Biologists have discovered the Typhoid Mary of the frog world - a little green hopper that is spreading a deadly fungus disease to other frogs and amphibians in the Sierra while remaining largely immune to the infection itself.

The fungus, known as chytrid, has killed frogs, toads, salamanders and newts in the Sierra and is the same fungus that has wiped out hundreds of frog species throughout the world in what many biologists have termed a "mass extinction."

In California, two San Francisco State University researchers have discovered that the common Pacific chorus frog, an amphibian barely an inch long, appears to be the guilty animal. The chorus frogs are heavily infected with the killer fungus, but almost never show symptoms of the disease, biologist Vance T. Vredenburg and Natalie M.M. Reeder, a recent graduate student in his lab, reported in Monday's online journal PloS One.

Not so for the frog's neighbors, the yellow-legged frogs that also inhabit Sierra lakes are rapidly being infected and dying with a litany of severe symptoms, including "weight loss, lethargy, excessive skin shedding, muscle spasms and loss of reaction to stimuli," the researchers said.

The chorus frog is common to California. Its noisy "ribbit" call can be heard from dawn to dusk around lakes from the Bay Area to the High Sierra, but Vredenburg said in an interview that no studies have been done to determine whether the fungus it carries is affecting Bay Area amphibians.

Population crash

Worldwide, the chytrid fungus has spread to nearly 600 species of frogs, Vredenburg said, and has probably driven more than 200 species to extinction.

"It's the worst population crash of animals in history," he said.

Reeder, a UC Berkeley graduate with a major in integrative biology, backpacked across the Sixty Lakes Basin in the High Sierra east of Fresno to study the chorus frogs and the endangered yellow-legged frogs that inhabit the same lakes. She brought back samples of both species to Vredenburg's lab.

Throughout Sierra lakes, the yellow-legged frogs have been in catastrophic decline for many years, according to Vredenburg. In her backpacking travels, Reeder observed that with their sticky toe pads, the infected chorus frogs are able to move overland from lake to lake, spreading the chytrid fungus.

Back in the lab, Vredenburg said he determined that as the infected chorus frogs swim through water, their skin releases thousands of "zoospores" that disperse the fungus spores and infect the yellow-legged species.

The fungus spores attack the yellow-legged frog's skin, causing it to thicken and preventing the animal from absorbing crucial water and salts like sodium and potassium. In chorus frogs, however, researchers found patches of both normal and infected skin.

"The patches of healthy skin appear to function normally in the chorus frogs," Vredenburg said, "and that's what probably protects them against the infection." The normal skin holds elements of the chorus frog's immune system, he said.

Thwarting the fungus

Vredenburg believes that further analysis may provide a path toward thwarting the chytrid epidemic. The normal skin holds many species of bacteria, and among those bacteria could be some that destroy the chytrid fungus, he said.

The scientists are also studying the chorus frog's genome in search of genes that could be responsible for their immunity to the fungus, he said.

"We've figured out one piece of the puzzle," he said, "and understanding the genome will help us to understand differences between those two species of frog."

Allan P. Pessier, an immunologist at the Wildlife Disease Laboratories of the San Diego Zoo's Institute for Conservation Research, is a co-author of the report.

David Perlman is a San Francisco Chronicle science editor.