08/21/2020

Should We Conserve Parasites? Some Scientists Say Yes

16:54 minutes

six sucking lice under a microscope
Six chipmunk sucking lice (Hoplopleura arboricola), one is still clinging to rodent hair. Credit: Kayce Bell

Learn more about how parasites drive ecosystems, and see images and parasite-inspired anime characters in a SciFri SciArts article!


The idea of a parasite—an organism that needs a host organism—has always captured our attention and has been the theme of countless movies, from the sci-fi horror film Alien to the Oscar-winning movie Parasite. But a group of scientists say that parasites undeservedly get a bad reputation, and that some of them should even be conserved. They published their 12-point parasite conservation plan in the journal Biological Conservation.

“There are millions of parasite species that don’t have negative impacts on people or domesticated species or wildlife,” says Skylar Hopkins, a parasite ecologist and an assistant professor at North Carolina State University in Raleigh. “We know very little about their individual ecologies.” 

We are most familiar with the parasites that harmful to humans—parasites that cause disease or decimate crops. A few parasites have caused some species to become endangered, explains Hopkins, but adds that there are usually other drivers as well, such as habitat loss or disease epidemic.

a woman with a safety mask and gloves removes parasites with tweezers from a rodent
Kayce Bell conducting an endoparasite exam on a rodent in Panama. Credit: Joseph Cook

“By definition, a parasite is something that does harm,” says Kayce Bell, assistant curator of terrestrial mammals at the Natural History Museum of Los Angeles County. “But for the most part, most things have co-evolved with their parasites to the point that they can tolerate them. So even though they’re doing some level of harm, very rarely are they lethal or causing enough damage to inhibit population growth.”

At the museum, Bell studies and catalogues species of blood-sucking lice and pinworms of chipmunks and ground squirrels. “Something like two-thirds of all chipmunks have at least one louse and one pinworm parasite, so that means that there’s got to be a pretty high level of tolerance, because it co-evolved with them,” she says. One hypothesis is that the parasites have co-evolved to help keep the immune system in shape—preparing the host to fight off other parasites that may be more lethal. Other parasites are even known to directly benefit humans, such as parasitoid wasps that help control populations of agricultural pests.

“We’ve been trained to think of parasites as a bad thing. People often discount them or don’t include them when they’re trying to think of conserving mammals or studying a mammal in an ecosystem,” says Bell.

a green caterpillar on a branch with a bunch of white eggs hanging from its back
A tobacco hornworm—a pest to tobacco, tomato, and potato crops—attacked by a parasitic Braconid wasp that has laid eggs on the hornworm’s back. When the eggs hatched, the larvae fed off of the hornworm and then cocooned themselves on its back for their pupal stage. Credit: Jeff-o-matic/flickr/CC BY-NC-ND 2.0

“If we didn’t have them in ecosystems, ecosystems would look very different and probably for the worse, in most cases,” says Hopkins. 

While they may not be the most charismatic creatures, they are still a part of our world, says Hopkins: “I think people can both think something is gross and want to conserve it.”

Skylar Hopkins and Kayce Bell, who are both authors on the recent article, talk about the role of parasites in the ecosystem and how a conservation plan might work. 


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Segment Guests

Skylar Hopkins

Skylar Hopkins is an assistant professor of applied ecology at North Carolina State University in Raleigh, North Carolina.

Kayce Bell

Kayce Bell is an assistant curator of terrestrial mammals at the Natural History Museum of Los Angeles County.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Parasite won the Oscar for Best Movie last year and why not? Movies have always played on our fears of parasites. Whether it’s an alien cocooning inside your body and bursting out, chasing us around a spaceship, [Movie Audio] or pods ready to snatch your body when you fall asleep. [Movie Audio]

Despite the horror of the parasites in Invasion of the Body Snatchers and the Hollywood representations, a group of scientists says real parasites in nature get a bad rap and we should even consider protecting and conserving them. That opinion was published this month in the journal Biological Conservation, which outlines such a plan.

My next guests are here to make the case for parasites. And they’re both authors on that article, Skylar Hopkins is an assistant professor of applied ecology at North Carolina State University in Raleigh and Kayce Bell is assistant curator of terrestrial mammals at the Natural History Museum of Los Angeles County. Welcome both to Science Friday.

