A deadly and contagious fungal disease first struck Mexican salamanders in the 1970s, found a new study. From there, it spread through Guatemala and Costa Rica over the next two decades.
As the first study among salamanders to document the history of an epidemic of the illness, the research helps confirm the fungus (known as Bd, for Batrachyochytrium dendrobatidis) as a major cause of widespread amphibian collapse in recent decades. Some 40 percent of frogs, toads and other amphibian species are currently in decline.
The findings could also lead to better ways of slowing or preventing the spread of Bd and similar outbreaks in the years to come.
"This really shows how devastating this disease can be," said lead author Tina Cheng, a graduating masters student in ecology at San Francisco State University. "Up until now, it was not known that this pathogen had any impact on salamanders, and many are highly threatened right now."
"Conservation efforts can now look at the impacts of this disease," she added, "and correct for that."
Amphibian numbers have been dropping for decades as a result of all the usual threats, including habitat destruction, pollution, invasive species and climate change. In the 1970s, researchers started to notice animals disappearing from pristine and protected areas, as well, which led them in 1999 to Bd.
Animals that become infected with the fungus develop chytridiomycosis. They shed their skin and become lethargic. Sickened salamanders lose their tails. Frogs lose weight and become so unresponsive that they fail to turn themselves over when put on their backs. Death comes in a matter of weeks.
Since the discovery of Bd, researchers have linked the fungus to the collapse of frog and toad populations in California, Australia, Panama and Peru. Some species have already gone extinct because of it.
As much as they know about Bd, scientists still debate how, why and even when the fungus first swept through Central and South America. Some experts have hypothesized that the fungus has always been around at low levels and that something triggered it to spread. Others have argued that its invasion was sudden and therefore devastating to vulnerable populations that had no resistance to it.
To investigate, Cheng and colleagues looked at hundreds of salamanders and frogs from museum collections held at the University of California, Berkeley, and the University of Texas, Arlington.
Instead of taking tissue slices for painstaking analysis, as old methods would, the researchers rubbed cotton swabs along the skin of each creature. Then, they analyzed the samples for DNA from Bd. With the new method, they could scan 100 samples in six hours -- a dramatically quicker approach.
Their results showed clearly that Bd was non-existent in samples collected in southern Mexico before the early 1970s, the researchers report today in the journal Proceedings of the National Academy of Sciences.
But after the fungus' arrival in that part of the world, it then seems to have spread to Guatemala and Costa Rica by the 80s. The appearance of Bd in museum samples coincided with known periods of rapid decline among amphibians in each area, confirming the disease as a major cause of the die-offs.
Scientists aren't yet sure how Bd came to the Americas, but the leading theory is that it traveled with African claw-toed frogs, which were brought over from Sub-Saharan Africa for medical purposes, Cheng said. These frogs, which carry Bd but don't get sick from it, were once used as human pregnancy tests. In a tank with a pregnant woman's urine, females of the species begin to ovulate in response to human hormones.
The new study -- and especially its new technique -- has opened the way for even more research that will help piece together an accurate history of a disease that continues to devastate amphibians all over the world, said Karen Lips, an ecologist at the University of Maryland, College Park.
"Just being able to get Bd off of museum specimens is huge, and I sort of hesitate to say it's going to revolutionize things, but there are going to be a ton of papers using this technique," Lips said. "This is one of the big problems here in the East. There are all sorts of anecdotal reports about how things went down, but no one was watching. No one saw the big die-offs happen."
So far, there is no cure for Bd. But researchers have been investigating symbiotic skin bacteria that appear to protect some amphibians from the disease. Sampling specimens for that bacterium could offer new insights into why some groups have survived the disease and which populations will be most vulnerable to it in the future.
"Even if we don't have a specific way to stop it right now," Lips said, "knowing where it's been and where it's going helps us, if nothing else, plan to grab animals and put them in storage, or search for this bacteria, or try to get geneticists to come up with a way to turn this fungus off and stop the destruction."