The US Centers for Disease Control and Prevention reports that as of 10 September 2024, there have been eight cases of Eastern equine encephalitis (EEE) in the country. All so far have been neuroinvasive.

The cases were reported in New Hampshire (1), New Jersey (1), Massachusetts (4), Vermont (1) and Wisconsin (1). An additional human case has been reported in Ottawa, Canada. This increase in EEE cases arrives immediately after a spike in Dengue virus (DENV) cases in New York and New Jersey, and may be connected to the regional mosquito vectors’ response to the seasonal heat.

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Northeastern states in which EEE can be found have also been vigilant about the presence of the disease in local horses and birds. There have been reports of EEE in horses in Connecticut, Maine, Michigan and New York. New York has also detected the virus in emus. Rhode Island has currently only detected the EEE virus in local mosquito populations.

The EEE virus can be transmitted to humans and horses by Culex, Aedes and Coquillettidia mosquito species. EEE virus infections in humans are rare, partly because the virus is mostly found in Culiseta melanura mosquitos, which prefer to bite birds rather than mammals. The disease can only be spread via mosquitos, not by contact with infected horses or humans.

Approximately 95% of infections with the EEE virus are asymptomatic. In symptomatic cases, signs will begin to emerge between four and 10 days after infection. These include headache, fever, vomiting, seizures and drowsiness. Serious complications include encephalitis, coma and death. EEE currently has a 30% mortality rate and has been responsible for at least two deaths so far in 2024. Approximately half of survivors will develop long-term neurological symptoms.

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Global heating is a potent driver of spread

EEE is a rare disease, with only five cases in 2021, one in 2022 and seven in 2023. However, an increase in cases does cause alarm, considering the severity of the disease. Mosquito season is projected to last until the end of September, so more cases may be reported.

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A potential driver of the caseload of EEE in 2024 is the increased global heat. A 2020 study by West and colleagues from the Institute of Food and Agricultural Sciences in Florida inspected the ability of C. melanura to transmit EEE year-round. They found that months with a mean temperature of 28C (82.4F) yielded the highest population density, which is linked with increased EEE transmission, but 30C (86F) reduced survival. This trend is also present in Culex mosquitos, the species responsible for transmitting not only EEE but also the West Nile virus, which has been present in the northeastern US since 1999.

The Aedes species, another bridge vector of EEE, has also expanded its territory and is approaching the northeastern US. This has resulted in increased cases of both Dengue virus and Chikungunya virus in the US in recent years, such as the spike in New York and New Jersey recently reported. In 2023, GlobalData epidemiologists predicted more than 70,000 cases of dengue virus and more than 170 cases of chikungunya virus in Mexico, and those numbers are likely to rise in coming years.

A vaccine for the EEE virus is available for horses, but no vaccine is currently approved for humans. Best actions against the virus include wearing trousers and sleeves, using DEET [diethyl-meta-toluamide]-containing bug spray, installing or repairing window screens and removing any stagnant water to reduce breeding sites for mosquitos. Public health officials in New England also recommend avoiding the outdoors during hot summer evenings.