West Nile Virus confirmed in Weld County horse

The Colorado State Veterinarian’s office has confirmed the first case of equine West Nile Virus of 2023 in a gelding residing in Weld County. The horse presented with sudden neurological symptoms including weakness, stumbling and recumbency and was euthanized as a result of his condition. The age and vaccine status for this gelding are unknown.  

WNV is a reportable disease in both humans and animals, and the first human case in Colorado this year was confirmed by the Department of Public Health and Environment on July 24 in La Plata County. Additionally, mosquito pools from Arapahoe, Boulder, Delta, Denver, Larimer, Pueblo and Weld counties have tested positive for WNV this summer. Due to the heavy snowpack last winter and a wet spring, mosquito levels have been high throughout the state. Weld, Larimer and Boulder counties have seen especially high activity, with record mosquito numbers in some areas. 

“The positive human case, equine case and mosquito pools are a reminder that West Nile Virus is actively circulating in Colorado and continues to pose a risk to people and unvaccinated equines,” said Colorado State Veterinarian Dr. Maggie Baldwin. “Horse owners should work with their veterinarians to determine the appropriate vaccination schedule to keep their animals healthy.”

WNV cycles between wild birds and mosquitoes and can sometimes affect other species like people and equines (as dead-end hosts, meaning they cannot pass the virus on further).

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