A deadly disease is devastating frog species around the world, foreshadowing an age of global pandemics for humans and the animal kingdom alike.
Dotted with lush trees and enveloped in frothy cloud cover, the Panamanian highland’s Fortuna Forest Reserve’s enchanting rainforest ambiance is rivaled only by the Amazon jungle. The 50,000-acre reserve hosts some of the most biodiverse flora and fauna on Earth. It is one of the few places that seems to remain unmarred by the Anthropocene, the current geological period characterized by humans’ influence on the natural environment and climate. The reserve has been preserved by the Panamanian government with painstaking care: it protects not just wildlife but a central watershed above the Edwin Fabrega dam—the country’s single largest energy source.
University of Maryland’s Dr. Karen Lips is no stranger to Fortuna. A professor of biology, her Ph.D. studies led her south of the equator to study Central American frogs in the mid-1990s. On one of her surveys, she noticed something strange. Hundreds of frog carcasses littered the stream beds of the reserve.
“I realized this is not really normal,” said Dr. Lips. At the time, she was aware of frogs disappearing in Costa Rica, where she had studied before coming to Panama, but no one understood the significance. Autopsy reports from the dead Panama frogs indicated they had died of a fungus growing on their skin. It wasn’t until 1998 that she and her colleagues came to the groundbreaking conclusion that the fungus wrought an infectious disease called Chytridiomycosis.
The lifelessness along the streams of the reserve, a place formerly characterized by the croaking of túngara frogs and the yellow flashes of the Panamanian Golden Frog, signaled a new era of amphibian existence: a sneak preview of what would become the deadliest pathogen on Earth.
Chytridiomycosis: The Frog Killer
What biologists and ecologists initially labeled as “enigmatic population declines” affecting frogs in the Americas, Europe, and Oceania were, in fact, Chytridiomycosis die-off events.
In the decades leading up to Dr. Lips’ chilling finding, scientists in Australia had observed these die-offs but were unaware if they represented location-based freak events or were part of a global phenomenon. Beginning in 1998, during the first generation of amphibian decline, Dr. Lips would meet with fellow scientists every year to discuss and compare their findings on global amphibian health. They still meet yearly.
The findings from Lips’ team, the Australian team, and research from Central America coalesced around a central explanation: Batrachochytrium dendrobatidis, or Bd—a fungal pathogen infecting hosts with a disease called Chytridiomycosis, or Chytrid for short—was killing the frogs.
The damage from Chytridiomycosis is remarkable but, until recently, there was no cohesive, centralized study demonstrating the pathogen’s global impact. Dr. Ben Scheele, an ambitious researcher at the Australian National University set out to change that. He assembled a team of over 40 researchers from around the world to compile decades’ worth of data on frog population trends. They published their findings in Science in 2019.
The study demonstrated a decline of at least 501 species of amphibians (indiscriminate toward frog, toad, and other amphibious species) out of a global total of over 6,000 over the last 50 years. Of those 501 species, 90 are presumed to be extinct, about 60 show signs of recovery, and about 200 are experiencing an ongoing decline. The panzootic (the wildlife version of a pandemic) hit hardest in the Americas and Australia, with 40 species on the decline and seven extinct in the latter. Chytridiomycosis has exacted the greatest toll on the biodiversity of any disease known in history.
Information about the fungus Bd and Chytridiomycosis is still sparse, but scientists have come to definitive conclusions about its lethality. A waterborne pathogen, Bd disperses spores throughout an environment, usually tropical, and enters the pores in the skin of its hosts. It then takes anywhere from days to months to kill its host by building up around the skin and preventing the amphibian from absorbing the nutrients it needs.
“You would slowly make your way through the wetland, up to over where the frog is … and you catch it, and only realize as you bring your hand over to your face to look at it that the frog was actually dead. It had just died, so rigor mortis hadn’t set in yet—it hadn’t gone stiff yet. It was sitting up on the vegetation, floating on the water body in a calling position.” -- Dr. Ben Scheele, Research Fellow, Australian National University
Chytrid is a nasty disease for a few reasons. Like COVID-19, hosts do not always express symptoms. Dr. Scheele noted that it is common for very sick frogs to act normal until the last days of their lives. Observers, such as Dr. Scheele and his team of researchers, were not always able to tell which frogs were infected just by looking. Instead, they used polymerase chain reaction (PCR) tests and waited on lab results, just as we do for COVID-19 test results for asymptomatic people.
So sudden is the demise of asymptomatic Chytrid-riddled frogs, field researchers have had harrowing experiences coming across the lifeless animals. “I had this situation that was quite haunting, really,” said Dr. Scheele, remembering a late-night research expedition to collect frogs in the Australian High Country.
“You often see their eyes shine with your head torch, and so you could catch them like that. You would slowly make your way through the wetland, up to over where the frog is … and you catch it, and only realize as you bring your hand over to your face to look at it that the frog was actually dead. It had just died, so rigor mortis hadn’t set in yet—it hadn’t gone stiff yet. It was sitting up on the vegetation, floating on the water body in a calling position.”
