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Why losing parasites could have devastating effects on our ecosystem

A “family” photo of another species of parasitic isopod, this one from the genus Anilocra, attached to the body and fins of the fish host. (Nico Smit)
A “family” photo of another species of parasitic isopod, this one from the genus Anilocra, attached to the body and fins of the fish host. (Nico Smit)

Parasites. Cause of terrible diseases. Big on the yuck-factor. It’s easy to think of them as doing no good.

“The public perception … when you use the word ‘parasite’ is that it’s bad,” says Nico Smit.

But when it comes to parasites, gross is good.

They’re one of the most common organisms on earth and they’re critical to sustaining a healthy ecosystem.

And now, those ecosystems are even more fragile because parasites themselves are reducing in number.

“Parasite loss is probably the biggest biodiversity crisis we’re facing,” says Chelsea Wood.

” … I fail to understand why something that’s slimy or gross is less valuable as a species than something that’s adorable and endearing.”

Today, On Point: Saving the parasites.


Chelsea Wood, parasite ecologist. Associate professor in the School of Aquatic and Fishery Sciences at the University of Washington. (@DrChelseaLWood)

Also Featured

Nico Smit, professor of ecology at North-West University in Potchefstroom, South Africa. (@NicoJSmit1)

Dr. Frank Richards, physician, epidemiologist and infectious disease specialist who focuses on parasitic diseases. Senior advisor at The Carter Center.

Peter Nejsum, professor in the Department of Clinical Medicine at Aarhus University in Denmark. (@PNejsum)


The famous tongue replacement isopod, Ceratothoa famosa, in the mouth of a Cape seabream.

Ceratothoa famosa and the fish, Cape seabream, both alive in the water.


MEGHNA CHAKRABARTI: What do you think of when I say the word parasite? I’m betting it’s maybe something slimy or gross. Well, there are a lot of different kinds of parasites out there. And parasite ecologist Chelsea Wood spends a lot of time looking at them and studying them, and she joins us today. Chelsea, what do you think of when you think of parasites?

CHELSEA WOOD: The word parasite evokes for me this beautiful netherworld of organisms that most people are unfamiliar with, that exists just under the surface of everything that’s familiar and that are beautiful. They play by their own rules. They do things that are out of a sci-fi movie. And yet they’re more common than pretty much any other organisms that surround us.

CHAKRABARTI: Wow. Well, Chelsea Wood is an associate professor in the School of Aquatic and Fishery Sciences at the University of Washington. And Professor Wood does study parasites. So I’m delighted to have you today because in a sense, the fact that parasites occupy this netherworld as you talk about, means that we need to pay more attention to them. So first of all, let’s start with some of the basics. When we see parasites, how should we define them? What part of the kingdom of life are we talking about?

WOOD: Well, that’s a special thing about parasites. It’s a term that describes a group of consumers, a way of living. So it’s not necessarily about one particular branch of the Tree of Life, but about a lifestyle that’s evolved many, many times all across the tree of life. I define parasites as organisms that live in or on a host and cause that host a fitness class basically.

They take energy from their host, and that means that the term parasite encompasses everything from viruses and bacteria to fungi and protozoa all the way up to the multicellular parasites, the animal parasites. And that means that they can take a whole variety of different body forms.

CHAKRABARTI: So the fitness course there is sounds like it’s the important distinction. So that would mean that organisms that live on or in a body host. But symbiotically so or without any kind of fitness course, you wouldn’t classify those as parasites.

WOOD: Exactly. Yeah. So there are three different kinds of symbionts things that live in or on a host. One are the parasites and they’re the ones that are causing a fitness class. One are the mutualist who cause a fitness benefit to their hosts. Think anemones and clownfish have a mutualistic relationship with one another. The clownfish live inside the anemone, and they provide fitness benefits to that anemone host. And then there are these organisms that sit in the middle, commensals, which cause no impact on their host fitness. So parasites are defined by their negative fitness impacts on their hosts.

CHAKRABARTI: But they encompass everything, as you mentioned, from viruses, which can be just a little bit of genetic material in a protein casing all the way to those very complex multicellular organisms that in some cases I was reading about parasites that can infect blue whales. They can be massive. I mean, can you describe a couple of your favorites and what they look like?

