Will the rainy weather raise the risk of mosquito-borne illnesses in New England this summer?

The rainy weather continues, with showers and heavy rain expected this weekend across Massachusetts, it may heighten another summer hazard: mosquitoes that carry West Nile Virus and Eastern equine encephalitis (EEE).Stuart Cahill/Pool

Relentless rains have hammered New England recently, causing floods, forcing evacuations, and prompting a federal emergency declaration in Vermont.

With more heavy showers over the weekend, the conditions may heighten another summer hazard: mosquitoes that carry West Nile Virus and Eastern equine encephalitis.

The state detected West Nile Virus in a mosquito sample for the first time this season on July 7 in Brookline.

Many mosquitoes thrive in hot and humid conditions, and climate change is shifting their range, driving many species farther north from the tropics. For the first time in 20 years, the United States reported locally transmitted cases of malaria in Florida and Texas over the last two months.

“We are concerned these changes in weather will lead to an increasing number of people being infected [with mosquito-borne diseases],” said Dr. Cassandra Pierre, associate hospital epidemiologist and medical director of public health programs at BMC and assistant professor at Boston University’s Center for Emerging Infectious Diseases.

Not all mosquitoes multiply after heavy rains, but the ones that carry EEE do.

EEE tends to be more prevalent during wetter seasons because the mosquitoes that are vectors for that disease breed and survive in larger wetland habitats, said David Lawson, director of the Norfolk County Mosquito Control District. He added that the disease has been especially recurrent in Southeastern Massachusetts including in Bristol, Plymouth, and southern Norfolk counties.

But even with the wet weather, a resurgence of EEE this season is unlikely.

The disease, which carries symptoms that range from a mild fever and headache to more serious and even fatal conditions, typically strikes Massachusetts every 10 to 20 years and lingers for two to three years. But the last big outbreak was in 2019, continuing into 2020, according to the Massachusetts Department of Public Health. Last year’s drought, which took a toll on the wetlands, also makes an outbreak this year unlikely, said Lawson.

While heavy rains can promote the spread of EEE, they tend to limit the spread of West Nile Virus. That’s because the mosquitoes that carry West Nile breed in standing water such as in puddles, catch basins, and backyard containers get flushed out and killed during downpours.

“In a summer like this one, every time we get a heavy rain, shower, or thunderstorm, water flushes all those mosquitoes out of areas where they would normally be breeding in a drier year,” he said.

However, mosquitoes are unpredictable, and so are the diseases associated with them.

That’s why experts recommend the public take preventative measures against mosquito bites.

Folks who are immunocompromised or are over 50 have higher risks of infection. But everyone should protect themselves, said Pierre.

“We have seen young healthy people experiencing complications from these illnesses,” she said.

Therefore, experts recommend staying away from wetlands, if possible, reducing exposed skin, and wearing insect repellent. The EPA website offers specific repellant recommendations based on how long you plan to be outdoors.

The Massachusetts Department of Public Health also encourages people to empty any standing water from around their homes and to repair window and door screens to keep mosquitoes out.

To help keep the public safe, Massachusetts also conducts mosquito surveillance, stepping up efforts as we approach late summer and early fall, the period when most human infections in Massachusetts occur, according to the Department of Public Health.

Long-term trap sites are set up throughout the state and monitored. Once collected, mosquito samples are sent to the Massachusetts State Public Health Laboratory and tested for West Nile and EEE.

The testing of female mosquito specimens ― only female mosquitoes bite ― generally begins in June and continues until October, “unless there is a need to extend testing season due to increased risk of infection,” according to the Northeast Massachusetts Mosquito Control and Wetlands Management District.

At Norfolk County Mosquito Control, the goal is to try and minimize the use of pesticides, said Lawson. They begin by eliminating standing water, by cleaning ditches in the fall to try to keep the water flowing, and treating wetlands with larvicides to mitigate the problem at its root.

“If you cannot treat them in the water, then we do what most people associate with mosquito control, the truck-mounted [pesticide] spraying,” he said.

Given the current low risk for transmission of EEE, there are no plans to conduct aerial mosquito spray, according to the Department of Agricultural Resources.

The state uses data from this program to keep the public updated about the risk of infection, said the Massachusetts Department of Public Health. This year, mosquito testing started on June 12.

No human or animal cases of West Nile virus or EEE have been detected so far this year, the department said, but cases usually peak in early September.

Material from Globe wire services was included in this report.