Mosquito-carried Virus EEE Arrives on Cape Cod

Aug 15, 2019

Bart Morris of the Cape Cod Mosquito Control Project holds up a net filled with mosquitos.
Credit Sarah Mizes-Tan / WCAI

Massachusetts has been seeing an uptick in the number of mosquitoes testing positive for Eastern Equine Encephalitis. The virus can be transmitted to humans and is considered dangerous. There's been one confirmed human case this year, in Rochester.

Numbers are up, scientists say, because a warm fall, followed by a rainy spring, has given mosquitoes a chance to thrive. The virus-carrying mosquitoes have been found on the South Coast, and now on Cape Cod, in Falmouth and Bourne. 

On a misty summer morning, Bart Morris is doing what most people might be afraid to do these days. He’s taking a quick hike into the woods and trapping mosquitoes.

"We found a positive mosquito in this swamp that we're going to right now, that's why I'm going to put on a little spray before we go there and check the trap," he said. 

Morris is with the Cape Cod Mosquito Control Project, and in response to the mosquitoes that tested positive for EEE recently, he’s laid out mosquito traps in the area near Devil’s Lane in Woods Hole.

He showed me a sample trap in the back of his truck. It’s a mechanism with a bucket of dry ice with a light hanging underneath it.

"That bucket's mimicking a person breathing, and the light is just attracting them even more," he explained. 

Under the light there’s a small vacuum, which will suck the mosquitoes into a net.

On this day, Morris was going out to collect a trap he'd laid out the previous morning. He hiked into the underbrush, and then he pointed out a branch with a trap on it, near a shallow pool of water. He pulled the net off and exmined the mosquitoes it had caught – about 30 of them, all clustered near the top. 

"You can see they're still alive, which is what you want. A nice healthy specimen to send to the lab," he said, pleased. 

These mosquitoes will be frozen at the mosquito lab in Falmouth, and then send via UPS to a lab in Boston, where they'll be tested for EEE. It's impossible to tell which mosquitoes are carrying EEE just by looking at them.

Morris had more traps to collect that day. The Mosquito Project has expanded its trapping area after spots in Woods Hole and Falmouth tested positive for EEE earlier this month.

Mosquitos that could be carrying the Triple E virus, captured in Woods Hole, MA.
Credit Sarah Mizes-Tan / WCAI

State epidemiologist Catherine Brown said that EEE outbreaks aren’t uncommon in Massachusetts. The last outbreak was in 2012.

"So EEE occurs in cycles in Massachusetts, and so what generally happens is you'll have two-to-three years of increased activity, followed by multiple years with sometimes no activity," she said. 

EEE is initially spread through birds, who can carry the disease but don’t get sick from it. Mosquitoes will first bite a bird infected with EEE, and then can pass it to a human.

Mosquitoes that test positive for EEE have tended to cluster around the South Coast, near Plymouth and New Bedford. It isn’t usually found in mosquitoes on the Cape, but Brown said that the overall higher mosquitoes population in the state has led to the virus spreading.

"The intensity of the activity in southeastern Massachusetts, in combination with large mosquito populations in Falmouth, [means] we just had some infected birds that travelled from southeastern Massachusetts down to the Falmouth area," Brown said. 

In order to kill adult mosquitoes that may be carrying the virus in the area, the town of Falmouth organized two trucks from central Massachusetts to spray roads between Woods Hole and Falmouth town hall. This was the first time in Falmouth history this has been necessary. The two trucks had what looked like a large fan propped up on the back, and they blew out a liquid pesticide called Zenivex. 

The trucks sprayed neighborhoods on Wednesday evening, between 9pm and midnight, to minimize harm to pollinating insects and people. Gabrielle Sakolsky of the project said she hopes this will be the only time Falmouth has to do this.

"Crews have been working hard to get out to the swamps," she said. "But we already have adult mosquitoes that are carrying the disease, so if we can knock those down we can reduce the risk of transmission in this area."

The mosquito spraying truck, with a back-facing fan that sprayed pesticide in Falmouth.
Credit Sarah Mizes-Tan / WCAI

The pesticide is said to break down within half an hour. Nonetheless, a number of people have been concerned. Out of the 40 linear miles that the trucks sprayed, 12 properties opted out of the spraying.

The project will be setting and checking more traps this week into next week to see if mosquito spraying was effective.