Guardian of the Frogs

A deadly fungus is killing the world’s amphibians. Professor Taegan McMahon is determined to save them.

By Amy Martin

rowing up in southeastern Connecticut, young Taegan McMahon loved to play with frogs. She and her brother—and sometimes her neighbor, then-Professor of Botany Scott Warren, and his family—would explore the outdoors for hours, marveling at the plants and the creatures that inhabit the lush woodlands in and around Connecticut College’s campus.

“When I was younger, you could go to any pond in the spring and find frog eggs, tadpoles, things like that. I fell in love with nature, and that’s how I fell in love with science,” says McMahon, now an associate professor of biology at Conn, having joined the faculty in 2020.

But as the budding scientist became more interested in conservation, she began noticing declines in the populations of her amphibian friends.

“That really broke my heart,” she says. “They just—disappeared.”

The phenomenon wasn’t limited to Connecticut, either.

Across the world, amphibian populations have been decimated by a pathogenic fungus, Batrachochytrium dendrobatidis, or Bd. One of the deadliest organisms in the world, Bd has been linked to the decline of more than 500 species of frogs, toads and salamanders, including 90 that are now presumed extinct or functionally extinct in the wild.

Primarily found in freshwater systems, the fungus effects the skin of frogs, stealing nutrients and breaking down cells. Since amphibians do some of their breathing and regulate water and salt concentrations through their skin, the diseased animals can end up with an imbalance that leads to heart attack and death.

Since 1993, when Bd was identified as the cause of a mass die-off among frogs in Queensland, Australia, scientists have been racing to better understand the unusual parasite within the amphibian system and save infected populations. So far, they’ve had little success.

But McMahon is taking a different approach. And she just might be the one to finally save the frogs.

AS A CONSERVATION DISEASE ecologist, McMahon likes looking at issues across the biological scale—from individuals to populations to ecosystems. Her approach is multifaceted, combining molecular lab work, fieldwork and data-driven modeling to better understand the true complexities within a given system.

She also thinks differently.

“I have some pretty severe learning disabilities and pretty strong neurodiversity. Which can be frustrating, and I did not always feel welcome in science. But over time I have learned to be flexible and kind with myself and to create a space where I absolutely thrive,” she says.

“Because my brain works differently, I tend to ask the questions no one else is asking.”

One thing that puzzled McMahon: Even in remote areas where entire amphibian populations were completely eliminated, the fungus, which can only survive a few days to a few weeks without food, still persisted.

“In the Caribbean, they have these huge, awesome-looking frogs called mountain chickens, and they were wiped out from these isolated ponds. A year or two later they were reintroduced, and Bd wiped them out again,” McMahon explains.

“Typically, if there is no host, there is no parasite. So the question is: How is Bd surviving with no amphibians?”

McMahon began to wonder if Bd could survive on non-amphibious animals, and if they could be spreading it to others.

At first, the scientific community was skeptical. For more than two decades, the focus had been solely on the amphibians. But it didn’t take long for McMahon to confirm her hunch.

In crayfish, for example, McMahon and her team found Bd persisted at low levels, with about 10-20% of the population infected in the wild. They handle the fungus fairly well, so it lives in the crayfish and they release the infectious spores, called zoospores, into the water. Those spores can then infect other organisms, including frogs.

We’ve shown that we can vaccinate tadpoles and frogs the size of a black bean—the group that has the highest mortality. The idea that we can protect them is huge.

— Taegan McMahon, Associate Professor of Biology

The discovery meant conservationists would have to completely change course. Moreover, McMahon has found the fungus interacts with each organism differently, further complicating efforts to minimize its impact on amphibians.

“Each time I look at another group, I have to figure out a new story. It takes longer to develop in some than others. Sometimes a group is wiped out completely, and other times they have these weird mutualistic, predator-prey type relationships, where they eat each other, but they’re also supporting each other, which is unusual to say the least,” she says.

“I love that it’s making me think about these systems and challenging our traditional views and biases. It’s easy to say, ‘Well, no, that’s not how it works.’ But it is working. As scientists, we build our expectations off observations, and these new observations are showing us that we need to expand how we think about things.”

