Prior posts have noted the decline in the frog population in California, among other locales worldwide. As with colony collapse disorder of honeybees, there have been numerous theses regarding the cause. Researchers now propose that the fungus Batrachochytrium dendrobatis ("Bd") is the cause.
South African clawed frogs (Xenopus laevis) were once used for pregnancy tests. In California, some either escaped or were released, and these frogs are now deemed an invasive species. It has been known for some time that X. laevis is a subclinical carrier of Bd. [Subclinical means the asymptomatic (without apparent sign of disease) carrying of an infectous agent (such as a microbe, fungus, intestinal parasite, or virus).] Researchers sought to ascertain if X. laevis could be infecting native species with Bd.
The historical presence of Bd in the indigenous African population of X. laevis is well documented. However, there are no reports documenting the presence of Bd in wild X. laevis populations in the U.S., particularly in California where "introduced" populations are well-established. To examine if this "common knowledge" was correct, researchers assessed178 archived specimens of 6 species of X. laevis collected in Africa from 1871–2000 and 23 archived specimens (all wild-caught Xenopus laevis) collected in California, between 2001 and 2010.
The overall prevalence rate of Bd in the tested X. laevis was 2.8%. The earliest positive specimen was X. borealis collected in Kenya in 1934. The overall prevalence of Bd in the X. laevis collected in California was 13% with 2 positive specimens from 2001 (collected in San Diego) and one positive specimen from 2003 (collected in the lily pond next to San Francisco's Golden Gate Park). Unfortunately, due to California's budget problems, efforts by the California Department of Fish & Game to kill the X. laevis frogs present in the lily pond and eliminate the fungus have ceased.
The results would thus appear to support the thesis that Bd infected wild X. laevis frogs are present in California, and that Bd may help explain the demise of frog populations not only in California, but elsewhere due to the international live amphibian trade.
The research can be found at: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0063791. A newspaper story on the issue can be found at http://www.sfchronicle.com/science/article/African-frogs-tied-to-U-S-frog-die-off-4529835.php.