Public Service and Agriculture

Equine Encephalitis case confirmed in Marion County


horse vaccination
Sean Eastman (left) and a technician vaccinate a horse. Vaccination and mosquito management are essential to prevent deadly diseases like Eastern Equine Encephalitis and West Nile Virus.

COLUMBIA, S.C. — The first case this year of deadly Eastern Equine Encephalitis (EEE) has been confirmed in a South Carolina horse, Clemson University Livestock Poultry Health (LPH), a state regulatory agency, has announced.

“The infected horse, a 15-month-old Tennessee Walking Horse stallion from Marion County, had not been vaccinated and had to be euthanized,” said Sean Eastman, a veterinarian and LPH Animal Health Programs field services director.

Wild birds serve as a reservoir of the fast-acting EEE virus. Mosquitos transfer it to horses, where symptoms usually develop from two to five days after exposure.

Symptoms include stumbling, circling, head pressing, depression or apprehension, weakness of legs, partial paralysis, the inability to stand, muscle twitching or death.

Although not communicable from horses directly to people, humans are also susceptible to the mosquito-borne virus. The U.S. Centers for Disease Control report only a few cases of the disease in people each year, but nearly a third of people who contract EEE die and many survivors suffer ongoing neurologic problems, according to the agency.

“We have effective equine vaccinations available against diseases like EEE and West Nile Virus (WNV) and it is crucial that horses be vaccinated,” said Boyd Parr, South Carolina state veterinarian and LPH director. “Horses don’t get SAR2-CoV-2, but they do get EEE and WNV if left unvaccinated and these are very deadly to them.”

“Horse owners should check with their veterinarians to be sure their horses’ vaccinations are up to date,” Parr said. “South Carolinians have done an excellent job of preparing against these diseases in recent history. This case reminds us how important that vigilance is.”

South Carolina reported only a single case of WNV and just five EEE cases in all of 2019, he said. In contrast, in 2013 the Palmetto State led the nation in cases of the disease with 49 EEE-infected horses, all unvaccinated. Of those, 48 died.

“These diseases have a very high mortality rate in infected, unvaccinated horses – between 30 and 40 percent for West Nile and 90 percent for EEE,” Eastman said. “The only way to prevent cases is through effective vaccination and mosquito management strategies.”

Following a relatively mild and wet winter — something often associated with high mosquito populations in the summer and fall — mosquito management is especially important in South Carolina this year, Parr said.

Clemson’s online Home and Garden Information Center offers information on controlling mosquito populations on homes and farms.

Clemson Veterinary Diagnostic Center currently provides testing for EEE and WNV at no charge thanks to an agreement with the S.C. Department of Health and Environmental Control (DHEC).

In addition to EEE and WNV, other neurologic diseases, including rabies and EHV-1, can infect horses. Any livestock that display neurologic symptoms — 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.

A list of reportable diseases, along with other resources, is published on the LPH website at


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