SKYLAR HOPKINS: And thanks for having us.

KAYCE BELL: Thanks. It’s good to be here.

IRA FLATOW: Skylar, so you think that parasites don’t deserve a bad rap?

SKYLAR HOPKINS: Yes, so of course everyone thinks that parasites are gross, and that’s OK. A lot of people think that earthworms are slimy, but most people probably realize that we need some earthworms in our gardens because they’re good for the soil, right? So we know that parasites are kind of gross and yucky but they’re playing important roles in ecosystems.

IRA FLATOW: Kayce, I imagine you agree?

KAYCE BELL: That’s right. I think parasites, they might have a bad rap but they have a lot of things that they can contribute. So it’s important to keep them around.

IRA FLATOW: Has anyone ever tallied up how many parasites are out there?

SKYLAR HOPKINS: It’s something that’s hard to estimate because there are many species that we haven’t identified yet. And so our best guess is that there are probably millions of parasite species. Alone there are parasites that are distributed across 15 different animal phyla, so it’s this really hyper diverse group and we’re not exactly sure how many parasite species are out there.

IRA FLATOW: Skylar, your group came up with a 12 point plan for conserving parasites. Please give me your reasons why we should conserve parasites.

SKYLAR HOPKINS: Sure, so you can use two different arguments, one would be about intrinsic value. So if we’re going to argue that species have intrinsic value and so they should be conserved, then we should be conserving parasite species. So something like 40% to 50% of all animal species are parasitic. So that’s a really big portion of global biodiversity. And so that has a lot of intrinsic value on its own.

There’s also the utilitarian argument, which is that parasites do things for us which are important. So for instance you know that parasitoid wasps are parasitizing and controlling insect pest populations. And as adults they also do some pollinating. So they are saving us billions of dollars per year in the agricultural sector.

IRA FLATOW: But can you make a case for parasites that do affect human health and cause disease the same way you’re talking about those parasites?

SKYLAR HOPKINS: No, so we certainly don’t advocate for conserving human parasites. So there’s a list of about 1,400 parasites and pathogens that are known to infect people and that is just a tiny drop in the bucket of total global parasite biodiversity. And so we think that those parasites that infest people should certainly be controlled and we’re talking about parasites in wildlife that don’t infect people.

IRA FLATOW: So you have to determine the difference between a bad and a good parasite and choosing what to conserve?

SKYLAR HOPKINS: Yeah. I would say that that’s true.

IRA FLATOW: How does a conservation plan work and what does it look like. Do you go pick parasites and put them in jars or do you leave them on their hosts or what do you do with that, Skylar?

SKYLAR HOPKINS: Yeah, so our overall plan has 12 different steps and they’re divided into these four different themes or categories and so one of those themes is just data collection and synthesis. So we just need to know more about the parasite species that are out there and we need to do a better job of using that data to understand the threats that parasite species are facing.

The next part of that is risk assessment and prioritization, so once we know what parasite species are out there, which ones really need our conservation efforts. For instance, we know that parasite species are really specialized to use host species that are endangered are probably also endangered because if their host species decline and go extinct then they’re likely to decline or extinct as well. So how do we figure out which parasite species are at risk and then can we put them on Red Lists like the IUCN Red List.

And then once we know what parasite species we’re trying to protect, developing conservation practice to do that. So can we develop protocols so that when we are dealing with endangered host species. We don’t accidentally wipe out the parasite species that use that host species. So that’s something that we accidentally did with the California Condor. When we brought it into captivity we deloused it, we probably drove one of its louse species extinct. So one of the protocols that we had developed to avoid doing that in the future.

And then finally, there’s education and outreach. So we just need more people to know that parasites are playing important roles in ecosystems. And so, we can do a better job through museums, and K-12, and college education and also just educating conservation practitioners that parasites are gross and they do harm their host species in many cases, but they are important parts of ecosystems.

IRA FLATOW: Educate me why delousing the California Condor was a bad idea. If you’re getting rid of a parasite that’s attacking a bird?

SKYLAR HOPKINS: I think it really depends on what the parasite is doing. So in many cases even though a parasite is using some of its host resources, it’s not really using enough resources to cause noticeable harm to that host species. And so in the case of the California Condor louse, that louse, might not really have been doing anything to the birds. But we got rid of them anyway just in case. And so that might have been a just in case step that we didn’t need to do.

