State Vet Confirms Western Slope Equine West Nile Virus Case


Broomfield, Colo. — The total number of horses in Colorado with confirmed West Nile Virus (WNV) cases has risen to 21, with a first case of equine WNV confirmed in Mesa County. Additional horses in Weld, Larimer, Morgan, and Douglas counties have also tested positive for WNV. Of the confirmed positive cases, five horses have died or have been euthanized.

“The West Nile Virus vaccine has proven to be safe and effective in preventing disease. Horse owners should work with their veterinarians to determine the most appropriate vaccination schedule for their horses. It is important to note WNV is a core vaccine recommended by the American Association of Equine Practitioners,” said Colorado State Veterinarian Dr. Maggie Baldwin. 

Mosquito pools from Weld, Larimer, Boulder, Adams, Arapahoe, Delta, Denver, Mesa, and Pueblo counties have tested positive for WNV this summer. There have been fifty-eight human cases of WNV in Colorado in 2021 resulting in two human deaths. CDPHE publishes data on human West Nile cases and positive mosquito pools on the CDPHE WNV website.  

Vaccination for WNV is an effective way to protect horses from the virus. If a horse has not been vaccinated in previous years, it will need the two-shot vaccination series. Horses need an annual booster to protect against WNV. Visit the AAEP website for a comprehensive list of vaccination recommendations. In addition to vaccinations, horse owners should also work diligently to reduce mosquito populations and their possible breeding areas where horses are located. Recommendations include removing stagnant water sources, using mosquito repellents, and keeping animals inside during the bugs’ feeding times, typically early in the morning and evening.

Any time a horse displays clinical signs consistent with neurologic disease, a complete veterinary examination is warranted. All infectious or contagious equine neurologic diseases are reportable to the Colorado State Veterinarian’s Office at (303) 869-9130. A chart of reportable animal diseases in Colorado can be found on the CDA website.

WNV is a viral disease that cycles between wild birds and mosquitoes, and can sometimes affect other species, like people and horses, as dead-end hosts. Clinical cases in horses are typically characterized by anorexia, depression, and neurological signs, which may include ataxia, weakness or paralysis of one or more limbs, teeth grinding, aimless wandering, convulsions and/or circling. For information on human WNV symptoms and prevention see West Nile virus and your health | Department of Public Health & Environment.

Below you will find the current count of WNV in horses (through Sept. 20, 2021):

Colorado County 

Number of WNV Positive Horses























The Colorado Department of Agriculture exists to support the state’s agriculture industry and serve the people of Colorado through regulation, advocacy and education. Our mission is to strengthen and advance Colorado agriculture, promote a safe and high-quality food supply, protect consumers, and foster responsible stewardship of the environment and natural resources.