EEE in South Carolina: One Horse Euthanized

The Equine Disease Communication Center has reported Clemson Livestock Poultry Health’s confirmation of the mosquito-borne disease Eastern equine encephalitis (EEE) in a 3-year-old Quarter Horse mare on July 26, 2019; this is South Carolina’s second reported case of EEE this year.

The mare’s vaccination history is unknown.

Clemson Livestock Poultry Health reported on its Facebook page that the mare resided in Marlboro County. Officials instructed South Carolina residents to check with their veterinarians that horses are up to date on their EEE, West Nile virus, and rabies vaccinations.

EEE 101

A viral disease, EEE affects the central nervous system and is transmitted to equids by infected mosquitoes. Clinical signs in horses include:

  • Moderate to high fever;
  • Depression;
  • Lack of appetite;
  • Cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing);
  • Behavioral changes (aggression, self-mutilation, or drowsiness);
  • Gait abnormalities; and
  • Severe central nervous system signs, such as head-pressing, circling, blindness, and seizures.

The disease can progress rapidly, with death occurring two to three days after onset of clinical signs despite intensive care in some cases. Fatality rates can reach 75-80%. Equids that survive might have long-lasting impairments and neurologic problems.

Studies have shown that vaccines can help significantly reduce the risk of a horse contracting EEE. Horses vaccinated in past years will need an annual booster shot; in areas with a prolonged mosquito season, veterinarians might recommend two boosters annually—one in the spring and another in the fall. Horses unvaccinated in previous years or those with an unknown vaccination history should receive the initial two-shot vaccination series within a three- to six-week period. Immunity takes several weeks to achieve.

In addition to vaccinations, owners should work to reduce the mosquito populations and possible breeding areas and horses’ exposure by: