First report of equine West Nile Virus in Colorado for 2020

The viral disease spread by birds claimed the life of a horse in Jefferson County.

The Colorado State Veterinarian has reported the first case of West Nile Virus (WNV) in a horse in that state in 2020. According to the Equine Disease Communication Center (EDCC), the single case was reported on October 1 in Jefferson County, Colorado, and the horse was deceased at the time of the reporting. No other details were provided in the EDCC report.

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. 

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

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. <<

For the week of September 25 to October 2, cases of WNV were also reported to the EDCC from the following locations

• Canyon County, Idaho

• Lake County, Florida

• Kings County, California

• Stanislaus County, California

• Weber County, Utah

• Madison County, Kentucky

• Utah County, Utah

• Carter County, Oklahoma

Among these cases, there were no reported deaths. 




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