Horses should be vaccinated to prevent infection with Eastern Equine Encephalitis, says State Veterinarian Boyd Parr.

Horses should be vaccinated to prevent infection with Eastern Equine Encephalitis, says State Veterinarian Boyd Parr.
Image Credit: Clemson University

COLUMBIA — A serious horse disease carried by mosquitos has spread across South Carolina, making it essential that horse owners have their animals vaccinated, according to officials with Clemson University Livestock-Poultry Health.

Recent cases of Eastern Equine Encephalitis (EEE) have moved beyond the coast to the Midlands and the Upstate, said State Veterinarian and Livestock-Poultry Health Director Boyd Parr.

“One of these cases is in Greenville County and is the first confirmed case in the Upstate in 2016,” Parr said. “The Greenville County case was a 10-year-old quarter horse. The second new case was a five-year-old quarter horse in Horry County.  Neither of these horses had been vaccinated according to label instructions and neither survived.”

Eleven cases have been confirmed by Clemson Livestock-Poultry Health so far in seven counties — four in Horry County alone. Also in the coastal plain, Berkeley, Colleton, Dorchester, Marion counties have reported cases, as has Kershaw County in the Midlands.

“Confirmation of the cases in Greenville and Kershaw counties provides solid evidence that the risk to South Carolina horses from EEE is not confined to our coastal counties but is indeed statewide,” Parr said. “It reminds us of the importance of taking immediate precautions to prevent the spread of mosquito-borne diseases.”

A simple vaccine will minimize the risk of EEE, but the disease is almost always fatal to exposed, unvaccinated horses. Though less common, the mosquito-borne illness can also affect humans.

“It can get out of hand if we don’t vaccinate horses and control mosquitoes,” Parr said. “Last year’s floods across much of South Carolina made conditions favorable for mosquito breeding. The best defense for horse owners is to maintain current equine vaccinations for Eastern Equine Encephalitis, West Nile virus and rabies for their horses.”

Parr also recommends that all South Carolinians take steps to reduce mosquito breeding by eliminating common sources of still water in which the insects lay their eggs. Draining or washing receptacles like bird baths, wading pools and garden ponds on a weekly basis can disrupt the mosquito breeding cycle. Likewise, removing open containers that collect rainwater will help reduce the mosquito population and hence the threat of disease.

The EEE virus is maintained in nature through a cycle involving the freshwater swamp mosquito, Culiseta melanura, commonly known as the blacktailed mosquito. Two to three days after becoming infected with the virus a mosquito becomes capable of transmitting the virus. Infected mosquitoes can transmit the disease when they bite horses and humans.

South Carolina led the nation in EEE cases in 2013 with 49 infected horses. Of those, 48 died. None of the horses infected during 2013 had been vaccinated effectively, according to a review of Livestock-Poultry Health vaccination history.

According to the Centers for Disease Control and Prevention, Eastern Equine Encephalitis is a rare illness in humans. Most persons infected with it have no apparent illness. Severe cases begin with the sudden onset of headache, high fever, chills and vomiting. People who are concerned should contact their physicians.

In horses, symptoms of EEE usually develop from two to five days after exposure. These include stumbling, circling, head pressing, depression or apprehension, weakness of legs, partial paralysis, inability to stand, muscle twitching or death.

Any livestock, including horses, that display these signs must be reported to the state veterinarian at 803-788-2260 within 48 hours, according to state law.

Information on animal diseases and reporting requirements can be found on the Livestock-Poultry Health website,