A recent surge in cases of eastern equine encephalitis (EEE) in the northeastern United States, likely brought on by warm weather–induced faster disease spread, has prompted warnings and preventive measures, according to several state public health departments. Recent human cases have set off a wave of mitigation efforts.
In August, the Massachusetts Department of Public Health announced plans for aerial or truck-mounted spraying in counties deemed at high or critical risk for EEE. The spraying was scheduled in response to the first human case of EEE in the state this season, according to a press release.
Although EEE is rare, it is a serious and potentially fatal infection that can affect humans of all ages, as well as animals. The virus is carried by mosquitoes, so protecting oneself against mosquito bites is essential if EEE is identified in the community.
In the press release, public health experts recommended using insect repellant and protective clothing and draining areas of standing water such as unused plant pots or wading pools.
"EEE occurs in outbreak cycles that are likely driven by a combination of the gradual evolution of the virus, mosquito populations, and weather factors," said State Epidemiologist Catherine Brown, MD, in an interview. "Both heat and precipitation generally increase mosquito populations, but heat also speeds up the processing of the virus that occurs in mosquitoes so that they are able to spread the virus faster," she explained.
The state of New York also experienced a surge in EEE over the summer among horses in several counties, but no cases in humans had been confirmed as of late August. In a press release from the New York state Department of Health, experts emphasized that EEE is transmitted from mosquito bites and does not spread from horses to humans and other horses. However, they advised horse owners to get their animals vaccinated and recommended that individuals take protective measures such as wearing long sleeves and pants during the dusk-to-dawn time of greatest mosquito activity, using insect repellants with N,N-diethyl-meta-toluamide, and checking whether screens at home are in good shape, with no rips, tears, or holes.
"The factors that can lead to increased EEE activity are complex but are likely related to ecological factors such as seasonal weather patterns, temperature, and rainfall," said Danielle R. De Souza, MSc, public information officer for the New York State Department of Health, in an interview. "There have been no human cases of EEE in New York State in 2024," she told Medscape Medical News.
However, clinicians should be aware of the potential symptoms of EEE in humans, which include fever, headache, vomiting, diarrhea, seizures, behavioral changes, and drowsiness. "Approximately 30% of people who develop severe EEE die, and many survivors have ongoing neurologic problems," De Souza said. "There is no specific treatment for EEE; clinical management is supportive," she noted. Individuals older than 50 years and younger than 15 years are at the greatest risk for severe disease, she added.
Recognizing a Rare Infection
As for other aspects of clinical presentation, "EEE is likely to result in encephalitis or meningoencephalitis after a variable febrile prodrome," Brown told Medscape Medical News. "It is important for the patient workup to include rule out of other more common causes of viral encephalitis/meningitis, especially as some of them may have specific treatments or management recommendations," she added.
The Centers for Disease Control and Prevention (CDC) website states EEE is a nationally notifiable condition, meaning that all cases should be reported to state or local public health authorities.
According to CDC, serologic testing is the primary method of diagnosis for EEE, and a positive EEE virus–specific immunoglobulin M test should be confirmed by the CDC or a state public health laboratory with neutralizing antibody testing.
The importance of mosquito bite prevention is the essential conversation for clinicians to have with patients about EEE, said Brown. Prevention recommendations include routinely using a mosquito repellent that contains an Environmental Protection Agency–registered active ingredient when outdoors during mosquito season, covering up to reduce exposed skin when weather permits and rescheduling evening outdoor events to avoid peak mosquito biting times when risk is greatest, she said.
"The risk from mosquito-borne illness will start to decrease as the weather starts to cool and populations of the mosquitoes most likely to spread EEE begin to die-off," said Brown. "However, although the population of mosquitoes will be lower, the infection rate in the mosquitoes actually increases, so any individual mosquito is actually more likely to be carrying virus," she said. "There will be some risk from mosquito-borne disease until there has been a hard frost," she added.
Brown and De Souza had no financial conflicts to disclose.
