The first cases of equine and human West Nile Virus in Colorado have been confirmed.
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 from the Colorado 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.
Horses that have not been previously vaccinated will need the two-shot vaccination series administered three to four weeks apart (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. Emptying and refilling water troughs at least every 4-5 days can prevent mosquito larvae from developing. Biological mosquito control like Bti (Bacillus thuringiensis israelensis) can also be used in water troughs.
Enhanced Surveillance
CDA and the Colorado Department of Public Health and Environment (CDPHE) recently launched enhanced vector-borne disease surveillance for equines and camelids, focusing on Western Equine Encephalitis Virus (WEEV), Eastern Equine Encephalitis Virus (EEEV), and West Nile Virus (WNV).
Recent years have seen record high WNV cases in horses and people in Colorado through routine animal and public health surveillance. Currently, there is no active surveillance system in mosquitoes or animals for WEEV or EEEV.
This year, with the start of vector season in Colorado, we initiated an enhanced One Health surveillance program and are requesting submission of samples from neurologic equines and camelids. Please submit samples to the CSU Veterinary Diagnostic Laboratory in Fort Collins.
Free testing is available for a limited number of submissions and pathogens. To maximize the program’s effectiveness, we encourage submission of samples from unvaccinated animals (for WEEV/EEEV/WNV and/or rabies).
Remember: neurological disease is a reportable condition in equines in Colorado.
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.
Submission Instructions
Fill out the Equine Neuro reportable disease form (for equines) or the Reportable Disease Case report form (for camelids).
Contact CDPHE for any questions on testing criteria at [email protected] or 303-692-2700.
Submit a CSU VDL general submission form and write: “EEE/WEE/WNV PCR testing for CDPHE surveillance program” in the history section at the bottom of the front page of the form. Mail or drop off samples to the CSU VDL.
If rabies is a concern, please use a rabies testing request form. Specimens requested:
- Antemortem: Cerebrospinal fluid (CSF) preferred, whole blood accepted
- Postmortem: brain and/or CSF
Note: If you are submitting serum for WNV IgM ELISA, these specimens may be banked for future EEE/WEE surveillance testing.
CDA and CDPHE will coordinate with the reporting veterinarian for any positive results.
While encephalitis caused by WEEV, EEEV, or WNV are reportable diseases in Colorado, please be aware that these diseases are reportable in animals for monitoring purposes only. Positive findings through this initiative will not trigger regulatory action or herd-level interventions beyond potential recommendations for management in consultation with the submitting veterinarian.
Thank you for your consideration and collaboration in these important surveillance efforts. Enhanced monitoring of neurologic disease in horses not only helps us assess risk to equine population, but also provides critical insight into potential human health risks, which may prompt further investigation. Equine outbreaks of these diseases often precede human epidemics, making this surveillance vital for early warning and informing targeted mosquito control strategies.
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.
Dr. Eryn McGrath
Assistant State Veterinarian
Colorado Department of Agriculture
