Amphibians Versus Fungus: Saving the Yellow-Legged Frog

17:40 minutes

A mountain yellow-legged frog. Credit: Emmanuele Biggi
A mountain yellow-legged frog. Credit: Emmanuele Biggi

The mountain yellow-legged frog of alpine California is one of hundreds of species devastated in recent years by the spread of chytrid fungus. The fungus, which irritates amphibian skin to the point where it sloughs off, can rapidly kill off entire populations of some species—including the yellow-legged frog.

But scientists in the Bay Area are working on one solution: immunizing young frogs so they can live with the disease long enough for resistance to evolve. Jessie Bushell, the conservation director at the San Francisco Zoo, and San Francisco State biology professor Vance Vredenburg explain how exposing yellow-legged frogs to small amounts of fungus could save their species—and perhaps other amphibians threatened by chytrid around the globe.

Learn more in this video from KQED’s “Deep Look:”

Segment Guests

Lauren Sommer

Lauren Sommer is a science and environment reporter for KQED Public Radio in San Francisco, California.

Jessie Bushell

Jessie Bushell is the Director of Conservation at the San Francisco Zoo.

Vance Vredenburg

Vance Vredenburg is an associate professor of biology at San Francisco State University in San Francisco, California.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow in San Francisco. Our next story is about California frogs like these.


Yep. That’s the sound of the mountain yellow-legged frog. It lives high in the Sierra Nevada, in alpine lakes. And it’s the subject of a bold experiment. Can we immunize amphibians against a deadly fungus by letting them catch it? Joining me now to tell the story is Lauren Sommer, KQED environment water and energy reporter. Good to have you with us. Welcome.

LAUREN SOMMER: Hi, Ira. Welcome to California.

IRA FLATOW: Tell us of this tale of the frog and the fungus.

LAUREN SOMMER: Yeah. In recent decades, this fungus, the chytrid fungus has become public enemy number one for amphibians across the globe. It’s wiped out thousands of animals here in California alone. And it’s particularly nasty because it attacks their skin, something amphibians depend on to breathe underwater and absorb nutrients.

Jessie Bushell is conservation director at the San Francisco Zoo. And she’s been working on fighting this fungus along with my other guest, Dr. Vance Vredenburg, as an associate professor of biology at San Francisco State University. He worked on the first survey of this fungus in Asia. And he’s looking at several possible solutions for preventing its spread in the long run. Welcome both of you to Science Friday.

JESSIE BUSHELL: Hi, Lauren. Thanks.

DR VANCE VREDENBURG: Thanks for having us.

LAUREN SOMMER: So, Vance. Tell us, just what is this fungus? And why is it so bad for amphibians?

DR VANCE VREDENBURG: Well this fungus, it’s a chytridiomycete fungus, which is a group of fungi that are microscopic and live in the water. And these fungi are really unusual in the sense that they infect amphibians and are really the worst case that we know of a single disease in human history.

So for example, what’s happened with amphibians makes what happened with humans and the Black Plague look like a drop in the bucket.

LAUREN SOMMER: And it’s not new, right. But why is it so bad right now? How is it spreading and why is it so prevalent?

DR VANCE VREDENBURG: Well, it’s a really hard question to answer actually. So it’s spreading in many different ways. But we still don’t know all the ways. But one thing we do know is it looks like humans definitely are involved with this. So this fungus is being moved around, probably on the backs of amphibians that we have let go by accident. Things like American bullfrogs or African clawed frogs. But they’re probably moving around in other ways as well.

There’s an example in the Caribbean where frogs were in bananas that were shipped from one island to another and caused the decline of the mountain chicken frog, which was really a terrible, terrible case.

LAUREN SOMMER: And you’re one of the few scientists that has actually seen what it looks like when this fungus can hit a lake or an area. I mean, what have you seen? What does it look like on the ground?

DR VANCE VREDENBURG: Well, it’s devastating. It truly is. It’s devastating. One thing that’s interesting is it’s microscopic. So you can’t actually see the fungus itself. But what you do see is when the animals get sick, especially en masse, you see sick and dying animals that die in at a scale that’s really not something we’re used to seeing in nature.

So for example, in the Sierra Nevada, when I witnessed the die-off of mountain yellow-legged frogs from this chytrid fungus, I saw literally tens of thousands of dead frogs littering the shorelines of these beautiful pristine lakes.