WOOD: Sure. My very favorites are the tapeworms. … And the thing that got me so excited about tapeworms when I first started learning about parasitology was how gorgeous they are under a microscope. Now, lots of people have probably come across tapeworms in their day to day lives. They’re not uncommon as parasites of dogs and cats and in some parts of the world of people too. So if you’ve seen a tapeworm, it’s probably in your dog’s poop. Not the most beneficial way to interact with a particular parasite. And when you have a parasite in the hand, I grant you, it’s pretty gross.

Slimy, flaccid, wriggling, unappealing traits. It’s when we look at these parasites at their own scale, under the microscope, that we get to see how truly gorgeous they are. And the cestodes take the cake. My favorites are from this order Tropeano Renko. … And these are shark tapeworms. They live in the guts of sharks. When they’re larvae, they live coiled up inside a little cyst. And I often find them when I’m dissecting fish.

And the larvae of these … tapeworms have no mouth, no eyes, no gut. They absorb all their nutrients right across their body wall, but their heads are armed with these four curlicue tentacles that they can evert quickly right out of the head end. And each one of those tentacles is equipped with thousands of backward facing spines. So when you look at these things under the microscope, the spines catch the light and refract the light and make it look kind of rainbow. And with those spines on these curlicue tentacles, I can’t tell you I’ve seen anything prettier under scope.

CHAKRABARTI: I’m looking at some I think they look like scanning electron microscope images of this shark tapeworm. And I see what you mean. It almost looks like almost like a plant-like umbrella structure. What impact does it have on the shark?

WOOD: There isn’t that much impact on what we call the definitive host, the host that’s hosting the adult stage of the tapeworm. Those tentacles actually have a use. What they do when they land in the gut of a shark is they’ll invert a tentacle out the front of their head end and lay it on the gut lining of the shark. And then those little recurve spines kind of dig in and the parasite can retract its tentacles so that it anchors itself onto the shark’s gut wall.

And that’s its way of maintaining its position in the gut. Even while the shark is continuously passing material through that gut tract, would be easy otherwise for the parasite to be kind of swept along on that tide of poop and pooped out of the shark. But this way it holds on to its place, and there it just absorbs a tiny bit of the nutrition that the shark takes in each day, a very, very small fraction. So it’s having a fitness impact on the host. Yes. But that fitness impact is very small and amounts to stealing tiny bits of the shark’s food.

CHAKRABARTI: Is there a place on the planet Earth where there are no parasites?

WOOD: As long as there is life, there will be parasites. Anywhere where you can find organisms, there will be parasites on them.

CHAKRABARTI: So what percent of sort of the Earth’s biomass do parasites represent?

WOOD: Oh, I don’t know that anyone’s ever done a tally of parasite biomass, but we have attempted to count up species and quantify the proportion of species that are represented by parasites. And when you do that, you find out that about 40% of all animal species are parasites. That’s a very conservative estimate. It’s probably closer to 50%.

But when you think about that, it’s pretty wild, right? Because no one learns about parasites in their biology classes. I almost got through an entire undergraduate degree in biology without learning about parasites. Somehow I almost had a degree in the life sciences and didn’t know about almost 50% of animal species. It’s a staggering number.

CHAKRABARTI: So that forces the question, which I wanted to ask you, Professor Wood, how did you get into studying parasites?

WOOD: Totally by accident. I don’t think anyone is born a parasitologist. You know, a lot of kids grow up wanting to be marine biologists. And I was one of those. I wanted to be the next Sylvia Earle. And I wound up at a college where there wasn’t a lot of marine biology happening. So as I was working my way through college, I worked in an engineering lab. I ground up bark beetles for a while. I worked on Lake Zooplankton.

And then as I was getting toward the end of my degree, I realized that I wanted to get my feet wet in marine biology. I asked my mentor to set me up with a marine biologist for an internship, and I wound up working with George Byers at the University of New Hampshire, and he just so happened to be working on parasites. And I went into this job like a little mercenary. I was not interested in parasites. I thought they were gross, but they were too small to be important. I was just there for the marine biology experience, but somewhere along the way they got under my skin.

CHAKRABARTI: … We’re going to talk in depth about your research and why you think it’s so important for us to recognize in much more detail the role that parasites play in various environments. Well, just on planet Earth as a whole. But I also just want to take a quick moment to note that, you know, when most people think about parasites, what they’re thinking about are not just the sort of gross factor, but … human disease and lack of well-being. And I just wonder how that factors into your research and how you view parasites.

