The chytridiomycosis that is devastating amphibian populations around the world, came to the United States in the 1960s by way of the African clawed frog (Xenopus laevis), which was used to improve human pregnancy tests, according to a paper published last week in the PLoS ONE Journal. The frogs, which were used in pregnancy tests up to the early 1970s were imported from Africa, and when the frogs were no longer needed, released by area hospitals when the test that involved their use became obsolete, according to the paper. The frog, which is resistant to the disease's effects, was first known to be a carrier in the 1930s, but was never tested in escaped or released frogs in North America until now.
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The African Clawed frog (Xenopus laevis). Photo by Michael Linnenbach/Wikipedia.
San Francisco State University amphibian ecologist Vance Vredenburg (lead author of the study) and his colleagues Stephen A. Felt, Erica C. Morgan, Samuel V. G. McNally, Sabrina Wilson, and Sherril L. Green tested 23 Xenopus frogs from museum specimens that were collected in California in 2000 and 2001 and found three infected frogs, supporting the notion that humans were responsible for the spread of the disease in North America by introducing an invasive species that is a carrier of the disease. The Xenopus frogs, unlike most frogs that contract the disease don't die when they become infected.
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It is still unclear how the disease spread worldwide, but Allan Pessier, a pathologist at the San Diego Zoo's Wildlife Disease Laboratories told
Nature.com that detailed genetic fingerprinting could be used to trace the diseases exact routes. Pessier supports the captive breeding of frogs for the pet trade to ensure these frogs don't carry any diseases, as well as the enforcement of existing international laws issued by the World Organisation for Animal Health in Paris that mandates the testing of all frogs for Batrachochytrium dendrobatidis, before being shipped out of any countries as food items, lab animals or pets. He doesn't think the U.S. Department of Agriculture is enforcing these laws and not every shipping point has the capability to test for disease.
The African clawed frog is an invasive species in the United States and is known for its voracious appetite, eating virtually any animal that it can fit into its mouth. It is highly resistant to disease, and spends the majority of its life, if not all of its life in the water. It has a smooth skin that is slimy to the touch and claws on its hind legs. It grows to about five inches in length. Xenopus laevis is currently regulated in Arizona, California, Kentucky, Louisiana, New Jersey, North Carolina, Oregon, Virginia, Hawaii, Nevada, and Washington.