Power to the Parasites with Chelsea Wood

In this epsiode I had the pleasure of talking with Chelsea Wood, an Associate Professor at the University of Washington and a dedicated parasitologist. We talk about her middle-grade book, Power to the Parasites. The book provides a comprehensive, scientifically rigorous look at the oft-misunderstood creatures living in, on, and around us. Wood shares how her academic focus shifted from marine conservation to parasitology after discovering the intricate and vital roles these organisms play in global ecosystems.

The core thesis of Wood’s book challenges the assumption that parasites are merely harmful anomalies; rather, they are ancient, ubiquitous, and integral to the natural order. Key insights from the conversation include the evolutionary logic behind complex, multi-host life cycles, the mechanisms by which parasites like Toxoplasma gondii actively manipulate host behavior, and the surprising reality that human-driven ecosystem changes are driving many parasite species toward extinction rather than proliferation.

Key Topics Discussed:

• The true definition of parasitism and the evolutionary logic of the r-selection strategy
• Complex, multi-host life cycles (e.g., the Lancet Liver Fluke’s journey from snails to ants to grazing animals)
• Behavioral manipulation and "mind control" by parasites like Toxoplasma gondii
• The unexpected role of parasites in ecosystem balance and wildlife conservation
• The hidden crisis of parasite extinction driven by habitat loss and overfishing
• Tracking historical parasite populations by dissecting century-old museum specimens

Find Chelsea Wood:

• Chelsea's website
• Chelsea on Instagram
• Chelsea's science book recommendations

Book Science:

• Website: https://www.trippcollins.com/podcast.html
• Show notes & transcripts: https://www.trippcollins.com/show-notes
• Check out the Book Science Book Shop on BookShop.org
• Instagram: @booksciencepodcast

Transcript

Tripp Collins 0:00

Today I am talking to Chelsea Wood, who is an associate professor at the University of Washington. Chelsea is a parasitologist and she wrote an awesome middle grade book called Power to the Parasites. Everything you ever wanted to know about the creepy crawlies hidden in your home, your food, your pets, and maybe even you. My son was a perfect age for this book and I had a blast reading it with him, and I had an awesome time chatting with Chelsea who just bubbles with enthusiasm for creatures that freak most people out if you find the topic Interesting. Stay tuned because Chelsea is set to publish a book on parasites for an adult audience to come out sometime next year. So without further ado, enjoy this conversation with Professor Chelsea Wood. All right, Chelsea, thanks for joining me. So this book is about parasites, which is uncomfortable topic for many. The life of a parasite is strange, stranger than I knew. In fact, I learned that from your book, and maybe even gross to some people. We don't like to imagine that maybe there are parasites living within us right now. But this book sort of turns all that on its head because the weirdness is what makes parasites cool or amazing creatures and interesting to study. So maybe just to start, can you tell us what a parasite is and what is this book all about for you?

Chelsea Wood 1:54

Yeah. Well Tripp. Thank you so much for having me. I'm so excited to talk about power to the parasites with you and your audience. I define a parasite as an organism that lives in or on a host. That host suffers a fitness cost as a result of the relationship. This is not a one-off relationship. It's not something that happens quickly and then both partners move on. This is something that happens over a long period of time in a really intimate configuration with the parasite living on the host's skin or in its feathers or hair or inside of its body. One of the necessary conditions of parasitism is that the host is hurt by that relationship. It's fitness is lower as a result of that relationship than it otherwise would be. So that's what a parasite is. And as for why I thought they were important to write about, they are the center of my professional life. And I honestly never thought that I would say that. When I got into the field of ecology. I wanted to be a marine biologist. I wanted to swim with dolphins and, do dolphin conservation. I have my own research vessel and chase dolphins all around the world. What I didn't realize is that the coolest thing about a dolphin is what's living inside of its gut. I learned that over the course of my education. And, I wanna bring that same insight to folks who read the book.

Tripp Collins 3:12

So would you say, are there clear distinctions between parasites and other creatures, or is it a bit fuzzy?

Chelsea Wood 3:20

It can sometimes get a little fuzzy. Like anything in nature, parasites don't like being put into pigeonhole. It's, it's often hard to define hard boundaries from one thing to the next when we're talking about nature. Sometimes a parasite. Can grade into what we call a commensal or a mutualist. Under certain circumstances, mutualists are organisms that live in or on other organisms, they're hosts, but benefit that host organism. the host's fitness is higher as a result of that relationship than it otherwise would be. And for commensals, same thing, but they have a neutral impact on their host. The host has no reaction to whether the commensal is there or not. And there are some examples of, organisms that can move across those boundaries. depending on the circumstances, particular gut bacterium might be a commensal most of the time having no effect on its host. But then when the host's immune defenses are low, it's able to opportunistically take advantage of. A breach in the intestinal wall and become a parasite and cause a reduction in that hosts fitness maybe by gnawing through the intestinal wall or eating the mucus of the intestinal wall or even puncturing through the intestinal wall. depending on the circumstances, you might see one outcome or the other. there can be some bleed from one category into the next. in general, parasites stick in their box of causing harm to the host. Within that box, there can be gradations, right? You can have everything from a tapeworm in your intestine that is probably gross to think about, but it's taking a tiny fraction of the total energy you take into your body. And that's in general something that you'll never notice. And it can go all the way to something like an nematode, burrowing through your brain, which can have obvious negative consequences.

Tripp Collins 5:24

Yeah. that's really interesting because, my son, he's eight, so this book was perfect for him. It kind of pushed him a little bit. And, we had a lot of cool discussions one of them was trying to work this out, one of his questions was why aren't the cells of our bodies considered a type of parasite? I didn't really have a great answer, one of my answers was, well, I thought that parasites had to be, a separate creature in and of themselves where a cell is part of your body. but maybe you can explain a little bit better, an answer to that question.

Chelsea Wood 5:52

So in general, you hit the nail on the head in that, an individual cell. Is the same organism and the same species as the body that it's a part of. part of our definition of parasitism is that these are different individuals, a parasite in its host and different species, the parasite in its host. The other thing is that one individual cell contributes to the fitness of the larger body, right? Like it's, it is there for a reason. It has a purpose. It serves that purpose in the body and helps that body to survive and reproduce. if you did wanna define it, it would be a mutualist. But given that it is the same organism and the same species, doesn't really fall into the category of what we call symbiance, the umbrella term that captures commensals, mutualists, and parasites.

Tripp Collins 6:40

And, you mentioned bacteria. So they don't necessarily have to be in the animal kingdom. it could be bacteria, it could be fungi, it could be, virus. does it, it spans all kingdoms essentially.

Chelsea Wood 6:52

If we're defining a parasite as an organism that lives in or on another organism and causes that host fitness costs, then that encompasses everything from viruses to animals, including all the things that you've mentioned, as well as fungi and protozoa.

Tripp Collins 7:06

Okay. yeah, and it's interesting to think about where parasites come from. I mean,'cause of course they evolved the same way any other creature evolved and these relationships also evolved. So it's not like parasites just appeared one day and animals are getting sick because of parasites. these are relationships that evolved over millions of years. it's just one other type of codependency or interconnection which occur for every creature in varying degrees in ecosystems. is there anything you wanna add to that comment?

Chelsea Wood 7:38

No, you've done a beautiful job of making an important point, which is that parasites aren't something new. they weren't invented a couple decades ago. They're not a result of human interference in the environment. Like sometimes we think about them, like we might think about chemicals or toxins in the

Tripp Collins 7:52

Mm-hmm.

Chelsea Wood 7:53

This like scary new thing that's on the rise that threatens our health. And that's not the case at all. They are as old as life itself.

Tripp Collins 8:01

Or so, yeah, like in the same category as something that shouldn't be there, but in reality it's just another part of nature. one thing I wanna ask you about is, the origin story of the book. I read in the acknowledgements you thank someone at, Godwin books, who heard you on NPR. So well first how was it that you found yourself on NPR and then, what happened next? Did the publisher like directly reach out to you, or how did that work?

Chelsea Wood 8:24

Yeah. I often get reporters querying me about the research results that my lab publishes. I was doing a radio interview and filling the host in about our latest research results, and it was very much like a, a. an interview for an adult audience, right? It was about a scientific paper published in the peer reviewed literature with a result that was important for our understanding of nature. that's how I conducted that interview. a couple of days later, I got an email in my inbox from Laura Godwin at Godwin Books who, was email, honestly, I thought was a hoax at first. So she asked me if I'd ever thought about writing a children's book, and I get lots of people writing me weird emails, asking me for parasite themed stuff. I didn't think it was real, but I looked up Laura and she was legit. So I got back to her and we ended up having a conversation and I was just kind of puzzled by her ask, because when you talk about parasites, you talk about. Sex and goop and death a lot that's part and parcel of Parasitism. I was confused about why she thought that would make good content for a children's book. I don't have kids myself, and I am a very enthusiastic aunt, but you know, that's my interaction with children is through, my friends and relative's kids. and when I, I asked her like, why do you think this content would make a good children's book? She said like, that's what kids love. They wanna hear about poop. it took a little bit of convincing and we had a couple conversations where she like brought me around to the idea that parasites could make good children's book content. once I started writing, I saw exactly what she meant. Basically the book is my upper level undergraduate parasite ecology course. Tweaked slightly for an elementary level audience, it was really easy to write. I sat down at the computer in the morning and the stories just tumbled out the way they do when I'm lecturing in that course. and I'm so glad that she talked me into it because it's one of the most fun projects I've done in the past couple of years.

Tripp Collins 10:35

That's awesome. you sort of touched on, I have like three upcoming questions and you sort of touched on each one of them, so maybe I'll just kind of, Ask you to elaborate, a little bit. So one of'em was the level, so it came out of stories that you teach at the undergraduate level. and you say that they're actually changed very little. That's great. I mean,'cause people are smart and curious and they can handle it. even little people. Right. but can you talk specifically about the level of the book and how you approached maybe say like rigor and language and having fun with it?

Chelsea Wood 11:04

the book is a chapter book for kids between second and fifth grade, so eight to 12 years old. the beginning and ending chapters are like bookends that explain why I think parasites are cool. How I got into parasitology and why I think. Everyone else should learn a little bit about parasites too. And then the internal chapters, each one kind of illustrates a different concept that I think is important in parasite ecology with an eye toward making sure that we get, get good, like geographic representation, representation from different groups of, of organisms. And then I know from having taught my parasite course, like what are the parasite topics that people find really exciting? And I made sure to incorporate all of those themes that my undergraduate students go wild for. so that's kind of the structure of the book. And as for how it was adjusted for its audience, I've honed these stories by lecturing for students who are in their late teens and early twenties, People are people, no matter how old they are, people don't really differ from one another in terms of what they find exciting what makes a good story, what keeps them on the edge of their seat, what makes them curious, what surprises them. those tweaks that I've made over the years to try and make my lectures more compelling, also work on the page for this book, the only things that were really changed were the language I had to make sure that I wasn't using too much sophisticated biological language in the book. I made sure to substitute words that were, age appropriate, but also accurate. And, then there were, there were a couple, like more upsetting parts of parasitology that I toned down for an elementary level audience. There's such a thing as overdoing it in a kid's book. as far as making sure things were accurate, I made sure to send out each chapter to a subject matter expert, at least one. this is a peer reviewed children's book where folks have looked at the, details of the parasitology I present and made sure that everything is totally up to snuff. I owe a ton to my peer reviewers who ensured that even for parasites that I'm not familiar with, We've got all the accurate details included.

Tripp Collins 13:18

Awesome. And you said it was, fun to write. And I believe it.'cause it was really fun to read. It reflects, in the writing. And, yeah, I was gonna ask you about that, because, it was so fun to read. I was thinking, man, she must have had a good time writing this. or maybe she was suffering like the rest of us and she's just hiding it. But, sounds like it was a blast to write. can you describe a little bit about what a writing day is for you? do you structure your day differently than a normal day at work? or how does that go?

Chelsea Wood 13:43

Writing is a big part of my job, whether I'm writing a kid's book or not. I'm very much a morning person. My brain works great first thing in the morning, and my IQ declines linearly over the course of the day. So if I'm gonna do something that requires my brain, I always make sure to schedule it first thing in the morning. And when I was writing this book, it was, COVID times. I, would get up first thing in the morning, make myself a cup of tea, and sit down with the chapter that I was working on for the book. each chapter took me, maybe weeks to write, maybe a little bit less than that of a couple hours first thing in the morning. And like any typical workday, I'll write for a few hours and then I'll turn to other things that don't require quite so much of my brain power in the later parts of the day when I'm a little bit less functional. But I spare all the good brain power for the most important tasks, including writing this book.

Tripp Collins 14:34

And there are illustrations, a number of really great illustrations. what was the process of finding an illustrator and how did you determine, the number of illustrations and how they were integrated in the book? there's a continuum between a graphic novel and a book with illustrations so maybe just speak to that

Chelsea Wood 14:50

Yeah, well, we knew we needed illustrations in this book because parasites are so unfamiliar to most people that we don't even have a frame of reference for. When I say like tapeworm, unlike, you know, bird fish, frog, everyone's got a, a prototype for that in their minds. We knew that we needed to provide really biologically accurate, but also compelling illustrations in this book. Early in the process, the publishers showed me Dave Mottram's work and. I just knew as soon as I saw his stuff that he was the right person for the job because Dave has two really important elements for this particular project. Number one is he's worked a lot with animals, and you can tell from the way that he draws'em, that he understands how they're put together. you can draw a really accurate crow, let's say, but if you can draw a crow that's full of personality and has some anthropomorphic features, but you can still see all the crow details you know that person understands how that bird is put together. he had that in his prior work. His biological accuracy was deep and beautiful. on top of that, he was able to imbue his characters with so much liveliness and character. I knew we were gonna need that for the worms because they're hard to empathize with. Under normal circumstances, they needed like an extra dose of personality to get people to be able to connect with them. So I knew Dave was the person immediately. Unfortunately, Dave is a very popular illustrator and we had to wait a while and there was some question about whether we were gonna be able to put the book off for long enough to secure Dave as the illustrator. And ultimately we, we decided it was worth the wait and he was a hundred percent worth the wait. His illustrations are spectacular. As for the number, the numbers kind of just determined by the publisher and they gave me a lot of latitude and deciding like what were the most important things to illustrate. I provided Dave with some cues, like my own little sketches. I'm not a good illustrator but I can point to things that are important or draw out a life cycle. I brought in models he might want to draw from, pictures I found on the internet or my own photos that he could use as inspiration or a template to draw from. and figured out like where a course across the course of the book, we most needed visual support for a concept being, explained in the text. and then tried to fit that into the number of illustrations that the publisher thought were appropriate for a book of this length. I never had the opportunity to work directly with Dave. It's always through the publisher that an author works with an illustrator, at least for this particular arrangement. he did such a spectacular job. I had really high expectations going into this, and he exceeded even those very high expectations.

Tripp Collins 17:40

and as far as the publisher goes, it sounds like when they came to you asking about a children's book, they already have in mind a specific structure for a book, right? Like, did they give you number of chapters, word count and number of illustrations? These were sort of preset from the beginning?

