State health officials warn of potentially tough 2020 EEE season

SHARON, Mass. — Entomologist Kaitlyn O’Donnell is wearing long pants and a long-sleeve shirt as she hikes into a dense strip of woods. But, like everybody else when it comes to mosquito bites, she’s not a fan.

“Definitely wear bug spray,” said O’Donnell. “Always wear bug spray.”

O’Donnell, who works for the Norfolk County Mosquito Control District, sets and checks traps at numerous locations, including one about fifty yards from the busy rail corridor that runs through Sharon.

“We’ve had pretty high numbers of mosquitoes so far this season,” said O’Donnell.

She attributes that to heavy rains that have fallen on Norfolk County lately.

“We’ve had some river flooding, so we’ve been seeing some flood-plain species coming off,” said O’Donell. “We’ve also had really high numbers of Culex species, that is the primary vector of West Nile Virus. And we’ve had high numbers of Culiseta melanura, which is the vector of EEE (Eastern Equine Encephalitis).”

Culiseta melanura transmits EEE between birds, while Coquillettidia perturbans is the “bridge vector” that transmits the disease from birds to mammals, including humans.

So far, O’Donnell has not found any West Nile or EEE positive mosquitoes in Norfolk County, but the Massachusetts Department of Public Health found two in recent days while conducting surveillance in Franklin County.

View the state’s town-by-town EEE risk map

Normally, EEE infected mosquitoes don’t turn up until early August or late July at the earliest, O’Donnell said. Still, given the pattern so far this year, she did not find the recent discoveries all that surprising.

"We had a pretty mild winter and a lot of rain, it was a very wet spring," O'Donnell said. "So things were happening kind of early. Mosquito larvae were found pretty early this year. The emergences were a little early. We've seen some other species coming off earlier than average this summer."

What does the early detection of EEE portend for human cases? It’s hard to say.

“We really can’t speculate about what the year will look like,” said Monica Bharel, commissioner of the Massachusetts Department of Public Health. “What we can do is look back at the patterns that we’ve seen in the past. And what we’ve seen from previous years is that EEE tends to be in two to three-year cycles.”

Those cycles coincide with the gradual build-up of immunity in host birds. Since most bird species don’t succumb to EEE, once infected, they become immune. Over time, in a given geographic area, the number of available hosts for the virus thus diminishes.

Bharel noted that while it’s impossible to predict the number of cases of EEE the state could see in 2020, it’s a rare but serious disease.

In fact, about 1/3 of those who contract EEE don’t survive. In 2019, mortality figures in Massachusetts were even worse than that - with half of the 12 who came down with the disease dying.

Last year’s outbreak was unusual in that it affected communities in central Massachusetts, though EEE had long been associated geographically with the southeastern part of the state.

“We don’t really know why exactly that’s been happening,” O’Donnell said. Some have speculated the expanded range of the host birds could be a manifestation of climate change.

What has not changed is the advice on how to stay healthy during EEE season:

• Use an insect repellant containing DEET or picaridin.
• Consider wearing clothing treated with the insecticide permethrin, which is derived from chrysanthemum flowers.
• Get rid of standing water around dwellings.
• Repair screens.
• Avoid outdoor activities at dawn and dusk.

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