WOOD: Yeah, Parasites are a serious concern for human health. It’s easy to forget that in the developed world where parasites are not really part of the pantheon of diseases that we have to worry about as humans. But in much of the global south, parasites are a reality of everyday life. The neglected tropical diseases are a suite of infectious diseases, many of them caused by parasites that take a human toll that’s completely out of proportion with the amount of research money that’s spent on trying to find ways to circumvent their life cycles and spare people from becoming infected with these parasites.

They range from river blindness, which we’ll hear a little bit about later, to a disease that I study called Schistosomiasis, which is a trematode parasite of people that you can get infected by when you’re swimming in contaminated water is the parasite actually penetrates the skin and takes up residence in the circulatory system. And more than 200 million people are infected today, despite the fact that we have a really good drug to treat. … Now, I don’t want to downplay the seriousness of those diseases. They are serious. They deserve to be tackled with everything that we’ve got.

And part of my research program is actually dedicated to finding ways to reduce the burden of schistosomiasis on people. But that said, only 4% of all the parasite species that have been described have any impact on human lives or livelihoods. And that’s, you know, probably a great overestimate of the total proportion of parasite biodiversity that affects people. The rest of those parasites are out in the wild doing important things that we need to understand more about.

CHAKRABARTI: So, Professor Wood, you were part of a team that published recent evidence about what’s happening with some parasites in the beautiful Puget Sound of Washington State. So can you tell me the story, first of all, of how and why you and your team even started thinking about Puget Sound fish and parasites?

WOOD: You know, we were curious about this intuition that we sense in a lot of people today, this intuition that parasitism is on the rise. Now, post-COVID, everyone can be forgiven for feeling like we’re living in an age of greater and greater infectious disease. We’ve seen lots of outbreaks in the past couple of years that have had really negative impacts on human populations. And we’ve all sort of got this sense that parasites are ascendant. My team was interested in testing that with real data.

Unfortunately, there aren’t that many data to track what parasites have been up to over the past century. And that’s particularly true for parasites of wildlife. We have some data on human infectious diseases, but for wildlife, there’s almost nothing from the past to give us a sense of whether what we’re experiencing today is normal or exacerbated compared to its historical state.

We decided to create those data ourselves, and the way that we did that was by going into museum collections, finding fluid, preserved fish specimens, and dissecting those specimens to identify and count the parasites that were infecting each of those fish at the time of their deaths. This is a powerful approach because every museum you’ve ever been into, you’ve seen in the public spaces a tiny fraction of the total number of specimens that are held by that museum. In the basement, there will be shelving unit after shelving unit of fluid preserved specimens of fish, amphibians, reptiles, sometimes even birds and mammals.

CHAKRABARTI: It’s the best stuff, usually, I would say.

WOOD: Absolutely. So we went into those specimens, and we assembled a time series of eight different fish species in Puget Sound that were collected between 1880 and the present day. We dissected all those fish. We collated all the data and we looked to see how parasites had changed through time.

CHAKRABARTI: Some 700 fish. And I see that you collected some 17,000 parasites from those preserved fish. And what did you find? How have the parasites or the populations changed over that century or so?

WOOD: What we were expecting to find was both winners and losers. That’s often what you see when you get long time series of any organisms’ abundance, that some species might be benefiting from. The changes brought about in the past century and others would be diminishing as a result of their vulnerability to those changes. Instead, what we found was decline. Those declines were concentrated among those parasites with the very most complex life cycles. Parasites can either be transmitted from one host to another host of the same species.

We call those directly transmitted parasites, or they can be passed from one host to another host of a different species, often in a sequence that can involve up to five obligatory required host species. And we found that those parasites with the most complex lifecycles that required the most different host species to complete their life cycles, they were the ones that were nosediving. And we further found when we looked for reasons for this decline, that it wasn’t pollution, it wasn’t changes in the density of their hosts, but instead it was sea surface temperature that did the best job of predicting whether those parasites would decline or not. And so we think that the declines we observed were driven by climate change.

CHAKRABARTI: So help me understand that a little bit more because, of course, my mind first went to what you said, like maybe there were key hosts during that. The parasite relies on doing various parts of its their life cycles that the population of those hosts declined. But you’re saying that’s not the main driver. Why is it potentially that climate change is having this impact? Because are there times in the parasite sites life cycle where they’re like free floating in the ocean or what?

