Using a deadly fungus against itself, scientists may have discovered a way to immunize amphibians against a devastating disease. According to a new study published in Nature this week, three frog species can acquire resistance to the killer fungus after repeated exposures. And at least one has also learned to avoid the infectious fungus altogether.
Chytridiomycosis is caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), which has already been implicated in the recent global decline of amphibians. Infected animals grow skin that’s several times thicker than it should be, affecting their ability to breathe and regulate water, causing dehydration and heart failure.
Now, Jason Rohr from the University of South Florida and colleagues discovered how three species can protect themselves against infection using behavioral or immunological responses. They placed oak toads (Bufo quercicus, pictured right) into a test chamber with the fungus on one side. At first, the toads spent the same amount of time on either side. Then the researchers removed the toads and killed the fungus using heat. When these previously-infected toads were placed back in, they spent less time on the side with the fungus, Science explains, suggesting that they can learn to avoid the pathogen.
The disease is also known to block immune responses, but the team found that frogs can develop a resistance that overcomes the fungal-induced immunosuppression.
In a trial where the frogs couldn’t avoid the fungus, their immune responses improved with repeated exposure. Cuban treefrogs (Osteopilus septentrionalis, pictured above) were introduced to live Bd up to four times; between each encounter, they were cleared of their infection using heat. With each exposure, immune cells called lymphocytes proliferated and strengthened their response -- roughly doubling their numbers by the fourth time the frogs encountered Bd. Although only 20 percent made it through the first exposure, Science reports, more than half of them escape their fourth bout with the fungus.
"Fungal counts went down from around 40,000 to 8,000 fungal cells per gram," Rohr tells New Scientist. Reducing fungal growth increases the likelihood that they'll survive subsequent infections. This was also the case for the endangered Booroolong frog (Litoria booroolongensis).
Furthermore, inoculating frogs with dead fungi resulted in a similar magnitude of resistance as infection with live fungi. "Exposure of waterbodies or captive-bred amphibians to dead chytrid or chytrid antigens might offer a practical way to protect chytrid-naive amphibian populations and to facilitate the reintroduction of captive-bred amphibians to locations in the wild where the fungus persists,” USF’s Taegan McMahon says in a news release. Hundreds of thousands of threatened species have already been removed from places were Bd is present, and if a vaccine is developed, they could be successfully reestablished in the wild.
More ambitiously, Rohr hopes to vaccinate wild amphibians by spraying the dead fungus into their habitats. Disease-causing fungi pose a greater threat to biodiversity than any other parasitic group. The technique, according to Rohr, "holds promise against white nose syndrome in bats and lots of other diseases, such as those affecting snakes and bees."
Images: Joseph Gamble (top) & Wikimedia (middle)
