Climate Change Bites: The Proliferation of Vector-Borne Diseases

According to the CDC, the spread of vector-borne diseases (those spread by blood-feeding bugs like mosquitos, ticks, and fleas) is linked to climate change. Rising temperatures and humidity influence breeding rates and can extend the range of disease-spreading bugs, bringing diseases to areas that have never seen a case. What are the ways that climate change can influence the spread of disease? How can we best track this spread to get ahead of it and avoid worse impacts? To find out what we need to know about the relationship between vector-borne diseases and climate change, we sit down with two experts in the field: Dr. Erin Mordecai, professor of biology and senior fellow at Stanford's Woods Institute for the Environment and Dr. Manisha Kulkarni, professor in the School of Epidemiology and Public Health at the University of Ottawa.

Follow us on Twitter, LinkedIn, Facebook, and Instagram.

Contact us at contact@climatenow.com

Visit our website for all of our content and sources for each episode.

Transcript

James Lawler: [00:00:00] Welcome to Climate Now, I'm your host James Lawler. Today we're speaking with two infectious disease experts whose research explores the relationship between climate change and vector-borne diseases. The World Health Organization, WHO, defines vector-borne diseases as "human illnesses caused by parasites, viruses, or bacteria that are transmitted by vectors." The WHO describes vectors as living organisms carrying infectious material, pathogens, between human hosts or from an animal host to a human host. For example, vectors include mosquitoes, ticks, flies, and other biting insects. Today, more than 6 billion people live at risk of vector-borne diseases, and there are more than 300 million vector-borne disease cases and about 700,000 vector-borne disease deaths every year. In our conversations today, we'll explore how global temperature shifts influence the distribution of vectors like ticks and mosquitoes across North America, along with the various roles urban and natural environments play in the spread of these diseases. To shed light on [00:01:00] how changing temperatures are shifting the patterns of vector-borne disease transmission, Dr. Erin Mordecai, Associate Professor of Biology and Senior Fellow at  Stanford University's Woods Institute for the Environment, will start off today's episode. After our conversation with Dr. Mordecai, we'll hear from Dr. Manisha Kulkarni, who is Associate Professor in the School of Epidemiology and Public Health at the University of Ottawa, to better understand her work tracking changes in vector-borne disease distribution. 

Dr. Erin Mordecai, welcome to Climate Now.  

Dr. Erin Mordecai: Thanks for having me.  

James Lawler: Erin, if you could tell us about your background and what you currently do.  

Dr. Erin Mordecai: I am an ecologist who studies how human impacts on the environment are affecting infectious diseases. What that means is studying how changes in the climate, changes in habitat, changes in deforestation, and human movement around the world are affecting the spread of mostly infectious diseases, but also how they're affecting biodiversity and ecosystem function. 

James Lawler: [00:02:00] So, for someone who hasn't thought about this at all, explain why climate change should have any impact at all on disease. I think we normally think of these things as two very different sort of challenges.  

Dr. Erin Mordecai: I think a lot of times when people think about the impacts of climate change, they think about species that are kind of the poster children, like polar bears being stuck on tiny icebergs that are melting. But, you know, not just polar bears, but every other organism on earth has its own set of environmental requirements. And, in particular, for things like mosquitoes, which can transmit lots of different parasites that cause diseases like malaria, or dengue, or yellow fever, or West Nile, those mosquitoes are small bodied and cold-blooded insects. So their body temperature, unlike ours, matches the temperature of the environment all around them. Now, because mosquitoes are cold blooded and their body temperatures are constantly changing with the environment, the rates at which they have their metabolism, how often they need to feed, how quickly they're able [00:03:00] to develop and complete their life cycle, and ultimately their capacity to transmit parasites is dependent on the temperature of the environment. So these are kind of a unique class of pathogens because they have this environmental component of the life cycle that takes place in the mosquito, so that's the key link between climate and this particular group of infectious diseases transmitted by mosquitoes or ticks or flies, which are called vector-borne diseases. 

James Lawler: So your work is really focused on figuring out how sort of the patterns of vector-borne disease might change under different climate scenarios. So, what might we see in terms of diseases that are mosquito borne and perhaps give us some examples beyond mosquitoes.  

Dr. Erin Mordecai: We'll start with mosquito borne diseases. When you think about mosquito borne diseases, you might think of malaria. At the beginning of the time when people were starting to think about the biological impacts of climate change, the immediate fear was, oh, climate change is going to make malaria much worse because we know that that mosquito life cycle is dependent on temperature. And we know [00:04:00] that it takes a while for the parasite, once it's ingested in a blood meal, the parasite, you know, the mosquito is not just a flying syringe that injects parasites into people. What it actually does is the parasite has to break through the mid gut barrier of the mosquito, disseminate throughout the body, and eventually bind to the salivary glands and that's when it gets injected into the next host. So that process actually takes days to weeks, depending on the temperature. So, 

  when the temperature is warmer, we know that that incubation process is faster, and so mosquitoes are able to transmit malaria at a higher rate. But that doesn't just continue indefinitely as you warm the temperature, because eventually mosquitoes start dying. So once you exceed temperatures of, say, 28 degrees Celsius,  for a malaria transmitting mosquito, those temperatures are becoming highly lethal, and so those mosquitoes don't live long enough to really transmit malaria at very high rates.  

James Lawler: For our U.S. audience, 28 degrees Celsius is equivalent to about 82 degrees Fahrenheit. 

Dr. Erin Mordecai: So what we would expect there is that places that are currently kind of on the edge of [00:05:00] temperature suitability, think about places like highland areas in the tropics, those are places where climate change is likely to push malaria higher in elevation into new populations that historically hadn't dealt with malaria. We might also start to see expansions of malaria transmission in more temperate zones. We've seen some cases of malaria in the United States, for example, this year. But it's important to know that that's a really complicated process because historically we've also had malaria in these regions and we've been able to eliminate malaria through controlling the mosquitoes in the environment. So it's not just temperature that's the only driver. But, we can expect to see increases in malaria transmission, particularly in those cooler areas that are at the fringes of where we have malaria now, but at the same time, what our research has shown is that the optimal temperature for malaria transmission is 25 degrees Celsius, which is 78 degrees Fahrenheit. So that's not actually that hot of a temperature. Most places that have a lot of malaria now are already at that temperature or above. So [00:06:00] further climate warming in already malaria endemic places for the most part is actually not expected to increase malaria transmission.  

James Lawler: And may do the opposite?  

Dr. Erin Mordecai: It may do the opposite, but it's important to know that different mosquitoes have different temperature preferences. So the mosquito that transmits dengue and yellow fever and Zika and chikungunya viruses is called Aedes aegypti. It's a much more urban mosquito vector, so it breeds in urban environments and it has a warmer thermal tolerance. So  it's optimum for transmission is around 29 degrees Celsius or 84 degrees Fahrenheit. And so climate warming is pushing more and more of the globe into high suitability for transmission of diseases like dengue and Zika and chikungunya. So thinking about the mosquito life cycle, it's really this balance between how quickly the mosquito is able to get through the parasite incubation cycle versus how much temperature starts to be detrimental for the mosquito itself and its survival and its biting. But if we shift gears to think about tick borne diseases, diseases like Lyme disease or Rickettsia, Rocky Mountain Spotted [00:07:00] Fever, these are transmitted by ticks. Ticks have a very different life cycle than mosquitoes. For the most part, they go through several year long cycles of development. So they'll feed once on an animal and then wait a year before feeding again. So this is a very different cycle of transmission. And tick-borne diseases rely on what we call zoonotic reservoir hosts, or basically animals that carry pathogens. So for a tick-borne disease to be transmitted, the tick has to bite an animal that's carrying that pathogen, go through a year of development, and then bite a human and transmit to that human. So there's a very different set of processes involved, and a lot of the key limiting factors on the tick-borne disease transmission cycle are things like, can that tick survive the winter or can it develop quickly enough that it's able to have a stable population given the temperature. So that's why we've started to see tick borne disease range expansions into places like Canada, where historically tick-borne diseases like Lyme disease weren't present because the winters were too cold to allow the tick to complete its life cycle. [00:08:00] Now the winters have warmed up to the point where stable transmission can occur in Canada. And so that's where we're starting to see that fingerprint of climate change driving expansions of Lyme disease.  

James Lawler: What worries you most when you look at the spectrum of diseases and their sort of, their characteristics, which diseases are scariest to you in the context of a warming climate? 

Dr. Erin Mordecai: Well, I think right now, probably the viruses that are transmitted by Aedes aegypti are a really big concern. It's often called the yellow fever mosquito. So it can transmit yellow fever, it transmits dengue, it transmits chikungunya, it transmits Zika. The reason those are worrying to me, there's a few reasons. One is that they do have that higher climate optimum, that higher temperature optimum, so they're transmitted better at high temperatures. The mosquitoes that transmit these viruses are also globally expanding. There's Aedes aegypti and it's so-called sister species, Aedes albopictus, which are invasive around the world, including in temperate parts of North America and Europe, as well as Asia, Latin America, [00:09:00] Africa. So almost every continent has this mosquito already. And right now the temperatures are limiting for transmission, but these viruses, like dengue, have been really growing exponentially throughout the tropics and subtropics over the last 30 years or so. So what's particularly worrying is these mosquitoes can transmit a lot of other viruses that haven't yet had these major worldwide outbreaks, but very much could. 

James Lawler: The WHO reports that vector-borne disease accounts for about 17 percent of total infectious diseases and results in over 700,000 deaths globally each year. Meanwhile, the National Institutes of Health reports that vector-borne diseases are on the rise. My question is, can you attribute the growing case count for vector-borne disease to a changing climate, or is it a much more complex set of factors here? 

Dr. Erin Mordecai: I think the case for attribution in vector-borne disease is hard because there's a lot of factors involved. I think climate change is certainly playing a role already in the transmission of diseases like dengue, and it's going to play an even bigger role [00:10:00] as climate change continues to accelerate. But I think the other roles that are really important are urbanization, so the mosquito that transmits dengue can breed in urban environments, and it can breed in human made containers as small as a bottle cap. So imagine any tiny little plastic container that can hold water. That's Aedes aegypti mosquito breeding ground.  

James Lawler: What would make an urban environment more hospitable to this particular species of mosquito? 

Dr. Erin Mordecai: This mosquito has a strongly evolved preference for breeding in containers, as opposed to kind of natural habitat like puddles or ponds. It also has a really strong preference for biting humans over biting other animals. So urban environments provide this perfect setting, especially urban environments with poorly planned infrastructure, so maybe they don't have good sanitation service or they don't have good, reliable piped water. These types of settings are really good for this mosquito in part because it doesn't face as much competition with other mosquitoes and it doesn't face as much predation. There's not sort of like an intact ecosystem in place that could help to limit the growth of this mosquito. And it's a mosquito that's [00:11:00] so strongly preferring humans that, you know, humans are highly accessible in this type of environment. So when we think about transmission of dengue by Aedes aegypti, it's really this kind of socio environmental process where you have human dominated habitats that really favor this mosquito over other mosquito species that are more common in forest environments. 

James Lawler: And I imagine as urbanization spreads and climate changes, different animals interact and that could result in new pathogens. Is that right?  

Dr. Erin Mordecai: One of the most challenging problems in this field is what are the unknown pathogens that are already out there that, given the right place and time, could end up emerging and causing pandemics? West Nile virus is actually an example of this. It's called West Nile virus because it's from the Nile region of Egypt. It's a virus that's transmitted by mosquitoes, and it doesn't really go from human to mosquito to human. It's really what we call a zoonotic virus, which means it goes, mostly circulates within animals, wild animals, that then occasionally spills over into people. West Nile [00:12:00] virus, for reasons that aren't really totally known yet, it got introduced to New York City, of all places, in 1999), and it got established in the mosquitoes and in the bird populations. It killed off a lot of birds, and then those birds were basically kind of incubators for the virus, you know, the mosquito would feed on the bird, it would acquire the virus, and then if it fed on a human, the human would get West Nile virus. And from 1999 onward, we've seen West Nile sweep across the United States. It's now in, you know, most of the contiguous United States and it does continue to kill birds, but it also cycles. It's become endemic in wildlife populations, in birds. So now we have this virus that cycles between birds and mosquitoes and humans, and it can be a very severe disease. It can cause what we call neuroinvasive West Nile disease, which can be really severe and even fatal in some cases, so, like many of these diseases, we only really see the most severe cases, but actually, if you look at the fraction of the population that's been exposed to West Nile virus, in [00:13:00] some places it's like 20 or 30 percent of people. So, it infects a lot of people and it can cause really severe disease. This is a case where a virus, you know, through this really sort of random, haphazard path made its way from Egypt to New York City all the way across the United States and now it's endemic in our bird populations and it's a conservation threat to birds as well as being a health threat to people. So it's kind of this feedback between the environment and mosquitos and human health.  

James Lawler: So it seems like there's a lot we don't know, but climate change does compound factors like urbanization, which increases disease transmission. What preventative measures can we take to help slow the spread of these vector-borne illnesses? 

Dr. Erin Mordecai: Well, I think there's a lot we can do on multiple fronts. The first front that's very important is climate mitigation, climate action, controlling climate change so that conditions don't just keep becoming more and more suitable, especially in temperate zones, for mosquito-borne disease transmission. The second one is, you know, eliminating land use transitions like [00:14:00] deforestation that can create places where vector-borne disease transmission can occur. That's another frontier where you tend to see a lot of vector-borne disease transmission is at these places where land use is being converted from forest to agriculture or from agriculture to urban. So incorporating knowledge about vector-borne disease into our planning when it comes to land use is important. Mixed surveillance, you know, really testing and tracking vector-borne diseases, making sure that cases can be detected and reported, doing regular vector surveillance to see what pathogens vectors are carrying, is also really important because it can kind of give us an early warning when a new pathogen is emerging, hopefully even catch it locally before it becomes a nationwide epidemic like West Nile, for example. And then I think, you know, the, the vast majority of the burden of this problem is going to fall in places that are already underdeveloped and under resourced and, and kind of still suffering the harms that were caused by colonialism. So, I think investing in equitable infrastructure in places, [00:15:00] especially where there's large amounts of unplanned urban development, so thinking about building higher quality housing, access to piped water that's reliable, and sanitation services that are reliable. Lifting people's quality of life overall and providing sustainable livelihoods and sustainable housing I think will also go a really long way to reducing the vector-borne disease burden because it'll reduce people's exposure to mosquitoes and reduce the amount of habitat that's available for mosquitoes. So combining that kind of investment in social infrastructure, as well as surveillance, as well as kind of reducing the hazard that's presented by climate change and deforestation in general, I think all of those things together are the most important things we can do.  

James Lawler: Next, we speak with Dr. Manisha Kulkarni about her ongoing surveillance research tracking infectious disease cases. But before we do, just how impactful are these diseases today? Malaria is cited as the deadliest vector-borne disease in the world, and claimed an estimated 620,000 lives in 2017, with a majority of those [00:16:00] deaths concentrated in Africa. Dengue is the second deadliest vector-borne disease, and killed roughly 40,000 people globally in 2017. While vector-borne diseases disproportionately impact poor populations around the globe, the spread of these illnesses in higher income countries is also increasing.  

In the United States, instances of vector-borne disease have doubled in the past 20 years, and  the CDC reported that tick borne illnesses, including Lyme disease, increased by 25 percent between 2011 and 2019. Dr. Manisha Kulkarni is here to explain some of the methods that her team uses to track these illnesses within the context of a changing climate and to create predictive models of where and how the diseases might spread to help minimize vector disease risk. Dr. Kulkarni is associate professor in the School of Epidemiology and Public Health at University of Ottawa and is a trained medical entomologist, someone who studies insects. At the university, she directs the Interdisciplinary Spatial Informatics for Global Health, or INSIGHT Lab. Her team combines fieldwork with mathematical models to connect factors like land use, [00:17:00] temperature change, and urban development to paint a realistic picture of where disease will most likely spread. Dr. Kulkarni's research relies on elements of disease, ecology, and spatial epidemiology, which is the study of geographic variations of disease, to account for socioeconomic factors driving the emergence of vector-borne diseases in new and high-risk areas. Here's Dr. Kulkarni.  

Dr. Manisha Kulkarni: So some of my initial research was really done in East Africa, looking at malaria transmission at different altitudes. So by sampling mosquitoes in this case from the environment, testing them to see their infection rates, looking at biting rates and, and density, and from that, we're able to derive measures of transmission intensity to understand what are the factors that are associated with higher transmission rates. So I'm applying similar approaches in my lab at the University of Ottawa, where we're doing a lot of work studying tick borne diseases, like Lyme disease, and also arboviruses like West Nile virus. And on the tick borne disease side, we're doing an [00:18:00] environmental monitoring and surveillance in the field to collect ticks from different locations across the city, across the region of Eastern Ontario, to characterize the environment in detail and to sample the small mammal population so we can see, using statistical models and using geospatial models, what are really the factors that are, that are driving higher population densities, higher infection rates and can we use these types of spatial models to identify hotspots where we need to target more surveillance and where we need to target preventive activities perhaps. So we're doing spatial modeling, connecting the field data with satellite remote sensing data to develop these types of predictive models.  

James Lawler: Is this working? Like, how close are you to have sort of a generalizable process for predicting the incidence of Lyme and other tick-borne illnesses?  

Dr. Manisha Kulkarni: So we've had quite a bit of success in my lab in terms of developing local scale predictive models of tick [00:19:00] distribution. We've used our field sampling data in combination with data from satellite remote sensing sources of land cover. So, different forest types, agricultural land use, climate variables, looking at precipitation, degree days above zero, as well as other types of factors like distance to roads, distance to water, things that may affect host population movement and dynamics. And we've been able to develop these types of models at a quite fine spatial scale, less than a hundred meter by a hundred-meter kind of pixels. And what's important is we've been able to validate them with independent field collected data. So, what it shows us is that there are some areas that are predicted to be highly suitable for ticks based on the climate and the environment. There are some areas that are less suitable for ticks. What's interesting is that not all of the areas that are highly suitable have high tick populations yet. So what we can do is use these types of models to look at areas where we're likely going [00:20:00] to see emergence in the future. And that's where we can start to do better surveillance, start to monitor and start to build capacity in terms of public knowledge, healthcare provider knowledge, on tick borne diseases. So they paint a nice visual picture of where we are likely to have risk of these tick-borne pathogens in the future.  

James Lawler: Yeah. And for North American listeners, how do you see vector-borne diseases spreading over the next decade or so as we face increasing changes from the changing climate?  

Dr. Manisha Kulkarni: We are likely to see the ongoing northward spread of different vector species because of the warming climate at higher latitudes. We're likely to see intensification of transmission of tick borne and mosquito borne diseases in regions where they're already occurring because of the longer warm season, the shorter winter, and the warmer summer. And we're likely to see the emergence of new vector-borne diseases because of new vector species that are able to [00:21:00] survive in our climate, and the introduction of new pathogens. So, we were already seeing species like Aedes albopictus, which is a mosquito species that can transmit a number of different viruses, including dengue, chikungunya, Zika virus. We've seen it move northward in the United States, and be detected in southern parts of Canada. We've seen malaria cases emerge in the United States with local transmission. So, with warmer temperatures, with changing environments that create vector breeding sites and modify the way that humans interact with the environments and are exposed to mosquitoes and ticks, we're likely to see the ongoing increase in vector-borne diseases and, and in their importance in terms of public health in the years to come. 

James Lawler: So, how bad do you think it will be? I know that, you know, a focus of your work is prevention and education on this topic, so I'm really just trying to understand how worried should communities be about this? Is this like, this is going to be sort of [00:22:00] pandemic-scale type outbreaks that are going to lower the average life expectancy in the Northeast in a significant way? 

Dr. Manisha Kulkarni: I think what we can expect is that we'll see increasing epidemics in terms of their frequency and their intensity, of things like West Nile virus, which we do see year to year variation, very dependent on weather patterns. We'll see the increase in the incidence rate of tick borne pathogens, lyme disease, as well as some of the newer emerging tick borne pathogens, like Anaplasma, Boston virus, Babesia. What the main concern for me is that, you know, the capacity of people to protect themselves from these types of diseases, it really, there's a lot of inequity in terms of healthcare access, in terms of public health knowledge of prevention and control measures, so the impacts are likely to be felt unevenly in the population. And so it's really important that there are [00:23:00] educational tools, that that there's increased capacity for public health prevention to reduce people's exposure. And that areas that are less able to implement these measures have the appropriate resources so that we can reduce risk for those most vulnerable. 

James Lawler: You know, on this podcast, we speak with a lot of people who are doing great work studying specific issues within the larger climate change umbrella. And when you think about all of these things together, you know, and their cumulative effects on society and on our ability to function, it's, it's not a pretty picture. So, you know, you combine smoke inhalation and you have vector-borne illness that is going to be, you know, increasing, you have climate migration issues. Do you ever think about all of these things together or is it just too overwhelming to do so?  

Dr. Manisha Kulkarni: That's a great question. I mean, I think as a scientist and as an educator, we have to think about all of these things together. There's an interaction [00:24:00] between all of the different climate change impacts and adaptation strategies and mitigation strategies that we're hoping to implement, you know, in East Africa, looking at malaria. Well, we can't really look at the impacts of climate change on malaria transmission without looking at the impacts on food security and agriculture, or on water security, recognizing that strategies to adapt to one impact, such as storing water to withstand drought conditions, can exacerbate other issues like vector-borne diseases if they're creating breeding sites for different types of mosquito species. So we need to be looking at this in terms of a network, right, in terms of an interacting, interrelated system, and prioritize those areas that are most vulnerable in terms of societies that don't have the adaptive capacity to withstand the effects, but also, you know, areas where we really need to to target our resources to have maximum impact. 

James Lawler: What should communities [00:25:00] be doing to better prepare, better adapt?  

Dr. Manisha Kulkarni: Well, if we take the case of vector-borne diseases, certainly enhancing and strengthening surveillance systems can be one of the best adaptation strategies to ensure that we are detecting things before they become a huge problem. So doing surveillance of mosquitoes and ticks, as simple as it sounds, is one of the best early warning systems we can have for new emerging vector-borne diseases. 

James Lawler: What does that look like? What does good surveillance look like?  

Dr. Manisha Kulkarni: So, I mean, good surveillance has good geographic coverage, and it's timely, and the information is usable. We have systems in place using kind of citizen surveillance methods now, in Canada. We have  eTick, which users can submit a photo of a tick that they've detected on themselves or in the environment that can be identified quickly, so this can be used to identify new invasive species. We have, you know, active surveillance. We have a pan Canadian sentinel surveillance network [00:26:00] for ticks and Lyme disease. So we have regions across the country where university teams are going out to collect ticks every summer, and we can really get a portrait of this evolving emergence of ticks and tick-borne pathogens across the country. So, we need to have good data. And that can be used to drive the models that we have to understand what the future impacts may be, but we can also work together in this type of multisectoral, multidisciplinary way, work with urban planners, work with, you know, conservation biologists, work with our public health counterparts and the environment sector to really take a unified approach to disease prevention and control because a lot of that needs to happen outside the health sector. 

James Lawler: If you had to point to one example where surveillance either has been done or is being done in an exemplary way for communities, regions, that want to do a better job at this, where would you point them?  

Dr. Manisha Kulkarni: [00:27:00] Sure. So in, in Ontario, for example, Public Health Ontario has a West Nile Virus Surveillance program which combines data from all of the participating public health units to monitor and track mosquitoes at a weekly interval, to look at the number of positive pools for West Nile virus and other arboviruses. These data are then rolled up at the national level with all the provincial agencies to really get a national portrait of what's happening with West Nile virus over time. With Lyme disease, we have similar type of surveillance going on in this sentinel approach with sentinel regions across the country in different provinces through the Canadian Lyme Disease Research Network in collaboration with public health units, provincial public health authorities, and the Public Health Agency of Canada. So, we're really taking this integrated approach to ensure that the knowledge isn't staying in academia and just being used to generate papers, but it's really actually being used for public health decision [00:28:00] making.  

James Lawler: Excellent. Well, thanks so much for your time. It's been great to have you on Climate Now. 

Dr. Manisha Kulkarni: Thanks for having me. 

James Lawler: That's it for this episode of the Climate Now podcast. Thanks for tuning in to today's conversation about how climate change is impacting the spread of vector-borne diseases around the globe, and how surveillance measures can help mitigate risk. Special thanks to Dr. Mordecai and Dr. Kulkarni for sharing their work on climate change and disease transmission. 

We love hearing from our listeners, so please don't hesitate to get in touch with us by emailing us at contact@climatenow.com. For a full transcript of this episode and a list of reference sources, you can always go to our website, climatenow.com. We hope you'll join us for our next conversation.