COLUMBIA, S.C. — The first confirmed cases of Eastern equine encephalitis (EEE) virus in South Carolina for the 2025 season have resulted in the deaths of two horses, state officials announced today.
According to the Clemson University Department of Livestock Poultry Health, a five-year-old unvaccinated stallion in Jasper County succumbed to the disease, while a five-year-old mare in Aiken County — whose vaccination status was unknown — was euthanized after exhibiting neurological symptoms. The National Veterinary Services Laboratories confirmed both diagnoses on Thursday, July 24, 2025.
EEE is a mosquito-borne virus with a mortality rate approaching 90 percent in unvaccinated horses. These recent cases serve as a critical reminder to equine owners across the state to remain vigilant during peak mosquito season.
Vaccination Is Essential
South Carolina State Veterinarian Michael Neault strongly urged horse owners to consult with their veterinarians to ensure vaccinations are up to date not only for EEE, but also for West Nile virus (WNV) and rabies.
“As is always the case with South Carolina summers, we have had a lot of rain and warm temperatures, which means that mosquito populations can be extremely high,” Neault said. “It is always important for horse owners to stay on top of equine vaccination schedules, but it is particularly critical in summer months.”
While South Carolina has historically maintained lower case counts compared to neighboring states, largely due to vaccination compliance, the risk remains elevated through late summer and early fall.
Recognizing the Symptoms
Symptoms of EEE in horses generally appear within two to five days after exposure and can include:
- Stumbling or incoordination
- Circling or head pressing
- Depression or apprehension
- Weakness or partial paralysis
- Muscle twitching
- Inability to stand
- Sudden death
Horse owners are advised to monitor animals closely and seek veterinary attention at the first sign of neurological behavior. Suspected cases of EEE, WNV, rabies, or equine herpesvirus (EHV-1) must be reported to the state veterinarian’s office within 48 hours, as required by South Carolina law.
Integrated Prevention Strategies
In addition to vaccination, mosquito control remains a crucial defense. Both EEE and WNV are spread primarily through the freshwater swamp mosquito Culiseta melanura, also known as the black-tailed mosquito.
To reduce mosquito exposure, owners should:
- Eliminate standing water around barns and paddocks
- Use equine-safe repellents
- Stable horses during peak mosquito hours (dusk to dawn)
- Install and maintain screens in stables
A complete list of reportable diseases and additional biosecurity resources are available at www.clemson.edu/public/lph/ahp/reportable-diseases.