Chelsea Wood 17:58

they gave me an approximate word count, but the more important thing was that they sent me a couple of other similar books they had published in the past few years to let me tune in on the format and tone length, they were envisioning for the book. And that really helped me to like, in my mind, adjust what I anticipated was going to be involved for a book like this one. Then they just let me have at it. They gave me a lot of latitude to decide the structure of the book I didn't have a clear, chapter by chapter structure before I started writing. I knew roughly what stories I wanted to cover based on, wanting to get some diversity in terms of geography and parasite taxonomy and knowing the themes that I wanted to hit. but the book took shape as I was writing it because I wasn't intending for it to be as personal as it wound up

Tripp Collins 18:52

huh.

Chelsea Wood 18:53

If you read the book, you'll see like, there's, there's literally a photo of an art project I did in seventh grade and that was not part of the plan, As I was writing it, I just couldn't find a way to tell the story of why parasites are interesting without telling the story of why I think parasites are interesting. that story is very bound up in my own development as a scientist. So that wound up being an important part of the story in a way that I hadn't anticipated. So there was some structure from the publisher. There was some structure that I imposed from the outset, and then some serendipity.

Tripp Collins 19:23

did they require of you, some kind of proposal or because they contacted you, were they just like, this is the idea, have a go?

Chelsea Wood 19:31

It was just game on from beginning.

Tripp Collins 19:33

speaking of your story in the book, you mentioned that your path to being a professor of parasites, was not a straight one you mentioned that you thought maybe you were going to be a marine biologist, like, many of us do when we're younger, and then you say you fell in love, and love is what, changed your path. what's that story?

Chelsea Wood 19:50

Yeah, well, it starts in college. I was born a marine biologist. I never thought about doing anything else. but when I was choosing a college, I didn't have a ton of guidance. And I wound up at a college that was great, but had no marine biologists or marine biology content on offer. That was okay with me in the first couple of years. I worked in an engineering lab for a little while. I worked in a bark beetle lab at ground up bark beetles, which was pretty cool. I worked on Lake Zooplankton and then as I was getting toward the end of my time as an undergraduate, I wanted to do marine biology research because I knew if I wanted to cuddle whales as my job, I needed experience as an undergraduate and I wanted to start getting that experience. So I talked to the person who I was working for the Lake Zooplankton ecologist, and I asked her like, are, do you have any friends who are marine biologists who I can work for? And she had had an office mate years before who was working at a nearby university, and I ended up working for him. He's a marine biologist, but he at the time was working on trematodes flatworm parasites in snails. And I went into his lab. Like a mercenary, not interested in the parasites at all. And in fact, I thought the parasites were gonna be pretty gross and like small and uninteresting. And I, I didn't wanna work on anything that like weird and off the beaten track. I wanted to work with dolphins, but I was willing to do anything to get my foot in the door. And I thought this marine biology job would help me, like ratchet myself up to cuddling whales someday. And I spent a couple weeks in his lab and I just was dumbstruck by how strange parasites are and how diverse and ubiquitous they're everywhere all the time. They do these alien things. And I, at that point was what, like 2021, something like that. I'd, I'd been studying biology for years and it was the first I'd ever learned about them. I couldn't believe that I could get that far in my education. Find this group of organisms that are 40% of all animal species that are literally all around us all the time. I was just now hearing about these improbable life cycles and ways of living. And that's what made me fall in love. The fact that they are so weird and common and yet poorly understood. It felt like I was getting to see a different dimension of life. it was that I found really intoxicating, getting to see something that I felt like no one else was getting to see. like waking up from the matrix and realizing the world is completely different than you thought it was. That's what made me fall in love.

Tripp Collins 22:34

That's awesome. I love that story. And so often we don't even know what we're interested in until we're exposed to it. I think part of the power of this book is maybe getting the word out about how cool and interesting and fun parasites are, and complicated. So you mentioned lifecycles. I knew, I guess in general what a parasite was. I had no idea how complicated their life cycles were. I think that's one place in the book where the illustrations were super, super helpful.

Chelsea Wood 23:02

Mm. Mm-hmm.

Tripp Collins 23:03

maybe it might be fun to talk through, a couple of parasites and their life cycles. in my notes I have, the story of the Lancet Liver fluke. Am I saying that right? is that a good place to start?

Chelsea Wood 23:16

Sure. That's actually a perfect place to start. I often start with the trematodes when I'm discussing complex life cycles. trematodes are flatworm, parasites. And most of them start their lives in snails. diaper C Dendritic, the Lancet Liver Fluke is a terrestrial trematode that uses terrestrial snails as it's first intermediate host, the first host of larval parasites. so these guys start out in land snails. And inside of those snails, they reproduce asexually. They basically clone themselves. And, those asexual stages are emitted from the snail in the mucus that the snail leaves behind. in this particular group of snails, that mucus tends to agglomerate into slime balls after it's left behind And those slime balls are a delicacy for ants. ants love to eat those slime balls, and that's how they become infected with the next larval stage of the parasite. Which, forms cysts inside of, the ant's heads and manipulates their behavior. These ants that are infected, tend to climb up onto vegetation and then clamp onto the very end of that vegetation and just weight there, which is a, not a normal behavior for an ant, but for an infected ant. Makes perfect sense because this parasite is trying to get its ant into the gut of the final host of the parasite, which are grazing animals like sheep and cows. Once that ant is eaten by a sheep or a cow that is just, grazing on grass, it's not trying to eat ants, but it accidentally might chomp down on one as it's grazing, that worm can now migrate into, the liver of that grazing animal where it. Finds another worm sexually reproduces, produces eggs, and those eggs are pooped out into the world by that sheep or cow. a snail eats that poop gets infected with the parasite and the lifecycle is complete. And that three host lifecycle snail to ant to grazing animal like sheep or cow, is classic in the trematodes. You see this, whether they're in marine or freshwater or terrestrial environments, not all of them, but most of them have this three host lifecycle. And it seems so improbable, right? Like, why make life this complicated for yourself? But that kind of lifecycle lets this parasite get the best of all possible worlds. It clones itself to huge numbers inside of its invertebrate hosts. and it uses, its vertebrate, hosts the sheep or the cows to move great distances, thereby getting its babies to spread far and wide. So it's really a way of optimizing multiple different, aspects of its own fitness.

