Around the world, frogs, salamanders and other amphibians are disappearing — and much about their demise has been a mystery. Now, in an episode of amphibian CSI, biologists have used decades-old museum samples of frogs, toads and salamanders to track the relentless path of a killer fungus across Mexico and Central America over the last 40 years.
The findings, published online Monday in the journal Proceedings of the National Academy of Sciences, strongly link the amphibians’ disappearance to the fungus and suggest that the disease was an alien invader rather than a native disease let loose by climate change.
By some estimates, about 40% of amphibian species are in decline. The main suspect is a skin fungus, Batrachochytrium dendrobatidis, known as Bd. The disease causes salamanders to lose their tails and frogs to lose weight, killing the animals within about three weeks.
Though scientists have known for decades that amphibians were mysteriously dying, by the time the they realized the scope of the problem in the 1990s, it was too late for many species. Costa Rica’s golden toad, for example, went extinct within three years in the late 1980s.
The fungus, discovered a decade later, was eventually identified as a suspect. However, researchers needed more information from the past.
“It would be great if we could go to these areas and study this disease,” said lead author Tina Cheng, a graduate student at San Francisco State University. Confirming the fungus’ relationship to the creatures’ deaths, and understanding how it traveled, could help researchers learn how to contain it. “But,” Cheng added, “the sad fact of the matter is that most of the animals are not there for us to study anymore.”
The extensive collection of amphibians at UC Berkeley’s Museum of Vertebrate Zoology provided a solution: It contained a trove of frogs, salamanders and other amphibians collected from sites around the world. The researchers could look for evidence of the fungus on the skin of creatures that were jarred and pickled decades ago, at times when the fungus was just emerging and later on, when it had spread to epidemic status.
Using traditional methods — cutting up pieces of skin and looking for the fungus through a microscope — would have been too difficult and would have destroyed the specimens, Cheng said. Genetic analysis also seemed problematic because the formalin preservative chemically chops up DNA into little pieces. Nonetheless, Cheng realized that DNA analysis could work because the fragments of fungal DNA they were looking for were so small that the DNA dicing couldn’t harm them.
Analysis of swabs from the bodies of frogs and toads from Costa Rica and salamanders from Mexico and Guatemala revealed some striking patterns: The fungus emerged in southern Mexico in the early 1970s and spread to western Guatemala over the next two decades, then reached Monteverde, Costa Rica, by 1987. The fungus’ path matched the drops in population of a variety of amphibian species in those regions.
“The minute it shows up, things go bad pretty quick,” said Karen Lips, an ecologist at the University of Maryland who was not involved in the study.
The fungus appeared to have been be completely absent in the years before scientists first detected the disease outbreaks — making it unlikely that it was there all along and began to run amok because of some environmental change, such as altered climate patterns.
One theory, Cheng said, is that it was introduced by the African claw-toed frog, a carrier of the disease that was once imported from Africa for use in pregnancy tests.
Cheng said the next step would be to use the same type of DNA analysis to search for the fungus on museum specimens in other parts of the world to see whether Batrachochytrium dendrobatidis was wreaking havoc there too.
“There’s going to be a lot of people going to museums and following up on things because there’s a lot of unexplained [amphibian] disappearances around the world,” Lips said.