Deadly 'triple E' kills New Hampshire man — what is eastern equine encephalitis?

A mosquito-spread virus nicknamed "triple E" recently killed a man in New Hampshire and sickened several people in other Northeastern states. The Massachusetts town of Plymouth recently began closing its public parks between dusk and dawn to help cut the risk of people catching the infection.

So what is triple E, and is it new to the northeastern U.S.?

Triple E is another name for eastern equine encephalitis (EEE), a potentially deadly viral illness that people can catch from mosquitoes. The virus behind the disease is called the EEE virus, or simply EEEV.

The disease is called "equine" because horses can also catch the viral illness, and in fact, EEE was first discovered among Massachusetts horses in 1831. The first human cases of the disease were documented about a century later, and since then, sporadic cases and clusters of infection have occurred, primarily along the Atlantic and Gulf coasts in the summer. Although EEE is rare, it is considered endemic to the U.S.

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In recent years, there have been between one and 15 cases a year nationwide. 2019 was a notable exception, with 38 cases, but it's not clear why so many EEE cases were detected that year.

This year, various states — including New Hampshire, Vermont, Massachusetts and New Jersey — each reported their first case of EEE in several years. In addition, a person was hospitalized in Wisconsin after catching the virus in a New England state, The New York Times reported. A total of six cases have been reported so far in 2024, although only four have been listed on the Centers for Disease Control and Prevention (CDC) website so far. (This lag is expected because the CDC has to confirm each case.)