Ticks, mosquitoes, and rising disease risks

[00:00:23]

SARA WHITLOCK: Hi, everyone, and welcome to SciLine’s media briefing on ticks, mosquitoes, and rising disease risks. In this briefing, we’ll give context about how climate change is shifting the risk of infectious diseases, and we’ll dive into the details about the diseases carried by ticks and mosquitoes. We’ll also discuss how scientists study and track the spread of these diseases.

My name is Sara Whitlock, and I’m SciLine’s scientific outreach manager. A little background about what we do at SciLine. We’re an editorially independent non-profit based at the American Association for the Advancement of Science. We’re fully funded by philanthropies so that everything we do is free, and our team aims to make it as simple as possible for journalists to use scientific evidence and expertise as your reporting. A little scientific research can deepen your stories with evidence and context, whether you’re covering a topic that clearly involves science, like heavy rainfall, or one where science is less obvious, like funding for education or national parks. More of our resources are available on sciline.org, including interview opportunities and trainings. You can click the blue “I Need an Expert” button if you need to speak with a scientific expert for your story. We’ll look for a source with the right background to answer your questions, who’s available before your deadline.

A couple of notes before we begin. I’m joined here by three experts who have studied vector-borne diseases carried by ticks and mosquitoes. So I’ll let each of them introduce themselves and their topics of research. Dr. Mordecai, would you go ahead?

[00:01:41]

ERIN MORDECAI: Hi, everyone. Thanks for inviting me today. My name is Erin Mordecai, and I’m a professor of biology at Stanford University and a senior fellow in the Woods Institute for the Environment.

[00:01:50]

SARA WHITLOCK: Great, thank you. And, Dr. Tsao, would you introduce yourself next?

[00:01:53]

JEAN TSAO: Yes. Hi, everyone. Thanks for inviting me. I am a professor at Michigan State University in the Department of Fisheries and Wildlife and Large Animal Clinical Sciences, and I’m a disease ecologist studying mainly ticks and tick-borne disease.

[00:02:08]

SARA WHITLOCK: Great, thank you. And Dr. Bartholomay.

[00:02:11]

LYRIC BARTHOLOMAY: I’m Lyric Bartholomay. I’m a professor at the University of Wisconsin-Madison in the School of Veterinary Medicine, and I’m an entomologist who studies those mosquitoes and ticks that impact public health, and how to control them.

[00:02:25]

SARA WHITLOCK: Thank you. So I’ll ask each of our panelists some questions before we start taking audience questions. And journalists, you can submit those questions at any time during the briefing. Just click the Q&A icon at the bottom of your Zoom screen, and please let us know if you’d like your question directed at a specific panelist. We’ll be posting a recording of this briefing on our website later today, and a transcript will be added in the next few days. But with that, let’s begin.So, Dr. Mordecai, how is climate change altering the landscape of infectious disease risk in the U.S., particularly for those diseases that impact human health?

[00:02:55]

ERIN MORDECAI: Yeah, this is a really important question, and we’re starting to hear about it more and more in the news, every day. You think about diseases like West Nile or Lyme disease or even cases of local transmission of malaria or dengue happening in the U.S., which we’ve never really heard about in recent history. So let me explain, since I’m the first panelist, a little bit about why we think climate and climate change affects these diseases. These are a set of diseases where the pathogen has to be transmitted by a biting arthropod, like a mosquito or a tick. So it requires that part of the life cycle in order to get from one human to the next.

Now, a mosquito or a tick, it’s a cold-blooded organism, so it’s body processes, how quickly it digests its food and how long it lives and how long it takes to develop, all of those things depend on the temperature of the environment around it, and as it gets warmer, those developmental processes happen faster. You get more mosquitoes, more ticks. They’re more likely to survive the incubation period of the pathogen. And so transmission increases up to a point where eventually the mosquitoes start dying if it gets too hot.

So, mosquitoes are really sensitive to temperature. They’re also really sensitive to rainfall and humidity because, you probably know, mosquitoes have to have little pockets of standing water to breed in. That’s where we get mosquitoes. And so, because of increases in temperature, that we’ve already seen because of human impacts on the climate, the temperature is already getting more suitable for transmission of disease here in the United States and North America, and we’re already starting to see increases in transmission of diseases like West Nile, Lyme disease, maybe even to some extent dengue and malaria.It’s really a growing threat that we’re starting to hear about more and more, and you might even be noticing it in your everyday life, as you see more and more mosquitoes over longer periods of the year. So it’s not just the summer anymore.

[00:04:38]

SARA WHITLOCK: Thank you. That’s really helpful. And are there specific regions or populations in the U.S. that are becoming more vulnerable to climate sensitive diseases, and what factors are driving that vulnerability?

[00:04:48]

ERIN MORDECAI: Well, I can give you a few examples of that. One area is West Nile, as I mentioned earlier. It was first introduced into New York City in 1999. So it’s been around for about 25 years, 26 years, here in the U.S. And we have really good record of documentation of how many West Nile cases there are and where the mosquitoes that transmit West Nile are.

Looking at New York State, specifically, the original epicenter of West Nile, some of the research that we’ve recently done in my lab has shown that the season for West Nile transmission has gotten 25 days longer over the last 25 years. And that’s because it’s getting warm earlier in the year and it’s staying warm later in the year, in New York. That translates directly into greater infection prevalence in the mosquitoes that transmit West Nile and larger numbers of cases in humans in New York. So we’re already seeing climate change, increasing the amount of West Nile transmission we have in New York.

We would expect that to be happening in a lot of the Northeastern states where West Nile transmission happens and particularly where cold winters were historically more prohibitive for mosquitoes, we’re also starting to see more cases of local acquisition of dengue. That’s a virus transmitted by mosquitoes.

