Dr. Brian Byrd: Mosquitoes, diseases, and control strategies
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As of July this year, two states have reported domestically acquired cases of malaria, and ten have reported cases of West Nile virus. Mosquitoes transmit these and other diseases, and controlling their populations involves a complicated mix of surveillance activities and chemical and behavioral interventions.
On Monday, August 14, 2023, SciLine interviewed:
Dr. Brian Byrd is a professor of environmental health sciences at Western Carolina University. He discussed topics including: the lifecycles of mosquitoes and when and why they interact with humans; the different diseases mosquitoes can transmit between people; how human intervention and climate change alters where mosquitoes live and what diseases humans are exposed to; precautions individuals can take to protect themselves from mosquitos; and large-scale control strategies that communities can undertake to control mosquito populations.
Declared interests:
Dr. Brian Byrd serves on the Editorial Board of the Journal of the American Mosquito Control Association and is a member of the North Carolina Mosquito and Vector Control Association Executive Committee, both volunteer positions. He reports prior scientific funding support from Bayer and AllPro Vector Group.
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Introduction
[0:00:19]
BRIAN BYRD: My name is Brian Byrd. I’m a professor of environmental health sciences at Western Carolina University. By training, I’m a public health entomologist. And so, I study mosquitoes, ticks and other arthropods that can vector or carry disease.
Q&A with SciLine
What should people know about mosquito life cycles?
[0:00:42]
BRIAN BYRD: Mosquito life cycles are actually complex, they involve both an egg stage, immature stages, and adult stage. And so those egg stages are where small, little—what are called larvae hatch from—this has to happen in the aquatic or water environment. And so, they go through a series of molts, these larvae—kind of a common term for them are called wigglers. They develop in that water, go through a series of molts, and they turn into a pupa. And from that pupal stage after about a day or two, they’ll emerge into an adult. And that adult has wings. And that’s the business end of the mosquito—the female mosquitoes, the one that we’re typically more familiar with, as she’s trying to take a blood meal. So, they have a complex life cycle and one that I think many people forget about the aquatic stages of the mosquito.
When and where are people most likely to interact with mosquitos?
[0:01:44]
BRIAN BYRD: We know for most mosquitoes, they’re most common—they’re more abundant, if you will, during the summer months. So, we know that during the summertime, that’s when you’re more likely to come in contact with them. During the course of a day, it’s the morning and evening times for most mosquitoes you may come in contact with them. It’s where the relative humidity is higher, and the mosquitoes are often out looking for a blood meal. So, you may come in contact with them more often later in the day in the twilight periods or sometimes late at night, depending on the species. In terms of where around where you live, that can really vary, but most people most commonly come in contact with them when they’re outside in their yard or outside hiking or doing other activities—recreation.
What diseases can mosquitos transmit to people?
[0:02:41]
BRIAN BYRD: Mosquitoes are known to be very important vectors, if you will, have a number of different diseases. And this includes in parasites such as plasmodium species. These are the parasites that transmit malaria. From a global perspective that’s where the largest burden of mosquito-borne disease comes from. But also, particularly viruses, these are called arboviruses, or arthropod borne viruses. And these are ones like West Nile virus, La Crosse encephalitis virus, dengue virus, Zika virus, and the list goes on.
How do mosquitos transmit diseases to people?
[0:03:22]
BRIAN BYRD: The female mosquito needs a blood meal in order for her to lay eggs. So, it’s that female mosquito for most mosquito-borne diseases that we think about in terms of the risky business end of these diseases and vectors. And so, a mosquito has to actually become infected first. And that can happen a number of different ways. That mosquito may feed on a—what’s called a reservoir, such as a bird in the case of West Nile virus, or St. Louis encephalitis virus. Or sometimes they feed on another human that’s infected, like in the case of dengue virus or malaria. And those mosquitoes have to actually become infected. That’s that first step for them to become infectious. And that takes a little bit of time. Usually, five to ten days that parasite or that virus actually has to develop inside the mosquito before that mosquito becomes infectious. So, the mosquito has to get infected, typically by biting another infected animal or human. In rare cases, in the case of a mosquito-borne disease I study, La Crosse encephalitis, that virus can be passed on from one mosquito to another through the eggs. So, that’s called vertical or transovarial transmission from one generation to the next. And then there’s also a nuance in that mosquito-borne disease where uninfected males—sorry, infected males, males that became infected from their mother during birth actually can transmit that virus on to uninfected females through mating. So, that’s those are rare types of mechanisms. But most commonly, mosquitoes are infected because they’re feeding on another infected host.
