Horse Diseases:
Eastern Equine Encephalitis (EEE) & West Nile Virus (WNV)

524 horses in Florida became very ill or died in 2002 from WNV and EEE! 224 were diagnosed in 2003. Will yours be next?

 Have you vaccinated your horses for Eastern Equine Encephalitis & West Nile Virus this year?

  • If not, do it today.
  • If never, do it today and again in three weeks.

EEE and WNV in Florida Horses Again This Year

Eastern Equine Encephalitis (EEE) and West Nile Virus (WNV) are viral diseases that are spread by mosquitoes. The mosquitoes become infected when they first feed on infected wild birds and then later on horses. These viruses are not spread from horse to horse, but are carried by the biting mosquitoes. Both of these diseases are somewhat similar in the way they are transmitted and with the symptoms of the disease.

EEE and WNV cause a swelling of the brain. Symptoms of the diseases include: wobbly gait or staggering, weakness, difficulty in rising after lying down, depression or listlessness, facial paralysis or twitching, teeth grinding, blindness, and fever in about 1/3 of the cases. Of the two diseases EEE is the most life-threatening, but both diseases have killed a number of horses in Florida this year.

In 2001, there were 106 confirmed cases of West Nile Virus in Florida and 32 confirmed cases of Eastern Equine Encephalitus. Another 137 horses were suspected to be infected with either EEE or WNV. In 2002, there were 499 confirmed cases WNV and 25 cases of EEE in horses across Florida. 29 horses in Jackson County were diagnosed with WNV in 2002. In 2003, there were 207 horses with confirmed cases of EEE (2 in Jackson) and 117 cases of WNV (2 in Jackson) in Florida. To view the latest WNV and EEE state summary go to the UF/IFAS Pest Alert site

Most of the horses that were infected by EEE or WNV had not received a vaccination within the last six months. If you vaccinated last year, you still need to give a booster shot this year. These diseases are also a threat to people. Horses are much more commonly infected, but 28 cases of WNV and 1 case of EEE were diagnosed in people in Florida in 2002. As of September 8, 2003, there have been 11 human cases of WNV and two with EEE.

There is no treatment for these viruses once infected. The horses simply have to fight them off, so older and very young horses are most at risk of death. Veterinarians can prescribe treatments for the symptoms to lessen the severity of the infection, but ultimately the horse’s immune system has to fight off the disease. Still, you should call a vet as soon as you suspect an infection. Early detection may be the only way to save the horse’s life.

So what should you do to prevent these diseases in your horses and your family? The horses can be protected through vaccination. EEE vaccines can be purchased at most any livestock supply dealer. The Department of Agriculture is recommending giving three (3) EEE shots annually, instead of two (2), due to the concentration of the disease in our area. If you have not vaccinated your horse within the last year, you will want to also give a booster shot three (3) weeks later. Viral vaccinations require two doses initially 21 days apart to be effective, so the horse is not protected until it gets the booster shot. 

Recently a WNV vaccine was released for use in Florida. This vaccine is currently only available through a veterinarian. If you have not already vaccinated your horses, contact your vet and do so immediately. Again you will need to provide a booster shot to protect your horses this first time. The EEE vaccine provides no protection against WNV and vise versa, so you will need to purchase both to adequately protect your horses. As with any vaccine, protection is not 100%. Use of permethrin based insect repellents is still a good preventative measure as well.

Currently there is no vaccination for use in humans, so the only protection we have is to avoid exposure to mosquitoes and to use repellents when outdoors, especially around dawn and dusk. Anything that holds water around the house or barn should be kept dry or cleaned out frequently to prevent mosquitoes from hatching out.

More Information


For additional information about eastern encephalitis or West Nile Disease, contact the Extension Office at 482-9620 or e-mail Doug Mayo.