Frogs that were imported for pregnancy tests and set loose in California carry a deadly fungus responsible for wiping out vast numbers of amphibians worldwide, scientists have found.

Populations of African clawed frogs (Xenopus laevis) have thrived for decades in the state’s drainage ditches and ponds, but their link with the deadly Batrachochytrium dendrobatidis fungus was unverified until a research team from Stanford University and San Francisco State University recently tested museum samples for the fungus.

“Until this report, there had been no conclusive evidence that Xenopus laevis had been carrying the disease,” said Sherril Green, a Stanford veterinarian who was part of the research team.

The pathogen’s spread through amphibian populations is contributing to one of the greatest disease-caused losses of biodiversity in recorded history. Entire species of amphibians, which are among the oldest vertebrates on Earth, have been driven to extinction since the fungus first was described in 1999.


“This pathogen is like no other pathogen we have seen,” said Vance Vredenburg, a biologist at San Francisco State who led the research. The discovery in the African frog populaton in California “tightens the noose around the idea that humans really are responsible for moving this pathogen around,” Vredenburg said.

The fungus causes amphibians’ skin to harden, interfering with the regulation of electrolytes and eventually causing cardiac arrest.

The African frog’s role in the rapid decline of California’s amphibian populations is uncertain -- American bullfrogs, also introduced here, could have infected the African frogs, researchers say. And other factors have affected amphibian populations, including loss of habitat, climate change and human consumption. But the presence of the fungus in a hardy and thriving populaton that has proved difficult to eradicate has raised alarm among conservationists.

African clawed frogs don’t appear to succumb to the pathogen’s deadly effects. Native to an arid region of South Africa, the frogs are extremely hardy, can adapt to cold and drought, and can survive for several years burrowed in mud. Efforts to eradicate them here and elsewhere have proved fruitless.


“Once they’re established in the wild they’re very difficult to eradicate,” Green said. “I don’t know of any place in the world where they have been introduced that has had success in eradicating them.”

Two of 23 archived specimens of wild-caught frogs from California, preserved in 2001 and 2003, tested positive for the fungus, according to the research, published Wednesday in the online journal PLOS ONE. The frogs, in a Stanford collection at the California Academy of Science, were caught in San Diego and San Francisco counties.

Because they have a hormone system similar to that of humans, African clawed frogs were used for much of the last century to detect pregnancies -- the frogs ovulate in response to serum or urine from a pregnant woman. The practice ended in the 1970s, when more sophisticated blood tests were introduced. Many hospitals simply released their frogs.

“We weren’t as conservation-aware in those days,” Green said.


Because of its biological similarities to humans and its hardiness and prolific reproduction, the species remains a mainstay of biological research.

“There’s a cell biologist down the hall from me who has a colony of Xenopus laevis that she has had for years,” Vredenburg said. “Actually, they’re getting stem cells from them. This species is very, very important. We’re not saying this species should be banned. We’re saying we should be very, very cautious.”

Importation and use of Xenopus laevis are tightly controlled -- unlike the situation for American bullfrogs, which are still widely sold for food or as pets.

Solving the mystery of the fungus has vexed biologists, who consider it a potential harbinger of emerging diseases that could affect domesticated species critical for food, such as cattle and poultry.


“Science thrives on predictability,” Vredenburg said. “We have very little predictability in this. In the very same habitat, sharing the same lakes and streams, you have frogs that are surviving. They’ve been infected and are doing fine. For every rule that we put down we find an exception to it.”

Even within the same species, some frog populations appear to survive the fungus, Vredenburg said. A South African researcher found that the fungus was not causing mortality in the wild frogs that carried the pathogen. The fungus and frog populations there could be evolving to adapt, a common evolutionary path for pathogens and their hosts. The mechanics of such a process could be crucial to solving future pandemics.

“We really need to understand the fundamental biology here,” Vredenburg said. “That same biology is occuring in other kinds of pathogens.”