Deadly horse disease on the rise in Florida

A deadly horse virus is spreading rapidly through Florida, infecting more horses this year than in all of 2023.

Nineteen cases of Eastern Equine Encephalitis (EEE) have been recorded in 2024: 10 in June and three in July. There were only 17 cases in all of 2023. The virus can also infect humans, and the resulting disease can have a mortality rate of up to 1 in 3, according to the Centers for Disease Control and Prevention.

EEE is mainly transmitted by the bite of infected mosquitoes of the species Aedes, Coquillettidia and Culex. These mosquitoes act as a vector between birds infected with the viruses and other animals, with the virus settling in their salivary glands and being injected into a human or other animal when the mosquito bites.

The virus does not spread directly between horses, from horses to people, or from birds to people: the mosquito is needed to transmit the disease and the virus must come from a bird, as humans and horses are considered ‘dead-end’ hosts.

The Florida Department of Agriculture and Consumer Affairs’ reportable equine diseases map shows three horses tested positive for the virus in Holmes County, Florida, two tested positive in Alachua, Levy, Madison, Marion and Osceola counties, and one tested positive in Columbia, Duval, Highlands, Jackson, Polk and Suwannee counties.

In both horses and humans, the virus can be fatal. In horses, EEE can cause severe neurological signs, including fever, circling or head pressing, inability to swallow, severe depression and lack of coordination. According to UC Davis Veterinary Medicine, up to 90 percent of infected horses die from the disease.

“It’s not common to see super mild cases,” Brittany Welch, a veterinarian at the Equine Medical Center of Ocala, told Florida news station WCJB. “There have been reports of horses surviving, but it’s a fairly preventable disease and usually the cases we see are pretty severely affected.”

Only about 4 percent of people infected with the virus develop symptoms. However, those who do may experience mild flu-like symptoms, including fever, chills, and malaise. Symptoms usually develop about a week after the time of infection.

In some cases, EEE can develop into a serious neurological condition causing encephalitis (brain inflammation).

“Overall, only 4-5 percent of human infections with eastern equine encephalitis virus result in eastern equine encephalitis,” the CDC says.

Symptoms may include high fever, headache, neck stiffness, confusion, seizures, and coma. The disease can be fatal or lead to long-term neurological complications.

“Approximately 30-45 percent of people with EEEV encephalitis will die from the disease, making it one of the most serious mosquito-borne diseases in the United States,” according to Florida Health.

There are approximately 11 human cases of EEE reported each year in the U.S., with most cases occurring in the Southeast and along the East Coast. There have been no human cases reported as of 2024.

“Transmission of eastern equine encephalitis virus occurs most frequently in and around freshwater hardwood wetlands in the Atlantic and Gulf Coast states and the Great Lakes region,” the CDC said.

There is no specific antiviral treatment available for EEE and care is focused on relieving symptoms and treating complications. Although there is no human vaccine for EEE, there is a vaccine available for horses that can prevent infection.

“The vaccine is very effective at preventing disease. When we see cases of Triple E, most of the time it is in horses that have not received the vaccine or have been vaccinated incorrectly or inadequately,” Welch said.

For humans, using insect repellents and wearing protective clothing can help prevent infection.

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