When I was a child growing up on the East Coast, I would often fall asleep to a concerto of frogs croaking outside my window. But that comforting sound has been silenced by the decimation of frog populations across the globe. In addition to environmental pollution and loss of habitat, one of the culprits is a highly transmissible deadly fungus, which has been identified for the first time among frogs in California, including one in Golden Gate Park.
Stanford researchers examined 201 preserved specimens from the California Academy of Sciences in San Francisco, which has one of the oldest and largest herpetology collections in North America. Of the 23 specimens collected between 2001 and 2010 in California, three were found to be positive for the fungus – one from San Francisco and two from San Diego.
“Our goal was to document historically how far back this fungus might have existed in California. Until this study, there were no reports to substantiate that Xenopus laevis (African clawed frogs) in California were infected with this fungus,” Sherril Green, DVM, PhD, professor and chair of comparative medicine, told me. She is the senior author on the study, which appears online today in PLOS ONE.
Green, who is a recognized frog expert, says the frogs were imported from Africa to the United States in the early 20th century as they were routinely used in pregnancy testing. When the urine of a pregnant woman is injected into the frogs, the frogs begin producing eggs – the sign of a positive pregnancy. Though the practice was discontinued in the late 1970s, some frogs from the testing were released into the environment – an unfortunate move from a conservation perspective, Green says.
The African clawed frogs are large – as much as seven inches across – and are highly invasive, posing a threat to native amphibians. They are carnivorous and may devour fish and other frogs that cross their path and may be carriers of disease, particularly the deadly fungus known as Batrachochytrium dendrobatis. Researchers previously have established that clawed frogs in Africa were infected with the fungus as far back as the 1930s. In the latest study, the Stanford researchers identified the earliest reported case of a specimen with the disease, a frog found in Kenya in 1934.
The fungus is believed to be responsible in part for frog epidemics in many countries, including the United States, UK, France, Spain, Germany, Portugal, Italy, Brazil and Japan and throughout North America, Green says. Though there have been U.S. government proposals to further restrict the importation and transport of the African frogs, she says this likely comes too late, as the frogs are already widely dispersed around the globe. It’s hoped that over time native species will become resistant to the fungus and ultimately survive, she says.