Public health officials on Wednesday declared the mosquito-borne eastern equine encephalitis (EEE) virus to be an "emergent threat" in the U.S. after an unusually high number of cases have occurred so far this year.

Why it matters: While EEE remains rare, there are no vaccines or specific viral treatments available. The virus can attack the brain and sometimes cause death.

"It's interesting, because something like between 1831 and 1959, there were around 13 total cases documented. But, for the first time this year, there's at least 36."

— Anthony Fauci, National Institute of Allergy and Infectious Diseases director, to Axios

Background: EEE is a member of the alphavirus family, which tends to attack the brain. The virus is spread mainly by Culiseta melanura mosquitoes and various tree-perching birds found in forested wetlands, but it can circulate in small mammals, reptiles or amphibians as well, per the NIAID.

C. melanura mosquitoes don't usually bite people, but occasionally a mosquito that does, like the Aedes aegypti mosquito, will bite an infected bird and then transmit the virus to a person.

While human infections are rare and most people (96%) don't show symptoms, the death rate for those who do experience symptoms is "really high," at around 35% mortality while many others suffer permanent and severe neurologic damage, Fauci says.

Researchers do not know yet why the virus is sometimes able to breach the blood-brain barrier and cause damage, Fauci adds.

The latest: As of Nov. 19, there have been 36 confirmed cases and 14 deaths from EEE virus disease in eight states, per the Centers for Disease Control and Prevention. These were in: Connecticut (4), Indiana (1), Massachusetts (12), Michigan (10), North Carolina (1), New Jersey (4), Rhode Island (3), and Tennessee (1).

In a perspective piece in the New England Journal of Medicine published Wednesday, NIAID officials including Fauci catalogue the threat of the growing spread of vector-borne illnesses (from mosquitos or ticks) to humans, like EEE.

Four vector-borne human illnesses "have evolved" so that they now infect the A. aegypti mosquito, which feeds mostly on humans, the authors point out. These are dengue, yellow fever, Zika and chikungunya.

Threat level: "You don't want the Aedes to adapt to that [EEE virus]," Fauci says. It hasn't happened yet, he points out, but researchers are "keeping an eye on it."

What's next: Some CDC officials earlier this year called for a national defense strategy to coordinate health responses to all vector-borne illnesses.

There's also an EEE virus vaccine under development by the U.S. Army that's currently in a clinical trial.

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