Colorado State Vet Confirms First Equine West Nile Virus Case of 2021 

August 2, 2021

A horse residing in Weld County has been diagnosed with West Nile Virus (WNV), marking Colorado’s first confirmed equine WNV case in 2021. The horse, which had not been vaccinated against WNV, developed acute neurological signs on July 24, 2021 and is currently recovering.  

“Vaccines have shown to be an effective prevention tool for West Nile Virus in horses and Horse owners should work with their veterinarian to determine the appropriate vaccination schedule for their horses,” said State Veterinarian Dr. Maggie Baldwin. “West Nile Virus is one of the AAEP core vaccinations for horses, along with others such as rabies, tetanus, and eastern and western encephalomyelitis. All of these vaccines are important to protect the health of Colorado’s equine herds.”   

Horses vaccinated for WNV in past years will need an annual booster. If a horse has not been vaccinated in previous years, it will need the two-shot vaccination series. 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.

The Colorado Department of Public Health and Environment has not reported any human cases of WNV to date in 2021. However, mosquito pools from Weld, Larimer, and Boulder counties have tested positive for WNV, as well as four raptors in Weld County and one raptor in Larimer County. The WNV positive animals and mosquito pools are a reminder that WNV is actively circulating in Colorado and poses a risk to people. CDPHE has published data on human West Nile cases, as well as positive mosquito pools, available on the CDPHE WNV website.

Source: Colorado Department of Agriculture