Eastern Equine Encephalitis (EEE), a disease spread through the bite of a mosquito infected with the virus, has made the rounds in the news over the summer. Although temperatures are dipping and fall has officially started, mosquito activity can continue through November, meaning that animals are still at risk. Experts recommend vaccination as the best prevention strategy.

The disease primarily affects equines and can cause severe inflammation of the brain. Other types of animals like camelids (llamas and alpacas), species of farmed birds like emus and ostriches, and certain game birds are also susceptible.

“Horses with the disease typically have a fever and clinical signs related to the nervous system, including behavior changes, vision problems, circling, head pressing, twitching, paralysis, and convulsions,” Stephanie Brault, veterinary medical officer of the equine health team at the USDA Animal and Plant Health Inspection Service (APHIS), said in an email. “The profound depression seen in horses with this disease has led to it being called ‘sleeping sickness.’ Unfortunately, most severely affected animals die.”

APHIS tracks cases of EEE in horses. As of September, 61 cases of EEE in horses have been reported this year. This time last year, only 25 cases of EEE were reported.

“It does appear that we are seeing an increase in cases in horses this year compared to last year,” Brault said. “However, in certain previous years, we have seen many more cases of EEE in horses. For example, there were 184 cases reported in horses in 2019.”

Brault said several factors likely explain the cyclical increase in EEE cases, such as an abundance of birds that transfer the virus to mosquitoes, weather effects like longer, warmer seasons, and increased clinical awareness. 

The year 2019 also saw the largest outbreak of human cases in more than 50 years, with 38 documented incidences and 12 deaths. This year, 13 human cases have been reported across seven states, with two deaths occurring in New York and New Hampshire. The Centers for Disease Control and Prevention (CDC) report that an average of 11 cases are recorded annually. All 13 cases reported this year were neuroinvasive, meaning the virus infiltrated the central nervous system.

Ken Gordon, press secretary for the Ohio Department of Health, noted in an email that EEE is a “rare but serious disease” for humans. Most people infected will have no apparent illness, and less than 5 percent will develop severe EEE disease with brain inflammation. The Ohio Department of Health recommends a three-stage plan for reducing the risk of mosquito-borne disease: avoid mosquito bites, plan for mosquitoes while traveling, and stop mosquitoes from breeding in and around your home.

The state recommends wearing EPA-registered insect repellants, loose-fitting, long-sleeved shirts and pants, and clothing and gear treated with permethrin to avoid mosquito bites. Extra precautions should be taken when working outside from dusk to dawn, peak mosquito activity hours. Mosquitoes breed in standing water, so action should be taken to remove any potential sites where water can gather, like tires, wheelbarrows, buckets, and barrels.

When traveling abroad, visit the CDC’s Travelers’ Health website, where the latest health notices are posted about the risk of mosquito-borne diseases like EEE and malaria. However, it’s also important to consider mosquito activity when traveling across the US. Brault said that EEE is most documented around freshwater hardwood swamps in the Atlantic and Gulf Coast states and around the Great Lakes. Many states have their own surveillance systems. In Ohio, the Department of Health works with other state agencies, local health departments, and vector control agencies to conduct statewide surveillance for mosquito-borne diseases.

“This includes trapping and testing mosquitoes, monitoring for these diseases in horses and other animals, and conducting investigations into any suspected human cases,” Gordon said.

It’s important to know that EEE can’t spread from equines to humans.

“Horses are ‘dead end’ hosts for EEE; the concentration of virus in their bloodstream is typically insufficient to pass the virus onto another mosquito and continue the cycle,” Brault said. “Therefore, EEE infections in horses do not increase the risk for human infections.”

Although the risk for EEE in animals differs from state to state and year to year, Brault and other experts strongly advocate for vaccination. The American Association of Equine Practitioners recommends that horses receive the EEE vaccine annually, ideally in spring before mosquito season begins.

“The important thing for (equine) owners to do is to vaccinate. Equine owners should consult with their veterinarian and ensure their animals are vaccinated against EEE as well as other potentially fatal diseases,” Brault said. “If equines are vaccinated against EEE, they are at very low risk of developing this disease.”

Vaccines are available for other mosquito-borne diseases, such as Western Equine Encephalitis and West Nile Virus. These vaccines are only approved for use in horses, but there has been some evidence that they work in llamas and alpacas.

Brault added that horse owners can also apply mosquito repellents approved for equines and take other steps to minimize mosquito exposure to reduce the risk of EEE and other mosquito-spread diseases.

If EEE is suspected in a horse or other animal, owners should consult with their veterinarian immediately, even if the animal has passed away.