Deadly Eastern Equine Encephalitis on the Rise: LSU Diagnostics Confirms Surge in Fatal Mosquito-borne Virus in Horses

Behind the image of a horse brain cell infected with Eastern Equine Encephalitis (EEE) lies a sobering truth: this mosquito-borne virus is far more lethal than commonly understood, and pathologists, virologists, and researchers at LSU Diagnostics are working to better understand and fight it.

EEE is found in the eastern, Gulf Coast and north-central regions of the United States, as well as parts of Central and South America and the Caribbean. Horses in areas with dense mosquito populations—such as swamps, coastal marshes, and coves—are at greater risk of contracting the virus.

The sedentary “black tail mosquito” (Culiseta melanura) primarily transmits the virus to birds. However, other very active common mosquito species, referred to as "bridge vectors," can transmit the virus from birds to mammals such as humans and horses.

"As the summer goes on, we typically have more positive cases as the temperature goes up," says Alma Roy, Ph.D. (LSU 2000), interim director of LSU Diagnostics.

Louisiana is no stranger to West Nile virus, but Roy says LSU Diagnostics is seeing an unusual uptick in EEE this year. Though most common in horses, the disease can also strike humans, leading to deadly brain inflammation. In horses, it may cause inflammation of both the brain and the heart.

"Necropsy and serological testing at LSU Diagnostics have confirmed at least
fourteen positive cases in horses thus far. It's early in the year to see this many. We're seeing some West Nile, but more EEE than West Nile," Roy said.

Horses are especially vulnerable to EEE, with mortality rates ranging from 75% to 90%. Survivors frequently suffer lasting neurological damage. Humans fare somewhat better, but the disease remains serious—about 30% of people who develop severe EEE die from the infection.

"Many survive a West Nile virus infection, but EEE can be unforgiving. Be careful," warns Dr. Fabio Del Piero, pathologist at LSU Diagnostics and professor at LSU Vet Med.

Since treatment is limited and mostly supportive, prevention is critical. Every day, LSU Diagnostics works to make that prevention possible.

Roy reminds the public that vaccination and mosquito bite prevention are key: "Horses can be vaccinated. We encourage animals to be vaccinated for EEE and West Nile virus. And for humans, of course, it's the prevention of mosquito bites.”

EEE is one of several life-threatening diseases diagnosed by LSU Diagnostics. Our team provides rapid, accurate disease detection through tissue and fluid testing as well as post-mortem diagnostics. LSU Diagnostics also supports statewide mosquito-virus surveillance to protect both animals and people.

Together, LSU Diagnostics and LSU Vet Med play a critical role in diagnosing and responding to threats like EEE and West Nile virus — for the health of horses, livestock, wildlife, and the people who care for them.