Testing of a mosquito trap in Merrimac has yielded the first findings of Eastern equine encephalitis in Essex County this year.
Following the test results released on Friday, spraying was scheduled for Monday night in the northern portions of Merrimac and Amesbury, near the New Hampshire border.
“We’ve been fortunate — it could have shown up a lot earlier,” said Jack Card Jr., director of the Northeast Massachusetts Mosquito Control and Wetlands Management District, which serves 32 cities and towns, including 30 in Essex County, plus Winthrop and Revere. “Hopefully, the weather will quiet the thing down as we get cooler nights. Last year it took a while before we got a frost to slow the mosquitoes down. Summer just kept on going.”
The virus was found in a sample of the Culiseta melanura species, a bird-biting mosquito that will sometimes bite humans. It is usually a transmitter of encephalitis, Card said, but some species may also carry West Nile virus. “Nature doesn’t always go by the book,” he said.
Public health officials in Massachusetts and New Hampshire are concerned about mosquitoes because they are transmitters of both West Nile virus and EEE in this area. Both diseases are potentially fatal to human beings.
Although the encephalitis virus spreads less easily to people, it is more virulent. There were two human cases in Essex County in 2012, in Georgetown and Amesbury. In both cases, the people died.
On Monday, the state Division of Public Health Services announced that it had found the encephalitis virus in a mosquito pool in Exeter, N.H.
Exeter’s deputy health officer, Judy Jervis, said the high school had been sprayed, and spraying was scheduled on Tuesday for the Brickyard Pond ballfields and other schools.
Six mosquito pools in New Hampshire have tested positive for West Nile virus this year — two in Stratham, two in Pelham, and one each in Sandown and Nashua, according to Whitney Howe, coordinator for vector-borne disease surveillance.
In results posted late last week, Massachusetts Department of Public Health testing also yielded positive findings for West Nile in mosquito pools in Malden, Medford, Everett, Salem, Saugus, Lynn (for the second time), and Newburyport (for the second time).
Card said Newburyport was scheduled for spraying on Tuesday, with Salem tentatively slated for Wednesday.
Because Lynn and Saugus were sprayed after the mosquito samples were taken, he said, the district would wait for the next round of results before spraying in those areas.
By spraying to knock down the mosquito population, the district was trying to reduce the number of mosquitoes that bite birds as well as humans. Mosquitoes can transmit the virus to birds and receive it from them. Humans or horses, meanwhile, are “dead-end hosts,” who will not pass the virus back to the mosquito.
“You can look at it as a circle, and every once in a while, there’s an arrow that shoots out of the circle: a mosquito biting a human or a horse,” he said.
There is no spraying scheduled in Malden, Medford, or Everett, said David Henley, superintendent for the East Middlesex and Suffolk County mosquito control projects, which serve 28 communities, including Boston. He said the district planned to continue surveillance, and the communities will rely on public health advisories to remind residents to take personal precautions — wearing mosquito repellent with DEET, wearing long sleeves and pants, limiting activities at the peak mosquito hours of dusk and dawn, and monitoring their yards for standing water where mosquitoes can breed.
“We’ve found positive results for West Nile virus in every city and town [in the district] inside Route 128,” as well as several on the other side of the highway, said Henley. “The message is: West Nile is out there. We’ve had no human cases, but that may change.”
There have been no reported cases of West Nile virus or Eastern equine encephalitis in humans this year. There have been two cases of EEE in horses, both in Belchertown.
The traps have yielded relatively light mosquito populations in recent weeks, Card said, but people should still be aware.
“It may create a situation where people are not thinking they need to use personal precautions, because there are not many mosquitoes out there,” Card said. “But at this point, they should be thinking about it.”