Tripp Collins 26:10

Yeah, it's mind blowing, right? it's so complex and complicated. It makes you wonder is there a measure in biology for how complex a lifecycle is, or how unlikely it just seemed. So when you think, when I thought, when I first read that and I read, about the lifecycle, I thought, that seems very unlikely to happen. But then the key part is once it's eaten by a, grazing animal, there has to be another parasite already in there, the mating happens because a second parasite meets, up in the gut of a grazing animal. it just seemed so. crazy and unlikely, but it must be balanced by how effective it is, Otherwise, it just wouldn't work. I guess the question is, is there a way to measure how complicated these things are it seems counterintuitive, but it must work. I mean, it's working so well. is there something in evolution that's pushing parasites towards more complicated life cycles. it's just hard to imagine how this evolved into this area. Could, could you find question there for me?

Chelsea Wood 27:09

Yes, absolutely. Yeah. So the thing about parasites, and this isn't true for all parasites, but for many parasites, they sit on one end of a spectrum that ecologists sometimes call this R versus K continuum. And by that we mean, um. that either invest a ton into their offspring and have very few of those offspring, you know, like take really, really good parental care, spend a lot of time raising their offspring. think like elephant or human right. we only have a couple of babies and we take really good care of each one. We invest a lot into each individual offspring and we have few of them. That's one side, that's the K end of the spectrum. The other side is the r end of the spectrum where you just like blast out a bajillion babies and you invest very little into each one and you just trust in statistics. Some of them are gonna make it by random chance, not by any investment that I've made as a parent, but just because some of them will make it because they'll get lucky and a lot of parasites sit on that. R end of the spectrum and. The stage, for example, that we talked about the trematode in the snail, it's producing hundreds of thousands of a sexually produced cloned larvae over the course of, one snail lifetime. And that is classic R selection, right? Like those babies cost very little to make you just pump'em out. And yes, like 99.9% of them are gonna die. But even if that's true, that still means like a couple hundred of them are gonna succeed. so you just kind of rely on luck to make sure some of your offering or making it into the next generation in that kind of life history approach. now that's r versus K selection, but you also ask what is the evolutionary incentive for developing such a complicated life cycle?'cause you can imagine a lot of disincentives, right? Like a complex life cycle like that is so fragile. If any one of those links is lost, that's just game over and it seems improbable that all the dominoes would line up in exactly the right way to make sure that this parasite can actually complete that cycle. But if you think about it like over evolutionary time, think about like that ant to raising animal transition. Like what if way back when this parasite was just cycling between snails and ants and back again, like that's still a complex lifecycle, but it's much less complex. Some ants are sometimes gonna get eaten by grazing animals on accident, and that probably happens relatively frequently considering that ants and the food that these animals are grazing on coincide with one another. In habitat, you're a parasite and your host gets eaten and you die, that's a dead end for you. If you're a parasite and your host gets eaten and you find a way to infect that predator host, then that could be a major boon for you. And so it's thought that a lot of these complex life cycles where predator prey relationships are a part reflects pressure over evolutionary time for parasites, to find ways to infect hosts that otherwise would just be predators that kill them.

Tripp Collins 30:26

Right.

Chelsea Wood 30:27

And because predation is something that happens all over the world, there's been a lot of opportunities for selective pressure to drive the evolution of more complex life cycles that infect higher animals in the food web.

Tripp Collins 30:38

Let me ask you a more elementary question for an animal whose lifecycle depends on being eaten, how do they not get damaged in themselves? Is it just a difference in scale?

Chelsea Wood 30:51

sometimes they do get damaged, but many of them have, strategies to avoid, number one being chewed up, and number two, being dissolved in acid. often when they're being consumed, they're in a cyst stage, so dirac dendritic, the Lancet liver Fluke is inside a resistant cyst inside its ant host. that cyst is, a cellular, like non cellular tough material, kinda like an eggshell that protects that parasite as it passes through the dangerous parts of its next host gut tract teeth and stomach acid. usually the stomach acid is the cue that the cyst uses to hatch. So it feels that stomach acid, there's a delay and then it hatches and boom, it's in the intestine where everything's safe. The pH is a pH that it can survive. There's plenty of food around, there's mates. and it can go on with its lifecycle. Different parasites do it different ways. that's how the trematodes do it. The nematodes or the roundworms have a very tough cuticle that can survive a lot of abuse. We put these animals into very low pH hydrochloric acid in the lab and they are happy as clams. it depends on the parasite, but they have a variety of adaptations to make sure they can make it through that dangerous stage of getting to where they want to go.

