As Primates Go Extinct, So Do Their Parasites, Upsetting Ecosystems

IRA FLATOW: This is Science Friday. I’m Ira Flatow. No one thinks of parasites as friends, do they? Live inside or on your body. They come in all shapes and sizes, from fungi or viruses to lice and, of course, even worms, as uncomfortable as that thought may be. But a thing they all have in common is that they need a host, whether that’s a person, another animal, or even a plant to survive. Doesn’t sound very welcoming. But SciFri producer Christie Taylor is here with a sympathetic story about parasites in peril and why we should care. Hey, there, Christie.

CHRISTIE TAYLOR: Hey, Ira.

IRA FLATOW: What’s this about peril?

CHRISTIE TAYLOR: Well, Ira, I’m going to use a metaphor for something you actually like for this, something that you may actually consider to be a friend, orchids.

IRA FLATOW: Yeah, you got my soft spot.

CHRISTIE TAYLOR: Well, one thing we’ve talked about a lot on this show is how orchids have often evolved to have very, very specific pollinators. They might have a very special relationship with a bee or a moth that doesn’t pollinate any other flower. And maybe it can’t because it’s evolved to only pollinate things shaped like that particular orchid. What do you think happens, Ira, when that orchid goes extinct or that pollinator goes extinct?

IRA FLATOW: Well, it sounds like the bee is going to starve to death, or that flower is going to have trouble reproducing.

CHRISTIE TAYLOR: Exactly. And you can think of that same problem when it comes to endangered primates– apes, monkeys, and lemurs. They all have parasites– lice and worms and viruses and teeny little microbes. And some of those parasites only know how to live on one species of lemur or monkey or ape. And about half of the species of primates on Earth are considered endangered right now. So guess what that means for the creatures that hang out inside and on top of them?

IRA FLATOW: Well, the parasites are going to die. And isn’t that a good thing?

CHRISTIE TAYLOR: So I talked to Dr. James Herrera, a research scientist at the Duke Lemur Center in Durham, North Carolina. He just finished a big study that looked at all the species of primates and what we know about their parasites and concluded that as many as 100 species of parasite could be lost entirely if endangered species went extinct. And he’s here to say that that’s going to be bad if that happens. So here he is giving the first very important definition, what is a parasite?

JAMES HERRERA: With disease ecology, we generally take the ecological definition of a parasite, which is any kind of organism that lives on or inside of another organism, deriving their resources and their benefits at some cost to the other organism, which we call the host. For us, it also includes not only the worms that we typically think of or ectoparasites like ticks and fleas, but also viruses, bacteria, and fungi and protozoa that in the medical field, they usually refer to those as pathogens and parasites. The helminths we often call, like, a macroparasite because it’s big enough to see. For us, it also includes viruses and bacteria.

CHRISTIE TAYLOR: Why did it seem important to look at what might happen if the primates that host these parasites went extinct?

JAMES HERRERA: We know that species are going extinct around the world largely due to human activities. But we’re all often focusing on the big charismatic species, the flagship species that we can all rally behind and support, like tigers and pandas. But then there’s this whole hidden biodiversity that’s going extinct right alongside these better-known species. And those are the parasites.

We know that there’s thousands of species of parasites that have barely even been described by science. Probably there’s another couple thousand out there that haven’t even been studied yet. So that’s kind of why we started thinking about, hey, this could cause more extinctions than we realize.

So we investigated how the ecological network of host-and-parasite interactions would be affected by the removal of the hosts, particularly the threatened hosts. We used approaches that have been applied in networks of plants and their pollinators, as well as social networks of people. So this required combining and combing through all the literature of everything that’s been written on the interactions between wild primates and their parasites.

We then assessed how those host-parasite interactions would be affected under different simulations of extinction. So we compared the changes in networks after removing the endangered hosts to the effects of just randomly removing the same proportion of hosts but without respect to the endangered status. We estimated that as many as 250 parasites will be affected by the loss of their endangered hosts.

Of those 250, some still have other hosts that are not primates. So they might have some kind of ungulate or bat that they also infect. And that’s where we figured that it’s something like 176 parasites that are only known to affect these endangered hosts. So they probably don’t have any other hosts out there. They might, but it hasn’t been documented yet.

CHRISTIE TAYLOR: So if you were going to make like a Save the Parasites poster, who would be on these posters? Is it viruses, like you mentioned? Or is it things like ticks and louses and tapeworms?

JAMES HERRERA: That’s a great question. I wonder who would want to buy any of those T-shirts.

CHRISTIE TAYLOR: [LAUGHS]

JAMES HERRERA: But I would have to say– this is obviously my own bias but, it would be many of the helminth worms and some of the mites and lice because those are some of the most specialized. So those are the species that really seem to only infect one or a few hosts. And the reason is because of their transmission.

So they are transmitted typically by close contact, which means you’ve got to be coming into literal physical contact with your infected individual for you to pick it up. This typically isn’t happening between different species because different species aren’t huddling and sleeping together or grooming each other. So things like these lice and mites that literally spend their entire life on a single individual host, they are some of the most specialized. In fact, there’s several mites and lice that are only found on these endangered lemurs in Madagascar. And given that Madagascar’s an island and it’s been isolated for 60, 90, 100 million years, it’s pretty clear that they’re likely to be truly specialized.

CHRISTIE TAYLOR: Maybe this is a devil’s advocate question, but you what if that louse or, say, a pinworm does go extinct? Does that matter at all? Should we be sad about the end of that species, if the only ecosystem it was ever part of is an ape that is no longer on this planet either?

