Frogs may be ‘fighting back’ against deadly pandemic

A deadly disease that has ripped through frog populations around the world, contributing to huge declines in many species and the outright extinction of several others, has shown little sign of slowing its onslaught since scientists first detected it in the 1990s. But recent research indicates some frogs are showing increased resistance to the pathogen, giving biologists and conservationists hope that infected populations may be able to recover.

Chytridiomycosis is caused by Batrachochytrium dendrobatidis (Bd), a species from a group of fungi called chytrids. Members of this group are usually found on underwater decaying plant or animal matter, but Bd is different – it feeds on the skin of living amphibians, primarily frogs. Infection interferes with a frog’s ability to take in water and air through its skin, often leading to death.

Scientists believe Bd originated in Africa and first spread around the world due to the trade in African clawed frogs (Xenopus laevis), which are commonly used as laboratory research animals. American bullfrogs (Lithobates catesbeianus), which show low susceptibility to the disease and have become an invasive species in many parts of the world, have also been implicated as carriers. In addition, scientists detected Bd on bird feathers, opening up another wide route of transmission. Today, Bd is found on every continent where amphibians live.

“This pathogen infects many different amphibian species — sometimes without causing disease — and can survive in the environment outside of its host, so it’s not going away anytime soon,” said Allison Byrne, a doctoral student at the University of California, Berkeley who is studying chytridiomycosis.

Infection can be devastating to frog populations, killing some off completely. In Australia alone, scientists believe the fungus was directly responsible for the extinction of four species. Worldwide, Bd has been implicated in the decline or extinction of at least 200 amphibian species, and some biologists peg it as the driving force behind the largest disease-caused loss of biodiversity ever recorded.

But there may be hope for frogs faced with Bd. A new study released yesterday in the journal Science finds populations of several frog species in Panama appear to be gaining resistance to the pathogen. The study was conducted by scientists at research institutions in the U.S. and Panama.

“In this study, we made the exciting discovery that a handful of amphibian species – some of which were thought to have been completely wiped out – are persisting, and may even be recovering, after lethal disease outbreaks,” study lead author Jamie Voyles, a disease ecologist at the University of Nevada, Reno, said in a statement. “We wanted to understand how it was happening. Was it a change in the pathogen, the frogs, or both?”

Voyles, Byrne and their colleagues looked at pathogen and frog host samples collected in Panama before, during and after infection by Bd. They found that while the fungus is still as deadly as it was before the outbreak, frogs now appear to be more likely to survive after infection.

“The evidence suggests that the pathogen has not changed. It’s possible that the hosts have evolved better defenses over a relatively short period of time” she said. “We found that nearly a decade after the outbreak, the fungal pathogen is still equally deadly, but the frogs in Panama are surviving and may have better defenses against it. This suggests that some of Panama’s frogs may be fighting back.”

Amphibian skin secretions are full of antimicrobial substances that help ward off disease. When Voyles and her team looked at the skin secretions of wild frogs that survived a Bd epidemic, they found that they slowed the growth of the fungus much more effectively than secretions from captive frogs that had not yet encountered the pathogen. They say this indicates frogs may gain resistance only after being exposed to Bd.

This, say conservationists, may have important implications for gauging the impacts of Bd, as well as relocation and reintroduction programs for species in affected areas. One of these is the Panama Amphibian Rescue and Conservation Project run by the Smithsonian Tropical Research Institute (STRI), which collected healthy frogs before the outbreak in the hopes of releasing them back into the wild.

“We learned to breed them in captivity and are now releasing Atelopus varius in areas where the epidemic has passed, so it is extremely important for us to realize that the defenses of these frogs may be weaker than the defenses of frogs that survived the epidemic in the wild,” said Roberto Ibáñez, study coauthor, STRI staff scientist and in-country director of the Panama Amphibian Rescue and Conservation Project. “Captive breeding programs must consider breeding and releasing frogs with stronger defenses, and testing their skin secretions against the fungus is one useful tool to see which frogs are more resistant.”

This study isn’t the first to detect the emergence of Bd resistance in frogs. In 2016, scientists discovered that a population of endangered Sierra Nevada yellow-legged frogs in the U.S. appeared to have developed resistance to Bd after infection by the fungus (together with an influx of nonnative trout) nearly wiped them out.

The authors of the Panama study say their findings offer hope for the survival of amphibians around the world. But they caution that even if this resistance trend holds for most species, detecting the remnants that survive infection and helping them persist and proliferate will require extensive monitoring efforts.

“Clarifying how disease outbreaks subside will help us predict, and respond to, other emerging pathogens in plants, wildlife – and in humans,” Voyles said. “These are increasingly important goals in a time when rapid globalization has increased the rate of introduction of pathogens to new host populations.”

 

Citation:

Voyles, J., Woodhams, D.C., Saenz, V et al. 2018. Shifts in disease dynamics in a tropical amphibian assemblage are not due to pathogen attenuation. Science. 10.1126/science.aao4806

Banner image of Atelopus varius by Brian Gratwicke

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