Tripp Collins 32:11

Yeah. it's amazing. it's amazing how different parts of their life cycle are perfectly suited for either getting through that stage, getting through eating, being eaten, and getting through, stomach acids. once they transition, of course that stage would be susceptible, but now they're in a position where, they're safe from that. Yeah. it's crazy. let's talk

Chelsea Wood 32:33

been happening for a long had a many, many generations to shape all these complex life cycles, and it's one of the coolest things about the parasites in my opinion, are these baroque life cycles that just seem, they blow your mind when you first learn about them because they just seem so unlikely to work, and yet there they are working.

Tripp Collins 32:54

Exactly. let's talk about toxo. this is apparently the world's most successful parasite that we know of. approximately one out of every three people has at some point been infected with Toxo. what is it and, how does it affect us?

Chelsea Wood 33:10

Toxoplasma Gandhi, which I like to call Toxo for short, is a protozoan parasite. So just single cell. Uh, it's very closely related to the parasite that causes malaria. it is much more successful than malaria because you can find it on every continent, in every species of mammal in many birds. And in, as you mentioned, one out of every three people on the face of the planet toxo at its most elementary is a cat parasite. it reproduces sexually in the gut of cats. Every toxo has to pass through a cat at some point in its lifecycle. it's typical intermediate hosts are the hosts that, um, that are home to the larval stage of the parasite are rodents. And you can imagine how that passage happens, right? The cat poops out and egg the egg is ingested by rats. Rats eat cat feces, contaminated things all the time. the rat becomes infected with the larval stage of the parasite. And then when a rat gets eaten by a cat, that's how the lifecycle gets completed. The tricky thing about Toxo and one of the coolest things about Toxo is that it could wait around in the rodent host for that predation event to happen, for the rodent to be eaten by a cat, because that happens often enough. But natural selection is never happy to leave well enough alone. It's always tweaking the dials to make things a little bit more efficient. And Toxo has developed some really sneaky ways of. Making it likelier for its rodent host to wind up in the jaws of cats. Normal rodents are afraid of cats with good reason. They're the most common predators of rodents. And so normal rodents will, when they smell a cat, adopt behaviors that make it likelier they're gonna stay safe. They'll hide, they won't move around as much. They're definitely not gonna look for food. and they'll move away from the smell of cats Rodents that have toxo not only don't have any of those evasive behaviors, but they actually move toward the smell of their predators. They love the smell of cat urine. as a result, toxo infected rodents are far likelier to be eaten by cats. And when you think about it, that's like this is a single celled parasite and rodents wall. Maybe not the smartest. Mammals on the face of the planet are still pretty smart. And somehow this single celled parasite can pull the strings of its rodent host and cause this whole cascade of really complex behaviors that serve the parasites ends and not the hosts. Really impressive. Even more impressive when you learn that humans can get accidentally involved in this lifecycle. We, because we're not, cats are not a place where the parasite can reproduce sexually, but we are similar enough to rodents that we can serve as intermediate hosts. And so taking the place of rodents in that lifecycle talk so pulls the same strings in our bodies that it pulls in the rodents and as the same outcomes. rodents that are infected with toxo have slower reaction times and they're more reckless and less cautious. Same thing in people, humans who are infected. Are three times likelier to get into a car accident or a workplace accident because people who are infected with toxo have slower reaction times. There's a whole other suite of behaviors that differ between infected and uninfected people, and it's even more impressive that a single cell parasite is able to pull the strings of what we think is the most complex machine in the universe, but somehow they do it.

Tripp Collins 36:58

What do we know about the mechanisms by which it works?

Chelsea Wood 37:01

I wish I could go into the details, but unfortunately I'm not super familiar with the literature on all the biochemical mechanisms by which Toxo manipulates our behavior it's an ever evolving area of research we're rapidly learning new things about how Toxo does this. all I can tell you is that it has something to do with neurotransmitters.

Tripp Collins 37:21

Yeah. Okay. it's obviously something that developed out of evolution, right?'cause it's not like talk to is sitting around thinking about how it can control other creatures. it's just a mechanism that happened to work at some point, and then that helped it survive and, became somehow encoded in its genetics. but yeah, it's crazy and it's not alone in terms of parasites controlling the behavior of other creatures. is there any other examples that you like

