College of Agriculture, Forestry and Life Sciences; Public Service and Agriculture

Two South Carolina horses die from mosquito-borne disease


Eastern Equine Encephalitis (EEE) has resulted in the deaths of two South Carolina horses, according to State Veterinarian Michael J. Neault, Director of Clemson Livestock Poultry Health.

The National Veterinary Services Laboratories confirmed the diagnosis on Monday, August 21, 2023.

One horse was a two-year-old quarter horse gelding in Sumter County that was diagnosed with the disease at necropsy. The second was a five-year-old quarter horse mare in Lee County that was euthanized.

They are the first known cases of the summer. Another case was confirmed in a horse in Berkeley County in mid-January.

Neault reminded horse owners to work with their veterinarian to create an appropriate vaccination schedule to protect their animals from EEE, West Nile virus (WNV) and rabies.

“We had some heavy rains in areas across the state over the last month which means that mosquito populations can be extremely high. It is always important for horse owners to stay on top of equine vaccination schedules, but it is essential now considering the rain that we’ve had,” Neault said.

Borne by mosquitoes, 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. However, widespread vaccination has kept the number of cases comparatively low in South Carolina compared to nearby states.

Likewise, mosquito control is an important precaution. Both EEE and West Nile Virus are maintained in nature through a cycle involving the freshwater swamp mosquito, Culiseta melanura, commonly known as the black-tailed mosquito.

The EEE and WNV viruses are mosquito-borne and fast-acting. Symptoms of EEE in horses usually develop from two to five days after exposure. The symptoms include stumbling, circling, head pressing, depression or apprehension, weakness of legs, partial paralysis, the inability to stand, muscle twitching or death.

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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