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.