AUBURN UNIVERSITY, Ala. — In late-July, the Alabama Department of Agriculture and Industries (ADAI) confirmed the presence of Eastern Equine Encephalitis (EEE) in Covington and Mobile counties. Also known as sleeping sickness, this disease is one of the deadliest mosquito-borne illnesses within the horse world, with 90% of cases being fatal.

Transmission and Symptoms

EEE is a viral disease that attacks the central nervous system, gradually getting worse over time. The disease is spread by certain species of mosquitoes. Horses are terminal hosts of EEE, meaning that they cannot spread the disease to humans or other animals. While rare, mosquitoes can transmit the disease to humans. According to the Centers for Disease Control and Prevention, only a few human cases are reported in the United States each year. Approximately 30% of those cases are fatal.

In horses, symptoms of EEE can range from depression and incoordination to a high fever and hypersensitivity to sound and touch. Owners should watch for these and other symptoms, such as apparent blindness, wandering and paralysis. Courtney Heaton, an Alabama Cooperative Extension System equine specialist, said there is not much an owner can do once their horse develops EEE.

“The swelling caused to the brain and spinal cord causes permanent damage,” Heaton said. “Most affected horses pass within a few days or humane euthanasia is recommended. Remember that a horse with neurologic symptoms poses a physical risk to humans handling them, as well as to themselves, because of their uncoordinated actions.”

If a horse does survive the disease, it may show gradual improvement of function over several weeks to months. However, the horse may never recover fully.

Prevention

Prevention is the key to safeguarding horses from EEE. Owners can accomplish this by keeping their horse’s vaccinations up to date, as well as controlling mosquito populations.

Vaccination

Heaton, who is also a lecturer in the Auburn University College of Agriculture’s Department of Animal Sciences, said EEE is considered a core vaccine for horses. Other core vaccines for horses include Western Equine Encephalomyelitis, rabies, tetanus and West Nile virus.

“There is little reason for a horse to contract EEE because a safe and inexpensive vaccine is available,” Heaton said. “Horses should be vaccinated prior to mosquito season, so protective immunity is developed prior to periods of likely exposure. At minimum, you should aim to vaccinate your horse for EEE annually.”

In areas where EEE is more prevalent, professionals recommend that owners vaccinate their horse every six months. This is also the recommendation for animals that are categorized as high risk or immunocompromised. If a horse’s vaccination history is unclear, or if the horse has never been vaccinated for EEE, Heaton recommends speaking with your veterinarian about dose requirements.

“Often, a two-dose series — with a three-to-six-week interval between doses — may be required for full immunity,” Heaton said.

Controlling Mosquito Populations

Eliminating breeding grounds for mosquitoes is a great way to reduce the risk of exposure to EEE. These breeding grounds can be any source of water that sits for more than four days. Water can often collect in items such as buckets, wheelbarrows, old tires, potted plants and watering troughs. Store these items in a barn or garage or turn them over so they do not collect water. To prevent mosquitoes in watering troughs, Heaton said to empty out the water every few days or use a biological larvicide to reduce mosquito populations.

“I like using biological methods of mosquito control in my water troughs,” Heaton said. “You can buy products, such as Mosquito Dunks, in a donut or granular form. The active ingredient in these products only kills mosquito larvae and is safe for horses, dogs, cats and even birds to consume.”

More Information

For more information on EEE, contact Heaton or the animal sciences and forages regional Extension agent that serves your area. You can find this contact information in the directory at www.aces.edu.