It’s typically more associated with tropical regions, but because we now have the mosquitoes that can transmit dengue here in the United States, we’ve started to see local transmission in places like Florida, Texas, even California, and Hawaii over the last few years. That is because those mosquitoes are getting more and more well established as the climate gets more favorable. As travelers go back and forth into endemic regions, they might get infected with the virus, bring it back, expose the local mosquitoes and then you can set off local transmission cycles. When I mention these diseases, they can have pretty profound impacts on the people that are infected. Sometimes they cause a fever and you are in some pain and feeling really feverish and flu-like for several days, but sometimes they can really spiral into a much more uncontrollable disease that can really damage people in the long- term and then sometimes kill people. So it’s very important to start taking these mosquito-borne diseases seriously.

[00:06:52]

SARA WHITLOCK: Thank you. Yeah, that’s a scary set of things happening. So how does climate change interact with other drivers like biodiversity loss, land-use change or urbanization to shape the overall risk of disease emergence and spread?

[00:07:05]

ERIN MORDECAI: One thing that’s interesting about these mosquito and tick-borne diseases is that they do depend on people, of course. Mosquitoes like to be where people are and they can feed on people. But it also depends on other aspects of the environment. Mosquitoes need a habitat to live in. Ticks need a habitat to live in. Ticks, in particular, need small animal hosts that they feed on over their different life stages. I know Dr. Tsao is going to get into that a little bit more, later.

Land-use change, things like fragmenting forests or breaking up large forests into smaller forests can change what types of animals can live there. As you can imagine, smaller animals do better in smaller patches of forest. Large animals like wolves or coyote or deer sometimes need larger plots of forest. And different animals are better or worse at hosting the bacteria that cause Lyme disease. And so it can really change the amount of risk for Lyme disease as you fragment forest and convert land from either large swaths of forest or into subdivisions. Other factors like the availability of wetland habitat, water-breeding habitat, and also the movement of people. Like I mentioned, people moving back and forth between the U.S. and other endemic regions around the world, can be a source of introduction of new pathogens. So there’s a lot of complex factors, and it’s not just the warming of the temperature, either. It’s also changes in precipitation cycles and humidity that are important for these mosquitoes and ticks.

[00:08:26]

SARA WHITLOCK: That makes sense. So what tools or models do scientists like you use to understand and predict how climate change and these other factors will affect disease transmission? And how accurate are these models in real-world scenarios?

[00:08:38]

ERIN MORDECAI: Those are great questions. And for something like climate change and infectious disease, we really have to triangulate based on multiple lines of evidence. There’s not just a single experiment that we could do that would tell us how climate change is affecting mosquito-borne diseases, unless we could say, OK, half of the United States is going to get warmer temperatures and the other half is not, and we’re going to see what happens. That’s obviously not really feasible or ethical.

So instead, what we do is we can perform experiments in the lab. We can bring mosquitoes into the lab, and put them at different temperatures, and measure how their vital rates change. Then we can build those into mathematical models that tell us how all the different changes in the mosquito life cycle like, development, reproduction, survival, virus incubation, how all of that adds up to affect the transmission cycle. And that allows us to make a prediction. For, what are the temperatures that are most suitable for transmission. Then we can look in the real world at where and when human disease cases are being reported, match that up with climate information and ask, is this really true. And what we find is actually, these models do a very good job of predicting what temperatures are most suitable for transmission and when we’ll start to see temperature increases driving increases in transmission.

We’ve recently done this for dengue, for example, showing that the optimal temperature for dengue transmission is around 28 or 29 degrees Celsius, and that we get the largest impact of climate warming when the temperature is about 15-20 degrees Celsius. So as you’re going from marginal to just warm enough for a dengue transmission, that’s when you get the largest increases in dengue. As a result of that, we’ve already seen climate warming expanding dengue burden by 18% globally, which translates into tens of millions of dengue cases. And that’s just what’s happened already with the climate warming that’s already happened. So that’s just an illustration of how we can use models that are really based on foundational lab experiments to scale-up into the real world and understand how much impact climate change is already having.

[00:10:34]

SARA WHITLOCK: Got you. Thank you so much. So let’s move on to you, Dr. Tsao. To start off, what does your research tell us about how climate and environmental changes are affecting the ecology and range of ticks and the diseases they carry, like Lyme disease?

[00:10:47]

JEAN TSAO: I have focused mostly on Lyme disease, and Dr. Mordecai spoke a bit about it, so that’s great. But I did want to remind us that actually, because I do hear this from other reporters, that there are many ticks, and in the U.S., there are probably three to four big human biting ticks, but certainly the many of them are more on the East Coast, though certainly there are some on the West Coast, and the states in between. But certainly, the Lyme disease tick, so the blacklegged tick in the Eastern U.S. and the western blacklegged tick in the West.

In the Eastern U.S., the blacklegged tick has been expanding in its range, right, a lot. Then also, the lone star tick has been expanding in its range, and that has been expanding mainly from the south, northwards. The American dog tick, which causes Rocky Mountain spotted fever, mainly, that hasn’t changed its range that much. All right. And then we have the Gulf Coast tick, which is found mostly in the southern U.S., that’s been expanding northward. So there has been a lot of dynamic change in the range, and some of it is probably impacted, in part, by climate change, but much of it probably, is not. It’s never just one factor, right? So, in terms of research it seems that actually, not just from my lab but others, it seems like, just as Dr. Mordecai said, a lot of land-use change, here in the continental US anyway, mostly the changes in the range is probably due to land-use change affecting wildlife communities. Also management of wildlife population, such as the white-tailed deer. That plays a major role for many of these ticks.

All right. If we were speaking in Canada, if we were talking to Canadian reporters, I would say that climate change probably has more of an effect for the expanding range of the blacklegged tick, northwards. All right. But here in the U.S., where Dr. Bartholomay is from, in Wisconsin, these ticks, the blacklegged tick, was found in northern Wisconsin. Also, I’m in Michigan now, the Upper Peninsula, that’s where the ticks were found in the 1980s. And then in, the Northeastern U.S., it was a little more moderate, Cape Cod, Massachusetts, more moderate. But the ticks have been spreading northwards, yes, but they’ve been expanding inwards towards Ohio, also southwards. A lot of it’s southwards.

