The Colorado Department of Agriculture (CDA) and the Colorado Department of Public Health and Environment (CDPHE) are launching 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).
Disease information
WEEV, EEEV, and WNV are spread by infected mosquitoes and can affect equids and people. Humans and horses are considered dead-end hosts and cannot transmit the disease directly. Wild birds serve as reservoirs for these viruses. WEEV has not been detected in the United States since the late 1990s, while EEEV is commonly found in the Eastern United States and Canada, with cases identified further west in recent years (Arizona in 2023 and North Dakota in 2020). WNV has become widespread in the US since its introduction in 1999. Colorado experiences a high burden of WNV cases in both humans and equids during vector season.
Annual vaccination against EEEV/WEEV and WNV is a key preventative measure in horses. Additionally, implementing mosquito control strategies can help reduce disease risk. These strategies include eliminating standing water, utilizing fans in equine housing, use of equine-approved mosquito repellent, and limiting outdoor exposure during peak mosquito activity (e.g. dawn and dusk).
Clinical signs of these vector-borne diseases can range from mild, flu-like symptoms to severe, life-threatening encephalitis. Symptoms can include fever, severe depression (“sleeping sickness”), behavior changes, incoordination or weakness, anorexia, impaired vision, circling or head pressing, muscle twitches or tremors, excessive sweating, teeth grinding, inability to swallow, paralysis, convulsions, and even death. Currently, there are no specific treatments for these diseases; management focuses on supportive and symptomatic care.
One Health Surveillance Initiative
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 are initiating an enhanced One Health surveillance program and 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).
As a reminder, neurologic disease is a reportable condition in equids in Colorado. The CDA requests veterinarians submit an Equine Neuro reportable disease form for all cases which meet this criteria or are suspect for WNV, WEEV, or EEEV. For neurologic camelids suspicious for WNV or other reportable conditions, veterinarians can complete the generic Reportable Disease Case Report Form.
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:
- Ante-mortem: Cerebrospinal fluid (CSF) preferred, whole blood accepted
- Post-mortem: brain and/or CSF
Note: Serum submitted for WNV IgM ELISA may be banked for future EEEV/WEEV 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.
We, at CDPHE, CDA, and CSU, appreciate your partnership and your continued dedication to the health of Colorado’s animals and people.