LAUREN SOMMER: And this species, the mountain yellow-legged frog is the one that you guys are working on to really study, can we control this fungus? And some of the tadpoles of those infected frogs actually made their way to you, Jesse, at the San Francisco Zoo, right? And by helicopter?

JESSIE BUSHELL: Yes. Well, we have a couple of projects going on. But one of them was a helicopter rescue. So it was a salvage operation for a population that’s similar to how Vance described it. The biologist went into the field, this really remote, remote site, and found that there were no surviving adults. But there were lots of tadpoles. Basically that was the last remnant of that entire population in this one area that had been naive to the chytrid fungus previously.

So they scaled this operation with the National Park Service, with Fish and Wildlife Service, with the UC-Davis biologists that were working there, and they collected every last tadpole and helicoptered them to my zoo and also to the Oakland Zoo, one of our collaborators on the project.

It was an amazing thing to see this fire rescue helicopter arrive with basically just like buckets of tadpoles. And they were so excited. They were like, we’re saving a species. It was like a moment for them and a moment for us. I mean, it really is a collaborative effort. And we raised them and treated them. And this summer we released about 150 of them back to that lake.

LAUREN SOMMER: So yeah, tell me what happens when these tadpoles got to the zoo. What did you do exactly?

JESSIE BUSHELL: So, when we got them they were late-stage tadpoles. So basically the eggs evolved into tadpoles normally. They last about three years as tadpoles in these high elevation lakes because the winter season is really long. And so they don’t have as much time to develop. So when they came to us they were pretty late-stage tadpoles and became froglets within a couple of months.

So when they became froglets we treated them against the chytrid fungus. We treat them with an anti-fungal so that they would no longer have an active infection. That allowed them to be healthy, to grow. And so after about nine months we then reinfected them with the chytrid fungus.

So this is the whole idea of the immunization. We wanted to stimulate their immune system so that when we release them a couple of months later, they would their body would recognize the fungus as something to fight against. They’d have antibodies already in their system.

So we infected them. But the infection brew to a point that we felt was not life-threatening but would enable their immune system to kick on. And then we treated them with an anti-fungal. So a medication to basically get rid of the infection, to get it so it wasn’t there anymore. Give them a couple of weeks to then get healthy again, strong, eat. And then we released them, hoping that in the wild when they saw it, they wouldn’t die. They would just fight it off on their own.

LAUREN SOMMER: So kind of like a vaccine almost.

JESSIE BUSHELL: Exactly. Exactly. And that’s what we were hoping it will be like. And it would just be enough to get these guys to survive to adulthood and be able to breed so that the population would continue. That it wouldn’t go extinct.

LAUREN SOMMER: And so, yeah, what are you seeing, Vance, in the wild? Is this working? Can their immune system kind of be turned on in a way that when they get hit with this fungus, they’re able to fight it?

DR VANCE VREDENBURG: Well, what we’re hoping is that these animals will respond the way that we expect organisms to respond to a pathogen. Pathogens don’t usually drive their hosts all the way to extinction. And so there is some co-evolution usually that happens or adaptations that occur. And so we do know that there are lots of species that actually can handle this chytrid fungus. They tend to be animals that have low levels of infection where it doesn’t overwhelm their systems.

And so what we’re trying to do is, we’re trying to get them to the point here in California with these naive frogs– naive to the pathogen anyway– trying to get them to that point where they can survive. And so it looks like– we certainly have a situation possibly like that in parts of the Sierra Nevada, including where we’re reintroducing some of these frogs, where it looks like they really are surviving this pathogen infection.

LAUREN SOMMER: So the idea is that if they can get through that really bad first onslaught they can kind of almost evolve to live with it essentially.

DR VANCE VREDENBURG: Exactly. Yeah. That’s exactly the point.

LAUREN SOMMER: And how long does that take? How long does it take for them to really kind of evolve that ability?

DR VANCE VREDENBURG: Well that’s a really good question. And we don’t have the answer to that. But we do know that there are potentially different pathways to that. So one is an actual adaptation from the host itself, from the frog itself, where they may, for example, evolve the ability to create T-cells that attack.

Their immune systems are relatively similar to ours actually. They’re quite advanced. So they might be able to attack them with T-cells or with white blood cells or some other technique that they could use to sort of fight them off immunologically.

