Six states have reported a record number of infections of Eastern equine encephalitis (EEE), a mosquito-borne virus blamed for seven recent deaths.

Massachusetts, Michigan, New Jersey, Connecticut, Rhode Island and North Carolina have reported 27 cases of the disease — the largest number of infections since 2005, when 21 cases were recorded.

“EEE is one of the most dangerous mosquito viruses that we have here in the United States,” said Lynn Sutfin, spokeswoman for the Michigan Department of Health and Human Services.

Michigan has eight EEE cases, including three deaths, the second-highest number of infections recorded in a state this year. Massachusetts has reported 10 cases, including two deaths.

“Despite any and all mosquito control interventions that have been used, EEE infected mosquitoes are still present and will continue to present a risk until the first hard frost,” said Catherine Brown, the state epidemiologist for Massachusetts. “It is critical that people continue to take steps to avoid mosquito bites particularly by using mosquito repellent anytime they are outdoors and avoiding outdoor activities between the hours of dusk and dawn in the highest risk areas.”

On average, the United States sees about seven cases of EEE each year, according to the Centers for Disease Control and Prevention. Last year, six cases were reported.

“It’s not unusual to see spikes in the number of EEE cases from year to year,” said Maggie Silver, a health communication specialist for the CDC’s Division of Vector Borne Diseases. “We saw similar increases in 2004-2006 and 2010-2012. As with most mosquito-borne diseases there are several factors that contribute to years with higher than average case counts. This could include changes in the bird and mosquito populations, weather patterns, and even human behaviors.”

The CDC has reported 18 confirmed cases as of Sept. 19. Ms. Silver said the spread of EEE is a “rapidly evolving situation” and noted the agency’s case count might differ from what individual states are reporting.

The agency is expected to provide updated case numbers Tuesday.

To help halt the spread of the virus, some states have taken extra precautions.

Massachusetts has conducted aerial spraying in several of the highest risk areas, while Michigan is discussing this as another potential protection option.

Some states also have urged schools and communities to postpone or cancel outdoor events after dusk.

Joseph Conlon, technical adviser for the American Mosquito Control Association, said spraying is not the only answer, stressing that people can protect themselves by applying EPA-registered repellents; wearing long, loose-fitting clothes; dumping standing water; and maintaining window screens.

There is about a 30% chance that a person who catches EEE and exhibits severe flu-like symptoms will die. Children younger than 15 years old and adults older than 50 years old are at the greatest risk of developing a serious disease from the virus.

Mr. Conlon said birds in hardwood swamps carry the virus. Infected mosquitoes that feed on birds and mammals can spread the disease to humans. States have reported numerous EEE cases in horses that cannot pass the virus to humans.

While there is an EEE vaccine for horses, there is none for humans.

Now that fall is here, Mr. Conlon said the U.S. should see some relief since EEE mostly occurs in the summer. But he added that the mosquito-borne West Nile virus usually kicks in late summer or early October as birds start to migrate.

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