Chelsea Wood 37:52

so here's a great example of behavioral manipulation and this one comes to us from another flatworm parasite that uses snails as its hosts. this one's called U Hep Locus, California Ansys, and it is a marine trematode that is very common in the salt marshes of California. It's passed from snails onto fish and onto birds and fish that are, uninfected here. And here I'm talking about a California achille fish. They're little, couple inch long bait fish that you might find schooling in a salt marsh. They're the snack food of the salt marsh. They're eaten by a whole variety of predators, including the store birds and wading birds that use those salt marshes as overwintering grounds, places where they're kind of fattening themselves up. These killy fish, if they're uninfected, much like uninfected rodents who are trying to avoid being eaten by cats, they will kind of hang out close to the bottom. They won't move around all that much. They won't leave cover because they know that they're being hunted by visual predators. In the case of these kill fish, it's egrets and herons stalking them from above. And these birds are looking down into the water trying to see if they can see the fish so that they can grab them with their beaks. And the kill fish are smart, they wanna avoid this, and so they hang out close to the bottom. fish that are infected with Californians instead will venture out from their refuges. They'll swim close to the surface. They'll flash their shiny side upwards, and in general, just like make a spectacle of themselves. As a result, those kill fish are between 10 and 30 times more likely to be eaten by a bird. Now, none of that's surprising. You've already heard about behavioral manipulation, but the cool thing about this example is that while what the parasite is doing is obviously very bad for the kill fish. The kill fish would rather live and not be eaten by a bird. It's actually doing something kind of good for the bird because if there are parasites in a marsh, making fish 10 to 30 times more available for birds, the parasites are feeding the birds. They are making more fish available to the birds than otherwise would be available. And birds are something that we as humans care a lot about. We invest a lot of money into trying to conserve them. We're really worried about a lot of bird populations in California, especially right now Unacknowledged assist from parasites. And it's not just you have Locus Cal, California Ansys that does this. It's not just this one. trematode salt marches in California behavioral manipulation is really common across many species of parasite, which means that all around the world, all the time, if we care about conserving predatory animals, we are getting this secret and quiet assist from the parasites that are found in the prey of those animals. behavioral manipulation is cool because it's neat to think about the animals we see walking around actually being puppeteered by parasites. but it's also cool because it means that we as conservationists are being helped in. Unacknowledged ways by the parasites that inhabit the prey of the animals that we care about. And I think it's really neat to imagine what the world might be like without parasites. In some ways it might be better, but in some ways it might actually be worse. And that's sometimes surprising for people to learn about.

Tripp Collins 41:38

Yeah. the parasites are affecting the balance in an ecosystem, and we don't really appreciate that. when we're going to conserve an ecosystem, we also need to consider the role of parasites in order to do it properly.

Chelsea Wood 41:50

Mm-hmm.

Tripp Collins 41:50

Yeah. And actually, and it's possible, of course it's possible that parasites could go extinct just like any other creature. that was one fascinating thing from your book. There's, uh, I, I remember becoming aware of this because I was reading about the Carolina parakeet, a species of parrot, which roamed North America, especially the southeast for millions of years after, European contact, maybe within a hundred years, something like that, the Carolina parakeet went extinct. And what people may not realize is that, that Carolina parakeet carried a parasite. And because that parasite, was completely dependent on that animal, the parasite also went extinct. it seems that it's difficult for us to really get a grasp on parasite extinction because they don't leave great fossil records. can you talk about that a little bit?

Chelsea Wood 42:40

Yeah, this is something that I worry about a lot. A lot of the work that my lab does is focused on trying to understand how parasites are changing as the world changes. one of the surprising things that we've learned is that while it's certainly true that when humans mess with ecosystems, sometimes parasites are able to take advantage and cause an outbreak much more often. The trajectory of parasites, the way that they change over time as humans increasingly erode ecosystem integrity is they decline sometimes even to extinction, as you mentioned for the Carolina parakeet We worry a lot about that. There are some examples other than the parasites of host species that have gone entirely extinct, like the Carolina parakeet. One of my favorites is from the North Atlantic, where there used to be this amazing a spit ofan parasite. spit of gaste are closely related to trematode nodes, but a little bit different. They're flat worms and they have this crazy sucker on their bellies that has a bunch of divisions inside of it. So it kind of looks like a honeycombed sucker. There's still other spit of gas around, but there is one spit aasan parasite that used to exist in the North Atlantic that we now believe was driven extinct by overfishing of its shark and Ray hosts. none of these sharks or rays have gone extinct themselves, but they have been fished to such low levels that this parasite is now gone and we suspect gone forever. it is hard to document these things because parasitology is a relatively new science. We haven't been documenting parasites for much longer than the past, couple of decades to 150 years. if something wasn't described a long time ago, it could have gone extinct without us ever even giving it a name. as you mentioned, parasites don't fossilize well, so we don't have long-term records of their abundance like we might for things that have bone. but it is important for us to track parasites and their trajectories through time. And, and part of what my lab works on is doing that even where the paper records, you know, the people writing species descriptions don't exist and fossil records don't exist. for that we use. Museum specimens, fish and other animals floating around in jars. typically what's in the public space of a museum is only a fraction of what the museum actually holds. they'll have like basements and basements and basements full of other jars of less attractive fish, birds, mammals, amphibians, reptiles. And what we can do is go into those animals, perform dissections of them, pull out any parasites that we find, and identify them to figure out is this a parasite species that we have today? Or is it one that's extinct?

Tripp Collins 45:35

Wow. That's awesome. I'll tell you what I took away. I definitely got your love of parasites, you wanted to share that because just knowing, that parasites are something to study or that they're important for nature, you know, there's more to biology than, than than dolphins. All dolphins are cool too. and that even weird creatures play very important roles in nature. and we have to understand them to have a holistic understanding of nature. but I'd like to hear from you, what do you hope that this book does in the world?

