Veterinary officials in Colorado have reported the first case equine West Nile virus in that state in 2021. According to the Equine Disease Communication Center (EDCC), the Weld County horse developed acute neurological signs on July 24 and is currently recovering. He had not been vaccinated against the disease. 

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. <<

The EDCC reports only three other cases of WNV in the United States this year, with affected horses in Yakima County, Washington; Fresno County, California and  Orangeburg County, South Carolina.

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