So that can’t be explained by climate change, right? I mean, I’m sure climate change has affected it, just as Dr. Mordecai has said for mosquitoes. The growing season, we like to say, has expanded, all right, so some of the ticks are probably active a little earlier in spring, and they go a little later in fall. So they have much more time to complete their life, or several life stages. That probably, has benefited survivorship of the ticks, and then therefore the whole population size, and then how much they can expand. So I think climate change has affected that way, but certainly all their changes in landscape, as I mentioned before, has played a large role.

Different ticks are affected differently. So the lone star tick has been moving northwards, probably with the increase of deer populations, but also it probably does have less of a winter tolerance, I guess, to the colder temperatures. But really cool, there are some studies that people have dug into the notebooks of early Swedish naturalists in the 17-1800s, and it seems like the lone star tick was found in New Jersey, New York, in the 17-1800s. And a Michigan entomologist recorded, probably what was the lone star tick, in Michigan in the 1850s. And so now we are just getting re-established potentially by this lone star tick in southwest Michigan.

And again, maybe climate change is affecting that, helping that, but certainly, it seems like they were here before, and potentially. I mean, they were here, but the numbers, maybe they’ll be greater now. But certainly landscape, land-use, and wildlife communities, they play a large role in where these ticks can be. And that’s affected by humans.

[00:15:18]

SARA WHITLOCK: That makes a lot of sense. And you’ve talked about how these ticks are moving around. So how are the diseases actually being tracked locally, or across wider regions? What are the limitations to the surveillance systems that we have for tick-borne diseases?

[00:15:30]

JEAN TSAO: Right. So for ticks, the CDC does what it can to fund different state health departments to conduct systematic surveillance for ticks. They provide the guidelines. But it always ends up being a matter of funding, whether or not states actually receive enough to be able to do this standardized sampling that you need, to actually track whether or not ticks are expanding in different regions, or if one year is a big year, another year is a more quiet year for the ticks.

When states do collect ticks in a systematic fashion through the health departments, and somewhat, potentially, by academics, especially if funded through the CDC, for example, through their Centers of Excellence, these ticks can be sent to the CDC to be tested for pathogens. And in that way, they can find out more about what pathogens are present. Now, this is happening much more, so for pathogens transmitted by the blacklegged tick, but less so by the lone star tick and the dog tick. But I know that those researchers at the CDC, would love to be testing more ticks as well, if they could.

So that’s active surveillance. Then there is a lot of passive surveillance with regard to ticks, where state health departments, as well as academic entities have these services where people can submit, actually, the ticks themselves or photos of ticks, and then they can be identified. So these data can be then put together, although that’s one of the limitations, there isn’t a way actually to collate all the data from all of these different sources. So that’s an area for opportunity. But there are a lot of people who are contributing that.

So there’s the federal government, there’s state government, there’s academics, and there are some private entities that do that, too. And that passive surveillance often can turn up a lot of data for where ticks can be. And in some places, in some situations, what pathogens are present and where, but you don’t get ticks sent in from places where people don’t live. And so in part, as are a fewer. And so in part, that’s another limitation to this passive surveillance. Besides the fact that there isn’t a body that is necessarily collating all of these data, in a timely manner.

00:18:05]

SARA WHITLOCK: That makes a lot of sense. I think now we’re going to move on to Dr. Bartholomay. So can you tell us how are climate change-related trends, like rising temperatures and shifting rainfall, affecting mosquito populations, and then the transmission of viruses like West Nile, in the U.S.?

[00:18:21]

LYRIC BARTHOLOMAY: Thank you. So, I like to think about this in terms of the fundamental needs of an organism. And when we think about mosquito fundamental needs, food and shelter and we may all just immediately think mosquitoes just need blood, but there’s more to it than that. So adult mosquitoes need access to sugar. They actually move between nectar sources to get sugar for energy. The females at least need blood, and they need a safe place to land or rest after they’ve taken blood, and places to lay their eggs. Then immature mosquitoes need standing water and organic material or something to eat.

So in a situation like this year, where we’re experiencing record rainfall in many parts of the U.S. and record-high temperatures, it becomes a perfect storm for mosquitoes to have ample places to lay their eggs and for their immatures to develop. And as Dr. Mordecai suggested, when we have really warm temperatures, that duration of time it takes for an immature mosquito to go from egg to adult is shortened. So we’ve got lots of opportunities to produce a lot of mosquitoes.

To the question of how this impacts transmission of viruses like West Nile in the U.S., having a mild spring and a really wet summer and really warm temperatures, I think we’re heading into what could be a serious West Nile virus year with a lot of cases. In several places in the U.S. right now, our surveillance efforts are revealing high West Nile virus activity in mosquitoes.

There’s historical evidence, too, for how rain and high temperatures facilitate outbreaks of these kinds of mosquito-borne diseases. Folks who live in the Southwest, or maybe just winter there, may recall that there was a record breaking West Nile virus outbreak in Maricopa County, Arizona in 2021, and there were something like 100 people who died and 1,500 people who got really sick, which was a really serious outbreak. And the reason for that outbreak was, in part, because they had experienced monsoon rains that year and explosive numbers of mosquitoes.

So we have these situations where rain and temperature play in, and we have the possibility, too, that then, mosquitoes can find more places to live. And so we’re seeing, as Dr. Mordecai mentioned, too, an expansion in places where dengue virus is being transmitted in a local way. So there’s mosquitoes available to transmit the virus. Currently, there’s a small outbreak of locally transmitted dengue virus in Brevard County in Florida, so far north of Miami, where we know that there’s local transmission of those things. So rainfall and warm temperatures play in to these transmission and outbreak events in a really big way.

[00:21:25]

SARA WHITLOCK: That makes sense. And I know you’ve said a little bit about this already, but are we seeing any other changes in the seasonality of mosquito activity or mosquito-borne disease transmission?

[00:21:33]

LYRIC BARTHOLOMAY: Yeah, I think you’ve heard from both of the other experts on this call, that changes in seasonality are absolutely something we’re feeling already, and I think that the changes reflect changes we feel and experience in our surroundings. Gardeners may have noticed, for example, that there have been changes to the USDA growing zones and to the kinds of plants that we can have in our areas. Maybe folks have noticed, too, that the last date of a frost is coming later, and maybe the threat of frost in the spring is wearing away sooner.

