The Decibel - Why we need parasites

Episode Date: August 11, 2023

You’re never far from a parasite, no matter where you are. They’re responsible for illnesses like malaria that cause death around much of the world; the word itself is a derogatory term for something freeloading and disgusting.They’re also very important to the health of ecosystems.Parasite ecologist and University of Washington associate professor Chelsea Wood makes her case for parasite conservation, and why they’re actually complex forms of life that need to be protected.This episode originally aired on August 16, 2022.Questions? Comments? Ideas? Email us at [e-mail]

For our final day of Science Week on the Decibel, we're taking you inside the world of parasites. I know, parasites can seem kind of gross and horrible, but they're also complex and really important. Last summer, I spoke with Chelsea Wood. She's a parasite ecologist and a professor at the University of Washington. She tells us why parasites have an essential role in healthy ecosystems. I'm Menaka Raman-Wilms, and this is The Decibel from The Globe and Mail. Chelsea, it's great to chat with you. Thanks for being here. Yeah, you too, Menaka. I'm really excited to talk to you.

So I want to start with the very basics here. What exactly is a parasite? Parasites are organisms that live in an intimate and durable relationship with a host where that host suffers a fitness cost. So when I say intimate, I mean, parasites are living spatially closely with their hosts. They're either in or on the host. And when I say durable, I mean, temporally long. This isn't just like a one-off interaction. It's something that lasts a really long time. Okay. And can you give me some examples of things that people may have heard of or might know? Yeah, sure. The one that people are usually most familiar with is the tapeworm. I think this is because a lot of people have found them in their dog's feces. And that is an organism that's living inside its host. So in an intimate relationship, can be in there for years.

So that relationship is durable. And it's extracting energy from the host. So the host is suffering a cost as a result of that parasitic relationship. And so how common are parasites in the environments around us? They're absolutely ubiquitous. I was afraid you were going to say that. I know it's a disquieting thought, but even if you're living in a high-rise apartment that's scrupulously disinfected with no pets, there's definitely still a parasite within 10 feet of you right now. Within 10 feet. Absolutely.

Guaranteed. I know it's upsetting to hear, but parasites infect every organism on Earth. So wherever there are hosts, there will be parasites. And they are all around us all the time. They just are hidden away. And so, I mean, help me understand, you've developed a lot of time really researching these creatures, Chelsea. And I think when a lot of people think about tapeworms and things like that, I mean, it can be kind of gross. Like we don't always want to

think about these creatures. What makes you so interested in them? So many things. How much time do you have? There are a lot of reasons to be excited about parasites. One of the most unexpected, though, is that they actually are really beautiful organisms. You have to look at them at their own scale. When you get them under the microscope, they have these really intricate appendages and organs, things that are just absolutely gorgeous that don't seem to be of this world. The other reason that I think parasites are really cool is that they're essentially hidden puppet masters of ecosystems.

I use the word puppet master because I mean that they're hidden behind this curtain. We don't usually see them, but they are literally pulling the strings of ecosystems, making things go without getting any credit for that. Interesting. Let's talk a little bit about how these creatures actually survive. So you talked about, you mentioned a host before, but how do parasites actually survive? There are a whole variety of ways to live a parasitic lifestyle. There are really familiar ways like, you know, taking blood meals from the skin of the host being an ectoparasite or a parasite on the outside of the host. There's also a whole variety of ways to be an endoparasite or a parasite that lives within a host. Everything from also a whole variety of ways to be an endoparasite or parasite

that lives within a host. Everything from tapeworms, which are kind of absorbing the host's energy across their body wall, kind of taking nutrients from the intestinal tract of the host, to things like trematodes, which are parasites that use three different host species in the course of their life cycle. And one of those host species is usually a snail. That parasite eats the gonads of the snail and literally castrates its host. It'll penetrate the foot of that snail and kind of ride its circulatory system around until it gets to the coiled part of the snail that's usually up in the spire of the shell. And in that organ, which is kind of like gonad and digestive tissue all mixed together, it will asexually reproduce itself,

eating up the gonadal tissue of the snail as it goes and turning that energy into new little baby parasites. Now it does this in a super weird way because it essentially has two different stages in that snail. One stage is like a bag that feeds and moves around. Inside that bag are these little sperm-shaped parasite stages called cercariae. And once that bag is ready to release all the cercariae inside, it bursts open, the cercariae can then swim out into the water and they go in search of their next host snail totally castrated never produces snail babies again but it lives a normal snail

life you know moving around in the intertidal zone eating algae all it's doing is essentially serving as a factory for parasites um so that's just one stage. Yeah, that's stage one then. Okay. So then what happens? So after we emit this cercaria into the water column, it's swimming around. It's looking for its next host. In our example, let's imagine a little fish. The cercaria penetrates its skin, rides its circulatory system around, and then settles down in the place where it wants to hang out, which in this case is the surface of the brain of this fish. It settles there, it forms this resistant cyst, and there it waits. Because the final stage of the parasite's life cycle happens in the gut of a seabird. So wait, how does it get from the brain?

