Bay Area scientists believe they have discovered the Typhoid Mary of the frog world: a flat, feral creature that carried a deadly fungus from Africa to California's ponds and puddles through global trading.
Genetic analysis revealed that eight of 206 African clawed frogs -- caught wild or preserved in jars at the California Academy of Sciences -- carried the fungal plague called chytridiomycosis, which leaves them unharmed but kills native frogs in catastrophic numbers.
An infection was detected in a frog captured in Africa in 1934, supporting the theory that the fungus thrived there before spreading worldwide. Another infected frog, still alive, was recently trapped in Golden Gate Park's Lily Pond.
"It confirms our suspicions that this is one means of spread of the fungus into the environment, through frogs that were not native," said Sherril Green, professor and chairwoman of comparative medicine at Stanford University, who collaborated on the study with San Francisco State biologist Vance Vredenburg.
The African clawed frog resembles roadkill, but it has served an important role in medicine and research. It was first brought to the United States for use as a pregnancy test in the early 20th century, when it was routinely injected with the urine of female patients. The frogs were useful because they ovulate when injected with human urine, produce eggs all year long, and were easily imported from Africa in large numbers.
Although the practice is now discontinued, the African frogs likely were released into the environment by hospital workers, Green said. It was a well-intended gesture -- with deadly consequences.
"Today, these frog populations are often found in or near urban areas," said Vredenburg, associate professor of biology at San Francisco State. "It's amazing that more than a half-century after being brought to California, these frogs are still here, and they still carry this highly infectious disease."
The African frog may not be the only culprit. A University of Michigan study also has implicated bullfrogs, farmed as a food source in South America and shipped to America.
While the urban world and wild frogs have been at odds for a long time, the fungus is particularly tough on the thin-skinned creatures.
Called Batrachochytrium dendrobatidis, or "Bd," it kills frogs by clogging their pores, deranging their blood chemistry and causing their tiny brains to swell.
The infection has led to the recent decline or extinction of 200 frog species worldwide, from the Sierra yellow-legged frog to the exotic jewel-colored creatures that decorate calendars, postage stamps and National Geographic magazine covers.
Some species seem resistant, such as the Bay Area's common Pacific tree frog.
But scientists have not figured out a way to transfer such resiliency to vulnerable frogs. Vaccines, a routine type of human protection, are impractical.
To determine the African frogs' role in the spread of the disease, Stanford and San Francisco State scientists and students spent hours in the basement of the California Academy of Sciences, sorting through thousands of specimen jars filled with old frogs floating in ethanol since 1871. Swabbing DNA from the skin between the toes and around the claws, they found that of 178 frogs, five (2.8 ercent) were positive for the fungus, confirming that the fungus was present among indigenous populations in Kenya and Uganda before they were exported worldwide.
They also embarked on frog-hunting expeditions. Traps -- baited with supermarket chicken -- caught 28 African clawed frogs, of which three (13 percent) tested positive. One lived in Golden Gate Park.
"It was a bit of forensic detective work," Green said. The paper is published in Thursday's issue of the journal PLOS ONE.
The timing of the epidemic's spread fits with their theory, Green said. "It takes decades to see the effect of what an invading species does," she said. "We are beginning to see now what happened long ago."
Because the frogs are widely dispersed across the globe, Green and Vredenburg said containing the epidemic is a major challenge. The well-established fungus also can spread through water, wind and feathers of birds, Green said.
The frogs' use, sale and transport are now highly regulated in California, but the damage has been done, they said. (A pygmy version, a favorite of aquarium enthusiasts, is less hardy, so it's not considered a threat.)
"Now we need to be cautious about other introduced species," Vredenburg said. "There could be other animals out there that are carrying diseases that we don't even know about yet."
Contact Lisa M. Krieger at 650-492-4098.