The South Carolina Department of Health and Environmental Control (DHEC) today announced that Eastern Equine Encephalitis (EEE), a mosquito-borne illness, has been detected in 11 horses in 8 South Carolina counties in the Lowcountry, Midlands, and Pee Dee regions from July 10 to August 21, 2020.

South Carolina counties in which the virus has been detected include Aiken, Berkeley, Colleton, Florence, Horry, Jasper, Lexington, and Marion.


The EEE virus amplifies itself in nature during spring and summer by being transmitted between black-tailed mosquitoes (Culiseta melanura) and various tree-perching birds in forested freshwater swamps. Since black-tailed mosquitoes thrive in these swamps in the northeastern and southern parts of the state, EEE virus is more commonly detected in the Lowcountry, Midlands, and Pee Dee regions.

During late summer and fall, the virus is acquired by other mosquito species after having fed on EEE-virus infected birds. These other mosquito species are then able to transmit EEE virus to people and other animals, such as horses. The virus can’t be transmitted directly from horses to people or from horses to mosquitoes.

In horses, EEE virus takes two to five days to cause symptoms and has a 90 percent fatality rate. The virus causes stumbling and poor coordination, inability to rise, paralysis, head pressing, circling and convulsions. Veterinarians in South Carolina frequently recommend vaccination before likely exposure to mosquito bites and booster vaccinations to ensure appropriate immunity levels. Consult your veterinarian for vaccination guidance.

“The black-tailed mosquito is not often targeted for control because this species is not a major nuisance to people and due to the inaccessible nature of many freshwater swamps that makes ground-based control of larvae and adults difficult,” said Dr. Chris Evans, State Public Health Entomologist with DHEC’s Bureau of Environmental Health Services.“Efforts to reduce black-tailed mosquito populations are usually made when EEE virus is detected in an area, but virus transmission also can be mitigated by control of other mosquito species that transmit the virus from infected birds to people and other animals.”

In people, EEE virus takes roughly three to 10 days to cause symptoms. The virus initially causes fever, malaise, intense headache, muscle aches, nausea and vomiting. Neurologic signs of EEE, which may appear within 5 days of infection, include meningitis, encephalitis, seizures and coma. Ninety-six percent of people infected with EEE virus do not develop symptoms, however, of those who do, one-third or more die, and the others frequently suffer permanent and severe neurologic damage.

Dr. Evans states that people can protect themselves from mosquito bites by:

  • Using an EPA-registered repellent containing catnip or catmint oil (nepetalactone), oil of citronella, DEET, IR 3535, oil of lemon eucalyptus (OLE or PMD), picaridin or 2-undecanone. Oil of lemon eucalyptus should not be used on children younger than three years old. Repellents containing 30% or more of the active ingredient DEET do not significantly increase protection.
  • Avoiding the outdoors at dawn or dusk when mosquitoes are most active.
  • Wearing long sleeves and pants.
  • Keeping property clear of standing water in which mosquitoes can breed.

“Despite the cooler weather of the approaching fall season, residents still need to protect themselves from mosquito bites,” said Evans. “The risk for EEE and West Nile virus transmission to people is always greater toward the end of mosquito season, even after extended periods of cold weather.”