A team of Stanford and San Francisco State University (SFSU) researchers recently proved that a type of frog found in the Bay Area carries a fungal pathogen that has caused the decline or extinction of around 200 other amphibian species.

After testing 23 South African clawed frogs found in California, the researchers discovered that three of the specimens, including one from Golden Gate Park, were carrying the Batrachochytrium dendrobatis (Bd) pathogen. Researchers believe that the species, which isn’t harmed itself by the pathogen, played a large role in introducing the fungus to the United States, where it has devastated other frog populations.

Vance Vredenburg, a biologist at SFSU and the lead author of the paper, began studying the South African clawed frogs several years ago while doing conservation work on mountain yellow-legged frogs, which he said were “literally dying in front of my eyes” after being infected with the fungus.

“I was trying to figure out what was going on with these frogs and it turns out that there was a pathogenic fungus killing frogs all over the world,” Vredenburg said. “The big question was where did the fungus come from and why is it having such a dramatic effect on species?”

Professor of Comparative Medicine Sherril Green subsequently contacted Vredenburg two years ago, seeking help in testing archival specimens to investigate the source of the Bd pathogen. Though scientists previously believed that the clawed frog was connected to the Bd infection, the team’s discovery was the first confirmation of the species’ role in spreading the water-transmitted fungus around the world.

“These frogs evolutionally have adapted to be very hearty in all kinds of environments, so they can sustain themselves through periods of drought by hibernating in the dirt,” Green said. “They are a pretty adaptable species.”

South African clawed frogs in the United States

The South African clawed frog was first introduced to California in the early 20th century through imports from the Ivory Coast. By the 1940s, American scientists began to use the frogs for pregnancy testing.

“If you take urine from a pregnant woman and inject it into these frogs, if she is pregnant the frog will lay their own eggs,” Green said. “So that was a positive pregnancy test.”

Once more advanced pregnancy screening techniques were developed in the 1960s and 1970s, hospitals and labs released the pathogen-carrying frogs into the wild, where they spread the harmful fungus to other amphibians through water.

“There are hundreds of species that when infected will die within two weeks. They get massive skin infections,” Vredenburg said. “They try to shed the fungus from the skin, but their skin can be up to 40 times the width of a normal uninfected frog skin.”

While the fungus has killed off amphibians in America, species in Africa developed immunity to it, limiting the damage caused by clawed frogs in their natural habitat.

“They were not causing major die-offs in Africa because [the frogs] had been there for a while,” Vredenburg said. “Humans inadvertently moved the pathogens to a new place where they had not been and thus caused the havoc that we see.”

Dangers of non-native species

Though the frogs still play a role in biomedical research today, Vredenburg said that there are restrictions on how the frogs can be used and what happens to them when the research is complete.

“What worries me is that there are other species like the African clawed frog that people who aren’t scientists can buy and trade and sell,” Vredenburg said, noting that such people might release the species into the wild. “The research labs that have them are not allowed to let them go, there are all kinds of great safeguards in research labs so that the pathogens are not getting out into the wild.”

Vredenburg said that while African clawed frogs are no longer being released into the wild, the manner by which the frogs, a non-native species, were able to infiltrate and damage Californian ecosystems presents broader implications.

“We need to start paying attention to food markets that are trading live animals or pet stores,” Vredenburg said. “At different pet stores you can buy amphibians that were captured in Asia and if you want to, you can release them in Golden Gate Park. That’s not a good idea. We should get this straightened out now, not 20 years from now.”

As for the amphibians affected by the fungus, researchers hope that the survivors can adapt and eventually develop immunity.

“At this point there are so many established wild populations of non-native species in California, it would be impossible to collect them all,” Green said. “We hope that if the native species here in California get exposed to the fungus, the ones who survive will become resistant.”