So that all provides for, really, temperatures that are conducive to mosquitoes being out and on the wing. If it’s about 55 degrees, most of the species of mosquito we have in the U.S. can be out and flying around. So that gives you a sense for when they might be out and active. And in many parts of the country, as you heard from Dr. Mordecai, that mosquito season is just lasting longer, as is reflected in those frost and the USDA grow zone data.

[00:22:48]

SARA WHITLOCK: That makes sense. So what mosquito control measures are being used, and how well are they working as these mosquito patterns are shifting with climate change?

[00:22:57]

LYRIC BARTHOLOMAY: Generally, we think about mosquito control as preventive and reactive. Prevention targets immature mosquitoes in the aquatic habitat. It’s that standing water that I mentioned. And then reaction steps in when adult mosquitoes are already out on the wing and are a problem, the females taking blood. So there are a variety of ways that we control mosquitoes. For immature stages, we have an arsenal of controls that includes all kinds of microbials, things like hormone mimics, that disrupt how they grow and then more traditional insecticides. For adults, we have a few kinds of insecticides that can be used. Either from a backpack fogger or a truck or an airplane.

A community’s capacity to use those tools depends on its investment in controlling the mosquito issue at hand for them. So there’s a full spectrum of investment in the US. In some places, there’s no coverage at all for mosquito control. Elsewhere, mosquito control might be done as a small portion of the workload of people who are working in a public health, or parks and rec, or environmental health district or department.

In other places, mosquito control is covered by tax-funded districts or organizations that are solely focused on providing mosquito coverage to their community. The extent to which the control really works depends on the investment and the capacity of the community, and the extent to which the control gets to the mosquito. There was a study by an agency funded by the CDC to look at capacity across the US. And in 2023, only about 20% of the programs in the US are at full capacity to do all the things to get good mosquito control to larval and adult mosquitoes.

Another issue is that there are places where mosquito control products won’t work, or their use these to be tweaked a bit because mosquitoes develop resistance to the product. So that’s a lot of different factors that play into how well they might work with changing climates. And I’ll just also say that in that climate change lens, a big challenge is that seasonality issue that you asked about. A community may have a situation where they have excellent mosquito control but it only is scheduled for like, May to August. So we may need to really think about how we can shift that to cover the mosquito season and the fact that these things are staying out longer.

A colleague of mine in California shared that their operations used to span May to October, but now they have to operate from March to November, and they’re really verging on needing year-round mosquito control. My own experience here, in the Midwest, we usually hire a lot of student workers to work with us, and the time that we need them around really keeps shifting. Where we used to be okay with reduced staffing with students going back to class, we really, now, need to have more people onboard in order to be able to do all the work to trap and identify mosquitoes and test for viruses.

[00:26:12]

SARA WHITLOCK: That makes sense. Thank you all, for sharing your expertise. We’re now going to begin asking questions to our experts. So I want to remind the reporters on the line to please submit your questions using the Q&A box that’s at the bottom of the Zoom screen. But to kick off this open discussion, I want to ask all of our experts here about the news coverage you’re seeing on tick and mosquito-borne diseases. What advice do you have for reporters and what isn’t being covered that should be? So maybe, Dr. Mordecai, we can start with you.

[00:26:39]

ERIN MORDECAI: I think, for a lot of people, once you explain the life cycle of a mosquito and the fact that it depends on temperature and standing water, then it makes a lot of sense why these diseases are responding to climate. And people start to draw their own conclusions. Oh, maybe climate warming is playing a role. But a lot of people just don’t know, and they’re not that aware of mosquito and tick-borne diseases as a threat to our health. They might be a lot more familiar with things like influenza, where the main season is during the winter. So I think just explaining that there are these diseases transmitted by biting arthropods, mosquitoes and ticks mostly, and that they do thrive in the summer, that mosquitoes aren’t just a nuisance. They can actually transmit diseases to you and that it’s important to protect yourself from mosquitoes and ticks because these diseases can have quite serious consequences. I think those are the most important messages to get across.

[00:27:29]

SARA WHITLOCK: Great. Thank you. And Dr. Tsao?

[00:27:32]

JEAN TSAO: I guess what I would say, also, and I’m sure many of the reporters here online already know this, actually, but sometimes I get asked questions like, at the national level, and we do tend to focus on Lyme disease, and that is in part because Lyme disease causes the most disease. But I know, more and more, that’s not the case in many of the geographic areas. And in fact, and someone mentioned it here, I saw the questions. But as I mentioned before, the lone star tick is another, really, nuisance tick, and more and more people are becoming familiar with the tick-associated red meat allergy. And so I guess what I would say, is that for the reporters to just make sure that if they’re interested in a story on ticks, to talk to some local tick experts or public health, to know what might be the most important locally, as well as highlight some national trends.

[00:28:26]

SARA WHITLOCK: That’s a great point. Thank you. Dr. Bartholomay?

[00:28:29]

LYRIC BARTHOLOMAY: Thanks. I would say, in addition to echoing all the things my colleagues have said, that I think these are great topics for the press to take on. And I’ve fielded a lot of questions this year about the extreme numbers of mosquitoes people are facing and curiosities and concerns about ticks, as well. And I think a lot of times the press requests that I get will track with when there’s been a case of human disease. Maybe I could just suggest that there will be some predictable things for you to rely on. May is Lyme Disease Awareness Month. So if you’re in a state that has serious concerns about Lyme disease, May would be a really great time to reach out to trusted sources and get a press release out, and remind people to be vigilant about protecting themselves from blacklegged ticks and the pathogens that they might transmit. West Nile virus is really a July, August, September issue, so it might be good to just put that on your calendar and think about checking in with those folks again around that time so that you could get the message out, maybe before even people get sick, to remind them that these things are an issue. We really do rely on the press a lot, to make sure that people are hearing the important messages about prevention and these really serious diseases.