Such morbid findings have become more common as Chytrid continues to pillage amphibian populations around the world.
The Live Animal Trade Exports Panzootics
The intercontinental exchange of wildlife is largely responsible for the spread of Chytrid, but the trade is less sinister than one may imagine: “Almost everybody immediately imagines this secret, illegal trade. But what we're really talking about here is pet trade,” explained Dr. Lips. “If you go to a pet store and pluck out some frogs, there’s a good chance they’re infected with Chytrid.”
Whether animals are being exported illegally, for scientific research, or through the pet trade, there is shockingly little regulation required for their travel. Amphibians, reptiles, insects, crustaceans, most mammals, and small birds represent only some of the animals whose flight travel does not mandate special disease regulation.
“If they’re infected, they’re not being checked, they’re not being quarantined, and they’re not being treated,” said Dr. Lips, who lamented the lack of policy coordination between the international institutions responsible for regulating trade. While the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and the World Organisation for Animal Health both recognize that “the global dispersion of B. dendrobatidis has been attributed to international animal trade routes,” there has been little progress in tightening wildlife trade regulations.
For a disease such as Chytrid, these lax policies make for swift global transmission. Much like victims of COVID-19, infected amphibians can carry the disease asymptomatically, meaning they do not express any of the tell-tale side effects. Thus, healthy-seeming animals are transported across state, country, and continental lines every day, wreaking havoc on entire ecosystems.
Dr. Jonathan Kolby, a former U.S. Fish and Wildlife Services (USFWS) Coordinator, CITES policy specialist, and Chytrid researcher, recalled his time monitoring wildlife trade through USFWS at Newark Liberty International Airport and Port Newark as rife with opportunity for infection. He was particularly taken with lackluster disease screening of incoming bullfrog shipments.
“Any time you have a high density of animals shipped together, you’ve got stress, suboptimal health conditions, questionable nutrition, and it’s just a great situation for diseases to spread— especially for things like Chytrid, where these pathogens are highly transmissible even by touch.”
These likely infected animals do not just communicate disease in the contained environments of their shipping containers, however. Dr. Kolby recounted an episode at Newark Airport where a bullfrog crate broke open in transit and hundreds of frogs escaped into the wild, where they could have introduced and spread a foreign strain of the disease to amphibian populations in the Newark Bay wetlands.
Shifting Baselines and the Anthropocentric Future
Some have speculated whether a changing climate, yet another calamity that characterizes the Anthropocene, has influenced the spread of the panzootic. Some scientists have suggested that linkages between higher temperatures and lower cloud cover, both symptoms of a changing climate in many Chytrid hotspots, have given way to the emergence of Bd.
While Dr. Scheele rejects the notion that a changing climate onset Bd, he noted it could exacerbate the crisis. “Climate change makes them [amphibians] more vulnerable to threats like Chytrid fungus, even if it doesn’t cause the emergence or spread.” For example, the Thermal Mismatch Hypothesis suggests that when disease hosts are driven from their optimal temperature zones by changing climates, they become more susceptible to infection. Consider this: If you lived in the Southeastern United States, a global biodiversity hotspot, but suddenly got shipped off to Siberia; would it be surprising if you felt “under the weather” and got sick more easily?
Whether climate change has played a direct or inconsequential role in the appearance and spread of Chytrid, one thing is clear: humans are impacting the natural environment, and the consequences are cropping up in a big way. Our consumption, pollution, habitat destruction, and trade have ushered in an era of mass extinctions. The planet lost 173 species between 2001 and 2014—25 times the default extinction rate in the normal course of evolution—and 400 vertebrate species have gone extinct since 1920—100 times the default extinction rate.
For now, Chytrid is only killing amphibians; but such species die-offs may suggest an existential unraveling of epic proportions.
“It’s not about the frogs, it’s about the issue,” said Dr. Kolby. “It’s going to happen again. It probably already has, and we just don’t know it. What happens next time, when it’s an animal you care about?”
“Eventually, there will be a fungal disease that will become very problematic for humans.” --Dr. Alessandro Catenazzi, Assistant Professor, Florida International University
What happens next time when it’s us? The COVID-19 pandemic has demonstrated humans’ fallibility in the face of global outbreaks of deadly diseases. Pulitzer Prize for Explanatory Reporting Winner Ed Yong has covered this extensively for The Atlantic: Our most vulnerable are being left out of our patchwork healthcare system (at least in the U.S.) and inconsistent public-health decision-making. We are zooming through a panic-neglect cycle that leaves our agencies and institutions flat-footed for the next pandemic. We can’t even agree on basic facts and have the decency to acknowledge those grieving from the pandemic. Are we paradoxically becoming less prepared ourselves the more we know about how pandemics work and the more tools we have at our disposal to deal with them?
Dr. Alessandro Catenazzi, ecologist and Chytrid researcher at Florida International University, has thought about this question quite a bit. “[Diseases] are becoming increasingly common, and it’s going to be a problem for humans,” he predicted. “COVID is just one of many more to come.”
