This season’s first batch of mosquitoes detected with Eastern equine encephalitis, a potentially lethal infection, have been detected in Amherst, an area of Massachusetts that has not typically been plagued by the virus, state health officials said Tuesday.
Also concerning is that the mosquitoes are a species known to bite humans, said state public health veterinarian Catherine Brown.
Usually, the virus is first detected in mosquitoes that only bite birds, and then levels of virus slowly build during the season, eventually spreading to species that also pose a risk to people. Also, Eastern equine is typically concentrated in the Southeastern section of the state, home to the Hockomock Swamp, a prime mosquito breeding ground.
“The problem that we face here is that in the early days of EEE, there were never really large mosquito trappings [in Western Massachusetts], so we have a lot less historical data to understand today’s finding,” Brown said.
The sample of mosquitoes was collected in Amherst July 23. Now, officials plan more sampling this week to see if they can determine whether the Eastern equine finding was an anomaly or whether there appear to be many more infected mosquitoes. No decisions have been made about spraying pesticide or cancelling evening activities, pending further testing, she said.
“What we know from other cases, is that the potential for spread to humans exists,” Brown said.
The finding raises the infection risk level in Amherst to moderate. Belchertown, next door, is already at moderate risk because a horse was infected last year, an indication that the virus likely lurked in mosquitoes that survived from last season, Brown said. The remaining surrounding towns of Granby, Hadley, Leverett, Pelham, South Hadley, and Sunderland are considered to be at low risk.
There have been no human cases of West Nile virus or Eastern equine so far this year. But seven Massachusetts residents were infected with Eastern equine last year, one of the worst seasons in decades. The virus, which causes serious neurological damage and is fatal in about 30 percent of cases, is spread to humans through the bite of an infected mosquito.
Health officials urged residents to take precautions to avoid being bitten, including using insect repellent with DEET and avoiding outdoor activities during peak times for mosquito bites, dusk to dawn.Kay Lazar can be reached at firstname.lastname@example.org. Follow her on Twitter @GlobeKayLazar.