[00:29:59]

SARA WHITLOCK: That makes sense. Thank you. So to kick off our questions from reporters, we have one clarifying question from a freelancer in Albany, New York. This freelancer is curious about the degree to which climate is driving the expansion of these different diseases. It sounds like, with mosquitoes, there’s a lot of influence of climate change and the spread of these diseases borne by mosquitoes. But maybe, Dr. Tsao, is there less of an influence of climate change on ticks?

[00:30:23]

JEAN TSAO: Well, I think it varies. It depends on the tick species, as I said before. But speaking about the reporter in Albany, then there, certainly, Albany, I think, in the past 10 years and 20 years, has seen more blacklegged ticks. And they’ve been increasing from the east and probably from the south. Depending on where you are. In the mountains, in the Adirondacks there, perhaps as climate change may have helped influence where the ticks can live higher up in the mountains and such. But besides having warmer temperatures, ticks really dry out easily, so it has to be more moist up there, as well, right, can’t be really cold and dry up in the higher mountains. Climate change can certainly, I think, influence, or augment, the effect of land-use change in those areas, but it might not be the most important. That’s what I would say, in some of those areas. But as you get closer to Canada, then it might become more important, certainly.

[00:31:29]

ERIN MORDECAI: Can I add a little bit to that answer, too?

[00:31:31]

JEAN TSAO: Yeah, go ahead.

[00:31:32]

ERIN MORDECAI: Along with those really important points. I think, one thing to think about is that any mosquito or tick has its own, what you could think of as a climate envelope, the kinds of conditions that it can live in, that it can do well in. So it needs at least a minimum temperature and a minimum amount of humidity, but that differs across different mosquito species and different tick species and different pathogens that they can transmit. And so whether or not you see a really big impact of climate change is also going to depend on how close you are to those limits.

If you’re in South Florida where it’s really hot all the time and it’s really humid all the time, you may not need that much climate change to start being able to see dengue and malaria, and you may not pick up that strong of a signal of climate change. Compared to somewhere like, I mentioned the case of West Nile in New York, where, historically, winter temperatures have really kept the mosquito populations at bay and limited that transmission season. As you start to see those climate limits being lifted, as the climate is really increasing, that’s a situation where you really start to see large impacts of climate change happening very rapidly.

But there’s also interactions with other limiting factors. So it’s not just temperature, it can also be moisture or breeding habitat, or even just whether or not there’s good vector control, like Lyric mentioned, as well. that varies a lot from place to place. So climate change is going on in the background, and you’re going to see more or less importance for different diseases in different places. It doesn’t mean it’s not important, it just means that it can be difficult to detect the impacts of climate change depending on how close you are to the climate limits.

[00:33:02]

JEAN TSAO: Right. And I would say, for example, I mentioned how ticks dry out easily. So the blacklegged tick population, there’s definitely areas where they’re really large, down in the southern U.S., but they tend to be a little bit more spotty. And some of that is that in some of those areas, they’re drier, even though there’s a lot of humidity in certain areas, but certain landscapes aren’t as good. And so, with warming temperatures, if some of the areas in the south, for instance, don’t have as much precipitation, maintain their humidity, actually, the tick ranges down there can contract. So it’s the opposite end, just like Dr. Mordecai was saying about this climate envelope for that particular tick. And the lone star tick, also prone to drying out. So its a little more complex.

[00:33:54]

SARA WHITLOCK: That makes a lot of sense. Yeah, lots of different factors, it sounds like, going on with all of these different climate-related shifts. And while we’re a little bit on the topic of ticks and their diseases, we have a question from KOPN Public Radio, in Columbia, Missouri, asking if you can address the biological or biochemical mechanism behind Alpha-gal.

[00:34:13]

JEAN TSAO: I’m not an expert on that, but what I can say is from what I understand, it is that the saliva of several tick species, but it seems most predominantly with the lone star tick, that there’s a molecule in there that mimics this Alpha, and long word, but a sugar side-chain on non-primate mammalian meat. So not humans, not other monkeys and such, but, let’s say, pigs, cows, deer. They have this sugar side-chain, and something in the saliva of lone star ticks, looks like it. But people don’t know why it is that, for example, not everyone develops this red meat allergy. So I would assume that there are probably some host factors, just like not everyone develops a hay fever allergy or a bee sting allergy, but there may be something on the host side, and maybe there’s something on the on the tick side, too, that we just don’t know. But around the world, there are some other tick species associated with this allergy. In Europe, it is the cousin tick to the Lyme disease tick that’s here. And some people think that blacklegged can also be associated with this allergy.

[00:35:36]

SARA WHITLOCK: Thank you so much. I want to zoom back for a minute for a good, bigger picture question. This is from science versus- a common question is, Can’t we just get rid of all ticks and mosquitoes? So the reporter is wondering, do ticks and mosquitoes have positive impacts in the ecosystem? Are there unintended consequences of trying to really control these populations? Maybe, Dr. Mordecai, I’ll let you go first, and then we can go around the circle.

[00:36:01]

ERIN MORDECAI: That’s always a fun thought experiment. If you could wave a magic wand and remove all the ticks and mosquitoes from the world, would that be a good idea? I mean, I wish such a magic wand existed, but I think that an important nuance to that is that it’s not all the ticks and all the mosquitoes that are important. There’s a handful, like Dr. Tsao mentioned, there’s a handful of medically important tick species. There’s a handful of medically important mosquito species, maybe five of each. If you eliminated those five, you could really eliminate the vast majority of the world’s vector-borne disease.

So I don’t think we need to eliminate all mosquitoes, just the species that transmit these diseases. They often tend to be more human-associated species, particularly for mosquitoes. They like to breed in human made containers, for example. There are animals that eat mosquitoes, either as larvae or as adults, like birds and bats can eat the adults and other insect larvae that are in the aquatic environment, or tadpoles, can eat mosquitoes.

Often in places with higher biodiversity, we do see some limits on the abundance of mosquitoes because those other species are keeping them in-check. But that said, I think if you could choose those top five mosquitoes that are responsible for transmitting the most of the human disease and wave a magic wand and eliminate them from the earth, I think that would be worth the trade off. But I’m interested to hear others’ point of view on that.