It’s also important to note that getting rid of all of an organism’s parasite species isn’t necessarily a good thing for that organism. So we know that every animal evolved to interact with parasites, that’s why we have immune systems. And if you take away all the parasite species, that immune system might not be functioning properly or maybe when you put that host species back out in the wild again it’s going to have problems because its immune system isn’t working properly or because it doesn’t have any parasite species and suddenly it’s being exposed to a lot of parasites in the wild.

IRA FLATOW: Kayce Bell, I know you studied ground squirrels along with parasites. Please tell me what the relationship is between these two organisms.

KAYCE BELL: Yeah, so lately my research has mostly been on chipmunks, which are a type of ground squirrel, and there are ectoparasites, so the sucking lice that live on the outside of them and there are endoparasites, the nematodes that live on the inside.

And my interest in this is studying how they have co-evolved or how easy it is for parasites to move among different host species. And this is kind of coming back to some of what Skylar was talking about in trying to understand these natural relationships that have evolved and if things have co-evolved over long periods of time, they’ve probably evolved to this level of tolerance almost. While they might not be good for the chipmunks or OK, they’re probably not actually inflicting that much damage.

So sucking lice are actually similar to the head lice that humans have and they feed on the blood, and yes if you have a whole lot of sucking lice they can cause problems, but if there’s only a few sucking lice on there, they’re probably not actually doing that much damage to the host.

IRA FLATOW: What can the parasites tell us about the environment that the host is living in? Does it tell us about how healthy it is or it’s in trouble or anything like that?

KAYCE BELL: Absolutely. So one of the things that’s really interesting that we’re trying to get the message out there with this paper is there’s a lot of those things we actually don’t know. And so it would we’re trying to learn as much information as we can before some of these parasites go extinct. But one of the really important things that we can learn from parasites about the environment often has to do with what other species may be around.

So a healthy functioning ecosystem requires a lot of different players and a lot of different types of parasites actually require multiple different host species. So it will go for example like through a snail and then maybe another snail and then a fish and then a bird. Well, for that parasite to be in the ecosystem all of those other host species have to be present, otherwise it can’t complete its lifecycle. So just finding a parasite can often tell you a lot about what other species are there and the fact that the food chains are actually functioning the way that we think they are supposed to.

And so to study them I work in museums and we take whole animals, we take a whole specimen and we examine it for ectoparasites, which we save and we also examine it for endoparasites and all of these things go together in the collection so they can be archived to learn more about these interactions and where different things are occurring.

IRA FLATOW: Well, Skylar, let’s talk about some of the different strategies that parasites use. I understand there is a tongue eating isopod that is really weird. Tell me about that.

SKYLAR HOPKINS: Yeah, so I don’t study this parasite but I think it’s very cool. It is an isopod that you can find within a fish’s mouth and it literally chews off the tongue of the fish and you see it sort of replaces the tongue. So the fish no longer has a tongue. If you open up its mouth you will see an isopod inside of that and it’s very large. It’s not a teeny little parasite. It is a very big parasite inside the fish’s mouth.

IRA FLATOW: Does it take the place of what the tongue is doing or is the fish going to die because it has no tongue anymore?

KAYCE BELL: Yeah, actually they do seem to take the place of the tongue and function. There are some studies that suggest that actually fish with that parasite may actually be more efficient at feeding but I’m not sure that’s been well supported. I just know that there’s been some research looking into how well fish fare with those parasites and they seem to do OK.

IRA FLATOW: You know if you look at the movies that are out there, whether it’s my favorite, Invasion of the Body Snatchers or any other movie where the parasites take over, one of the things that parasites do is that they take over your brain or they change your behavior. Do they have a type of mind control? Are there parasites that can do that Skylar?

SKYLAR HOPKINS: Yes, so nematomorph parasites, which are sometimes called horsehair worms or Gordian knot worms, are these remarkably large worms that are stuffed inside the tiny body of a cricket. And they manipulate the cricket’s behavior and cause the cricket to jump into bodies of water, which is not something a cricket would normally do, they’re not aquatic, right? When the cricket gets in the water the worm will leave its body and actually you become free living. So it can swim through the water and it’s looking for other adult worms to mate with and continue its life cycle.