JAMES HERRERA: It might be hard to imagine, but some parasites might actually play important roles for the regulation of host populations, similar to the way predators do. So in that sense, they’re really important to stabilize populations and prevent them from exceeding the environmental carrying capacity. So in that way, you can think of the parasites shaping their host population dynamics kind of the way the wolves shape the ecosystem’s in Yellow Stone when they were reintroduced. I think it’s not an exaggeration to think of that kind of analogy.

One of the things we are hypothesizing is that endangered hosts seem to have fewer parasites than their non-endangered hosts. The parasites they do have are these specialists, that they’re only occurring in one or a few host species. And they’re not the generalists that are infecting tons of different species. The reason that we see this pattern now may be that we’ve already lost a lot of these parasites in the process of the host becoming endangered.

So what are the things that make a host endangered? Small population sizes that are fragmented so that they can’t interbreed. Those are also factors that prevent the transmission of diseases. It’s like our social distancing, right? So that may have actually already led to some extinctions.

Now, what effect does that really have with the lemurs? It’s hard to say. We really need to do a lot more research. But actually we’re seeing that in areas where the habitat is degraded by people, like people cutting trees and going to the forest to collect forest products, people can bring their parasites with them and introduce them to the lemurs.

And so common generalist parasites, things like Giardia and cryptosporidium, that are naturally not very frequently seen in the wild, pristine populations, you do start to see those infections in the lemurs that live close to humans and where there’s more likelihood of interaction with humans. Losing these wild parasites and acquiring these humanized parasites may really– that might be what’s really negatively impacting their health.

CHRISTIE TAYLOR: Hmm. That’s really interesting. Well, and I guess one of the questions I had is, correct me if my numbers are wrong, but something like 50% of primate species are considered endangered, 90% of lemurs like the ones you work with in Madagascar. Can understanding their parasites better help us in conserving those species and preventing them from dying off entirely?

JAMES HERRERA: You got the numbers pretty much right. Yeah, 90%– especially for lemurs, 90% of lemurs are considered threatened with extinction. The other thing I like to think about is that we just don’t know what many of these organisms are doing in their host. Many parasites are not necessarily causing disease in the host, and they may be performing really critical roles in modulating the immune system of the host.

So for example, helminth worms have a mechanism whereby they can tune down the immune system of the host so that they’re not getting kicked out right away. And there’s actually several examples in humans where we see negative impacts when people are de-wormed. And we see positive effects when introducing elements like in treating autoimmune disorders. So we were really just starting to dig into the meat of that question, which is, what does it really mean to be healthy?

We see lots of these organisms, individuals, who are simultaneously co-infected with 9, 10, 11 different kinds of intestinal parasites and ectoparasites. And it’s not like the animals are just falling out of the trees dying from disease. They’re foraging. They’re socializing. They’re finding mates.

And so when we see, for example, in captivity, where animals have now access to veterinary care and things, the animals still get sick, and they get sick with things like cryptosporidium and Giardia, which are these common generalist parasites that we know can really grow out of control when they’re not kept in check by other potential interactions with a parasite. But there is really a growing consensus that there’s more to it than just these parasites cause disease. And in fact, many of these, in fact, almost all of them, have co-evolved with their hosts, some for tens or hundreds of millions of years.

So if you think about a smart parasite, the smart parasite doesn’t necessarily want to kill its host because then its food source is gone. The old way of looking at it was this evolutionary arms race, or the Red Queen, where each organism is running as fast as they can just to try to keep pace with the other. And the parasite is evolving a new way to infect the host, and the host is evolving new immune defenses, more behavioral defenses. And really there may actually be more mutualism to it than we’ve given credit to in the past.

CHRISTIE TAYLOR: I want to take it back to primates, their endangered status, and again, this research that you just completed that found that we have maybe nearly 200 unique parasites that may also go extinct if these primates go extinct. What do we still need to know if the goal is overall the preservation of biodiversity?

JAMES HERRERA: Yeah, that’s a really deep question because at the most basic level, we still need the basic natural history information. We don’t even know how many species are going extinct because they’ve never been described. So we still need the basic natural history-type science to document, really, at a species level. Because oftentimes what we’re dealing with are data like a fecal sample from an animal that you can put under a microscope, and you can look for eggs of the helminths, for example.

But the eggs of two closely related species look almost identical. You can’t tell them apart. So if we really want to know, is this a unique species that’s only found in this host, you’ve got to go to the genetic level. You’ve got to go to the next generation genomic level, where you can really try to determine that the species, or what we often call an operational taxonomic unit level, because viruses and bacteria, what do we call a species gets really tricky.

So there’s the one aspect. But I think kind of one of the areas that’s like the next frontier on the horizon for studying in disease ecology is investigating questions of co-infection. So as I’ve been mentioning, there are all these studies that show a single individual might have 10, 12 different kind of parasites at the same time. How do those parasites interact within their host?

We don’t know anything about that in most animals. And that’s fundamental to going back to like, what does it mean to be healthy? What does a healthy wild population look like? It’s not going to be some sterile population with no parasites. That’s kind of the battle that humans are in now because we led this long war and campaign on germs. And we tried everything we could to sterilize our environment. And now we realize, oh, shoot, we need these bacteria that we now call the microbiome that keep us healthy. So the same is true for the wildlife. We need to understand what diverse communities of parasites exist in natural, normal populations.

CHRISTIE TAYLOR: Mm-hmm. Well, that sounds like quite the scientific adventure, I guess I should say, and maybe a bit of a dirty job.

JAMES HERRERA: [LAUGHS] That’s for sure.

CHRISTIE TAYLOR: James Herrera’s a research scientist at the Duke Lemur Center and the program coordinator for their conservation program. He joined us from Durham, North Carolina. I’m Christie Taylor. And this is Science Friday from WNYC Studios.

IRA FLATOW: Thanks, Christie. I think I see your side now. What a good case for the power of parasites.

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