WOOD: Yeah, it differs from parasite to parasite. We had 85 different species in our dataset and of those 44 were these complex life cycle parasites with three or more hosts in their lifecycle. And when we looked at the influence of host density on the abundance of those parasites, we were just looking at the hosts in which they were collected, the fish hosts, but they passed through a whole bunch of other hosts in one spin of a life cycle. So here’s how we think about the climate change connection. Do you know what a Rube Goldberg machine?

WOOD: So for our listeners out there, if you don’t know what Rube Goldberg machine is. Go Google it right now and you’re going to fall into a rabbit hole of cool videos of these machines, which accomplish a very simple task with an overly elaborate series of mechanisms. Now, imagine that we’re building a Rube Goldberg machine. If you want to make it more impressive, you want to put more mechanisms in it. And the thing that makes that impressive is that the more mechanisms you have, the more things there are to potentially break, the more potential failure points there are and the cooler it is when it actually works.

Same thing with these complex lifecycle parasites. We think that the more host species that you add into the lifecycle, the more vulnerable that machine is to breaking down, particularly when the conditions change. Imagine if we put our Rube Goldberg machine, which we’d built indoors, out into a rainstorm. Of course, some mechanism is going to break eventually, and the more mechanisms there are, the likelier that is to happen and the faster that’s going to happen. So in the same way, climate change is shifting the template on which all these life cycles are playing out. And of course, the parasites with the most precarious life cycles are the ones that are going to do worst in those conditions.

CHAKRABARTI: So to be clear and I’m sorry if this just sounds like a bio 101 type of question, but I want to understand this at a fundamental level. Are you saying that what you wonder, what your team wonders is if the impact on climate change? And we should point out that, you know, the oceans really are the sinks for the additional carbon that we’re putting into the into the atmosphere.

So it’s particular of particular importance that you were studying parasites that had aquatic life cycles but so that the climate changes disturbing the lives and survivability of we don’t know which hosts that these complex parasites rely on. And they’re but what we’re seeing is it is having an impact somehow. So therefore, there’s a reduction in the density of parasites that you found in those fish samples from more recent decades.

WOOD: Yeah, they have to pass through a number of other host species and it could be a decline in any one of those host species that results in a reduction in transmission and therefore abundance of the parasite. But it doesn’t just need to be the abundance of the hosts. It can also be their timing, you know, the timing of reproduction or migration. Parasites are finely tuned to their hosts comings and goings, and if climate change is going to mess up the timing and the overlap between host and parasite, that can also result in a decline in parasites, even if the host stays at the same abundance.

CHAKRABARTI: Okay, so it sounds like that finely tuned aspect of parasites makes them almost like an indicator organism about the overall health of the ecosystems that they are in. Important in and of itself, but in addition to that is a reduction in parasite biodiversity. Does it have an impact on those ecosystems?

WOOD: Well, that’s a big question that parasite ecology is tackling right now. I don’t have a firm answer, but we do have some really interesting hints that parasites might play important roles in ecosystems. I like to think of parasite ecology right now as being where predator ecology was in the 1960s and 1970s. Back then we had no idea how important predators were in ecosystems, and then we accumulated all of these really interesting case studies, including, you know, the one of wolves and Yellowstone.

Wolves were eradicated in the 1920s. Yellowstone’s ecosystems fell completely out of whack as grazers increased in abundance in the absence of their predators and ruined stream side vegetation that other species needed to survive. When wolves were reintroduced in the 90s, that ecosystem went back into balance. We are at the very beginning point of that string of studies for parasite ecology, but what we do know is that they serve some of the same purposes as predators. They can keep host populations in check just the way that wolves keep elk in check in Yellowstone National Park. But they also do some other tricky things, including pushing energy through the food web, which is a very unique parasite trait.

CHAKRABARTI: Meaning what? Pushing energy through the food web. What does that mean?

WOOD: Well, parasites are selfish. Many of them are in these complex life cycles transmitted from prey to predator at least one juncture in their life cycle. And they can wait around for that transmission to happen. You know, prey get eaten by predators all the time, or they can encourage that process along. And evolution has sculpted parasites to manipulate their prey hosts to make them clumsier more reckless, slower, in order to make them likelier to be preyed on by the predator. And therefore, for the parasite to be successfully transmitted to that predator host.

