S1 E3: Stuck on You with Chelsea Wood

Chelsea Wood is an Associate Professor in the School of Aquatic and Fishery Sciences. She is a leader in the ecology of parasites and pathogens in freshwater and marine ecosystems, the ecological drivers of parasite transmission, and human impacts on parasites in a changing world. Wood discusses the fascinating world of parasites, their “Rube Goldberg-esque” life-cycles, and her recent study - the world’s largest and longest dataset of wildlife parasite abundance - that suggests parasites may be especially vulnerable to a changing climate.

https://chelsealwood.com/

Chelsea Wood is W.M. Keck Foundation grant recipient for her work with historical reconstruction of infectious disease prevalence in wildlife.

https://environment.uw.edu/podcast

Transcript

00;00;00;00 - 00;00;12;21

Chelsea Wood

They are the Yellowstone wolves of Puget Sound. They are quietly, over the years, keeping a cap on the abundance of things that need to be held in check.

00;00;13;27 - 00;00;35;03

Sarah Smith

But this is field sound. I'm Sarah Smith. We're talking parasites with Chelsea Wood. Is here. Well, okay, so what is it about parasites then? What was like? What was the hook? No pun intended.

00;00;36;08 - 00;01;02;29

Chelsea Wood

I think the biggest thing is that they have these life cycles that just they shouldn't exist. They're just ridiculous. It's silly that they exist. They'll have three or four or even five host species that they absolutely require for just one turn of the life cycle. Until I learned about parasites, the most complex life cycle I had heard of was like butterfly to caterpillar and back to butterfly.

00;01;03;23 - 00;01;21;28

Chelsea Wood

And we think about that metamorphosis as magical. But imagine doing that metamorphosis five times just to reproduce once. That's what parasites do. They have these life cycles that are are so complicated. You just want to sit them down and ask them like, why? Why did you make your life so much harder than it needs to be?

00;01;22;01 - 00;01;45;26

Sarah Smith

Chelsea Wood is an associate professor at the School of Aquatic and Fishery Sciences at the University of Washington College of the Environment. Her research program explores the ecology of parasites and pathogens in the changing world. So we all have something in mind when we think of the word parasite. But how do they really operate? And what's their relationship with the host organism?

00;01;46;08 - 00;02;07;25

Chelsea Wood

For parasites, it's always one way. They're the definition of a parasite. Is that something that lives in or on a host where the host suffers a fitness cost? So by definition, the host is harmed by this relationship. And that's another aspect of parasitism that's really cool. The host is trying to kill the parasite at all times. They're trying to avoid becoming parasitized.

00;02;07;25 - 00;02;27;10

Chelsea Wood

And once the parasite takes hold, they are trying to kill that parasite either, you know, by grooming it off, if it's an actual parasite or by killing it with the immune system, if it's an end up parasite. And yet somehow they managed to persist. They live in an environment that is actively trying to kill them. You know, next time you're out in a rainstorm, it might feel hostile.

00;02;27;17 - 00;02;36;12

Chelsea Wood

Imagine if that rainstorm were actively trying to kill you. That's what a parasite lives with every day. And yet they keep on going.

00;02;40;08 - 00;03;01;21

Sarah Smith

How do you look into the past with marine parasite ecology? Well, the answer isn't so simple. Chelsea recently published a study on the history of parasites and made some remarkable discoveries in relation to climate change. As Chelsea explains, a new line of research was born out of this parasitic curiosity.

00;03;03;09 - 00;03;21;16

Chelsea Wood

It's not straightforward because no one really keeps data on parasites over long time series. No one cares enough, so there's no notebooks filled with parasite data from the early 1900s. You know, you can rely on that for things like fish that people care about, or at least things that are economically important. But for parasites just we have no clue.

00;03;22;09 - 00;03;44;13

Chelsea Wood

So I thought it was impossible for a long time until a mentor of mine suggested, You know what? Have you ever tried opening up a fluid, preserved museum specimen? You know, a fish floating in a jar in a museum? I hadn't. I tried it. There were the parasites. And this whole line of research was born. These fish floating in jars.

00;03;44;17 - 00;04;04;02

Chelsea Wood

You'll see some of them in a museum. They'll always take their prettiest ones out of storage and put them in the public space. But what exists down in the basement are millions of other specimens that are just waiting for their moment in the sun. They're there vouchers of biodiversity from a particular time and place. And there they are, parasite time capsules.

