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Environment & Science

South African Clawed Frogs spreading deadly fungus in California

Image of an African clawed frog, or Xenopus laevis.
Image of an African clawed frog, or Xenopus laevis.

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A species of frogs once used to test for signs of human life is now contributing to the spread of a fatal disease among a wide variety of California wildlife. 

In the late 1920s, long before modern-day pregnancy tests were developed, the South African Clawed Frog was used to help doctors test for pregnancy. It was discovered that if you injected the frog with the urine of a pregnant women, the frog would ovulate and lay eggs. 

RELATED: An animal once used as a pregnancy test may be 'the Typhoid Mary of the frog world'

The frogs were used widely in hospitals from the early 1930s through to the 70s. However, when labs using these frogs finally shut down, well-meaning hospital workers released the frogs into the wild, likely unaware of the impact they'd have on native species here in California. 

"At that time, we didn't quite have the conservationist approach to things and the awareness of the kind of impact that this non-native species might have when introduced into geographic locations that had never seen them before," said Professor and Veterinarian Sherril Green from Stanford University's School of Medicine. 

Scientists have now found that the South African Clawed Frog carries a fungus that in endemic to the species and difficult to detect, since the from shows no signs of sickness. The fungus is adding to one of the biggest losses of biodiversity ever seen, according to researchers at Stanford and San Francisco State University.

"It takes 10-20 years or at least that long before you can actually tell the impact of introducing a new species into a new area," said Green. "We're seeing this now, but the frogs have been here 50 years or so. I think that what we're seeing now is probably something that was introduced that long ago."

The fungus affects other aquatic amphibians, as it's passed through the water, by wind or on the feathers of birds. The fungal spores colonize the porous skin of other amphibians, causing the skin to thicken and making it difficult for them to regulate their electrolytes and body water. The affected animal ultimately die. 

"We've had a number of species in California that were already threatened by loss of habitat and pollutants and other things that now have this added stresser of being exposed to this fungus," said Green. "Their populations have been decimated. For example, the Red-Legged and the Yellow-Legged Frog in the Sierra Nevada were endangered for years, and right now, it's very hard to find those frogs at all. We fear they are extinct."

Green estimates that we've lost about 200-400 different species of frogs in the last decade, in part due to the spread of the fungus. Curbing the effects of the fungus isn't easy, because there's no vaccine and the South African Clawed Frog has been living and breeding in the wild for decades. Scientists now hope future generations of frogs will develop their own resistance to the fungus. 

"The ones that are exposed to it that don't die will, hopefully, survive and go on to produce generations of frogs that can learn to live with this. It's a tough situation; it's not something we're going to be able to stop," said Green. "I think more rules and regulations probably aren't going to do much at this point, but the heightened awareness the public has about the impact of releasing anything that isn't native to the area — frogs and other species, plants and animals of all kinds — can be significant, although you won't see it immediately."