Horses from multiple Colorado counties test positive for West Nile Virus

Horses from multiple Colorado counties test positive for West Nile Virus

Since the index equine WNV case in Colorado on July 27, 2023, horses from multiple Colorado counties have tested positive for West Nile Virus. One additional horse in Weld County, four horses in Conejos County, three horses in El Paso county,  two horses each in Douglas and Fremont counties and one horse each in Adams, Alamosa, Bent, Custer, Elbert, Garfield, Larimer, Lincoln, Pueblo and Rio Grande counties have tested positive for WNV.  This brings the total test positive cases in 2023 to twenty-three. For up to date case information, visit the Animal Health Division Equine West Nile Virus webpage.

Colorado county Number of WNV positive horses
Adams 1
Alamosa 1
Bent 1
Conejos 4
Custer 1
Douglas 2
Elbert 1
El Paso 3
Fremont 2
Garfield 1
Larimer 1
Lincoln 1
Pueblo 1
Rio Grande 1
Weld 2


Mosquito pools from Adams, Arapahoe, Boulder, Delta, Denver, Jefferson, La Plata, Larimer, Pueblo and Weld counties have tested positive for WNV this summer.  There have been thirty-six human cases of WNV in Colorado in 2023.  This should serve as a reminder that WNV is actively circulating in Colorado and poses a risk to people. CDPHE publishes data on human West Nile cases and positive mosquito pools on the CDPHE WNV website.

West Nile Virus (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.

WNV prevention in horses

Vaccines in horses have proven to be a safe and effective prevention tool for West Nile Virus.  Horses that have been vaccinated in past years need an annual booster. If the horse had not been previously vaccinated, they will need the two-shot vaccination series within a three to four week period (or according to vaccine labels).  Visit the AAEP website for a comprehensive list of vaccination recommendations.

In addition to vaccinations, horse owners also should work diligently to reduce the mosquito populations and their possible breeding areas.  Recommendations include removing stagnant water sources, using mosquito repellents, and keeping animals inside during the insects’ feeding times, which are typically early in the morning and evening.

Information for veterinarians

All cases of acute equine neurologic disease must be reported to the State Veterinarian’s Office, unless the cause is known toxicity or trauma.  Call our office at (303) 869-9130 to report suggestive signs of reportable diseases, even if the disease has not been confirmed.  If it is after-hours, our office phone message will indicate which staff veterinarian is on call.

Many equine neurologic diseases are clinically indistinguishable, so prompt reporting and diagnostics are important components of response for many of our reportable equine neurological conditions.

**Remember, ALL equine neurologic cases must be reported to the State Veterinarian’s Office**

Please visit our Reportable Disease Page for more information on reportable diseases and how to use our online reporting tool.  Here’s a direct link to our Equine Neurologic Disease webpage for more information on the reportable equine neurologic diseases, clinical signs, testing, and additional resources.

While awaiting test results, neurologic horses should be isolated and biosecurity measures shall be put in place, due to the potential for disease transmission.  Some neurologic diseases in horses are zoonotic, so precautions should be taken for veterinarians, owners, and anyone else interacting with the horse while waiting for results.  See the following resources:

NASPHV Compendium of Veterinary Standard Precautions for Zoonotic Disease Prevention (2015)

AAEP Guidelines: Suspected Case of Infectious Neurologic Disease