Mosquitoes have always been an annoyance, swarming and biting during summer outings, particularly in the evening. But now they come with a greater threat: Eastern equine encephalitis, a serious illness that has been detected in mosquitoes on Cape Cod.

Eastern equine encephalitis, also known as EEE, is a mosquito-borne illness that, while very rare in Massachusetts, is beginning to spike, particularly in Bristol and Plymouth counties, likely as a result of wet weather last fall that created the ideal breeding grounds for the mosquitoes that carry the virus.

“Last fall we had a lot of water in the swamps,” said Gabrielle Sakolsky, entomologist and assistant superintendent of Cape Cod Mosquito Control. “The types of mosquitoes that amplify and transmit the disease overwinter in their larval stage in the water in the swamps. We knew that we were going to start this spring with high populations of those mosquitoes, and last fall animals tested positive for EEE, so there were red flags already.”

Sakolsky said mosquitoes have tested positive in Falmouth, with multiple species at multiple sites, and Bourne, with one sample testing positive in mid-August in a bird-biting mosquito not known to transit the disease. The latest positive came out of Truro in a mammal-biting mosquito in the Head of the Meadow area, which concerned Sakolsky as there is a campground nearby.

“These mosquitoes that transmit the disease are out dusk to dawn. Most of us if we're being bitten by mosquitoes go in. If you're not going in, you've got your repellent on, and you're choosing the EPA registered repellent, not a natural, bought-at-the-farmer's market repellent, but something that's been proven safe and effective,” Sakolsky said. “But people who are tenting...”

Sakolsky said the disease is being carried around the state by birds that are either about to begin migrating south or are already in migration, which means a lot of influx of birds from areas of critical transmission, such as Bristol and Plymouth counties. Sakolsky also worries that as summer wanes, people will get lax in their concern regarding mosquitoes.

“The departments of health in these towns have worked with us closely and they're really great at getting the word out and making sure they're keeping people in their towns informed,” she said.

According to Boston Children's Hospital (childrenshospital.org), EEE is an arbovirus, or an arthropod-borne virus spread through the bites of blood-sucking insects such as mosquitoes and ticks. People who spend a great deal of time outdoors are at greatest risk of exposure. Symptoms begin showing up roughly four to 10 days after infection and can include high fever, headache, tiredness, nausea and vomiting, and neck stiffness. EEE can also cause seizures and disorientation depending upon its impact on the brain. In one third of cases, the encephalitis caused by EEE can be fatal or can lead to permanent brain damage.

“Of those who have the neurological disease, 40 to 60 percent die,” Sakolsky said. “Those that survive have ongoing issues that severely impact quality of life.”

The challenge with EEE is that because it's a virus and not a bacterial infection, there isn't much that can be done in the way of medical treatment.

“They can give you care to treat you, but once you have it, even though it's rare, it's serious,” Sakolsky said.

Fortunately there are steps that people can take to reduce their chances of getting bitten.

“Anything you can do to reduce the mosquito population around your house is good,” said Sakolsky. “The mosquitoes that lay their eggs in small ponds or man-made containers such as buckets or tarps that hold water after the rain, all of those mosquitoes are definitely out dusk to dawn. If you have a lot of mosquitoes dusk to dawn, a lot of those species don't fly very far, so they're probably coming from your yard or your neighbor's yard.”

Sakolsky urges people to start by checking on all areas where there might be standing water around the home, including rain gutters that are no longer level and therefore hold water, or are blocked and need cleaning. If water has collected in buckets, on tarps, in trash barrels, or other containers, dump the water immediately, and change the water in bird baths frequently.

“Mosquitoes lay their eggs and then their larva develop in standing water. As long as you dump out the water, you kill all the mosquitoes that were in it and you don't provide habitats for them to lay eggs and develop again,” Sakolsky said.

If you have a decorative garden pond, consider having someone from Cape Cod Mosquito Control visit and add a bacteria known as BTI, or bacillus thuringiensis israelensis, that helps kill mosquito larvae.

Sakolsky doesn't recommend pesticide use or use of the gas-powered mosquito traps as they do more harm than good.

“One is something that sprays a pesticide every so many days or hours. That's spraying a pesticide without even knowing you have a problem,” said Sakolsky, who added that many pesticides are contact killers that wipe out not only the mosquitoes, but also beneficial insects, such as bees and butterflies.

The gas-powered traps are also problematic as they actually attract mosquitoes in order to trap them, so unless the user has an extremely large yard and can situate the device in a far corner away from the home, mosquitoes will still be present.

“24 hours a day, seven days a week, you're calling in mosquitoes to your yard,” said Sakolsky. “And they'll have it right next to their picnic table or fire pit.”

When in doubt, Sakolsky said people are encouraged to call Cape Cod Mosquito Control to have an assessment done.

“They'll walk through and see if there's any standing water in toys, buckets, or what have you. They also use GIS technology to map areas where mosquitoes are heaviest,” Sakolsky said. “If there's a source of standing water, they can treat that with a bacteria that kills them.”

If no standing water is found or crews can't find the source of the mosquitoes, samples will be taken back to the lab for identification.

“I can identify them to species,” Sakolsky said. “There are about 25 species of mosquitoes on Cape Cod, approximate and common. The species are very specific about where they lay their eggs, the type of habitat, so I can tell if it came from a man-made container, or if it came from acidic water like the kind found in cedar swamps or cranberry bogs.”

Sakolsky advises people not to use private contractors for eradicating mosquitoes.

“There are a lot of private contractors that are out there right now, some of them benefiting from the fact that there are mosquito diseases in our area. I would caution people against hiring them,” she said. “I would make sure I knew what they were using on property, label, safety data sheet. Just because somebody says it's natural doesn't mean anything. That doesn't mean that it's effective. That doesn't mean it's not a chemical pesticide derived from a natural source. You want to make sure with disease around that you're using something effective and you want to make sure it's safe, and you want to make sure that anybody who is applying any pesticide to your property has a pesticide applicators license from the Commonwealth of Massachusetts. Every person. Not just the person that drives them there or their boss.”

If someone is going to be outdoors, especially in areas where mosquitoes are heavy, protection can come in the form of bug sprays with EPA-registered ingredients such as DEET, permethrin, picaridin, or oil of lemon eucalyptus, and/or wearing clothing treated with permethrin, ideally long sleeves and long pants, as well as shoes and socks.

Learn more about EEE and how to stay safe at www.mass.gov/guides/aerial-mosquito-control-summer-2019.