We can barely control a deadly disease affecting humans all around the world. What about the one that’s killing amphibians?
It was December of 1996 when Karen Lips turned up the first bodies—and finally felt an ember of hope. As a graduate student working in the muggy forests of Central America, she’d noticed that an as-yet-unnamed culprit had been stripping the area of its frogs. Regions that had once rung with a chorus of croaks were silent and still, but no one had found the carcasses that could speak to a cause. With those finally in hand, “I remember thinking, Wow, this might actually be helpful,” Lips told me. Surely, data would beget a solution; surely, the frogs’ declines would now be reversed.
More than 25 years later, Lips has felt much of that early spark of hope fizzle and flame out. Scientists did indeed go on to identify the amphibian-killing pathogen: the fungus Batrachochytrium dendrobatidis, or Bd for short. But Bd has not been stopped. Instead, it has spread to every continent where frogs and their close cousins are found. “If you pick up an amphibian here in the U.S., on average you have a 50 percent chance of picking up one that’s infected with Bd,” said Lips, who now runs her own lab at the University of Maryland. Eradication is no longer possible; the fungus has established itself in too many animals, in too many places. Lips sometimes imagines the planet coated in a layer of fungus that grows back when poked, prodded, or torn. “I’m not sure I have optimism,” she told me, not anymore.
Bd is the paragon of a pandemic. It has been described as perhaps the most devastating disease the world has ever recorded, in terms of its species scope and death toll. A pathogen that wriggles inside amphibian skin cells and causes fatal heart attacks, the fungus is estimated to have contributed to the decline of about 500 amphibian species, about 90 of which have been driven to extinction; more are expected to follow, sending ripples through countless food webs. Bd is also, outside of tight circles of amphibian enthusiasts, little known, and barely addressed. For the network of researchers who have devoted decades of their lives to combatting it, hope has long been hard to keep alive. And in the past three years, as another outbreak—this one, a plague of humans—erupted into public consciousness, their prospects for success have felt even dimmer.
Bd wasn’t always thought of as a permanent planetary scourge. When scientists first began to study the pathogen, “it was not looked at as a hardy organism,” Lips told me. Several antifungals, including a drug called itraconazole, can easily wipe it out in test tubes; so can potent chemicals released by multiple species of bacteria, including some that naturally reside on certain amphibians’ skin. Researchers actually have to fight to keep the finicky fungus growing in the lab: Even small perturbations in temperature or salt content are enough to nuke it, forcing scientists to start their cultures over from scratch. “We used to joke about how easy it was to kill,” Lips said.
Out in the wild, though, Bd rapidly proved itself to be far more formidable. Some research suggests that the fungus can linger in the environment for days or weeks, awaiting its next host; it is a fast evolver, too, with the ability to essentially “add or kick out chromosomes at will,” says Trent Garner, a biologist at the Zoological Society of London and University College London. The range of animals it can trouble is also staggeringly large: The fungus seems to be able to infect just about any of the 8,000-plus species of amphibians it encounters, transmitting directly through skin-to-skin contact, or by releasing sperm-shaped spores into water. It’s hardy; it’s ubiquitous; it’s impossible to permanently purge. Boot it out of one population, and it just moves into the next.
Researchers, having acknowledged that Bd’s threat will never completely dissipate, still try their best to mitigate its harms. Antifungals work, at least in limited contexts: About a decade ago, a team of scientists, including Garner, used them (along with disinfectants) to eliminate Bd from several ponds in Majorca, Spain. Some researchers are also experimenting with probiotics that can be slathered onto amphibians like “a topical yogurt” to imbue their skin with fungus-fighting bugs, says Molly Bletz, a disease ecologist and conservation biologist at UMass Boston who’s working on one such intervention. Other scientists are looking into Bd-focused vaccines, or selective breeding in captivity—even engineered genetic tweaks—that could make certain species less vulnerable to disease. Some researchers are trying to mobilize amphibians out of Bd-infested areas; chauffeur them into fungus-free havens; or seed their habitats with crustacean micropredators, such as water fleas, that might snarf Bd down.
The tricky thing with all of these tempering tactics, though, is that they’re ultra-laborious—with little guarantee that the effects will last. In zoos, frogs that are cleared of Bd with drugs get “reinfected all the time,” Lips told me. And that’s after researchers “treat them all,” a proportion that would be infeasible in the wild. The looming specter of fungal evolution also keeps herpetologists up at night. Obed Hernández-Gómez, an evolutionary ecologist at Dominican University, in California, has found that it can take as few as 15 generations for Bd to evolve resistance to the molecules made by certain probiotic bacteria; the case is probably comparable with antifungals, though the phenomenon hasn’t been well studied. Some also worry that any chemical, bacterial, or environmental intervention could come with serious consequences for creatures that coexist with frogs, or for the frogs themselves.