[00:37:19]

SARA WHITLOCK: Dr. Bartholomay, I’m curious if you have more you want to add with those mosquitoes and their positive roles in the ecosystem.

[00:37:27]

LYRIC BARTHOLOMAY: Uh-huh. Well, as I mentioned, there are these fundamental needs that mosquitoes have, and one of them is that they need sugar. And so they actually visit nectar sources, and they may be providing ecosystem services in places where there might not be other pollinators. I have done some work in northern Wisconsin, and in the springtime, there are these mosquitoes that emerge well before, like anything else, the larvae can actually live under a sheet of ice in these ephemeral pools in the woods. They emerge when it’s really cold and there are actually some flowers out, that I can’t imagine are pollinated by anything else but a mosquito. We know that this is true for some orchid species and other things.

So they play these ecosystem services, like Erin mentioned, right, like feeding other things and potentially doing this pollination work, too. So I think if we were to get rid of them, there would probably be unintended consequences. There’s a study that was done, that showed in a place where mosquito larvae were really well controlled with a totally non-toxic larvicide, just killed the larvae, that it had impacts on health and fledgling of birds. They need that protein source in order to thrive. So I shudder to think about ridding the world of all of them.

[00:38:51]

SARA WHITLOCK: That makes sense. Anything to add with ticks, Dr. Tsao?

[00:38:55]

JEAN TSAO: I think, with ticks, I can’t think of a good examples such as the pollinators of the mosquitoes with regard to how they might be benefiting the ecosystem so much. I mean, as we know, these ticks are expanding in ranges now. So I guess in part, we’d have to see, is there any beneficial or negative effect. We do know, for the most part, if there are effects of these ticks on regulating, let’s say, the host communities, they must be subtle. We don’t really see as much, but lots of other factors are changing at the same time. So it’s really hard to tell. Certainly, ticks can serve as food. Oftentimes, people talk about the oxpeckers in Africa, feeding on the ticks on the buffalo and such. But for some organism to really benefit from the ticks, it has to make up a large part of their diet. And so, I think for the most part, right now, that would be if there are some organisms, fungi or something like that, other arthropods, that really benefit from these ticks, that’s something to be discovered.

[00:40:02]

SARA WHITLOCK: That makes sense. Thank you.

[00:40:03]

JEAN TSAO: Well, as parasites, they might promote social bonding behavior amongst primates. So maybe that’s a benefit.

[00:40:14]

SARA WHITLOCK: That’s great. We have a couple of questions about mechanisms of control, now that we’re on the topic of should should we get rid of all of them. Sounds like, No. But a question from a freelancer in New York City, I’ll start with, is less about eliminating, but more about can you weigh in on the idea of mosquito repellent clothing? Does it work? Is it a good idea? Any thoughts, Dr. Bartholomay?

[00:40:39]

LYRIC BARTHOLOMAY: Sure. So our field teams, both in my lab, and I think in Dr. Tsao’s lab, too, are spending their summers wearing those clothes. So we can either purchase the clothes, already with the insecticide embedded in them, or we can spray them after the fact. And so they’re quite effective at keeping the ticks off of us and also repelling mosquitoes. We know that those are are really well used by our military as well, and are an important part of the arsenal of protecting people from the types of diseases they might get when they’re deployed.

[00:41:23]

JEAN TSAO: And I’ll jump in and say, I put up this tick suit behind me. In some ticky areas, we have that, and they are then sprayed with permethrin, right, but I give some to my students, all of them to use. But they are hot. You can’t see all the way down to the legs. So I can imagine, not just they, but other people may not want to wear long sleeve, long pants. And so then therefore, we say, Yeah, you should then use an EPA registered repellent for your exposed skin, and such. But all these measures are to help reduce the risk of getting a mosquito or a tick bite.

[00:42:02]

ERIN MORDECAI: I was just in Costa Rica on a mosquito sampling trip, and, of course, we were very concerned with getting bitten by mosquitoes that might give us dengue. And we found that if you go to a sporting goods store, there’s a lot of really lightweight clothing that are permethrin-impregnated, so you can buy it already with that, or you can douse your clothes in it, which is really nice. And that, combined with DEET on any of our exposed skin, did a really good job protecting us. The worst hazard we got was when I accidentally stood in an anthill and the ants got all over me and bit me. Apparently, they were not sensitive to the permethrin. Or at least it didn’t keep them from walking on me. Yeah, I think those things are very effective.

[00:42:40]

SARA WHITLOCK: That’s great to hear. So moving from controlling these insects when they’re around us, to our lawns. We have a question from The Day in New London, Connecticut, and that question is, What impact, if any, does the widespread application of lawn treatments that are supposed to kill ticks actually have on tick populations? Do they work? Is there any killing of non-target insects, and does that impact biodiversity? So, Dr. Tsao, I’ll give that one to you.

[00:43:03]

JEAN TSAO: I will talk about it, and then I’ll pass it to Dr. Bartholomay. But as far as I know, there actually aren’t very good studies looking at how your commercial pest control operators who spray, or apply, just regular pest control, I guess, products on lawns, how well they work overall in efficacy in the real world. Certainly, there are researchers who have applied these compounds, and there is an effect. But the question is whether or not the actual applications, the way they’re applied, like Dr. Bartholomay talked about earlier, about mosquitoes, how well they work. And with regard to different products, yes, there are people developing greener products to use. And so I’ll pass that to Dr. Bartholomay.

[00:43:58]

LYRIC BARTHOLOMAY: I’ll tag off of the first point that you made, which was that there aren’t good published studies out there on how well those control programs work. So one of the things we’ve done as a Midwest Center of Excellence for Vector-Borne Disease, is asked that exact question. And a master’s student who’s working here at UW-Madison, went out and followed one of the major pest control companies in the U.S., in backyards in northern Wisconsin, to ask whether their treatment protocols work.

