World’s Deadliest Pathogen On Record Devastates More Than 500 Amphibian Species—and We Don’t Know How To Stop It

The powerful fungus essentially ‘eats’ its victim’s skin alive, weakening the system before triggering cardiac arrest

The Toad mountain harlequin frog is one of more than 500 species endangered by the fungal outbreak Brain Gratwicke/Smithsonian Conservation Biology Institute

A fungal disease that attacks amphibians’ skin and triggers cardiac arrest is officially the deadliest pathogen on record, contributing to the decline of at least 500 frog, toad and salamander species. Some 90 of these species are presumed to be extinct, while another 124 have dropped in number by more than 90 percent and are unlikely to ever recover.

The chytrid fungus responsible this devastation—officially dubbed Batrachochytrium dendrobatidis, or Bd for short—was previously believed to be linked with the decline or extinction of around 200 amphibian species. New research published in the journal Science, however, suggests the actual number is more than double this estimate; in total, a team of 41 global researchers reports, the global outbreak, spurred mainly by Bd but to a lesser extent by a second fungus called Batrachochytrium salamandrivorans (Bsal), has impacted one in 16 of all known amphibian species.

“That’s fairly seismic,” Wendy Palen, a biologist at Simon Fraser University, tells Carl Zimmer of The New York Times. “It now earns the moniker of the most deadly pathogen known to science.”

According to Zimmer, scientists first noticed signs of inexplicably declining frog populations during the 1970s. Despite the fact that these amphibians lived in habitats largely untouched by pollution and deforestation, their numbers continued to drop, and by the 1980s, many were extinct or all but decimated. By 1998—the year researchers finally identified the fungus behind the culling—the damage was already done. Some 60 species had already gone extinct, Yong notes, and hundreds more were (and still are) headed in the same direction.

A genetic study conducted in 2018 indicated that Bd originated in the Korean peninsula, then spread across the globe via international trade. As The Atlantic's Ed Yong explains, infected animals likely stowed away on traveling ships or were exported as food, pets and even pregnancy tests. Today, Helen Briggs reports for BBC News, the fungal disease is present in more than 60 countries and has proven especially harmful in Australia, Central America and South America.

“Our new results put it on the same scale, in terms of damage to biodiversity, as rats, cats, and [other] invasive species,” lead author Ben Scheele, an ecologist at Australian National University, tells National Geographic’s Michael Greshko.

Bd targets the keratin protein found in amphibians’ thin skin, according to Popular Science’s Ula Chrobak. Moving from one unsuspecting victim to another through direct contact or infected water, the aquatic fungus essentially “eats” through this protein and subsequently its prey’s skin, slowing down the system before finally triggering a heart attack.

Unfortunately, Erik Stokstad explains for Science magazine, Bd appears to be an unbeatable disease: Since some amphibians, particularly those in the disease’s continent of origin, are immune to its effects, they act as a “natural reservoir” that not only keeps the pathogen alive, but enables its spread.

Writing in the Conversation, Scheele and study co-author Claire Foster, also an ecologist at Australian National University, point out that the newly published data, compiled with the help of published records, survey data and museum collections, places Bd’s global peak during the 1980s. Although some 12 percent of the 501 species catalogued in the study have shown signs of recovery since this time period, the “vast majority” remain in decline.

“Bd is but one more nail in the coffin for the state of amphibians globally,” Palen and Dan Greenberg, also of Canada’s Simon Fraser University, conclude in a related commentary piece published in Science.

It may be impossible, or at least extremely difficult, to manage the fungus once pathogenic strains have been established, but as the pair notes, scientists and policy makers still have time to take steps to combat habitat loss, exploitation and climate change, all of which can work in conjunction with Bd to further devastate Earth’s vulnerable amphibian populations.

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