More disease-carrying mosquitoes have been found in Southeastern Massachusetts, prompting state public health officials Wednesday to substantially expand the area targeted for aerial pesticide spraying, to nearly two dozen communities.
The decision comes a day after officials disclosed plans to begin mosquito spraying in the region.
Aerial spraying is slated to begin Friday evening, weather permitting, and continue Saturday night to prevent the spread of Eastern equine encephalitis to people in 21 cities and towns, officials said Wednesday.
The latest testing found two more batches of Eastern equine-bearing mosquitoes that bite people in Rehoboth, and state public health Commissioner John Auerbach said in a phone interview that he worries that the numbers will keep climbing.
Several other samples of infected mosquitoes have been identified in the past week in Easton, Carver, and Lakeville. The Eastern equine virus can cause serious disease and death.
“Rather than having to revise the plan a second time, we would prefer to simply enlarge the spray area and avoid last minute changes,” Auerbach said.
The communities to be sprayed include Acushnet, Berkley, Bridgewater, Carver, Dighton, East Bridgewater, Easton, Freetown, Halifax, Hanson, Kingston, Lakeville, Middleborough, Norton, Pembroke, Plympton, Raynham, Rehoboth, Rochester, Taunton, and West Bridgewater.
Aerial spraying often sparks concerns about health risks to residents and potential environmental damage to crops and waterways.
Kevin Cranston, director of the state’s Bureau of Infectious Disease, said officials have taken those concerns seriously in selecting sumithrin, a pesticide that is combined with piperonyl butoxide, a compound that activates the sumithrin. The pesticide is also known by the brand name Anvil 10+10.
“It was selected because of its effectiveness against targeted mosquitoes, and it breaks down very rapidly when exposed to sunlight,” Cranston said. “It has a very low residual effect and a very low toxicity to humans and animals.”
Cranston recommended that residents bring pets inside when their town is scheduled to be sprayed and to stay indoors during that time.
Auerbach said specialists are scrutinizing detailed information about the region’s sensitive environmental areas as they prepare aerial spray plans.
“The spraying occurs with a GPS system that includes lots of very specific geographic information and is programmed so spraying would cut off over a body of water for drinking or a certified organic farm,” said Auerbach.
Officials hope the spraying will begin Friday night, but Auerbach said that could change because the pesticide requires very specific weather conditions. “Mosquitoes tend to not fly at night if temperatures fall below 62 degrees,” said Auerbach. Planes begin flying at 9 p.m., he said, “and what happens is sometimes it may be in the upper 60s, and the airplanes have to literally monitor the temperatures because if they drop too fast, they have to quit and wait [till] another evening.”
He said the department will continue to post updated information at www.mass.gov/dph about schedules for spraying in each town. Auerbach stressed that residents should continue to take precautions, including using insect repellent with DEET and avoiding outdoor activities during peak times for mosquito bites, dusk to dawn.
No human cases of Eastern equine encephalitis have been reported this year in Massachusetts, but two people acquired the infections last year, including a Raynham man who died. That case and the large number of infected mosquitoes in the past two years led the Department of Public Health to adopt a more aggressive plan to fight Eastern equine and West Nile virus, which is also transmitted by mosquitoes.
Dan Murphy, an Easton selectman, said residents are relieved the state has decided to conduct aerial spraying.
“It can be nerve wracking for residents, particularly those with young kids,” said Murphy, the father of three youngsters. “We have town events, kids’ races every Thursday evening at the park, and we have had to end those early,” Murphy said, “but still a lot of families aren’t coming down at all because they are very concerned.”Kay Lazar can be reached at email@example.com. Follow her on Twitter @GlobeKayLazar.