PITTSFIELD -The state Department of Public Health has detected eastern equine encephalitis in a mosquito sample collected in the Williams Street area.

This is the first time EEE, a rare but potentially fatal virus, has been confirmed in mosquitoes in Berkshire County, according to the DPH. There have been no human or animal infections reported in Massachusetts this year.

The sample of mosquitoes was collected on Monday. After the sample was tested, the results were made public Friday morning.

The city, using a nozzle mounted on a truck, started spraying insecticides Friday night and planned to continue throughout the weekend, mainly after dusk and before dawn when mosquitoes are most prevalent. According to the DPH, spraying will take place in the areas of Williams, Elm, Newell, and East streets and Dorchester and Dalton avenues.

This method is the "most effective way to reduce the adult mosquito population" in an urban community like Pittsfield, according to the city's public health director, Meredith O'Leary.

O'Leary said sporadic findings of EEE in mosquitoes "occur across Massachusetts and do not necessarily represent significant risk of disease. Only southeastern Massachusetts has seen significant activity so far this year."

While EEE can infect people of all ages, those younger than 15 and over 50 years of age are at greatest risk for illness. EEE is spread to humans through the bite of an infected mosquito.

Symptoms of EEE include fever,

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often between 103 and 105 degrees, stiff neck headache and a lack of energy. The symptoms, according to the DPH website, often show up between three to 10 days.

More serious symptoms include swelling of the brain, which may lead to a coma and death.

Last year, two people contracted EEE in Massachusetts; one, a case in Bristol, was fatal.

"Since testing [mosquitoes] is relatively new in Pittsfield, it is very hard to interpret the data we are receiving," said O'Leary.

Absent historical data, she said, there is no way to determine if the presence of EEE is new "or has always been present, to an extent."

According to the state Department of Health, since EEE was first identified in the state in 1938, fewer than 100 cases in humans have been diagnosed. More than 60 percent of those cases have been discovered in Plymouth and Norfolk counties.

Tests already have confirmed the presence of West Nile virus in mosquitos in Pittsfield and in the Ashley Falls section of Sheffield.