CHAKRABARTI: Okay, well, now we’re getting into some of the almost like sci fi aspects of what parasites can do, But this seems to be a good point, Professor Wood, to take an important tangent here in the conversation, because we touched on it earlier. That for most people, when they think of parasites, what they’re thinking of are the parasitic diseases that infect human beings to great harm to millions of people around the around the world.

So river blindness, as you mentioned, is one such parasitic disease. It’s endemic in Africa and also found in several countries in Central and South America. And it’s caused by a worm parasite that’s transmitted by black flies. And when the worms get under the skin in humans, they cause terrible itching, skin discoloration and at times blindness.

FRANK RICHARDS: One person in Uganda, a woman actually brought out from her house a blouse, if you can imagine. It was worn thin. You could see through the material. And she said the reason it’s worn thin is from my scratching. And I didn’t just use my fingernails. I used the stone and I would have to scratch not just my exposed skin, but through this blouse.

CHAKRABARTI: That’s Dr. Frank Richards. He’s an infectious disease specialist and expert on vector borne parasitic diseases. He’s at the Carter Center in Atlanta, and he’s worked on river blindness for nearly 40 years. And throughout that time, his career has had its lows and its highs. And one of the high points? When Guatemala eliminated river blindness, a major achievement that was verified by the WHO in 2016.

RICHARDS: Being a part of getting rid of river blindness from Guatemala, Guatemala, which had the highest number of people infected in this hemisphere, from river blindness. To have been there when we got rid of it is sort of like a pinnacle for me.

CHAKRABARTI: Dr. Richards says the good news here is that there is a treatment for river blindness. That can’t be said necessarily for many other human parasitic diseases. Now, this treatment is a drug you might have heard of in recent years as it became a point of controversy in the COVID pandemic, talking about ivermectin from the former pharma giant Merck. Now, the drug maker announced back in 1987 that it would donate as much ivermectin as was needed to river blindness programs around the world.

And at the same time, the CDC, where Richards was working at the time, they assigned him to focus his work on Guatemala. Now, Richards remembers early on in his time there, there was a moment where his team was interviewing villagers in Guatemala about river blindness in their community, and he remembers that there was an older gentleman standing off to the side and watching this process. And Richards took a chance and asked the man what he thought the most significant disease was in his community.

RICHARDS: And he looked at me, and almost looking through me and said … Fix yourself on this poverty. Which can’t be escaped. That was his reply to the worst disease. And he was right. It struck me and I felt stupid, actually, at that point.

CHAKRABARTI:  So poverty being the worst disease. Now, that message has stayed with Dr. Richards for years. And he says, well, he can’t solve poverty. But eradicating a dangerous parasite is a meaningful step forward.

RICHARDS: When you live totally exposed to all these things, don’t have good water and sanitation, don’t have good food, don’t have good nutrition, don’t have protection from biting insects, etc., then you see the conditions that I work on. And when that goes away, these parasites that I work on largely go away. And that is why they are appropriately called diseases of poverty.

CHAKRABARTI: So overcoming the obstacles that poverty presents in efforts for disease control and prevention. And to declare river blindness eliminated, at least in Guatemala, those two things contained valuable lessons for global health, says Dr. Frank Richards.

RICHARDS: The notion going back to the early days of reaching the poorest communities that that are not usually places that people worry about is the key bit for me. For elimination eradication programs. And that’s equity. You can’t say anybody is too poor or too far away and declare success. And I really like that.

CHAKRABARTI: That’s Dr. Frank Richards. He has spent his career working to eradicate river blindness and other parasitic diseases. Professor Wood, we’ve got about a minute before our next break here. Just your thoughts on what Dr. Richards said there.

WOOD: I think his work is incredible and the work of the Carter Center in general. When the Carter Center was first set up, they were working on Guinea worm, which is another terrible infectious disease of people caused by a parasite. And I’m all for eradicating parasites that affect human lives and livelihoods.

CHAKRABARTI: And so then and at the same time, though, you have this concern about the overall drop in in biodiversity amongst parasites. And just to take a second to tell me how does that drop in biodiversity, parasitic biodiversity? Could it have an impact on humans?

WOOD: Absolutely. Through several mechanisms, including the control that parasites exert on populations of hosts that otherwise would grow out of control and through their ability to push energy from prey into predators and therefore prop up populations of predators.

This article was originally published on WBUR.org.

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