International trade of the invasive South African clawed frog (Xenopus laevis), a subclinical carrier of the fungal pathogen Batrachochytrium dendrobatis (Bd) has been proposed as a major means of introduction of Bd into naïve, susceptible amphibian populations. The historical presence of Bd in the indigenous African population of Xenopus is well documented. However, there are no reports documenting the presence of Bd in wild Xenopus populations in the US, particularly in California where introduced populations are well-established after intentional or accidental release. In this report, a survey was conducted on 178 archived specimens of 6 species of Xenopus collected in Africa from 1871–2000 and on 23 archived specimens (all wild-caught Xenopus laevis) collected in California, USA between 2001 and 2010. The overall prevalence rate of Bd in the tested Xenopus 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 and one positive specimen from 2003. The positive Xenopus (3/23) collected in California were collected in 2001 (2/3) and 2003 (1/3). These data document the presence of Bd-infected wild Xenopus laevis in California. The findings reported here support the prevailing hypothesis that Bd was present as a stable, endemic infection in Xenopus populations in Africa prior to their worldwide distribution likely via international live-amphibian trade.
Frog Once Used in Pregnancy Tests Spread Deadly Fungus
A species of frog that was used from the 1930s to the 1950s in human pregnancy tests is a carrier of a deadly amphibian disease that is now threatening hundreds of other species of frogs and salamanders.
The species, the African clawed frog, was shipped across the world for use in human pregnancy tests, until a different method evolved for determining whether a woman is pregnant. Released to the wild, the frogs are now proving to be a threat to other animals on multiple continents.
“There are populations here in Golden Gate Park, in San Diego, Los Angeles, Europe, China, nearly everywhere,” said Vance Vredenburg, a conservation biologist at San Francisco State University and one of the researchers involved in the study, which was published on Wednesday in the journal PLoS One.
The pathogen the frogs are spreading is a fungus called Batrachochytrium dendrobatidis, or Bd. It has led to the recent decline or extinction of 200 frog species worldwide, the researchers report. Researchers in 2004 found Bd in a museum specimen of an African clawed frog that dated to 1934. But the frog itself appears to be unaffected by the fungus.
“Evolution has run its course,” Dr. Vredenburg said. “The species probably at some point suffered, but the survivors have figured out ways to survive.”
For other species, the pathogen is “the worst disease in vertebrate history,” Dr. Vredenburg said. The disease infects the skin of frogs and salamanders and causes it to thicken 40 times greater than normal, Dr. Vredenburg said. Within a couple of weeks, the disease causes an electrolyte imbalance and the amphibians die of heart attacks, he said.
Dr. Vredenburg and his colleagues tested museum specimens of the clawed frog at the California Academy of Sciences in San Francisco, and found evidence in swabbed DNA of the fungus in the preserved frogs’ skin. They also tested specimens in Africa collected between 1871 and 2010 and found that the disease was present in populations of the clawed frog before they were exported worldwide.
Thousands of African clawed frogs were shipped from South Africa to labs and hospitals around the world before the middle of the 20th century. In those days, some pregnancy tests involved injecting a woman’s urine into a female frog. If the frog began ovulating within about 10 hours, there was a high likelihood that the woman was pregnant.
The frogs are no longer imported to the United States for pregnancy testing, though they are still used for scientific research.
Vance T. Vredenburg, Stephen A. Felt, Erica C. Morgan, Samuel V. G. McNally, Sabrina Wilson, Sherril L. Green. 2013. Prevalence of Batrachochytrium dendrobatidis in Xenopus Collected in Africa (1871–2000) and in California (2001–2010). PLoS ONE. 8 (5): e63791 DOI: http://dx.doi.org/10.1371/journal.pone.0063791