The warm, late summer temperatures have brought more than lazy days outside. Late summer has welcomed more mosquitoes and signaled the start of West Nile virus (WNV) and Eastern equine encephalitis (EEE) season for horses and humans.
About Equine Encephalitis Viruses
Equine encephalitis viruses—Eastern, Western, and Venezuelan equine encephalitis (EEE, WEE, VEE) and WNV—are spread by infected mosquitos and can cause severe brain inflammation in equids and people. These viruses are widespread in birds and rodents, making them reservoirs for disease.
EEE and WNV can be fatal in unvaccinated horses. The only treatment is systematic care which is rarely effective for EEE.
Confirmed WNV and EEE Cases This Year
So far this year (through Aug. 26), the Equine Disease Communication Center (EDCC) has issued 37 confirmed cases of WNV and 56 confirmed cases of EEE. The cases are submitted to the EDCC by state animal health officials and attending veterinarians.
Indiana leads the nation with seven confirmed WNV cases in 2024. California has reported four cases while Colorado, Kentucky, Oklahoma, and Utah have reported three each. Nevada and Wisconsin have two confirmed cases. Florida, Maryland, Michigan, Missouri, New York, North Dakota, Oregon, and South Carolina have at least one case. Canadian provinces Quebec and Ontario have had one each.
For EEE, Florida has reported 21 confirmed cases while New York has 14 confirmed cases. Alabama and Michigan have reported three cases each. Arkansas, Georgia, New Jersey, and Virginia have reported one case each. In Canada, Ontario has confirmed nine cases while Quebec has confirmed two.
Preventing WNV and EEE
EEE and WNV are considered “core vaccines” by the American Association of Equine Practitioners. The AAEP recommends that ALL horses be vaccinated for EEE and WNV annually, preferably in the spring before mosquito season. Initial vaccination is followed in four to six weeks with a booster; yearly revaccination is recommended after.
Other tips to limit and prevent infection include using insect repellents frequently; keeping horses in at night; eliminating or minimizing standing water; eliminating brush piles, gutters, old tires, and litter; and removing all equipment in which water can collect.
Horses residing in regions with a year-round mosquito presence may need EEE vaccinations two to three times annually for full protection. Horse owners should check with their veterinarians to determine the needed vaccination.
Additional Resources
For more information about West Nile Virus, please visit https://equinediseasecc.org/west-nile-virus. Additional information about Eastern Equine Encephalitis can be found at https://www.equinediseasecc.org/eastern-equine-encephalitis.
Related Reading
- The Future of Equine Infectious Diseases in a Changing Climate
- Disease Du Jour: West Nile Virus Refresher
- Researchers Identify Potential New Transmission Method for West Nile Virus
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