Mosquitoes are a common nuisance this time of year, but they're increasingly posing a danger here in the Northeast, cases of West Nile Virus and Eastern Equine Encephalitis are popping up from the Hudson Valley to Eastern Massachusetts and beyond. The rare EEE virus, which is transmitted to mammals by mosquitoes, is fatal in roughly one-third of people with severe symptoms. Laura Harrington is a professor of entomology at Cornell University who studies human and animal mosquito interactions.
Eastern Equine Encephalitis is also known as Triple E, and it's a virus. It's similar to West Nile Virus, but it's in a different group. And it is what we call a mosquito borne virus. So, it undergoes part of its life cycle in mosquitoes and then the other part of its life cycle in animals. And this virus is extremely rare, human cases are extremely rare, but every now and then we have years where we have a higher number of cases than usual, and that's what we're experiencing this year.
How does Triple E spread? And from what I understand, mammals can't spread it. It's really mosquitoes to mammals?
Yeah, it must be spread by the bite of a mosquito that's infected with the virus. And so, we have different types of mosquitoes. There are actually quite a few different types of mosquitoes. Only a small number are important in Triple E transmission. There's one mosquito that only feeds on birds, that is responsible for maintaining the virus and infecting new birds and sort of starting a cycle where the virus becomes more and more common in the bird population. And then there are a couple of species that will readily bite birds and also people. And those are what we call bridge vectors. Those are the mosquitoes that we're most concerned about as far as protecting public health, helping people prevent mosquito bites. Also, those mosquitoes are also ones that we monitor for infection rates.
Is there any way for people to tell what kind of mosquitoes are surrounding them? Do they live in different areas? Do they have any identifying features?
Yeah. So, the mosquito that is really involved in the bird-to-bird transmission cycle is found in hardwood swamps, and it has a very specialized life cycle where the immature stages of mosquito live and grow underneath what we call crypts. These are subterranean pockets in the water underneath hardwood trees, and so you won't find these mosquitoes too far from that kind of habitat, and the ones that are bridge vectors are in more swampy habitats. None of the mosquitoes that are important in transmission to people, as far as Triple E is concerned, are peridomestic mosquitoes are the mosquitoes that we traditionally focus on for West Nile that feed in containers. That's not the type of mosquito involved in Triple E transmission.
What makes Triple E different than West Nile? I know you said it's part of a different group, but what are the characteristics of the different groups?
Well, Triple E, primarily, is different from West Nile from our perspective as far as the symptoms in humans and the mortality rate. So, the fatality rate in humans can be very high, and for those people who actually survive, so it can be fatal. For those who do survive, they can have lifelong neurological symptoms. As far as the genetics and the biology of the virus, Triple E virus can change a little bit more quickly than West Nile Virus, but we don't really see significant changes in the Triple E virus populations that are being monitored.
You touched on it a few minutes ago. But is this year worse than other years in terms of the volume of mosquito borne illnesses?
No, not necessarily. When we look at West Nile Virus, for example, we are seeing cases, but they're not significantly above the baseline so far. So, for example, 377 lab-confirmed human cases have been reported to the CDC so far, in a typical year, we have anywhere from 2000-3000 cases, and so we're not seeing tremendous increases in West Nile, the activity in the end might be higher than this 10-year cycle, but it's not like Triple E, where we're seeing very, very high infection rates in mosquitoes that are either involved in that enzootic cycle, or are bridge vectors.
And on that, why is there so much infection of Triple E happening right now?
Well, we always see the number of infected mosquitoes increase as we get later into the summer, and it's really just a numbers game. So, if you can imagine, you're starting out with a certain number of birds that are infected and infectious, and as the mosquito population increases, they feed on those infected birds, and then they can transmit the virus to new birds, and so you get a much larger population of infected birds, and also with that, infected mosquitoes.
And what makes Triple E so serious and so fatal when it is human cases?
Well, you know, that's a good question. The virus is more harmful. You know, with West Nile, a lot of people get West Nile never even know it. And you know, with Triple E, because it's so rare, we don't know what the, we call those asymptomatic infections, we don't know what that rate is for Triple E, because it hasn't been studied as well. But the, you know, a decent proportion of people who do get infected can get very, very ill and potentially die, especially among individuals who have underlying, you know, conditions that may weaken their immune system and their immune response.
Is it just the northeast that sees these kinds of infections, or are they across the world, other parts of the country?
Well, unfortunately, our region tends to see the highest number of cases. There are also a fair number of cases in Wisconsin and in the Upper Midwest, but the northeast, you know, especially in New England states, tends to see the highest number of cases. Interestingly, we know that Triple E is abundant in states like Florida, and we think that that birds that over winter in Florida and other areas in the south can harbor the virus and then bring virus back into the northeast with the bird migrations in the spring.
What are the symptoms that people should look out for with West Nile and Triple E?
You know, symptoms can include a headache. It can include just general feeling unwell. Some patients with Triple E can slip into comas. They can have, of course, encephalitis, which is an inflammation of the brain and spinal cord. It's really important that people get in and get care as soon as possible if they have symptoms.
And how long could it take for symptoms to pop up? And are they different? There was one human death in New Hampshire and a horse death in New York recently. Do they differ between humans and animals?
No, the animals respond, horses have very similar symptoms to humans. Oftentimes, horses will exhibit a circling behavior, and that's a symptom of the neurological damage that the virus is causing. Typically, people won't develop symptoms for four to 10 days after they're infected with by a mosquito bite.
And how can people protect themselves from getting the virus?
Well, I think that it's really important. This is something that we always try to emphasize, as if you know for people to educate themselves about what the prevention strategies are, and they include avoiding mosquito bites as much as possible during the peak hours of activity for the certain vectors that we're most concerned about, and that's from dusk to dawn. And using repellents, we have some very, very good EPA approved repellents that are available for people to use. There are three in particular that are especially effective, including one newer product called oil of lemon eucalyptus that was recently added. We have picaridin, and then we have DEET, and all of them are very effective. People can also purchase clothing impregnated with permethrin, which is a very effective repellent, or they can impregnate their own clothing by soaking it in a permethrin solution that they can buy.
And how is the spread being monitored?
Well, there’s a lot of work that the general public isn't even aware of that happens from the from the early spring throughout the mosquito season, there are programs either at the state or local health department that are collecting mosquitoes and testing them for these viruses that we're concerned about and they do that all throughout the summer. And the information they get from infection rates in their mosquito populations is incredibly important because it usually indicates that there is a pending risk to the human population. And so that will stimulate ad campaigns and education campaigns to try to help people understand the risk and protect themselves.
And when does the season end?
Well, you know, the first killing frost will kill most of the mosquitoes that are still active. Now, many mosquitoes are entering hibernation for the winter. We call it diapause, and some of them have already started entering that state. We suspect that there are mosquitoes that are infected with Triple E and will potentially become active and infectious in the spring, when they come out of diapause, but there is not a lot of data on that yet we that's a concern that we have that we're addressing.
Laura Harrington is a professor of entomology at Cornell University, and studies human and animal mosquito interactions.
