A method of human pregnancy testing that was popular until the 1970s might be to blame for the spread of a mysterious fungus that has decimated frog, toad and amphibian populations throughout the world, researchers suggested Wednesday.
Before home pregnancy tests became commonplace, if a woman suspected she was pregnant, a urine sample would be injected into a South African clawed frog, explains Vance Vredenburg, a researcher at San Francisco State University and coauthor of a study published Wednesday in the journal PLOS ONE. The test was popular in doctor's offices worldwide.
"They are tropical frogs that breed year round, are easy to keep in the lab and have the same hormones as humans," he says. "If a woman is pregnant, the hormones in the injected urine will cause the frog to ovulate. ... We can look at its ovaries and tell if the frog is getting ready to lay eggs. If it is, she's pregnant."
According to a report on the subject published in 2000, "in the 1940s and '50s medical demand for pregnancy testing made [South African clawed frogs] very widely available in European and North American laboratories."
That wide use may have led to the spread of Batrachochytrium dendrobatidis (Bd), a deadly fungus discovered in 1998 that has been linked to the extinction of the sharp-snouted day frog in Australia and mysterious die-offs of hundreds of other types of amphibians over the past several decades. It often causes cardiac arrest in amphibians that it infects.
Since it was discovered, amphibian experts have desperately tried to find a cure, trace it back to its source or at least understand why some frogs are able to live healthy lives with the fungus. So far, they haven't learned much, until now. Bd has been discovered in healthy South African clawed frogs living in the wild in California, where they are an introduced exotic species.
The fact that many types of frogs began to die off in the United States in the mid 1970s, when frog pregnancy testing was still in use, coupled with the fact that Bd was found in museum specimens of South African clawed frogs from the early 1900s, means it's plausible that the disease originated in them.
"We don't know if this is the only way it's spread, but it's a likely way," Vredenburg says. "They carry it subclinically so you can't tell they're sick, and we've shipped them all over the world, where they've escaped from labs."
The species has since evolved to be able to live with the fungus, which could provide scientists with a model to study. Unlike almost every other parasite-host relationship studied, Bd seems to ravage entire populations of species before they have a chance to evolve to coexist. The bubonic plague, for instance, killed roughly 30 percent of humans who it infected. In some species, Bd has a near 100 percent death rate.
"Pathogens aren't supposed to drive hosts to extinction, because if they did that, they'd go extinct too," he says. "It's absolutely crazy what's happening. We're talking about 100 percent of hundreds of species. It's phenomenal."
In California's national parks, for instance, in what Vredenburg calls "pristine environment," 99.9 percent of the mountain yellow-legged frogs have died off due to the parasite.
"It's one of the most protected habitats in the world, there are no roads; you can't even ride a bike," he says. "And I've taken pictures there where there are hundreds of thousands of dead frogs in a lake."
Vredenburg says that it's no surprise that humans are likely behind the spread of the pathogen.
"This shows yet again that there is evidence humans were involved," he says. "We have an ethical reason to try to do something about the outcome here."