How has climate change altered where mosquitos live and what diseases they carry?
[0:05:15]
BRIAN BYRD: This is pretty tricky, sort of assigning causality to climate change directly. But what we do know is that—especially over the last 10 to 15 years—we’ve seen range expansion of mosquitoes. And what I mean is: Mosquitoes that heretofore were in more southern states, we’re seeing them move up. So, we’ve seen mosquitoes more commonly found historically in Texas move their way across and up the mid-Atlantic region. And so, that’s concerning. And that’s something we expect more of with climate change. From a disease perspective, it even gets a little more complex. And what we do know is that with the extreme weather variation and variability we expect to see, our jobs are going to be higher—harder, rather—because it’s not just a simple relationship between climate change and disease risk.
How have other human activities changed where mosquitoes live and what diseases they carry?
[0:06:15]
BRIAN BYRD: There’s a lot of evidence suggesting that as there’s been more development, more human encroachment on undeveloped land, we see spillover of mosquito-borne diseases into the human population. And this has been documented from malaria to arboviruses. And so, there are more humans putting themselves in closer contact with mosquitoes and the reservoirs—or hosts—of mosquito-borne diseases. We expect that trend to continue.
What precautions can people take to protect themselves from mosquitos?
[0:06:52]
BRIAN BYRD: I’d like to repeat the idea of what are called the three Ds—are, drain, defend and dress. So, all mosquitoes have an aquatic or water lifecycle—part of their lifecycle. So those immatures, the mosquito larvae and pupae are found in the water. And so, if there are containers around your yard, or your backyard, gutters, tires, buckets, bird baths that are holding water, you’re likely to be creating some of your own mosquito problems. And so, if you can drain them—or what we sometimes think of as tip or toss—you’re reducing the number of mosquitoes that you’re causing—you’re creating in your own backyard. The dress is one that’s a little tough to do sometimes during the summer. Dress means dressing appropriately, covering up skin, long pants, long sleeve shirts. But again, that that’s really tough. But if you’re getting mosquito bites, try to minimize the amount of exposed skin. And then the last one is defend. And what I mean by that is using EPA-approved and CDC-recommended—Centers for Disease Control-recommended repellents. And there’s a number of them, including DEET, IR 3535, oil of lemon eucalyptus, and picaridin. And these are these are sort of personal protection measures—these defense that you can take and kind of dial in based on the amount of time you’re going to be outdoors or at risk for exposure.
What are some large-scale control strategies that communities can use to control mosquito populations?
[0:08:24]
BRIAN BYRD: We know that personal protection measures are very effective—and so the drain, dress, defend. But sometimes challenge—the mosquito burden or mosquito abundance is a little bit more than what an individual can do. And these require community approaches. And we rely on experts—mosquito control experts and public health experts to do community-wide or area-wide mosquito control. But that’s not done in a vacuum. It’s done as part of what’s called an integrated mosquito management or integrated pest management approach. And part of that is figuring out exactly what mosquito or mosquitoes are causing the problem. There’s many mosquito species that could be in your backyard and your community, and some may be active at different times of day. And so that’s really important to understand in terms of control strategy. So, figuring out what the problem is is sort of step one. And then step two is a hierarchy of approaches that include things like maybe some environmental modification. Draining ditches so that they’re not holding water, which may require some help from your public works agencies in your local county. And these step up and in a very systematic approach to what may result in the use of pesticides or insecticides to reduce the mosquito population. And so, the two main approaches there are what are called larvicides. So, they apply chemicals, often biologic control agents that can reduce the number of mosquito larvae before these mosquitoes ever take wing—before they’re ever flying. And then sometimes they require area-wide control using—in today’s world using scientific-approach, ultra-low-volume pesticides.
