2 cases of West Nile contracted in Mass.

A 53-year-old Arlington man and a 48-year-old Worcester woman have been diagnosed with West Nile virus, the first known cases of the mosquito-borne illness contracted in Massachusetts this year, health authorities said yesterday.

Both the man and the woman, who were not identified because of patient confidentiality rules, became sick in August, and both are now at home recovering, said Dr. Alfred DeMaria, director of communicable disease control for the state Department of Public Health.

Health authorities in Massachusetts earlier in the summer diagnosed three people with West Nile, but in two cases, the patients were visitors who had been infected in their home state. The third case involved a Boston man exposed to the virus while in Montana.

The woman in Worcester probably was exposed to the virus in late August, that city's health commissioner said.

Christine M. Connolly, health and human services director in Arlington, said the man diagnosed with West Nile virus in her town was bitten between Aug. 15 and 25.

"But that doesn't mean residents here in Arlington can stop using mosquito repellant," Connolly said.

DeMaria agreed, while conceding there probably was little mosquito activity during the past few chilly nights. The species of mosquito that spreads West Nile rebounds quickly when temperatures warm, meaning that highs expected to reach the upper 70s later this week could put more mosquitoes into the air. Typically, only a prolonged stretch of freezing temperatures assures that the threat of West Nile has passed.

"If the temperatures were to stay cool, mosquitoes aren't going to be very active," DeMaria said. "But if it warms up, as it often does in late September, we may see more cases."

So far this year, nearly 1,400 cases of West Nile virus have been documented in the United States, with most reported in states west of the Mississippi River.

In the most severe cases, West Nile virus can cause a high fever, headache, neck stiffness, stupor, coma, tremors, convulsions, muscle weakness, paralysis, or even death. The US Centers for Disease Control and Prevention estimates that one of every 150 people infected develops severe symptoms.

To avoid contact with infected mosquitoes, the Massachusetts Department of Public Health recommends limiting outdoor activities from dusk to dawn, peak biting times for the West Nile mosquitoes. It also urges residents to wear as much clothing as comfortable and apply insect repellent such as DEET, permethrin, picaridin, or oil of lemon eucalyptus.

DEET should not be used on infants under 2 months and should be used in concentrations of 30 percent or less on older children. Oil of lemon eucalyptus should not be used on children under 3 years old.

Stephen Smith can be reached at stsmith@globe.com.