McMahon’s work on non-amphibian hosts is now supported by a large, multi-institutional National Institutes of Health grant she received after her initial discovery. Her team, which includes a research assistant and Conn student researchers and collaborators from the University of California and University of Colorado, are now identifying more non-amphibian hosts and working to better understand their role in the spread of the deadly fungus.

WITH SO MANY DIFFERENT ways for Bd to survive, eradicating it isn’t going to be possible. So, McMahon wondered, could frogs be protected before they are infected? Could they be vaccinated?

It may sound like science fiction and conjure up images of khaki-clad wildlife biologists traipsing through the jungle and chasing tiny, elusive frogs around with syringes. And maybe that’s exactly why no one had ever tried it before.

But McMahon gave it a shot. Which would have made for a good pun except in this case, the vaccine didn’t need to be a shot at all. Because Bd enters through frogs’ skin, McMahon and her team decided to see if they could induce an immune response that could provide protection from future infection by dousing frogs’ backs with low levels of live fungus and then clearing them. (The fungus cannot survive a heat chamber, but most frogs can, making it relatively easy to clear them.)

It worked. The next step in the research, funded by a second large grant from the National Science Foundation, was to see if the same results could be achieved using dead fungus instead of live infection.

One of the deadliest organisms in the world, Bd has been linked to the decline of more than 500 species of frogs, toads and salamanders.

“We found that was equally as effective. That was really exciting, because try convincing folks that they should infect endangered, wild-extinct animals with the same fungus that made them go extinct. That’s not going to be very popular,” McMahon says with a laugh.

The Bd zoospores use a chemical combination to break down a frog’s skin enough for the fungus to enter, so McMahon and her team then focused on whether they could activate the immune system and create an immune response using only those chemicals. That, too, worked, meaning the vaccine could be made using just the chemicals and no live or dead fungus at all.

“If we can go out and give these organisms that initial protection up front, then when they get exposed, they’re not as likely to die. They get less fungus, they release less fungus, there’s less fungus in the wild. The ones that are more tolerant are likely to have more babies, because they didn’t get as sick, and then hopefully eventually you’ll see populations that are more tolerant,” McMahon explains.

She and her team are now the only group in the world producing this Bd vaccine for amphibians. They continue to test it in the lab and have begun to test it in the field, too, in a series of isolated ponds in California for which they have Bd data for the last 20 years.

But even if the vaccine doesn’t work in the field, it could still save whole species.

“We know it works really well in the lab, and there are groups around the world maintaining thousands of animals for release back into the wild. We can take the vaccine, boost protection for these animals and then release them, giving them a fighting chance,” McMahon says.

“I got emotional this summer because we’ve now shown that we can vaccinate tadpoles and frogs the size of a black bean—the group that has the highest mortality. The idea that we can protect them is huge.”

STILL EARLY IN HER CAREER, McMahon has many more questions she wants to answer. But she’s just as excited about inspiring a new generation of conservation-minded scientists.

“I specifically picked a small liberal arts school because what drives my passion is that interaction with the students, and I want them to be able to take leadership roles in these projects,” she says.

“You have to create space for questions, you have to create space for failures and you have to create space and time for folks to learn the system, so they can find their place in it.”

Increasingly, McMahon hopes to create and expand opportunities for students who, like herself, are neurodiverse and have learning disabilities, and may become frustrated trying to fit into a mold created for a different demographic of human.

“I’ve done a lot of reflecting over the last few years and I’ve realized not only is it okay for me to let my brain be different and to think about things differently, but now I’m in a position where I can raise awareness and advocate for others,” she says.

“We worry folks who are different will have a hard time fitting the mold, and so we try to protect them. But I think we should be helping them find and create their own space instead. There are enough barriers for us; I’d like to open some doors.”

There’s no shortage of work left to be done on Bd either. McMahon jokes there’s an entire career in just understanding the dynamics of Bd in non-amphibian creatures like dragonflies.

“Dragonflies don’t do well with it. And we know dragonfly populations have declined in many areas, but nobody was looking for Bd,” she says.

“It’s a weird, weird fungus. It certainly keeps my brain agile.”