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A Middlesex County man in his 60s is the first person in Massachusetts diagnosed with West Nile virus this year.

The man developed an uncommon form of the illness affecting his nervous system; he is recovering at a hospital, the Massachusetts Department of Public Health reported Friday.

The Hinton State Laboratory Institute confirmed that the man had West Nile, which is transmitted by mosquito bites. Most people infected with West Nile have no symptoms. But about 1 in 5 develop a fever with other symptoms that can include headache, body aches, joint pains, vomiting, diarrhea, or rash.

In about 1 in 150 infected people, the virus invades the nervous system, which is what happened to the Middlesex man, causing his brain and the lining of his brain to become inflamed, said Dr. Catherine Brown, state public health veterinarian. The man first experienced symptoms Aug. 11 and was admitted to the hospital soon after.

“This individual got care promptly and reportedly is recovering well,” Brown said.


DPH is investigating where the man was probably exposed to infected mosquitoes.

Massachusetts has reached peak mosquito season, Brown said. “This is the time that we really need to pay attention to mosquitoes and the threat that they can pose to us,” she said. “I’d like everyone to feel empowered that they can take steps to prevent that,” by using insect repellent, covering up, and reducing outdoor activities at dusk and after nightfall.

In 2013, eight Massachusetts residents were diagnosed with West Nile virus.

This year, however, lower-than-normal temperatures have resulted in fewer mosquitoes and also a smaller proportion infected with the virus, Brown said. The West Nile risk is considered low across most of the state and moderate in Greater Boston. Even so, Brown said, “we have had enough virus circulating to get someone sick.”


Also on Friday, an adult from Conway, N.H., was diagnosed with another, more serious mosquito-borne virus: Eastern equine encephalitis. It is the first human case of Eastern equine in that state since 2009, said the New Hampshire Department of Health and Human Services. Conway is in the northern half of the state. New Hampshire authorities have detected the Eastern equine virus in five samples of mosquitoes.

Massachusetts has recorded no human cases of Eastern equine this year, although 13 mosquito samples tested positive for the virus. And last week in Freetown, Eastern equine was diagnosed in a deer that was found ill by the side of road and then died.

Eastern equine is carried by a different breed of mosquito than those that carry West Nile. The infection is extremely rare in people but “extraordinarily serious when it happens,” Brown said. Symptoms may include high fever, severe headache, stiff neck, and sore throat, which can progress to seizures, coma, and death.

Finding a deer with Eastern equine indicates that mammal-biting mosquitoes are carrying the virus. As a result, the risk level in Freetown was raised to high, which means town officials have advised rescheduling evening outdoor community events.

The risk levels for Eastern equine and West Nile virus for every community in the state can be found at westnile.

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer.