Curious Conversations, a Research Podcast

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Travis

You've probably heard the phrase, don't let the bed bugs bite. But how long exactly have these bugs been biting us in our beds? How has the number of bites ebbed and flowed in relation to the history of human civilization? And maybe most importantly, how has the bug evolved over time and what mitigation strategies do we now need to employ to help ensure that these bugs don't end up in my bed?

Thankfully, Virginia Tech's Lindsay Miles as an expert in this very topic and was kind enough to join the podcast to walk me through the answers to these questions and quite a few more. Lindsay is a postdoctoral fellow in the Department of Etymology and an affiliate with the Freilin Life Sciences Institute. Her research focuses on population genetics and molecular evolution to understand how urbanization impacts organisms, and she's currently studying how urbanization and pesticide use can influence the bugs.

Lindsay shared with me what she has learned from studying the genomic sequence of the bedbug, and also what she's learned about the relationship between human populations and the bedbug population, and how that's impacted some of our ability to mitigate against these creatures. So if you are invested in ensuring the bedbugs don't actually bite, I think this podcast will have a lot for you. I'm Travis Williams, this is Virginia Tech's Curious Conversations.

Travis

You're working a lot with bed bugs. Not maybe not... I should back that up. You're not working necessarily with bed bugs, but you're studying the history of bed bugs. And so I know you've recently done and worked on this large genomic sequencing of the bed bug. So I think a good place to start is simply, what is that?

Lindsay

So one of the things that we hadn't actually talked about earlier, but we also have a publication out where we got a chromosome level haplotype resolved whole genome for the bed bug, CIMEX leucotillarius. And so that involves grinding up a really nice fresh tissue sample of a bed bug, extracting out their DNA getting really high quality DNA, and then you send it off to some fancy sequencers, and they do a whole lot of work there. And then at that point, we get these chromosomes. And this is a really good resource that we're going to use for many of the projects that we work on when it comes to genomics and bed bugs. So that was one of the first things that I've done as a postdoc here is we released this genome. So now after we have this genome, Dr. Booth had previously gotten whole genome sequences of a handful of bed bugs. so basically, it's all of your DNA. That's what the genome is, right? So if you've ever heard of something like 23andMe or ancestry.com, right? and they take your DNA and then they tell you kind of your family history. And so this is all based on DNA. And the fun thing about 23andMe is that it's the 23 part is your 23 chromosomes, right? And so we're talking about chromosome, like genome evolution here. And so the bedbugs, they don't have 23 chromosomes, they have 15. And So we were able to pull out 15 chromosomes and bed bug instead of 23 and me. Once we do that, and again, Warren had some previously sequenced bed bug DNA. The problem is that there was not a really good genome. And when you have genome-wide data, you need that resource to be able to align each individual, right?

So we're talking about billions of base pairs. And if you try and do it without a good reference, they're like, I don't know, I might be here, I might be there. I just, I don't know where I go, right? So you have that really nice genome and it's like, I go here and you go here and you go here. And when we do that, we're able to confidently say, okay, since you go here and this site is different than our reference, we know for sure that that is what we call a SNP. It's a single nucleotide polymorphism. Okay? And so that's a variant site. And these variant sites are now what we're using as our data to look in the past.

Travis

Okay, so let me see if I'm understanding this because that was a lot of new information for me. It sounds like that you're trying to basically create, of figure out maybe the family tree for this bug and it sounded like you had some of the information but you needed this key to help understand it and that would be the genome. Yes. Okay.

Lindsay

Yeah, yes, exactly.

Travis

And so now you all are able to look, how far are you able to kind of look at this family tree? How far can you see back into the history of this bug?

Lindsay

So we can reach back to around 100,000 years ago. So if we had had more genetic variation, because bed bugs just don't generally have genome-wide variation like other animals do, if we'd had more, we could go back further. Some people are able to go back millions of years. But when you don't really have a lot of variation, you're kind of on these shorter time scales.

Travis

Okay, so they're not all that different then? Is that what I'm to understand by that? They're very similar?

Lindsay

Yeah, so any one individual is very similar to each other. And this is especially true when you have infestations, because a single bed bug infestation is usually by a single pregnant individual. She has her babies, and then they breed with each other, and they breed with each other. So you get these relatedness factors. Zero would be you're completely unrelated. One means you're super related, right? And we're getting relatedness factors of like 0.9. So they're close to being identical, right, within an infestation. So generally, this species just doesn't have a lot of genetic diversity.

Travis

Well, when you look back across, I guess, the history of bed bugs, are you all able to see, I don't know, when different mutations and different things happen within these sequences? Does that unlock a part of their story?

Lindsay

It does. So one of the things that we can do, we can't give an exact time estimate of a particular polymorphism. But what we can do is take in totality all of the polymorphisms, and we can see changes in effective population size. So effective population size is the number of breeding individuals. So it's not your census size, right? So for example, If you have identical twins, you have a census of two people, but they're identical, so you have an effective population size of one, because they can only give the genes that they have and they share the exact genes. So your effective population size is based on your genetic diversity, and it will change based on how much genetic variation you have as a species.

Travis

And so if we look back and we see a large amount of that, what does that tell us?

Lindsay

Yeah, so what we did was we used a really cool program called Stairway Plot and it tracks changes through time. And what we found was around the last glacial maximum, there was a huge decline in bedbug populations. And this is pretty typical of any kind of insect that you see in North America and Northern Europe because during the last ice age, there were ice sheets, right? And bugs just don't do very well in ice. And so what will happen is that those ice sheets came down and they took away habitat for all of these bed bugs. So the populations crashed. So, and in this timeframe, we also have looking at bed bugs that are in caves, right? And so after that ice age kind of retreats and humans start coming out of caves, we start to see just a tiny little increase in that effective population size because now they're able to get out of those caves, those ice sheets have retreated, the available habitat is growing, so they're able to kind of bounce back a little bit. What's really interesting is then humans started building cities. So like really early cities. So you can think, know, like Mesopotamia, Egypt, ancient Egypt. So we're starting to get these really high human population densities. So of course the bed bugs are also going to start getting high in high densities. So right around that 10,000, 12,000 years ago when the earliest cities were being formed by humans, the population of bed bugs skyrockets. And they continue to exponentially increase until modern time. And then within the last couple hundred years, our stairway plot analysis is not able to kind of get that fine tuning of such a short timeframe. And so that's when we start looking at what we know about what's happening with bed bug populations and we can make kind of inferences based on other things that we already know about bed bugs.

Travis

Well, I read somewhere that one of the things that you uncovered was like a possible mutation that made bed bugs resistant, which sounds like the creepiest thing in the world that we have a mutant bed bug and now it's super strong and it's hard to defeat, right? What happened there? What did we learn from that?

Lindsay

Yeah, so there's been a lot of other studies out there and I want to give credit to them that they have looked at what we call the KDR mutation and it's KDR because it's knockdown resistance, KDR. And that happened around the 1950s when we were spreading DDT. So it's a specific mutation that gives them resistance to DDT and other pesticides that have kind of similar chemical compounds to that. And so we know that that happened. We know we put that DDT down and we know that they got an increase in resistance to that through the KDR mutation. What's really interesting is that the RDL mutation, and this is the one that we found, has not yet been found in bed bugs. And one of the things that I think is really interesting about it is that this mutation is in common with cockroaches, it's in common with fleas, and it's in common with mosquitoes. So many different insects have this mutation. And we hadn't really expected to see it in bed bugs because it's actually resistant to a type of pesticide that we actually use in our households on our pets for flea medication, right? And so it's not being sprayed by pesticide groups. It's actually being put onto your cat or dog and it helps them not have fleas. And somehow they have been probably in bed with their owners. You give that little application and then your dog snuggles up with you at night. If you happen to also have bed bugs, now the bed bugs have this insecticide that has not previously been targeted at them, and now they're generating resistance to this pesticide.

Travis

Wow, so you found that they're resistant to something that people weren't even using, like against bed bugs. What does that mean for the future of bed bugs? Does that just make them stronger in general?

Lindsay

Yes. Yeah, so every time we try and do new formulations to get rid of these bed bugs, right, we use insecticide combinations that have different chemical compounds in it. And it's a really strong selection force. So it's a force of evolution, right? You die or you have a mutation and you survive. And so now we're starting to find that they've got mutation to things we haven't actually targeted them with, right? So that's going to start to eliminate the possible chemical compounds that we can target because if that mutation is already out there, we don't want to start using that insecticide because that resistance will just spread like wildfire.

Travis

Yeah, well, that's, I mean, that's kind of a bummer for all of us that we're hoping to get rid of dead bugs easier. I guess on the upside, how can we use the things that you all are studying with just the DNA and the history of the bed bugs? How do think we can leverage that in a more positive way for us? Not necessarily for the bed bugs, but for us?

Lindsay

Right. Well, one of the things I think is kind of understanding their ancient history with us, right? So we know that they are so closely associated with humans. There's a bat lineage that is actually the same species as the human feeding lineage. And so they were originally on bats, kind of dropped down into like early humans and  The bat lineage one, it declined in the last glacial maximum and it never recovered. So bedbugs on bats aren't doing so hot. so being able to kind of look at the difference between the bat and the human lineages, we're able to see like, okay, well the bat lineage, it's not recovering and it's probably because the bat populations are not recovering, right? We're taking out all of their available habitat and their food sources. So bats aren't doing good and therefore their parasite isn't doing good. Humans on the other hand, we're doing great, right? And we've got our kind of huge population growth. And as we continue to build cities, we're currently in the Anthropocene, which means we're going to build more and more cities and have more and more high human population densities. And if we look in our history, that's when bedbugs are able to take off. So our ability to kind of get rid of them is really intertwined with our history. So that's something we need to take into account when we're looking at these things. And modern human movement, we're now global and so are our bedbugs, right? So kind of understanding that they travel with us and they, as much as we try to eradicate them, they're going to try and survive. And so we have to come up with new and unique ways to kind of eradicate these infestations to try and temper their populations.

Travis

Yeah, so it sounds like I need less neighbors. Is that right? Less neighbors, less bedbugs.

Lindsay

You know, unfortunately, you could be in a very rural area. And if you happen to travel outside of your rural area and pick up a bed bug, you can bring that infestation home. It's just less likely that you'll then spread it to your neighbors. So like, if you're in a rural area and you stay home all the time, yeah, you probably won't get bed bugs.

Travis

Well, I don't know if that's quite the sacrifice that I'm willing to make, but I definitely don't want to get bed bugs.

Lindsay

Yeah, most people don't want to get bed bugs. And what's really fun with bed bugs is there's been no evidence that they can actually transmit diseases, right? So mosquitoes and ticks, can transmit diseases, but they're one of the only blood sucking insects on humans that don't transmit disease. Most people get the bigger ick out of the bed bugs though.

Travis

Yeah, that's true. think I do too. I think it's because they're in the bed. I think that's not, you know. Right, like you're supposed to get bed outside. Like that's where the bugs live. I don't want the bugs to come inside. They come inside. They're on our turf. In the bed, definitely you wouldn't want to have them in there. Wow, that is absolutely fascinating. So it just sounds like that we have a lot of work to do to better understand these critters and at the very least how we can mitigate some of the

Lindsay

That's supposed to be your safe space.

Travis

I don't know, itchiness and whatever you get from bed bugs.

Lindsay

Yeah, well, so they do kind of release a histamine that can cause you to itch, but not everybody reacts to it. And if you have bed bugs for long enough and you go untreated for long enough, they can actually reduce the amount of itchiness and red splotches so you don't even know that you've been bit. It's kind of like the variation between when people get bit by mosquitoes, some people have the little bump and some people swell up. The same thing happens with bed bugs and you would think like, you would know. You would know if you had an infestation. But some people just don't react to the bite. And so these people don't know that they have an infestation all the time.

Travis

So maybe we're just looking at this the wrong way. mean, maybe these aren't, these are just pets. Maybe that's what's happening.

Lindsay

You know, it's like really large groupings of pets. Yeah.

Travis

Well, that's the perspective that I may try to take, but I don't think anybody else in my family will agree to that.

Lindsay

Yeah, know, they, bed bugs definitely make me feel just not okay. I don't like working with live bed bugs. I love working with their DNA. Their DNA is so interesting, but the live bugs, no.

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Travis

And thanks to Lindsay for helping us better understand the history of bed bugs. If you or someone you know would make for a great curious conversation, email me at traviskw at vt.edu. I'm Travis Williams, and this has been Virginia Tech's Curious Conversations.

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