With High Mosquito Growth, EEE Could Return To MA In Summer 2024

SUDBURY, MA — Massachusetts may be on the verge of a particularly active summer for mosquitoes, with some parts of the state detecting above-normal populations of the bloodsuckers in early season surveillance this spring.

Heavy rain in 2023 and this spring have created ideal conditions for supercharged mosquito breeding, experts say. Many parts of the state received more than 50 inches of rain in 2023. This spring has also been rainy, with nearly 30 inches so far in the Boston area.

Those ideal breeding conditions combined with the emergence of Eastern Equine Encephalitis (EEE) last summer means a risk to humans in 2024. The last big EEE outbreak in humans happened in Massachusetts between 2019 and 2020 with 17 cases and seven deaths.

"There's a five to seven-year cycle where we see a big surge of cases of EEE or positive mosquitoes. This is predicted to be one of those years," said UMass Amherst professor Stephen Rich, a biologist who studies zoonotic disease.

The black-tailed mosquito, or Culiseta melanura, has especially boomed this spring in some parts of the state. That's the species responsible for spreading EEE between birds, leading to possible human cases later in the disease transmission cycle.

Sudbury Director of Public Health Vivian Zeng said that the East Middlesex Mosquito Control Project — which covers 26 communities north and west of Boston — began detecting an uptick in mosquitoes in that town in early May. Like many towns in the state, Sudbury has plenty of wetland areas with trees that hold standing water in crypts, perfect breeding grounds for Culiseta melanura mosquitoes.

Early mosquito counts are also high in the Bristol County Mosquito Control Project area, which includes 20 communities between Mansfield and New Bedford.

"Surveillance data for adult mosquitoes is showing about a 2X increase from the five and 10-year average," BCMCP Superintendent Priscilla Matton said this week.

It's also difficult to count mosquitoes. Todd Livdahl, a retired Clark University professor and ecologist who spent his career studying mosquitoes, said population estimates typically come from trapping and counting mosquitoes, and comparing those numbers over time. Those counts are just a relative measure of an entire population.

Livdahl said there are about 50 different mosquito species in Massachusetts, including newer additions like the Aedes albopictus, or tiger mosquito, that arrived in the 1990s. Many mosquito types don't typically carry diseases, although the ones that spread diseases like EEE are a concern. Those types of mosquitoes prefer to feed on birds, and Livdahl said there's some research indicating they may switch to mammals when bird leave New England closer to autumn. That theory accounts for why EEE tends to enter the human population later in the summer and into the fall months.

Bristol, Plymouth and Norfolk counties are traditional hotspots for EEE, but the disease can pop up anywhere in the state. In September, mosquitoes in southern Worcester County tested positive for the disease. During the 2019 outbreak, Middlesex County was a hot spot, leading to a 5-year-old from Sudbury surviving EEE.

"Prevention is key," Zeng said. "We are advising the public to adopt preventative practices such as wearing long-sleeved clothing during peak mosquito hours, remove standing water on their property and use EPA registered repellents when outdoors."

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Many mosquito control projects across Massachusetts will soon begin summer spraying operations targeting adult mosquitoes. Those same organizations also treated wetlands earlier in the spring with Bti, an insecticide deadly to larvae.

Meanwhile, the state Department of Public Health will officially begin monitoring communities across the state for EEE and West Nile risk beginning June 17. That's when the state will assign risk levels to every city and town based on mosquito samples, and the detection of human and animal cases of mosquito-borne diseases, if any occur.

According to Rich, state residents should also be vigilant about ticks, whose populations are increasing partly due to warmer winters. Deer ticks, also known as black-leg ticks, carry a host of diseases from Lyme disease to babesiosis. The lone star tick, a relative newcomer to the state found mainly in coastal areas, can transmit a dangerous meat allergy known as alpha-gal syndrome.

Like Zeng, Rich advised state residents to take steps to prevent bites. Permethrin spray can kill mosquitoes and ticks, although it can't be applied directly to skin, only to clothes, shoes, tents and other outdoor gear. Use only bug spray with either DEET or picaridin, Rich advised. Wearing light colored clothing and tucking pants into your socks can also help make tick prevention and detection easier, he said.

"During this next month, June and into July, worry about ticks," Rich said. "July into September, worry about mosquitos."