The rare but dangerous Eastern equine encephalitis virus continues to pose a threat to Massachusetts residents in Bristol, Plymouth, Middlesex and Worcester counties.
The EEE virus, which infects birds and is then spread by mosquitos, was first discovered in humans nearly a century ago in Massachusetts. It’s most often been found in and around Plymouth and Bristol counties. But this year, as it has in 2012, and again in 2019 the virus has spread to more distinct regions of the state. In addition to a wider distribution of the virus, EEE outbreaks have become more frequent in recent years.
The Department of Public Health’s state epidemiologist, Dr. Catherine Brown, said she believes both factors suggest climate change is increasing this threat by shifting bird migration habits and creating conditions for a longer mosquito season.
“EEE used to be a Massachusetts problem, and other states in the region considered themselves fortunate that they didn’t have to deal with it,” Brown said. “But as we have seen new emerging hotspots in Massachusetts, there have also been the emergence of new areas in other parts of New England.”
The virus remains rare — including the current outbreak, there have been a total of 120 cases in the state — but it can be disabling or even deadly because there is no treatment, and EEE can cause swelling of the brain. There have been 64 deaths total in the state. None of the four people infected in the current outbreak has died.
While Massachusetts may no longer be the clear epicenter of EEE activity in the Northeast that it once was, Brown pointed out that, within the state, the number of outbreaks is accelerating. There were just five outbreaks from the 1930s through the 1990s, but eight outbreaks since the 2000s.
| Eastern equine encephalitis in Massachusetts | ||
| Year(s) | Human EEE Cases | Human EEE Deaths |
| 1938-39 | 35 | 25 |
| 1955-56 | 16 | 9 |
| 1973-74 | 6 | 4 |
| 1982-84 | 10 | 3 |
| 1990-92 | 4 | 1 |
| 2000-01 | 2 | 0 |
| 2004-06 | 13 | 8 |
| 2008 | 1 | 1 |
| 2010-11 | 2 (plus 2 non-residents) | 2 |
| 2012 | 7 | 3 |
| 2013 | 1 | 1 |
| 2019-2020 | 17 | 7 |
In the town of Plymouth, where some of the first human deaths from EEE occurred in the 1938-39 outbreak, the state has been spraying insecticide. As an added precaution, the town closed its parks from dusk to dawn.
Plymouth Commissioner of Health and Human Services Michelle Bratti said they are waiting for the first hard frost, when the danger from infected mosquitoes should end.
“It really has to be a downward consistent trend in temperature,” Bratti said. “If it’s still in the 60s at dusk … the mosquitoes love that temperature at dusk. So you’re really looking for a downshift in temperatures to kind of regulate some of the risk in the community.”
In addition to the warmer weather keeping mosquitoes alive longer, Brown said, unfortunately, the lack of rainfall in many parts of the state isn’t doing much to decrease their population. She said that’s because most of the mosquitoes we’ll see this year have already hatched.
“However, [the drier conditions] could potentially benefit us next year, depending on how dry it stays,” said Brown, “because some of the water that occurs in late summer and fall in New England actually ends up as the breeding site for mosquitoes for next year.”