According to Dr. Catenazzi, our shelter from diseases like Chytrid is tenuous: “The only reason we’re safe is that Chytrid doesn’t like hot bodies. We’re too hot for Chytrid. Above 30 degrees Celsius, it just dies off. But that should worry us—we’re not too different from a frog in terms of our immune systems. Eventually, there will be a fungal disease that will become very problematic for humans.”
In fact, such a fungal disease has already arrived. Candida auris, a fungal infection detected in five continents in the last 13 years, is proving resistant to the standard classes of antifungal drug treatments.
Chytrid is far from the only panzootic ravaging the animal kingdom (currently salamanders, bats, snakes, wheat, and rice are threatened with massive decline by disease, directly and indirectly) as well. And, as Dr. Catenazzi pointed out, our own demise could be more imminent than we know. In the meantime, our collective failure to recognize our own connectivity to the natural world continues to push countless species to the brink.
For Dr. Scheele, it’s been painful returning to the habitats that he had monitored as a young scientist and finding that species he studied for years had gone locally extinct. “Something we’re all really vulnerable to as humans, as a species, is this idea of shifting baselines,” he said. Quiet tropics devoid of colorful, croaking frogs could become the new normal. To make progress against diseases like Chytrid, Dr. Scheele believes it will take more resources and a real investment in biodiversity preservation.
While there was a flurry of international reporting on Chytrid following the 2019 report, a coordinated response from the international community never materialized. The Australian bushfires beginning in January 2020 also presented an impetus for greater investment in wildlife protection (who knew that rainforests could burn down?), though the COVID-19 pandemic, despite its relevance to species decline, detracted international attention from both issues.
Dr. Lips painted a bleak picture of the future of frogs, considering how much is still unknown. She said the vast amount of data used in the 2019 study was only “the tip of the iceberg”—a relatively small snapshot of a massive problem that scientists have yet to quantify.
“I don’t think the cards are stacked in favor of massive recovery and thriving of amphibian populations. We’ll be lucky if we stay where we are,” she said. “Certainly, someone will figure it out and find a way to live with this thing … but we don’t know what that timeframe is. Couple years, dozens of years, 100 years. We don’t know.”
There is a Pathway to Recovery – If We Have the Will to Act Now
“Figuring out” the solution to the great amphibian panzootic will rely on bold interventions in public health and conservation policy. If the COVID-19 pandemic has taught us anything, it is that disease response requires timely international, multipronged responses. The game plan must include preventative measures targeting travel-borne transmission, damage control of currently infected populations, and long-term addressing of the direct and indirect conditions that allow for species decline by disease, such as climatic changes and habitat loss.
On the preventative front, the US and other countries can continue to ramp up amphibian trade regulations, as USFWS did by imposing restrictions on the trade of over 200 species of salamanders in 2016 to prevent the spread of Bsal- another pathogenic Chytrid fungus. This victory is a drop in the bucket, however, as the trade of only 2.5% of amphibian species is regulated by CITES as of 2021, while one-fifth of all amphibian species internationally traded are endangered, critically endangered, or even vulnerable to extinction.
To curb current threats of extinction, conservation ecologists and biologists need to revitalize infected populations’ dwindling numbers by cultivating successful captive colonies, reintroducing healthy individuals into the wild, and translocating populations. Researchers have made headway in treating Chytrid with antifungal therapies. Administering small doses of certain metabolite drugs onto the skin of captive frog colonies has proven to derail the stability of Bd cells, inhibiting their growth within the skin of infected frogs. Additionally, antifungal treatments can bolster amphibians’ immune responses against Bd. If these treatments prove successful in a lab environment, they could promise success for individuals reintroduced into their native communities.
As for long-term solutions, much of the impetus to end the panzootic is on the international organizations responsible for animals’ unrestricted global travel. It also falls on those institutions responsible for allowing the degradation of the natural habitats of these animals, bringing them stress, the need to relocate, and exposure to other populations, all conditions rendering them prone to contracting disease.
“Certainly, someone will figure it out and find a way to live with this thing … but we don’t know what that timeframe is. Couple years, dozens of years, 100 years. We don’t know.” -- Dr. Karen Lips, Professor, University of Maryland
The loss of life and color at the Fortuna Forest Reserve and amphibian habitats around the world bear a reminder of the insidious impact the Anthropocene has wrought on biodiversity. The road ahead, at first glance, diverges into two paths: the scientific community adopts a robust and comprehensive crisis response to Chytrid, with necessary backing from the international institutions and conventions governing the live animal trade; or the human race fails to enforce a strong disease response campaign or critical restrictions on trade and many, many amphibian species will go extinct.
But we don’t have to accept the amphibian apocalypse. We don’t have to accept the continued degeneration of the natural world at the hands of humans — until, that is, there is nothing left on Earth to degenerate. Nature is resilient; frogs can certainly outsmart its threats, hop over them, and rebound like they have done since the Jurassic Period — if we give them a fair shot. We can and must choose the path that favors species conservation and restoration, because in doing so, we may just be saving ourselves.