African Clawed Frog Responsible For Spreading Deadly Fungus
May 16, 2013

African Clawed Frog Culprit In Spread Of Deadly Fungus

Brett Smith for - Your Universe Online

For years, scientists have been on the trail of a slippery culprit responsible for a deadly fungus, and they´ve finally found the culprit.

The fungus, called Batrachochytrium dendrobatidis,“¯or ℠Bd´ for short, has played a role in the recent decline or extinction of 200 frog species worldwide. According to a new report in the open-access journal“¯PLOS ONE, long-suspected African clawed frogs have finally been confirmed as the amphibians responsible for bringing the disease to the US.

"We found that African clawed frogs that have been introduced in California are carrying this harmful fungus," said co-author Vance Vredenburg, a biologist at San Francisco State University. "This is the first evidence of the disease among introduced feral populations in the US, and it suggests these frogs may be responsible for introducing a devastating, non-native disease to amphibians in the United States."

Starting in the 1930s, thousands of the frogs were exported from Africa for use in pregnancy tests. To perform the test, doctors would inject a suspected mother's urine into the frogs. If pregnancy hormones were in the urine, the frog would begin to ovulate.

"Today, these frog populations are often found in or near urban areas, probably because hospitals released them into the wild when new pregnancy testing methods were invented in the 1960s," Vredenburg said.

To identify the frogs as culprits, a team of California-based researchers examined 23 specimens, collected locally between 2001 and 2010. They also looked at 178 specimens collected in Africa between 1871 and 2000 that were preserved in alcohol and donated to the California Academy of Science.

Of the African samples, 2.8 percent tested positive for the fungus. Of the California samples 13 percent tested positive. The earliest positive case was found in a sample collected in 1934 in Kenya, establishing that the fungus was prevalent in Africa long before large-scale exporting of the frogs.

"It's amazing that more than half a century after being brought to California, these frogs are still here, and they still carry this highly infectious disease," Vredenburg said. "This implies that there must be a stable relationship between the pathogen and the frogs, whereas there are other frog species, for example in the Sierra Nevada, which have been wiped out by the pathogen."

Co-author Sherril Green, a professor and chair of comparative medicine at Stanford University, said that containing the frogs and the fungus is a major challenge for conservationists.

"Right now people are still tracking what populations may be affected and may succumb a few years from now," she said. "Resistant species are surviving with chronic infections and will learn to live with it.”

“While it wipes out most frogs that are not native or indigenous to the source of the fungus, some survive and there is hope that surviving populations will adjust and become resistant to it,” she added. “But we don't have enough data to know indeed if that is happening.”

The US Fish and Wildlife Service has indicated that it is considering more restrictions on the importation and intrastate transport of the frogs, both of which fall under federal jurisdiction. However, Green said these new rules would have little impact on the current situation.

"In essence, it's like closing the barn door long after the horse has left," she said. "Further restricting“¯(African Clawed Frogs) at this point is probably going to have very little impact on the ongoing epidemic, but will certainly have a negative impact on biological and biomedical research."