And actually, those crickets that jump into water bodies turn out to be pretty important. So we know that in Japan those crickets are eaten by these endangered Japanese trout. I think they actually make up something like 30% of the trout’s diet and because the trout are eating these crickets, which are subsidizing a stream environment, they eat fewer of the invertebrates that are living in streams, which means that there are more of those invertebrates, which can do more consumption of leaf litter and nutrient cycling and things like that in the stream ecosystem. And so this one parasite manipulating cricket behavior has these rippling effects through the entire ecosystem.

IRA FLATOW: Kayce, there’s a parasite that can influence an ant to change the color of its body?

KAYCE BELL: Yeah, that’s right. So there’s a nematode or a type of roundworm that infects ants. And what it does is it gets into the abdominal region of the ant and it turns it bright red so it looks like a berry and then these ants actually will hang out in places that they’re really visible to birds and then birds think they’re berries, come along and eat them, so then the nematode gets passed on to the bird, so it can complete its life cycle. There’s a lot of great examples like that where parasites make a host make itself more available for the next host.

IRA FLATOW: Pretty tricky. Skylar, parasites have a complex life strategy I understand. Lots of animals have symbiotic relationships, but how does an organism evolve from symbiotic to parasitic?

SKYLAR HOPKINS: Yes, that’s a great question as to does something become symbiotic first and then become a parasite? That it’s not necessarily the same sequence of events for all parasite species. We actually know that parasitism has evolved independently many, many times in the animal kingdom. So like I mentioned there are 15 phyla within the kingdom animalia that contain parasitic species. And then within those 15 phyla, parasitism has cropped up again and again.

And you might think that these things might start from a mutualism, so two species are benefiting each other and then one species starts to cheat and be a parasite, but that doesn’t really seems to be the case actually. It seems like cheating can just evolve on its own pretty commonly.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios. Our bodies are filled with bacteria and viruses that live off our bodies. Should we consider a microbiome to be full of parasites?

SKYLAR HOPKINS: Yes, so we know of course that our microbiome can consist of both beneficial species and detrimental species, but there is a gray area where it can be really hard to tell if an individual microbe, or even a larger sort of ectosymbiotic organism is really good or bad for the host.

So there are examples with like staph, where a relatively high proportion of the human population has this potential pathogen on them all the time but it is not hurting them and for some reason, once in a while these infections end up being very detrimental. And so in general, the line between parasite and not parasite, is a little grainy, can be hard to determine.

IRA FLATOW: You know, I started out our discussion by talking about how parasites have been a source of inspiration for Hollywood. Skylar, what are some other parasites that have invaded pop culture?

SKYLAR HOPKINS: So you talked a little bit about the zombie parasites. There is a whole video game called The Last of Us, which is about Cordyceps fungi basically taking over the human population and turning us into zombies. Also, I’m not a huge Pokemon fan. I don’t know all the details. But I do know that there are Pokemon characters like Parasect, which are based on parasite species.

IRA FLATOW: Kayce, why do you think we’re so fascinated in our culture and when we talk about by this particular host-parasite relationship?

KAYCE BELL: I think some of it comes from being interested in things that we perceive that there is a little bit of a yuck factor. A lot of us are interested in things that others perceive as gross. But also I think it’s kind of the same reason we’re fascinated with horror movies, right? There is this like intrigue to these things that are mysterious and potentially dangerous. So at least from my perspective there’s a lot of different reasons that people can become interested in studying parasites or even just from a pop culture perspective, understanding how these foreign things could potentially impact our lives or the lives of animals or something.

IRA FLATOW: Well, we’ve run out of time. A fascinating discussion. I want to thank both of you for taking time to be with us today. Skylar Hopkins, assistant professor of applied ecology, North Carolina State University in Raleigh. Kayce Bell, assistant curator of terrestrial mammals, at the Natural History Museum of Los Angeles County. Again, thank you both for taking time to be with us today.

SKYLAR HOPKINS: Yeah. Thank you so much.

KAYCE BELL: Thank you.

IRA FLATOW: And we’ve got an article that shows some of the parasites we’ve talked about plus some parasite inspired anime by parasitologist Dr. Tommy Leung. It’s on our website at ScienceFriday.com/Parasite.

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