Earlier this week, New York State health officials announced the death of an Ulster County man after he contracted eastern equine encephalitis, a mosquito-borne illness last detected in a New Yorker in 2015. EEE is fatal in 30 percent of human cases, there is no vaccine, and those who survive often face serious, long-term health effects. Every year, mosquitoes in at least two or three counties test positive for EEE, but so far this year, mosquitoes in 15 counties have tested positive. (Human cases have also been reported in New Jersey, Massachusetts, New Hampshire, Vermont, and Rhode Island.) Since its first recorded outbreak in 1938, which killed 25 people, Massachusetts has had more cases than any other state in the country, says state epidemiologist Dr. Catherine Brown. I spoke with Dr. Brown to get a sense of how dangerous EEE actually is, the role climate change plays in all of this, and how public-health officials are managing the backlash to mitigation recommendations after the COVID lockdowns.
So first, how does this thing spread?
Migrating songbirds are what bring triple E into the state as they move northward. We have several very large swamp systems in Massachusetts that have both the birds and the mosquitoes that spread triple E to horses and people.
What constitutes an outbreak?
Anytime we have more than one human case, we consider that an outbreak. At this point, we’re at four human cases and 96 positive mosquito samples. That is still quite a bit lower than what we had in 2019, when we had 12 human cases, six deaths, and almost 300 positive mosquito samples.
Is climate change going to make EEE outbreaks happen more frequently?
We’ve seen a couple of changes in our historic pattern that I expect are at least partly due to climate change. In general, yes, we seem to be seeing cycles happening more frequently. We had a cycle in 2004 through 2006, and then we had another one in 2010 through 2012. So those were four years apart. Then we got a little bit of a nice break between 2012 and 2019, and now we’re back to four years.
That’s not the only change. In the 2004 to 2006 cycle, we had five cases. In the 2010 through 2012 cycle, we had seven. And then in 2019 and 2020, we had 12. So we’ve seen a steady increase in the effect on humans. I am concerned that that may also be a part of the climate-change effect. Also, before this year, all of those exposures occurred before September 15. I anticipate climate change is going to start moving that date further out.
Is climate change also why we’re seeing EEE pop up in more places this year?
The geographic locations where we’re finding our most intense activity is very clearly changing. Southeastern Massachusetts in Bristol and Plymouth County has been our hot spot since 1938, when the first outbreak was recognized. But we have seen an expansion to other parts of the state. My suspicion is that there’s some change in the migratory bird pattern that is resulting in movement of the virus around the state. It’s hard to say.
I would imagine those birds in Massachusetts are the same ones that are flying over the swampier areas of Long Island or the Hudson Valley.
The migratory birds bring triple E up from Florida in the spring. Some portion of them carries the virus up to Massachusetts and New York and New Hampshire and Maine. Once they bring it up, though, the amplification cycle that happens between birds and mosquitoes is going to vary by the individual area. It depends on how many bird species there are that year, how good the swamp habitat is, how much rain each swamp had, and also the size of the mosquito populations in those particular swamps that particular year.
So since the swamps play such an important role in all of this, will the changes in rainfall patterns have an effect on all of this in the future?
The type of mosquito that bites the birds that carry the disease breed in swamps, in pockets of water that accumulate under tree roots. The more rain you have, basically, the more water is in those pockets, which is better for mosquitoes. But there’s a caveat. If it rains a lot — like a lot, a lot — after the mosquitoes have laid their eggs, it can actually flood out those pockets of water. When that happens, the eggs end up on top of the surface of the ground and they dry out, and that will actually reduce mosquito populations. So it’s complicated!
I’ve seen people warn that malaria could become a problem in the Northeast. Do you worry about that?
Honestly, I spend much more time thinking about problems we have now that are very likely going to continue to be a problem, like West Nile virus and triple E. As we see increasing establishment of Asian tiger mosquito populations in the Northeast, it is possible that we could start to see increases in local transmission of malaria, dengue, chikungunya, and all the other viruses that people talk about. But unlike EEE, dengue is a disease of humans. There are no birds involved. In order for it to become a problem locally, we would need a relatively large influx of humans to become infected all at the same time.
Some states — including Massachusetts — have recommended that towns in areas where triple E has been detected in mosquito populations encourage residents to stay indoors after dusk. Is this the first time that municipalities have taken that kind of action?
It is not the first time that’s happened. There’s actually been sort of a standing recommended public-health mitigation measure for communities that are at higher critical risk of triple E. I think it’s gotten a little bit more attention this year, because of how challenging the COVID shutdowns were for some people. And so I think this feels to some people as if it’s the same thing. But this predates the pandemic.
I heard a rumor that the state is considering withholding information about EEE-related deaths until the end of mosquito season. Is that true?Well, there’s a need to share information to help people understand what the public-health risk is, but we also want to make sure that we’re protecting patient confidentiality. So right now, we’re providing information about the sex of the person, their age range, and then the location where we think they were exposed to the virus. When you start to add additional information, like whether the patient was hospitalized or their current status, including whether they have passed away or not, it makes it easier to identify people. Also, triple E is a very significant infection. People don’t always just die or recover in a very short period of time. And so it often takes time to really understand what the outcome is of all of our patients.
In your personal life, as you step outside, do you think about EEE?
Yeah, it’s a totally fair question. And so I will tell you that I actually live in an area that is at risk from EEE. And so the answer is yes, I absolutely do think about it. And I alter my activity based on EEE risk. I’m really careful about using mosquito repellent, and I don’t spend time sitting on my back deck where I would be a sitting target for those mosquitoes that could be carrying triple E.