00;04;04;02 - 00;04;24;19

Chelsea Wood

They they preserve all the parasites that were in the host at the time of his death. Which means that I can go to a jar of fish collected from 1880, open up one of those fish and know exactly what it was carrying in 1880. We then take enough of those little jars and put them together. We actually get a picture of how things have changed through time.

00;04;24;29 - 00;04;52;27

Chelsea Wood

It's tantamount to time travel. We just completed a project that's focused on Puget Sound specifically, and we picked Puget Sound because UW Burke Museum of Natural History and Culture has the largest collection of North American fishes anywhere in the world. And the representation from Puget Sound is fantastic. We had all the fish that we could possibly need in a building that's literally across the street from where my office is.

00;04;53;19 - 00;05;16;18

Chelsea Wood

We looked at a series of 700 fish across several fish species that spanned 1880 to 2019. And what we found was really surprising. I expected from my previous research in MPAs and open access areas that there would be some winners and some losers among the parasites. It's a common thing to find when you're looking at long term change.

00;05;17;04 - 00;05;37;03

Chelsea Wood

Instead, we found a lot of losers and not very many winners, and most of those losers were those parasites that I initially fell in love with, the ones that that I that brought me into parasite ecology, the ones with the most complex life cycles, the more complex it seemed, the more likely the parasite was to decline through time.

00;05;38;04 - 00;05;57;16

Chelsea Wood

We think that it's probably because climate change is reshuffling ecosystems, and the more hosts you have in your lifecycle, the more potential points of failure, the more chance you have of losing a host that you need and therefore the likelier you are to decline in response to climate change.

00;05;58;20 - 00;06;02;07

Sarah Smith

And what happens if we don't pay attention to parasite loss?

00;06;02;17 - 00;06;20;18

Chelsea Wood

The more complex their lifecycle is, the more precarious they are. And the way that I think about this, it's like a little hard to wrap your mind around, especially if you've not spent the past 15 years thinking about parasite life cycles. But the the analogy that I use are Rube Goldberg machines. You know, those.

00;06;21;14 - 00;06;25;08

Sarah Smith

So like The Goonies of The Goonies, like.

00;06;27;18 - 00;06;50;29

Chelsea Wood

If anyone listening doesn't know what a Rube Goldberg machine is. Google it right now. Look for videos there. Tons on YouTube of people's professional and homemade Rube Goldberg machines. But they're they're all basically machines that take a really simple task and use this overly elaborate series of mechanisms to complete that task where each mechanism depends on the one that comes before.

00;06;51;23 - 00;07;10;07

Chelsea Wood

And the the thing about Rube Goldberg machines is that they're impressive when they're complicated, and the thing that makes them impressive is that the more mechanisms you have in a rigged Goldberg machine, the more likely it is for something to go wrong. And when it works, it's just shocking and really cool to watch the parasites with the most complex lifecycles have the most potential points of failure.

00;07;11;00 - 00;07;33;05

Chelsea Wood

So you know, when things are going along as normal in the conditions in which these parasites evolved, everything is fine. They've got the hosts that they need. Those hosts line up with them in time and space. Everything's okay. But as soon as you start changing things, that's when these parasites get into trouble because they've got so many different things to keep track of, they've got to make sure that host number one is there and host number two and host number three and host number four.

00;07;33;13 - 00;08;00;14

Chelsea Wood

And that they're there at the right time as well. Climate change reshuffles the abundances of those hosts and the times when they're around to be infected. And it shifts us out of the conditions in which the parasite evolved, which means that at some point, inevitably something's going to break. The parasites that use just one host species seem to be fine, and even the parasites that use just two host species seem to be fine.

00;08;00;29 - 00;08;24;12

Chelsea Wood

But when you're trying to juggle so many host species in a world that's rapidly changing, that's when things break down and those are the parasites that we lose. Now, who cares, right? They're worms, they're ugly, they're slimy. No one likes them except me. Why do we care that they're gone other than me being upset about it? There are some good reasons to care about whether parasites are present in Puget Sound or not.

00;08;24;27 - 00;08;54;25

Chelsea Wood

One is that they're doing all these ecological functions. They are the Yellowstone wolves of Puget Sound. They are quietly, over the years, keeping a cap on the abundance of things that need to be held in check. And when we lose them, we're going to experience the same thing that Yellowstone did, which is that some species not previously a problem, are going to become overabundant and reduce the amount of resources available for some other species.

00;08;55;02 - 00;09;10;12

Chelsea Wood

Now, do I know what species that's going to be? No, I have no idea. But we do know that parasites do play this role and my suspicion is that we will find out in a very visceral way what they've been doing when they are gone.

