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Invasive frogs may spread deadly amphibian fungus

Imported African species implicated in B. dendrobatidis epidemic

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More evidence has just dropped into place suggesting that frogs once imported to the United States for pregnancy testing could have spread a fungus deadly to many native amphibian species.

From about the 1940s into the 1970s, medical labs tested for pregnancy-associated hormonal changes by injecting human urine into readily available and easy-to-work-with African clawed frogs. If a woman was pregnant, traces of hormones in her urine would accelerate egg development in the frog.

Frogs sometimes escaped or were set free, allowing African clawed frogs to get a foothold in North America.

A new study using museum specimens of free-living African clawed frogs (Xenopus laevis) collected in California finds that three out of 23 carried the dread fungus nicknamed Bd, says amphibian ecologist Vance Vredenburg of San Francisco State University. This species doesn’t succumb itself, but it can spread the fungus, Batrachochytrium dendrobatidis, to more vulnerable neighbors.

The fungus also showed up in about 3 percent of 178 specimens of six Xenopus species collected in Africa as far back as the 1930s, researchers reported May 15 in PLOS ONE. But it cannot be determined whether the strain in the African specimens matches that in the California frogs. Such a match would strongly suggest an African origin for the worldwide fungal epidemic.

The raging Bd fungus is now blamed for wiping out, or causing declines in, about 200 species of amphibians around the world. “It’s just shocking,” Vredenburg says. He’s observed two kinds of yellow-legged frogs in Sequoia and Kings Canyon National Parks dwindle from 500 populations in 1997 to just 12 in 2012.

Where the fungus that is blamed for this amphibian Armageddon originated and how it spread has inspired plenty of ideas. The fungus may have hitchhiked out of Africa, or out of Asia. And in their travels, milder strains of the fungus may have hybridized to create a more virulent form of the fungus. “We still don’t have a unified story that we’re all behind,” Vredenburg says.

In any case, the trade in African clawed frogs has clearly helped move Bd around the world, says Matthew Fisher of the Imperial College School of Public Health in London. He and colleagues have shown that African Xenopus frogs in a zoo breeding facility passed the infection to other animals that were later released on the island of Majorca. “It's not hard to believe that this has happened in the USA as well,” he says.

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