A Middlesex County man in his 60s has been infected with West Nile virus, becoming the first person in Massachusetts this year sickened by the mosquito-borne disease, state health officials reported Wednesday.
The man was diagnosed with West Nile in late July and remains hospitalized, but appears to be recovering, officials said. State health officials typically do not release identities of patients, citing confidentiality rules.
Health authorities have raised the threat level from West Nile to moderate in Arlington, Belmont, Cambridge, Somerville, and Watertown, based on information from the man’s family about where he may have contracted the disease, coupled with the number of infected mosquitoes collected in the Middlesex County communities.
This has been a particularly active year for mosquito-borne viruses in Massachusetts. Planes blanketed 21 communities in Southeastern Massachusetts last month with pesticide and returned this week to respray six of the communities to tamp down the risk from Eastern equine encephalitis, a mosquito-borne illness that often proves more serious than West Nile, killing up to one-third of those who develop symptoms.
“In the Greater Boston area, there’s a good risk for West Nile virus at this point,” said Catherine Brown, state public health veterinarian. “If you have been ignoring us all along, now is the time to use [insect] repellent and take precautions.”
That includes covering exposed skin and avoiding outdoor activities between dusk and dawn, when mosquitoes are most active.
Separately, officials have confirmed the first case of West Nile in a horse, in Ludlow in Western Massachusetts, raising the threat level of infection in that town to high.
Brown said the unusually warm winter and summer, along with periodic drenching rains, have accelerated reproduction of mosquitoes and amplified the amount of virus they are carrying.
Infected mosquitoes have inflicted misery across the country this summer.
Mayor Mike Rawlings of Dallas declared a state of emergency Wednesday, a step necessary to allow aerial pesticide spraying in the battle against West Nile, which has battered Texas this year. More than 100 people have been infected with West Nile in Dallas alone, with five deaths, according to city officials.
Earlier this month, the US Centers for Disease Control and Prevention said it had counted the most West Nile infections nationwide since 2004.
In Massachusetts so far during 2012, West Nile-infected mosquitoes have been found in 48 cities and towns, from Cape Cod to the Berkshires, health officials said. Last year, six Massachusetts residents were infected with the virus, and one horse was diagnosed with it. But in 2002, the year after the virus was first detected in Massachusetts, 22 infections were recorded, according to state records.
While the virus can infect people of all ages, people over the age of 50 are at higher risk for severe disease.
“The same is true for a wide variety of infectious diseases, and it probably has to do with the aging of our immune systems, which are not as strong as when we are younger,” said Dr. Paul Sax, clinical director of the division of infectious diseases at Brigham and Women’s Hospital and a professor at Harvard Medical School.
The West Nile virus is usually transmitted to humans through the bite of an infected mosquito. Most people who become infected have no symptoms. But some experience fever and flulike illness. In rare cases, more severe conditions can develop, including brain infections.
Symptoms signaling a severe infection include seizures, confusion, and a change in thinking patterns, Sax said.Kay Lazar can be reached at firstname.lastname@example.org. Follow her on Twitter @GlobeKayLazar.