00;09;12;27 - 00;09;14;17

Sarah Smith

Does Chelsea have a favorite parasite?

00;09;15;27 - 00;09;48;20

Chelsea Wood

They're all my babies. I know I'm not supposed to have favorites, but I do have a favorite. There is an order of tapeworms called the trypanorhynch. The order Trypano, Rhyncho. And these parasites are really special because they exist only in their adult form as they exist in the intestine of sharks in their adult form. It's the only place they can reach maturity and the only place they can achieve sexual reproduction.

00;09;48;22 - 00;10;17;17

Chelsea Wood

And they are just so beautifully adapted to that habitat. They're tapeworms. So they don't have any eyes. They don't have a mouse. They don't have a gut tract. But they do have a head. And that head is armed with these four tentacles that kind of spiral into the head, into these sheets when they're retracting. Now, the worm can actually eject these tentacles at will independently of one another.

00;10;17;17 - 00;10;41;01

Chelsea Wood

And it's amazing to watch them do it. They're very active. Each one of those tentacles can be shot out of the head. And when it is, you can see that each of those tentacles is armed with thousands of backward facing hooks. Now, the tentacles are just beautiful. They're these, like, sinuous coiled tentacles that move around independently of one another.

00;10;41;01 - 00;11;09;27

Chelsea Wood

And they have just this lovely movement. But the thing that makes them really gorgeous is that those spines, those hooks actually refract and reflect light. So you get this like rainbow effect. So imagine just this like rainbow unicorn parasite, like dangling its tentacles all over the place. It's wild to see. Now it's really beautiful, just esthetically, but it's also incredibly functional because the inside of a shark's intestine is slippery.

00;11;10;14 - 00;11;37;25

Chelsea Wood

It's not easy to hold on to. And there's tons of flow, right? The shark is digesting food and passing it out of its hands. And you don't want to get passed out into the environment because you'll die if you're that tapeworm. So these tentacles get laid down on the wall of the intestine and then the hooks anchor in, and then the worm is able to pull that tentacle back into its body to anchor itself to the wall of the sharks intestine.

00;11;37;25 - 00;12;04;28

Chelsea Wood

And they can actually kind of like move themselves around using those tentacles like grappling hooks almost. So not only are they just lovely to look at under the microscope, but they also are strangely, weirdly adapted to this alien world that we've barely ever imagined, but that they live in every day. So those are those are my favorite parasites of follow your gut.

00;12;05;25 - 00;12;06;21

Chelsea Wood

If you're.

00;12;07;12 - 00;12;08;28

Sarah Smith

Here just for fun today.

00;12;09;03 - 00;12;35;11

Chelsea Wood

Like the Fonz. Yeah. Go with your gut. Parasites are like a weird thing to like. People are probably not going to understand at first what the attraction is, but once you start learning about them, it's like getting to know the natural history of an alien planet. It's a totally unfamiliar place with so many surprises. They're still parasites that I run across today and today.

00;12;35;12 - 00;12;52;02

Chelsea Wood

You know, I'm pretty cocky. I like to think that I've heard them all. Every once in a while, I'll hear a new parasite story that just blows my mind. There's no other group of animals, no other group of organisms that can do that, that contained so much weirdness and so much novelty. That's the thing that I love about them.

00;12;52;02 - 00;13;06;04

Chelsea Wood

And, you know, people will maybe question your sanity, your choices as you explore this new world. But that's really just because it's so underappreciated and so underexplored. That means you're going in the right direction.

00;13;06;13 - 00;13;13;29

Sarah Smith

Interesting. Mm hmm. Is there any well, are there any, like, examples of that with humans and sort of parasites? Oh, people. Yes. Yeah.

00;13;14;29 - 00;13;39;23

Chelsea Wood

Yeah. I mean, you might not. Trigger warning in case you're worried about whether you carry a parasite. This one is in 40% of humans. So if that freaks you out, now's the time not to listen. Fast forward. The parasite I'm thinking of is Toxoplasma gondii, and it is a protozoan parasite that's really closely related to the parasite that causes malaria.

00;13;41;05 - 00;13;48;18

Chelsea Wood

But this parasite does something very, very different than its cousin. It's transmitted by cats, and it's.

00;13;49;23 - 00;13;50;22

Sarah Smith

Love parasite.

