Horse Diseases: EEE and WNV

​​​​White horse in fieldEastern equine encephalitis (EEE) and West Nile Virus (WNV) are potentially fatal diseases of horses transmitted by mosquitoes. Both diseases affect the nervous system, and effective vaccines are available for both. DATCP encourages horse owners to work with their veterinarian to vaccinate their horses against these viruses to prevent the high level of fatality associated with them. EEE is fatal in more than 90% of clinical cases in horses while WNV is fatal in 30-40%.​​


WNV and EEE may cause encephalitis, or inflammation of the brain. Symptoms of encephalitis in horses include depression, appetite loss, drooping eyelids and lower lip, fever, weakness, twitching, paralysis or lack of coordination, aimless wandering, circling, and blindness. Horses may also go down, be unable to rise, exhibit seizures, or become unresponsive, especially with EEE, which can be fatal within 24 hours.

Vaccination Process​

Horses that have never been vaccinated will need two doses of the vaccination initially, and then boosters at least annually. It takes at least two weeks to build up enough antibodies to protect the horse and the vaccine will not protect the horse if given after the horse is infected. Horse owners should check with their veterinarian to ensure their horse’s vaccines are current.​

Limiting Exposure to Mosquitoes

Besides vaccination, other steps to limit a horse’s exposure to mosquitoes during warm weather include:​

  • Removing items from surrounding property that could collect stagnant water such as old tires, tin cans, plastic containers.
  • Keeping rain gutters clean and draining properly.
  • Cleaning and chlorinating swimming pools, outdoor saunas and hot tubs, and drain water from pool covers.
  • Turning wading pools and wheelbarrows upside down when not in use.
  • Emptying and replacing water in birdbaths at least once a week.
  • Keeping horses in the barn from dusk to dawn, if possible, as this is when mosquitoes are most active.
  • Discussing the use of equine mosquito repellents with your veterinarian.​
Because the viruses follow mosquito populations, the threat varies depending on the weather but normally starts in mid- to late summer and remains until the first killing frost.​

Human Health

While humans may also be infected by WNV and EEE, the viruses do not pass directly between people and horses. Mosquitoes carry the viruses from infected birds and the only route of transmission is from a mosquito bite.​ For more information, visit the Wisconsin Department of Health Services website and search "EEE".