So we had that question, Is it just how researchers deploy the chemicals or can the technicians who are working for the companies spray it and control the ticks. And we saw, actually, a really good reduction in the number of the nymph stage of blacklegged ticks, which is sort of the teenager stage of the tick, that’s most important in terms of public health and transmission of the pathogens that can be transmitted by that tick. And so we did see that it worked quite well to reduce the numbers of that life stage of the tick. That was with a a synthetic pyrethroid. So that’s an insecticide that’s based on a compound that’s produced by. So it’s synthetic, but also has its roots in natural chemistry.

We’re also really interested in the possibility that there might be green chemicals or natural things that people could use in their yards to control ticks. And so one of the things our lab is doing right now, is to look and see what products are available that are sold as botanicals and natural products, and how well do they really control the ticks. We haven’t yet found one of those that works as well as the more traditional insecticides, but we’re hopeful, and designing and doing those studies to ask that very question.

[00:46:00]

SARA WHITLOCK: That’s really interesting.

[00:46:01]

JEAN TSAO: The question that the reporter raised, though, however, it is the case that thus far, these compounds will not target only ticks. So then it becomes, and the pest control operator community knows this, they try to administer these chemicals in a way that can minimize the non-target effects, but it’s not going to not target those other animals. And then the other thing I would say is that there was one large study which showed that there can be a large reduction of, I think, 60%, maybe, reduction in blacklegged nymphs, as Dr. Bartholomay was saying, along that ridges between properties and the woods, this was in Maryland, New York and maybe one other state. But they didn’t really see a reduction in the number of people that got bitten by Lyme disease ticks or infected with Lyme disease.

So there is still a little part of the question, Where do people actually pick up these ticks. And so when I answered your question from the reporter, it was about products treated to the lawn. Generally speaking, lawn is not necessarily the best place for these ticks. That’s why those people before, in that study, they applied these chemicals to the ecotone, or the edge between the lawn and the woods, which is where most of the ticks are coming in from the woods. But a colleague of ours from Minnesota would say, Well, the ticks don’t necessarily read the handbook. There are ones that are on the lawn. And even if there are fewer of them in the lawn, because that’s not their best habitat, if people spend more time in the lawn, then actually along that ecotone, those become the more dangerous ticks. And so that is still, then, a question out there with regard to can we eliminate those ticks that are in the areas that people recreate in their immediate backyard.

[00:47:58]

SARA WHITLOCK: That’s interesting.

[00:47:59]

LYRIC BARTHOLOMAY: I could also just add that there’s a big difference between what the professional pest control operators do in backyards and what the larger organized agencies do. In the case where you’ve hired a company to come to your backyard, they’re using a backpack with a spray, but the control districts use airplanes and trucks and other things to deliver the pesticides. Those work differently and maybe have different impacts on biodiversity and non-targets or pollinators because they are sprayed in very, very teeny droplets, and they’re only sprayed at night. So maybe there’s less environmental impact of mosquito, and well, just mosquito control in those communities where the spraying is being done that way.

[00:48:57]

SARA WHITLOCK: And a quick follow-up on the question of all these different types of sprays they will be using to control mosquitoes from KOPN Public Radio in Colombia. What are some of these chemical compounds that are being used? You mentioned some that come from chrysanthemums. I know there’s some more traditional ones, if you could say a few more words about that, Dr. Bartholomay?

[00:49:14]

LYRIC BARTHOLOMAY: Though the vast majority of products that are used are these synthetic pyrethroids, which are the chrysanthemum-like compounds, and so there are a lot of different named brand products that people use that would have some type of pyrethroid in them. And some places will use other kinds of organophosphates, like malathion and naled. But I think in a lot of places, the products would be those pyrethroids.

[00:49:45]

SARA WHITLOCK: Great. Thank you. And so I want to circle back to a little bit broader scale questions. We have a question from Minnesota Public Radio News about, How has 2025 ranked in disease outbreaks compared to previous years? Or if that data isn’t available yet, how did 2024 compare? So, Dr. Mordecai, maybe you can jump in on this one.

[00:50:04]

ERIN MORDECAI: Well, as was mentioned earlier, July, August, September, are the time of the year for West Nile in the United States. So we haven’t quite seen that yet. We don’t quite have the data yet. Although, as Dr. Bartholomay suggested, it’s looking like maybe it’s going to be a very high West Nile year. In terms of dengue transmission around the world, 2023 was the largest dengue year on record. With 5 million cases reported in the Americas, followed in 2024, 10 million cases reported in the Americas.

So we saw this huge, exponential increase in dengue transmission. Part of that we think has to do with climate and increasing climate suitability for dengue. Part of it probably also has to do with the COVID lockdown years and a rebound after that, as people started moving around again and leaving their homes again. But once one of these really large dengue outbreaks gets set off, it really spreads throughout the entire Latin America, Caribbean region, as well as Southeast Asia and South Asia, as well as Africa and other parts of the world. That’s when we also start to see imported cases and even local cases in places like North America and Europe.

So we don’t know yet for 2025, whether it’s going to be as big as 2024 or whether we’re going to see an abating. It’s not just climate that drives these outbreaks. It also has to do with which strains of the virus are circulating and how susceptible the population is to those virus strains. We often see these multi-year cycles where you’ll have a really big outbreak year, followed by a few smaller outbreaks, followed by a big one again. So we don’t yet know about 2025, but 2024 was definitely the largest on record, at least for dengue.

[00:51:42]

SARA WHITLOCK: Related to dengue, we have a question from a freelancer in Albuquerque, New Mexico, who says that, In recent months, I wrote a project about the threat of dengue fever in a changing climate, and a doctor reached out to criticize that he’s saying that dengue fever and other mosquito-borne illnesses were a drop in the bucket, say, compared to the flu or something else like that. So do you have any advice for how to respond to people who are minimizing the public health impact of tick and mosquito-borne diseases?