But there are other pathways as well. For example, there are some species that we think might actually alter their behavior. And in altering their behavior, they’re doing something that’s basically not good for the fungus. So they might be sort of moving into sunnier, warmer spots where the fungus can’t grow as quickly. So they’re sort of limiting the growth of the fungus.

Other things that could happen that we’re also curious about is the skin microbiome. So all these organisms that live on the skin– we’re talking about mostly bacteria– we found that some of the species of bacteria on frogs and salamanders produce antifungals that actually kill the chytrid fungus as well.

So it’s a complicated place out there in the wild. And those things can sort of– we’re not really sure which ones are the most important. But being conservation biologists, I think we’re working in this crisis situation where we’re trying all of them as much as we can, sort of full-court press to see if we can get them over that hump of that first initial infection and the big outbreak that happens, to sort of dampen down that outbreak and give these animals a chance to basically get used to having the skin infection that doesn’t need to be fatal.

LAUREN SOMMER: Because obviously this is a huge amount of work for this species. Right. I mean, is it really a last ditch effort to save these mountain yellow-legged frogs?

DR VANCE VREDENBURG: They are in bad shape. They’re in really bad shape. I mean, the decline of the species is dramatic. And one of the things that’s so important when we’re talking about mountain yellow-legged frogs is that– think of this. Most of the habitat that they’re in is some the most protected habitat that we have anywhere in the United States, in the continental United States.

And so it’s pretty alarming to think that we’re losing these animals in places where you can’t even ride a bike. You can hike and you can ride a horse, but there’s no structures, there’s no roads. This is as wild as it gets. And yet we’re losing some of these species that have been there for millions and millions of years.

LAUREN SOMMER: And Jessie, you lived with these frogs for a while, right? A year or more. And to release them, it’s almost this amazing frog expedition, right. They get packed into these tiny little Tupperware containers and put into backpacks and hiked up the side of the mountain. What’s that like to release them in the wild?

JESSIE BUSHELL: Yeah. It’s an adventure for sure. One of our sites is– actually you don’t have to helicopter in, which is nice. And we do, we pack them up at like 3:00 in the morning into these individual containers, put them into canisters, put them on backpacks, hike three or four hours up.

But you know, it’s amazing. Releasing them and knowing that they are going to survive in that lake, and that they are doing well. And then being able to go back and see them again, and knowing that we were instrumental in making sure that there are still frogs in those lakes is amazing.

And, I have to give kudos because the California Department of Fish and Wildlife have been working really hard to make these lakes fish-free. Because non-native fish are another issue that really threatens this species. And they’ve cleared those fish out of those lakes so that the frogs can exist. They’re not being predated upon. Their eggs aren’t being eaten by these fish.

And they come on these expeditions with us to release the frogs. And a lot times they’ve never seen the frog and they’ve been working 10 years in these lakes to clear the fish and restore the habitat. And for them to be able to release the frog is almost, for me, even a better experience. Because I know how hard and how long they’ve worked.

Of course, for me it’s amazing too because they’re like my little kids I’m releasing out into the wild. But knowing that we’re recapturing them and that they’re out there still. And we just are waiting to make sure that they can reproduce out there and that they’re continuing their genetic line. We’re hoping that some of those offspring then will survive and just keep that population moving forward.

LAUREN SOMMER: And you mentioned the fish. These are invasive fish, right? Fish that aren’t supposed to be in these lakes.

JESSIE BUSHELL: A lot of these high alpine lakes never had fish. They were fish-less lakes. And so the frogs developed as kind of the top predator in those lakes. Fish were introduced for recreational fishing in a lot of lakes. And in a lot of lakes they’re still going to be tons of fish for recreational fishing. But in some of these lakes that we want to restore for the frog, there doesn’t need to be fish there. So few people go there. But yes, they’re non-native.

LAUREN SOMMER: And so, Vance, I mean, this is the big question, right. This method that you guys are experimenting with, can it be used to save other frog species around the world?

DR VANCE VREDENBURG: Well, I think the jury’s still out on that. But this is in an excellent place to test that idea. And before we go forward and try to do it in other places, I think we really have to have the flexibility to try these kinds of experiments. But I think we’re not going to be able to inoculate every single frog species in the world, because there are thousands, over 6,000 species of frogs alone in the world. So we’re not going be able to do that.