00;13;51;02 - 00;14;13;11

Chelsea Wood

That's the the natural lifecycle for this parasite is that it has its sexual stage in the gut of a cat. It gets pooped out as an egg in cat poop. And then the egg can be very resilient. And either it stays in cat poop or the cat poop breaks down and it winds up on vegetation or garbage or whatever.

00;14;14;13 - 00;14;43;03

Chelsea Wood

And then a rat accidentally eats that egg either from cat poop or from something that cat poop has contaminated. The rat becomes infected. And then in order for a Toxoplasma to make it from this rat host into the cat host, the rat has to be eaten by the cat. So we've now set up the situation where Toxoplasma can either just hang out, enjoy its time in the rat, and hope that one day it gets eaten by a cat or it can take matters into its own hands.

00;14;43;03 - 00;15;04;13

Chelsea Wood

And of course that's what evolution has sculpted it to do. So what Toxoplasma then does in its rat host is it like pulls the strings of its cognition a little bit. Rats, under normal circumstances are afraid of the smell of cat urine because usually where there's cat urine there are cats and they know that in order to avoid cat predation, they have to avoid places where cats go.

00;15;05;11 - 00;15;29;11

Chelsea Wood

But rats that are infected with Toxoplasma actually are attracted to the smell of cat urine. And not only do they like, move toward it, it's they actually find it sexually arousing. So it's a very strong impulse for these rats to move in the direction of cat urine, which of course, is the direction of death for these rats. There there are other impacts as well.

00;15;29;11 - 00;15;53;08

Chelsea Wood

The parasite makes them clumsier and slower so than when the cat goes after them. They're unlikely to make it away alive. And that's how the parasite gets along in the world. That's how it completes life cycle. Now, where do humans come into this? The thing is, we like to think that we're very different from other mammals, and in some ways we are, but in other ways we're not.

00;15;53;08 - 00;16;19;28

Chelsea Wood

And the biochemistry over that is pretty much the biochemistry of a person. Now, people can become infected with Toxoplasma the same way rats do, which is by eating things that are contaminated with cat feces. Think vegetables in a field that is visited by an infected cat. Think letting your cat poop in a litter box and then letting it walk over kitchen counters, snuggling your cat.

00;16;19;28 - 00;16;43;01

Chelsea Wood

The these ooze sister eggs can get everywhere and then they can be ingested even without you ever realizing it. The other way that people can become infected is by eating another intermediate host. Cows can be infected by Toxoplasma and then the parasite insists in their muscles. And if you then eat undercooked cow me, you can become infected in turn.

00;16;43;28 - 00;17;08;23

Chelsea Wood

And then once the parasite is in your body, it basically treats you like a rat. It thinks it's inside a rat. It pulls all the same biochemical strings that it does in a rat. And it can have some of the same effects on human behavior. One really well-established effect is that humans who are infected are three times likelier to get into a car accident than people who are uninfected.

00;17;09;06 - 00;17;31;19

Chelsea Wood

It just slows down your reaction times just enough that it makes you likelier to get into an accident instead of being able to avoid one. And then there are some personality changes as well. They differ between men and women. Women become more gregarious, men become more paranoid and isolated. And then there are some kind of fringy ideas about how they might influence human behavior.

00;17;31;20 - 00;17;56;11

Chelsea Wood

These are not well substantiated, but there's some suggestion that humans become more tolerant to the smell of cat urine when they are infected, which would explain the tendency of cat people to acquire more cats, which of course is good for Toxoplasma. Not saying that that's scientifically proven that is, I have a hypothesis only, but you know, we like to think that we're these complicated, you know, top mammals, right.

00;17;56;25 - 00;18;07;09

Chelsea Wood

That our brains are are the most sophisticated computers in the universe. And then along comes a single celled parasite that can play us like a fiddle.

00;18;09;07 - 00;18;25;27

Sarah Smith

Yes. Oh, wow. Well, that's yeah, kind of disturbing. But also. Yeah, I mean, we've we've talked about this in the office. If you take we're all animal people here. So we talk about, you know, just like I don't know how obsessed with our with our pets and.

00;18;26;20 - 00;18;28;00

Chelsea Wood

Yeah, I mean, now there's a reason.

00;18;28;01 - 00;18;51;13

Sarah Smith

Now there's a reason. It all makes sense now. So yeah, weird. But it's it's just you made it through this episode with all the puns and all the parasite talk. I'd like to thank my guest today, Chelsea once. Thanks for listening. See you next time.

00;18;53;26 - 00;18;55;22

Chelsea Wood