Vaccines could be a more lasting intervention, with fewer environmental ripple effects. But effective immunizations don’t yet exist. Cold-blooded amphibians are also a challenging group to vaccinate. “Their immune systems are really slow,” Bletz told me, especially when temperatures dip. Even vetted vaccines wouldn’t pass protection down through the generations, requiring scientists to make regular trips into the field. Interventions in captive contexts, too, may serve only as a stopgap. The idea is to “breed them, then return them to their habitats,” says Ana Longo, a herpetologist at the University of Florida. “But if the pathogen is still there, is it worth it to spend all this effort?”
People, too, could get their act together. Humans seem to have ferried the fungus, once restricted to parts of Asia, around the globe, via imported or stowaway amphibians. Better regulation of the international trade in these animals could reduce the global burden, but Bd has already spread to nearly all frog-inhabited corners of the world, save for maybe Papua New Guinea and a few nearby island outposts, and its ubiquity is seen by many as a foregone conclusion. Researchers have also been distracted, for the past 10 years or so, by another fungal outbreak caused by a sister species called Bsal that mainly targets salamanders. Bsal hasn’t yet been detected in North America, the “hot spot” of salamander diversity, Hernández-Gómez said, and the effort to keep it out has gobbled up herpetologists’ attention, pushing Bd to the sidelines. And among some policy makers, there’s been a pervasive attitude of “what exactly do you want us to do?” Lips told me. “It’s already here.”
That sentiment has seemed particularly familiar of late, Bd experts told me, now that the world is grappling with another pandemic-caliber disease, this one trained on humans. COVID has forced a reckoning with the same sorts of questions as the frog fungus, and produced similar stalemates: What level of suffering is sustainable, or tolerable? What do you do when a disease is still raging but many people seem to have tired of fighting it? As with Bd, the coronavirus has no silver-bullet solution. Both are here to stay.
Lips has been gathering data that could draw more direct connections between amphibians’ well-being and our own. She and her colleagues recently published a paper proposing that the decline of amphibians in Central America may have led to a boom in populations of mosquitoes—typical frog fare—and raised the risk of malaria among people. Though even infectious threats to Homo sapienscan be easy to ignore. Our response to the coronavirus pandemic, in particular, felt like “a slap in the face,” Hernández-Gómez said. “If humans don’t even care about a disease that’s killing off their own,” Bletz told me, “how are they going to care about something that affects amphibians?”
In broad strokes, much of the rest of the Bd and Bsal story may feel written: More populations will dwindle; more species will disappear, many of them far from human habitations, where they may, once again, escape the notice of most. Perhaps more species will ultimately adapt to resist or tolerate Bd, and so the struggle continues to “keep populations in the wild for as long a time as possible, to give more time for natural selection to act,” says Ben Scheele, a disease ecologist at Australian National University who’s working to save his nation’s corroboree frogs. But even on an evolutionary timescale, there are no guarantees: Where frogs go, the fungus seems to follow.
“There’s almost nothing we can do, in a way, and that’s the sad part,” says Timothy James, a chytrid-fungus expert at the University of Michigan. Lips has held dying frogs in her hand, each of them sluggish, discombobulated, and weak, sometimes to the point where they can no longer muster the energy to try to wrest themselves free. “They just sort of sit there, even if you bend to pick them up,” she told me. Their deaths are slow, subtle affairs—agonizing fades that have become, like so many other infectious endings, a kind of background noise.
Some of the experts I spoke with told me there is still plenty of room for optimism—that the efforts of the few could still turn the tide, especially against the less pervasive Bsal. Others, although far from giving up on the Bd battle, feel more conflicted. At the start of the COVID outbreak, Lips felt another wellspring of hope burble up in her chest. She gave talks. She told people, “This is not my first pandemic.” Maybe, she thought, there would be a surge in interest in infectious disease; maybe, she thought, people would understand the importance of conservation, and keeping ecosystems intact. That’s not what happened. “I had hoped COVID would be our success story,” she said. “But I went from ‘This will be a motivating factor to do better!’ to ‘Wow, we’re kind of losing momentum again.’”
Lips still remembers what Costa Rica’s tropics looked like in the 1990s, before Bd was truly known. She recalls the feeling of becoming enamored of the spectacular green coloring and the nubby spiked skin of the region’s Isthmohyla calypsa tree frogs. Isthmohyla calypsa is now no longer in Costa Rica: Bd has driven it out. And Lips no longer does much fieldwork. A lot of pain comes with confronting the froglessness—trying to count creatures that she and others worry will no longer be countable in a few years’ time. Lips’ current research—some of it geared toward influencing policy, and buoying biodiversity as a whole—does keep her going. But as the frogs continue to vanish, so too does the work of the scientists who study them. “Where do I go?” she said. “Where are the frogs?”
All Rights Reserved for Katherine J. Wu