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Can amphibians bounce back from the brink?

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A live boreal toad from waters known to harbor the deadly chytrid fungus. bberwyn photo.

Research suggests some species can develop or acquire an immunity to deadly fungal pathogens

By Summit Voice

FRISCO — Florida-based researchers say they may have some answers for the puzzling wave of amphibian deaths that’s been wiping out populations of some species. At least some frogs and snakes may be able to develop immunity to the deadly chytrid fungus that’s been implicated in the die-off, University of South Florida biologists said this week.

Their findings could be good news in general for biodiversity, as emerging fungal pathogens are seen as posing the greatest threat of any parastic pathogens, contributing to declines of  amphibians, bats, corals, bees and snakes.

The new study, published in the journal Nature, shows that amphibians can acquire behavioral or immunological resistance to the chytrid fungus.

“Acquired resistance is important because it is the basis of vaccination campaigns based on ‘herd immunity’, where immunization of a subset of individuals protects all from a pathogen,” said Jason Rohr, an associate professor of integrative biology who led the research team with Taegan McMahon, a USF alumnus who is now an assistant professor of biology at the University of Tampa.

One experiment in the study revealed that after just one exposure to the chytrid fungus, frogs learned to avoid the deadly pathogen.

In subsequent experiments in which frogs could not avoid the fungus, frog immune responses improved with each fungal exposure and infection clearance, significantly reducing fungal growth and increasing the likelihood that the frogs survived subsequent chytrid infections.

“The amphibian chytrid fungus suppresses immune responses of amphibian hosts, so many researchers doubted that amphibians could acquire effective immunity against this pathogen,” Rohr said. “However, our results suggest that amphibians can acquire immunological resistance that overcomes chytrid-induced immunosuppression and increases their survival.”

Rohr also noted that “variation in the degree of acquired resistance might partly explain why fungal pathogens cause extinctions of some animal populations but not others.”

“The discovery of immunological resistance to this pathogenic fungus is an exciting fundamental breakthrough that offers hope, and a critical tool for dealing with the global epidemic affecting wild amphibian populations,” says Liz Blood, program officer in the National Science Foundation’s Directorate for Biological Sciences, which funded the research through its MacroSystems Biology Program.

Conservationists have collected hundreds of amphibian species threatened by the fungus and are maintaining them in captivity with the hope to someday re-establish them in the wild. However, reintroduction efforts so far have failed because of the persistence of the fungus at collection sites.

“A particularly exciting result from our research was that amphibian exposure to dead chytrid induced a similar magnitude of acquired resistance as exposure to the live fungus,” McMahon said.

“This suggests that exposure of water bodies or captive-bred amphibians to dead chytrid or chytrid antigens might offer a practical way to protect chytrid-naïve amphibian populations and to facilitate the reintroduction of captive-bred amphibians to locations in the wild where the fungus persists.”

“Immune responses to fungi are similar across vertebrates and many animals are capable of learning to avoid natural enemies,” Rohr emphasized. “Hence, our findings offer hope that amphibians and other wild animals threatened by fungal pathogens – such as bats, bees, and snakes – might be capable of acquiring resistance to fungi and thus might be rescued by management approaches based on herd immunity.”

Rohr cautioned, however, that “although this approach is promising, more research is needed to determine the success of this strategy.”

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