show few, if any, signs of any illness. Horses newly exposed to the virus might develop a low fever and listlessness for a few days, but most are able to fight off the infection and recover fully.

However, in roughly 10 percent of cases, WNV crosses the blood-brain barrier to attack the central nervous system. In those cases, within five to 15 days horses will begin to show more serious signs of illness, including elevated fever, muscle weakness and incoordination, loss of appetite, muscle twitching of the face, behavioral changes and paralysis and recumbency. The most striking signs are incoordination, constant waves of muscle twitching and major changes in personality—most often with exaggerated fear responses.

A vaccine against WNV is considered a “core” immunization, recommended for every horse in the United States at least annually, by the American Association of Equine Practitioners.

Click here to read about research on how local climate conditions can affect the spread of WNV. 

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