[00:52:05]

ERIN MORDECAI: Yeah. I mean, I think that’s a great question. I’m really glad the reporter asked that, because maybe dengue fever, in particular, doesn’t seem like that big of a threat because we’re not very familiar with it in the United States, but our best estimates are that about 60 to 100 million cases happen every year, so it’s widely under-reported. But dengue fever is nicknamed, breakbone fever. People who have had it often say, It’s the sickest I’ve ever been. I was in so much pain. I had horrible joint pain, fever. And that’s just for regular dengue fever. It can also progress to hemorrhagic fever or even other types of really serious complications that can be life threatening.

Just because we’re not familiar with it in the United States doesn’t mean that it’s not a large threat worldwide. In the United States, we don’t really have immunity to dengue fever, we haven’t been exposed much before. So if it does start to increase and you’re unlucky enough to be the one that gets it, locally transmitted in your state, it’s a really painful and really trying disease. And likewise, for West Nile, we can see really severe cases.

We saw Tony Fauci hospitalized with West Nile, last year, for example. That’s just one famous example of someone who had a really severe case, and he said it was the sickest he had ever been. So these diseases really can cause some severe consequences. And maybe Dr. Tsao can speak more to the consequences of Lyme disease and other tick-borne diseases. But many people have long lasting consequences of those diseases also. So yes, even though we have a flu season every year, and there’s other pathogens that circulate, these are really important emerging threats. And if you live in a place that’s endemic with these diseases, they can really cause large burdens of disease.

[00:53:45]

SARA WHITLOCK: That makes sense. Anything you want to add related to ticks, Dr. Tsao?

[00:53:48]

JEAN TSAO: Yeah, I would just say that the numbers of cases of Lyme disease have been increasing. The ones reported to the CDC, it was a big jump. I’m looking at the graph from 2023. And to the reporter, I would just say, because of the way Lyme disease and other tick-borne diseases are reported, we’re always about a year and a half to two years behind. But if you also look, CDC has this tick tracker database. I’m pulling data from the emergency visits with regard to cases coded for ticks. This year is a big one in some areas in the Midwest, it’s so far the largest over, let’s say, since 2017. In the Northeast, it seems like 2017 was still higher.

But, yeah, there are just more and more cases of Lyme disease and tick-borne disease, and in part, it’s because of the expanding range of these ticks. And I would say that Lyme disease, right, it can cause some longer lasting symptoms, hopefully with greater knowledge and outreach to both the public and physicians, healthcare workers, we can nip the disease earlier in the bud, or prevent it in the first place, because the sooner you get treated, the more probable that you won’t have longer sequelae or clinical signs occurring afterwards.

But there are some other diseases that are rarer, but that can have much greater consequences on health, such as Powassan virus, and also from the lone star tick, there’s Heartland virus and Bourbon virus. And so, as Dr. Mordecai said you just don’t want to become infected. You just don’t want to tick bite, but you don’t want to become infected with any of these tick-borne diseases.

[00:55:39]

SARA WHITLOCK: Thank you. And we have one more question for our experts, which will give all of them a chance to quickly cover the most crucial insights that we’ve shared in this briefing. But first, I want to flag for journalists that you’ll receive a short email survey when you sign off of this briefing, and if you could take even 30 seconds to give us the feedback that you have, it would help us to design these services to give you what you really need for your reporting. So for our final question, in about 30 seconds, what is one take-home message for reporters covering tick and mosquito-borne diseases? And, Dr. Bartholomay, I’ll start with you.

[00:56:10]

LYRIC BARTHOLOMAY: I think I’ll just reiterate that there are some knowns that we can predict and that these are hot topics to report on. There are a lot of really great people that you can reach out to to get information on mosquito and tick-borne diseases, in virtually every state. And it’s good timing if you’re in the right places to think about May, and June, July as times when you might want to really get out the message to prevent mosquito and tick-borne illness.

[00:56:42]

SARA WHITLOCK: Great. And, Dr. Tsao.

[00:56:44]

JEAN TSAO: So what I would say, and this addresses someone’s question, perhaps, a Ms. Amy Mathews Amos, I would say, in addition to the articles that would help prevent tick and mosquito-borne disease in real time, I would say, besides those, to try to get at the root causes, which really happened to be how humans are changing the environment, if there can be other stories, articles, to investigate how, for instance, municipalities decide to allow farmland or rural areas to become parks or to become subdivisions, what coordination is there between that part of government and then then DNR, the agencies that are managing wildlife.

And then to think about those changes in the context of how they could cascade down to affecting human health and such that we can try to understand, more holistically, these interactions, then perhaps through journalism and informing the public, then perhaps people can be more involved to try to come to some solution. We need everyone’s input of people, not just vector biologists, public health folks, but others to really try to tackle this problem.

[00:58:07]

SARA WHITLOCK: Thank you. And Dr. Mordecai.

[00:58:09]

ERIN MORDECAI: I would leave you with a big picture concluding statement that, Climate change is really profoundly affecting human health. And we might be more familiar with some of the impacts on things like heat waves or hurricanes or extreme flooding or droughts or wildfires. But increasingly, we’re recognizing that climate change is affecting infectious diseases as well. And some of these mosquito and tick-borne diseases are ground zero for climate change starting to affect our health. Diseases that were formerly, mostly elsewhere, are now increasingly becoming common here, in the U.S. Depending on where you live and where your audiences live, West Nile virus is actually pretty common around the entire United States, so that’s a threat everywhere. Tick-borne diseases, like Lyme disease, are particularly common in the Northeast and upper Midwest, and that’s where Lyme disease will be an important story to cover. And then emerging diseases like dengue or Zika or even malaria are going to be more important in your southern states like, Florida, Georgia, Texas, Louisiana, California, Arizona. So I think also looking geographically at what the largest mosquito and tick-borne disease threats are in your area can really help you to expand the coverage and help people protect themselves and become informed so that they are less likely to be exposed.

[00:59:22]

SARA WHITLOCK: Thank you so much. So I want to say a huge thanks to the panelists here, for sharing so much information and wisdom, as ticks and mosquitoes are increasing in their prevalence, and the season is underway, and related diseases are on the rise. From all of us here at SciLine thanks to all the journalists who logged on to gain insight and contexts that will enrich your coverage. So I hope we’ll see all of you at our next briefing. Thank you.