Tiffany Kosch/The New York Times

Espada’s marsupial frog, near the Gocta Waterfall in the Chachapoyas province of Peru. As a threat to wildlife, an amphibian fungus has become “the most deadly pathogen known to science.”

April 6, 2019

Scientists Uncover the Most Devastating Disease-Afflicted Biodiversity Loss Known to Science

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A recent paper published in Science revealed the shocking results of a global disease transmission assessment: A fungal disease affecting amphibians has been identified as the most devastating recorded example of biodiversity loss attributable to a single disease. The analysis was made possible by an extensive collaboration involving experts from 36 institutions — including two Cornellians.

Over the past half century, the amphibian chytridiomycosis panzootic, an infectious disease that affects amphibians worldwide, has resulted in 90 presumed extinctions as well as the decline of at least 501 amphibian species.

This fungus was identified in amphibian populations about 20 years ago as the cause of death and species extinction at a global scale. The last similar analysis that assessed global amphibian decline was published in 2007 but was mainly focused on the regions that suffered the most decline.

One of the two Cornellians affiliated with the study, Prof. Kelly Zamudio, ecology and evolutionary biology, has worked in Panama, Brazil and the United States studying the effect of frog-killing chytrid fungus Batrachochytrium dendrobatidis using population genetics.

Zamudio said the idea for the global epidemiological analysis started from a conversation going around the amphibians researcher community: “How bad is this? How much have we really lost?”

After comprehensive review of the scientific literature from the past 20 years, scientists studying amphibian decline worldwide classified species by abundance, severity and location.

The most drastic amphibian declines due to chytridiomycosis were cited in Australia, Mesoamerica and South America. These results support the hypothesis that B. dendrobatidis spread from Asia into the New World and suggest that development and trade have facilitated the spread of deathly diseases like chytridiomycosis.

“This study provides synthesis, which I think both makes a strong case for the magnitude of chytrid-related biodiversity loss and provides an example of the extent to which globalization may influence contemporary disease transmission,” said Cait McDonald grad, the other Cornell affiliate.

McDonald was involved in assessing North American species affected by the chytrid fungus and contributed data to the global analysis that quantified the extent and severity of declines, status of species recoveries and species distributions and habitat.

The results of the global epidemiological analysis revealed that chytridiomycosis, caused by two fungal species, B. dendrobatidis and B. salamandrivorans, has played a significant role in Earth’s sixth mass extinction, which is currently ongoing and estimates have already revealed an exceptionally rapid loss of biodiversity over the last few centuries.

“There are 8,001 frog species known to science, 501 declining plus 90 extinct is 6.5 percent of that diversity. The human population is 7.7 billion people.  If you were to take 6.5 percent of the human population that would be 455 million people, more than the population of the US. That’s the impact,” Zamudio said.

B. salamandrivorans, despite being largely restricted to salamanders, is not yet present in North America, according to McDonald. However, this doesn’t mean it will never pose a problem.

“Researchers are quite concerned that if it does invade, we could face a pandemic in salamanders similar to what we’ve already seen in frogs with B. dendrobatidis,” McDonald said.

McDonald is researching salamander functional genomic responses to the disease, as well as the role that genetic diversity and pathogen-mediated selection may play in host susceptibility. She hopes to understand how scientists can use this information to guide B. salamandrivorans disease prevention and management strategies.

Despite the decimating effect of chytridiomycosis, research has cited that 20 percent of the 292 surviving species have shown initial signs of recovery after the decline. However, this doesn’t mean that the whole species is bouncing back.

“However, even for the species that are showing any sign of recovery, the recovery generally is at the population level,” Zamudio said. “We are really interested in the populations that are showing signs of recovery because that might tell us something about how species recover.”

Zamudio hopes that in the future the scientific community will gain a better understanding of host-disease dynamics.

“We have never seen an obligate pathogen of vertebrates that is this generalist,” Zamudio said.  Obligate pathogens must cause disease in order to be transmitted and infect a host in order to survive. “Usually pathogens tend to have a narrower focus, attacking one particular species,” Zamudio said.