Yeah, it's in the brain of the fish there. So how does it get from there to the bird? In a very tricky way. It can either wait because the bird is a predator of the fish, but these parasites, you know, they exist in a world where natural selection can draw out behaviors or physiological adaptations that let the parasite get to where it's going more efficiently. And what natural selection has drawn out of this parasite is the ability to manipulate its fish host.

The parasite insists on the brain of the fish because from there it can induce these behaviors in the fish host that make it more likely to wind up in the gut of the bird. So is it kind of like taking over the brain of the fish in a way? Like it's making it do things that it wouldn't otherwise? Pretty much, yeah. So a normal fish will kind of like hang out at the bottom, you know, be very inactive, particularly during times of day when birds are active. It won't draw attention to itself. But an infected fish will swim up to the surface. It'll flash its shiny side upward. It'll kind of generally draw attention to itself. And as a result, the infected fish are between 10 and 30 times more likely to be eaten by a bird than the uninfected fish. So the parasite, rather than

just waiting around for something to happen, it actually induces its fish host to wind up in the beak of its bird host, which is precisely what the parasite wants. Okay, so then it is in the beak of the bird. What does it do then? What happens? Slides down the gullet, winds up in the stomach, and then it hatches and it can become its adult form. They'll mate, they'll produce eggs, and those eggs will pass out of the bird with its feces and hatch in the intertidal zone where they go on to infect a snail. And the life cycle is complete. So you can see how just dizzyingly complex these life cycles are. And this is one of the things that makes me really excited about the parasites. It feels like something that precarious, that delicate really shouldn't exist, right? It seems so unlikely that they're going to

actually complete this long journey. And yet there are tens of thousands of trematode species that do exactly this life cycle. And there are hundreds of thousands of other parasites that do similar complex life cycles. There's a lot of species, it sounds like, that could be affected by parasites. And the thing is, I guess it seems like these species are harmed by the parasite often, right? Like, why are parasites important? Like, are they doing any good at all in this whole cycle? We think of parasites as bad because we always take the host's perspective. And by definition, parasites are bad for the host.

But when you look at things from a broader perspective, for example, at the host population level or at the community level or the ecosystem level, you start to see how parasites are actually really good and even necessary in ecosystems. And that's because they do things like hold host populations in check. In the absence of parasites, we would get a whole bunch of free-living species that were way more abundant than is good for the ecosystems that they occur in. And that's because parasites, just like predators, can keep host populations at the level that they're supposed to be in. Parasites also, through their behavioral manipulation, like the trematode that I just mentioned, are all over the

world, all the time in every ecosystem, quietly pushing energy from prey into predators. In all kinds of different ecosystems, we have parasites that are manipulating the behavior of their prey host to make it likelier that that prey is going to be preyed on by a predator. And that essentially means that parasites are subsidizing predators. It means that they're making energy available to predators that otherwise would not be available to them. And I consider that really important because a lot of predators are endangered. Parasites are actually secretly behind the scenes helping us do that predator conservation. And because of the importance of predators and ecosystems, parasites are helping

to keep those ecosystems intact all over the world as we speak now. Yeah, it sounds like they actually do have or they can have a very important role in a certain ecosystem then. Part of the work you're doing is actually trying to protect parasites and conserve them. How do you do that? Well, it's tricky because for most people, parasite conservation is still an oxymoron. Our reflexive view of parasites is that they're bad and they deserve eradication. And if people have heard of parasites, it's probably in the context of an effort to get rid of them. So I think one thing that we need for parasite conservation is broader understanding of what parasites are and their roles in ecosystems. Parasites are just barely covered in K through 12 and even undergraduate biology curricula.

And that is a problem because we can't conserve what we don't know and we can't conserve what we don't understand. So I think one thing that we need is sort of the renaissance that predators underwent over the past couple of decades. If you think just, you know, a generation ago, predators were considered vermin. They weren't revered. They weren't lionized. They weren't things that were the target of conservation action because they were things that threatened our families and our livelihoods and that deserved to be persecuted are eradicated. And we saw things like the Discovery Channel come out with tons of beautiful documentaries that showed predators in their natural habitats doing what they were selected to do. And as a result, over the past couple of decades, we've gone from seeing predators as vermin to seeing them as really valued members of ecosystems that we seek to conserve. I think we need the same thing for parasites.

Parasites are just as important as predators are in the ecosystems where they occur. They're doing many of the same functions that predators are doing. And I think part of the reason that people can't get on board with parasite conservation is that they don't understand what parasites are or what they're doing. I think that parasites are overdue for the sort of public relations overhaul that predators have been undergoing over the past couple of decades. So yeah, so we should really be maybe looking at the discovery specials about the tapeworms or the snail-eating worms that you were talking about to kind of reframe the way we think about these

things. I think it's long overdue. You know, we've got a shark week. Why not a parasite week? Chelsea, thank you so much for taking the time to speak with me. It's my pleasure. That's it for today. I'm Mainika Raman-Wells.

Our summer producer is Nagin Nia. Our producers are Madeline White, Cheryl Sutherland, and Rachel Levy-McLaughlin. David Crosby edits the show. Adrienne Chung is our senior producer, and Angela Pachenza is our executive editor. Thanks so much for listening, and I'll talk to you next week.