COLUMBIA — State veterinary officials are urging South Carolina horse owners to vaccinate their animals following the discovery of the first South Carolina case of Eastern Equine Encephalitis in 2017.

The case was identified Friday, July 21, in a horse from Dillon County, said Boyd Parr, state veterinarian and director of Clemson University Livestock-Poultry Health.

Eastern Equine Encephalitis (EEE) is a serious, mosquito-borne illness in horses that can also affect humans. In unvaccinated horses, it is almost always fatal. The horse in Dillon County was not known to have been vaccinated and did not survive.

The disease will kill 90 percent of exposed, unvaccinated horses. Symptoms usually develop in horses 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.

A simple vaccine will minimize the risk to horses and other equine species from EEE.

“It can get out of hand if we don’t vaccinate horses and control mosquitoes,” Parr said. “The best defense for horse owners is to maintain current equine vaccinations for Eastern Equine Encephalitis, West Nile virus and rabies for their horses.”

South Carolina led the nation in cases of the disease 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 Clemson Livestock-Poultry Health vaccination history.

Following that outbreak, increased vaccinations helped reduce the number of cases to seven in 2014 and six a year later. However, 2016 saw another spike in cases, ending the year with 15 infected horses.

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 it. Infected mosquitoes can transmit the disease when they bite horses and humans.

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, Parr said.

Rains at crucial times across much of South Carolina made conditions favorable for mosquito breeding again this year, Parr said.

Any livestock, including horses, that display neurologic signs, such as stumbling, circling, head-pressing, depression or apprehension, 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,