Chelsea Wood 46:04

I hope the book opens folks' eyes to the life that's beneath the life that you can see. I think in the past couple of years, as we've talked more and more about the microbiome, folks have realized that there are whole ecosystems inside of the animals that occupy most of our attention in the field of ecology and biology more broadly. But there are things beyond just the bacteria that inhabit our guts. There are lots of other organisms, animals, bacteria, but also viruses, fungi, protozoa that have interesting effects on their hosts, but are also interesting in and of themselves. the special thing about learning parasites to me was that I got to peer under the hood of life and it kind of opened my eyes to the fact that. What is quotidian in our everyday life is just the surface level and that it's possible to go deeper. Learning about parasitology showed me this kind of portal into a different dimension, and then I could take that portal anytime I wanted. So that's the most important thing for, for that. I hope folks will take away from the book this opened door that they can then pass through again and again throughout their lives, anytime it ps their curiosity to know what's happening underneath the surface of what's familiar.

Tripp Collins 47:32

Yeah, exactly. there's more out there than what meets the eye. are there any science books you would like to recommend? Either, if people are interested in learning more about parasites or just more generally science books that you like? I.

Chelsea Wood 47:45

Absolutely. Folks who enjoy power to the parasites. Might also enjoy Never Home Alone by Rob Dunn. Now this is a book that's geared toward adults, but the content in the book and it tickles the same spots as Powered to the Parasites does the premise of Never, never Home Alone is, that are houses are inhabited by a whole menagerie of creatures that usually go unacknowledged. And I thought that as a professional parasitologist, there would be very little in this book to surprise me, but I was shocked. my jaw was on the floor multiple times as I read that book. it was really eye-opening for me and I won't spoil any of the amazing stories that Rob tells in that book, but I'd strongly recommend folks who are curious about the life that exists under the surface. check that one out. Another science book that I read in One Go Couldn't Put Down is The Chimp and the River by David Kwaman. This is a really short, book about the origin of HIV, which sounds like it could be a pretty dry story, but in Kwame's hands it is a thriller. I literally sat down with this book one morning and did not get up again until I finished it. And again, I thought that I, I had actually picked up the chimp in the river'cause I was in the middle of doing some research on the origins of HIV for another project I'm working on. I'd already read all the peer reviewed literature, I'd read a lot of the scientific books, and so I thought that I knew everything there was to know. But Kwaman has this way of just bringing all that science to vivid life and making it into this totally compelling page turner of a story. so those are two science books that I've read recently that I just could not put down.

Tripp Collins 49:39

Awesome. And what's next for you? what are you most excited about exploring in the future?

Chelsea Wood 49:46

I'm actually working on another book project right now. I'm under contract with, basic books at Ette and the book is due to my publisher this coming fall and will probably be on bookshelves in early 2027. It's called Netherworld, the Past, present, and Future of Parasitism on Planet Earth. it's a trade book geared toward an adult lay audience. the book essentially asks which way parasites are going as the world changes. Are we gaining parasites? Are we losing parasites? And what does that mean for us?

Tripp Collins 50:22

Is this growing out of the book you already wrote, is this like the adult version of power to the parasites or, is this something that is completely different in some way? can you talk about that?

Chelsea Wood 50:34

Yeah, it has many of the same themes as power to the parasites, but where power to the parasites is kind of like a romp through the field of parasites. Um, to introduce folks to the major themes and players. netherworld, grapples with a central question in my field of parasite ecology, which is as we mess up ecosystems, are we bringing ourselves outbreaks of parasites? Are we driving parasites extinct.

Tripp Collins 51:06

Mm-hmm.

Chelsea Wood 51:07

it grapples with that question through both the story of my interest in the question, kind of like power to the parasites, but it also introduces folks to peer reviewed scientific findings that fall on both sides of that ledger and comes to a conclusion about what we should expect in the coming years in terms of parasites that affect people and parasites that affect wildlife. So, netherworld is much more of a introduction to a, a key question in a scientific field and reveals as a professional in that field what answer I've come to. power to the parasites is really more about sharing my, pure joy at the weirdness and coolness of parasites.

Tripp Collins 52:00

Yeah, that sounds fascinating. Can't wait for that one to come out and if people wanna follow along, how do people best find you and support your work?

Chelsea Wood 52:08

You can check out my work at my website, which is www.chelsealwood.com. you can also find me on Instagram at Parasite Enthusiast.

Tripp Collins 52:21

Cool. Chelsea, thank you so much. This has been great. the book was really fun. This conversation was really fun. if you're out there listening, this is great for you, but it's also a great, gift for the kids in your life. this has been Chelsea Wood, and we've been talking about power to the parasites. Chelsea, thanks again.

Chelsea Wood 52:36

Thank you so much for having me Tripp.

52:52

Hey, trip here. One more thing before you go, I want to thank you. Yes, you Thank you for listening to Book Science. If you got something out of today's conversation. Please share with a friend. That's the best way you can help spread the word about the show. If you wanna learn more and keep the conversation going, head over to trip collins.com. That's T-R-I-P-P collins.com. There you'll find show notes and transcripts and our full R archive. I'm Trip Collins and this has been book Science. Your invitation to think deeply. Stay curious, get off the scroll and get out into the world. Take care. This show was produced on the outer banks of North Carolina on the traditional lands of the Roanoke Andin Peoples.