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, roughly 40 percent of amphibian species are in decline. The main suspect thus far is a skin fungus, Batrachochytrium dendrobatidis, known as Bd. The disease causes salamanders to lose their tails and frogs to lose weight. The animals die within about three weeks.
Although scientists have known for decades that amphibians have been mysteriously dying, by the time 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 a three-year span in the late 1980s.
The fungus, discovered a decade afterward, was later identified as a possible 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's 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 the University of California, 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 on to Monteverde, Costa Rica, by 1987. The fungus's path matched the drops in population counts 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 had been there all along and had begun 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 imported from Africa for use in former times 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 if 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.
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