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.
This article originally appeared in The New York Times.