Residents of 10 communities in Massachusetts have been warned to stay indoors from dusk to dawn. Some local parks are closed at night. A person in New Hampshire has died. Pesticides are being sprayed from trucks and aircrafts.
What’s causing all of the alarm? A mosquito-borne virus called Eastern equine encephalitis, which is so rare that most infectious disease experts might never see a case. This year, at least four states have reported human E.E.E. infections.
Here’s what you should know.
Is this an unusually bad year for E.E.E.?
The virus is endemic in North America, which means that it’s consistently present on the continent. The United States sees a handful of infections every year, mainly along the East Coast and around the Great Lakes. In 2019, there were a record 38 cases, the largest U.S. outbreak in recent history.
As of Tuesday, the Centers for Disease Control and Prevention had reported four human cases, in Wisconsin, New Jersey, Massachusetts and Vermont. In addition, officials in New Hampshire have reported one death.
But experts say it’s too soon to know whether this will turn out to be a particularly severe year. Mosquito-borne illnesses usually spread widely until around October, or when it’s cold enough for water to freeze, said Dr. Jonathan Abraham, a clinical infectious disease specialist at Harvard Medical School.
Until then, health officials in the affected areas are urging residents to take precautions.
Who should worry?
The main risk factor for contracting the virus is exposure to mosquitoes, said Dr. Cameron Wolfe, a professor of infectious diseases at the Duke University School of Medicine.
Those who spend extended periods of time outside — including people who have outdoor jobs, who are homeless or who enjoy hiking or biking — are at higher risk of contracting the virus simply because they are more likely to be bitten by mosquitoes.
They should consider taking extra precautions, like wearing long pants and shirts and using bug spray. Notably, not all outdoor areas are equally risky.
Walking around in downtown Manhattan is probably much less risky, Dr. Wolfe said, than spending time outside in a swampy, lowland area of South Carolina, for example — the sort of environment that mosquitoes love.
Where does the virus circulate?
In the life cycle of the E.E.E. virus, humans are sort of an afterthought. In swampy ecosystems, mosquitoes feed on infected birds and then pass the virus on to healthy birds.
Some species of mosquitoes feed on both birds and larger animals. These are the biters that infect people. Humans cannot transmit the virus to other humans, and mosquitoes don’t usually become infected after biting humans because people don’t have high viral concentrations in their blood. That’s why humans are considered “dead-end hosts.”
So, too, are the other animals that may fall ill with the infection, like ostriches, llamas and, as the name suggests, horses. The E.E.E. virus was first identified in the 1930s using cultures of infected horse brains.
What’s the illness like?
Most people who are infected with the E.E.E. virus have no symptoms or have flulike symptoms that resolve within a few weeks, according to the C.D.C. But if the virus reaches the brain — which happens in less than 5 percent of cases — the prognosis becomes dire.
Once in the brain, the virus attaches to proteins on the surface of neurons and kills them, often causing lasting neurological damage. The body’s immune response, which leads to swelling and inflammation, may also injure the brain, said Dr. Kevin Gobeske, a critical care neurologist at the Yale School of Medicine.
About 30 percent of patients who develop neurological symptoms die, according to the C.D.C. Survivors may be left with severe, long-term conditions like paralysis, intellectual impairment and personality disorders.
Is there a treatment?
There are no treatments that target the E.E.E. virus, so doctors focus on managing symptoms with pain medications or, in severe cases, using steroids to control swelling. “We don’t have many good options,” Dr. Gobeske said.
It’s not clear why some people recover fully while others are hospitalized with E.E.E. infections. There is not enough evidence to say whether particular groups are at higher risk of severe illness, Dr. Abraham said.
While there are vaccines for horses, none are approved for humans. The disease is rare and complicated enough that there hasn’t been a push to develop a vaccine for people, Dr. Gobeske said.
The best preventive strategy is to limit exposure to mosquitoes by dumping out standing water, repairing damage to door or window screens and, in some high-risk areas, avoiding outside activities after dusk.
Even if E.E.E. has not been reported in your state, Dr. Wolfe recommends taking these precautions to help avoid all mosquito-borne illnesses, like West Nile virus, that peak around this time of year.
“I don’t know a good, healthy reason you should be happy about being bit by a mosquito,” he said. “It doesn’t matter if it’s malaria, dengue or Eastern equine — there’s no particularly good outcome.”