But I think it is possible in some places, where we may be able to sort of stem the tide in a sense and really keep the fungus from overwhelming these animals. It may be something that we can do. I mean, zoos around the world right now are working to take on threatened species of amphibians from all over the place. And they have many of them in captivity already. And so it’s sort of like, we should try. We should try this, because if we don’t, we just have these animals in captivity and we’re not moving forward.

LAUREN SOMMER: I’m Lauren Sommer and this is Science Friday from PRI, Public Radio International.

Yeah, Vince, you mentioned these frogs that are in captivity, right. I mean, what’s happened there? Have people kind of been rescuing them from the wild? And maybe those are the cases where this might work the best?

DR VANCE VREDENBURG: So in North America we have only a few. Sorry, in the United States we have only a few species that are extinct in the wild. And one of them is the Wyoming toad. So the Wyoming toad is actually extinct in the wild but it’s alive in captivity. So of course the idea is, we want to get those back out into the wild.

The mountain yellow-legged frogs here in California have actually gotten pretty close to that level of threat. So their numbers have plummeted dramatically in the field. So by using the zoos to help us raise these animals that are coming from these really threatened populations, we’re trying to keep the lines alive and keep them going.

But in other parts of the world– for example, in Panama, there was a group of scientists that actually collected a bunch of animals, a bunch of species, in front of a chytrid wave of death. And they brought them into labs. And the chytrid wave swept through and caused the extinction of all the remaining animals in the wild of those species.

So at one point– there has been an estimate that we need the capability to keep alive something like 400 to 600 species of amphibians around the world. There are that many that are that threatened right now. Of course we don’t have the capabilities for that at this time. But the zoos have– I don’t know the exact number, but I know it’s more than 10 species and probably less than about 50 that are alive in captivity. And I hope we hear more about that in the future.

LAUREN SOMMER: You can’t really release them without a plan, right? But what’s also interesting about this fungus is that not all frogs just die immediately, right? There’s variation out there.

DR VANCE VREDENBURG: That’s the thing that’s been so confounding to scientists, is trying to figure out why? Why is so different? Here in California, in the Sierra Nevada, the yellow-legged frogs are highly susceptible to the fungus. And this share a habitat with the Pacific chorus frogs or tree frogs. And the Pacific chorus frogs can actually handle really high levels of infection.

In fact, they compete with those yellow-legged frogs. And then when the yellow-legged frogs get killed off by this infection, the chorus frog numbers have actually increased. So, yeah, the susceptibility of species is highly variable. And it can even be variable within a species.

So I think a lot of this is, we need to understand the context of what’s happened with this pathogen. As it sweeps into these new populations, or these naive populations of hosts, we need to know something about what’s the timing of that? Did it happen two years ago? Or did it happened 40 years ago?

And it turns out in California we have examples now that we’re putting together that we know approximately when the fungus invaded different parts of California. Interestingly, there are other parts of the world where the fungus seems to have been around for a very long time. And in those areas, we don’t have evidence of mass die-offs from this fungus.

LAUREN SOMMER: And so, some people love frogs because, they’re pretty cute I would say. But what’s the importance of saving frogs? I mean, remind us why we care here?

DR VANCE VREDENBURG: Well, frogs are really important. I mean, for one, they’re a group of vertebrates that over 360 million years old. So they’ve been around for a really, really long time. They’re survivors in the general sense.

But they’re also really embedded in food webs. So they’re one of the most important connectors, for example, for energy between aquatic and terrestrial systems because they have this multi-biphasic life cycle. But they’re also really important because they link together all these different species that are feeding on them.

LAUREN SOMMER: And Jessie, what’s next for this project? What’s next for you guys?

JESSIE BUSHELL: Well, obviously we’re going to continue– we’re continuing to do the immunizations. So we have some recent animals that have come in from populations that have been hit by a similar situation. The adults are gone. We’re trying to restore these tadpoles, bring them up to an immunization size and then release them. And we’re going to continue to do that as needed.

But we’re hoping that this is not a permanent solution. We’re not going to be able to immunize all these frogs forever. But we’re just getting– as they need to be done, we’re going to be available to help with this.

LAUREN SOMMER: We’re out of time. But thank you guys. It’s been fun, guys. Dr. Jessie Bushell is director of conservation for the San Francisco Zoo. And Vance Vredenburg is associate professor of biology at San Francisco State University.

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