A common sight throughout the northwest, scientists say this Pacific chorus frog is a carrying a deadly amphibian infection.
Joyce Gross
Vance Vredenberg has spent 17 years watching yellow legged frogs die in the the Sierra Nevada Mountains. Now he thinks another species of frog may have helped make them sick.
The yellow legged frog was once a common sight in California's alpine lakes and streams, but its population has declined by about 95 percent. Vredenberg, a biology professor at San Francisco State University, says a microscopic fungal disease called chytridiomycosis swept through the Sierra Nevada, killing frogs.
“It makes the black plague look like nothing," Vredenberg says.”
Yellow legged frog/ USFWS
Since the 1980s, the disease (which biologists call chytrid for short) has sickened at least 200 species of amphibians worldwide, particularly in Australia and Central America, and as many as 600 species may be affected.
But Vredenberg says chytridiomycosis doesn't affect all frogs equally. At his study areas in the Sierra Nevada, he noticed survivors.
"There was this really interesting pattern in the field," Vrandenberg says. "Yellow legged frogs were dying off by the tens of thousands. But Pacific chorus frogs were doing just fine."
Even if you've never heard of the Pacific chorus frog, you've probably heard one before. From Mexico to British Columbia, they are the frogs kids often catch and grown-ups hear ribbiting in chorus outside campsites and along roadside ditches.
Vrendenberg and a grad student brought dozens of the chorus frogs back into the lab and observed them for four months. They found the frogs were infected with hundreds of millions of zoospores of the fungus that causes chytridiomycosis -- surprisingly intense infections -- but they didn't die. Patches of the chorus frogs' skin appeared to resist the infection, allowing them to survive with what would otherwise be lethal levels of the disease.
In a paper published this week, Vredenberg and his colleagues suggest that the partially immune chorus frogs may act as a perfect host for the disease, helping it spread from pond to pond and travel upstream.
"The findings help explain the pattern and speed of the chytrid epidemic in the Sierras," he says.
Vrandenberg is concerned the Pacific chorus frog could help the disease spread in other remote mountain ranges.
"They have an enormous distribution, and the number of species they overlap with is huge," he says. "Salamanders, red legged frogs-- there are dozens of dozens of species that they could be putting at risk."
Pacific chorus frogs recorded in Crater Lake National Park
It's not yet clear what Vredenberg's research means for frogs and toads in the Northwest. Scientists have found chytridiomycosis present throughout the region, but it hasn't caused any major outbreaks in the region.
Explaining how and why outbreaks of chytridiomycosis have appeared suddenly worldwide is a tricky business. Some scientists like Vredenberg believe the disease originated in Africa and has only recently been spread around the world by the trade of African clawed frogs. They also carry the host pathogen without getting sick, and were common in labs and pet shops and even used in an early pregancy test.
Other scientists believe that chytridiomycosis has been present in the Americas for a long time, but some new environmental factor is making it more virulent in particular watersheds.
"It could be anything. It could be pollutants in the water, or a changing climate, that's turning this thing on," says Andrew Blaustein, a professor of zoology at the University of Oregon.
Pacific chorus frog/Jason B. MacKenzie
Blaustein and other scientists agree that while the fungus has been a major factor in amphibian declines in the Sierra Nevada and Colorado, it has had little impact so far on sensitive species in the Oregon Cascades.
"We don't see lots of dead and dying animals here," Blaustein says. "But we do see the chytrid in almost every species."
Blaustein says it's not yet clear what has prevented a major outbreak of the amphibian disease in the Northwest, or whether the Pacific chorus frog could help it spread here.
