The deformities are caused by a flatworm parasite called Ribeiroia ondatrae, which infects several species of frogs just as they're developing their limbs. Infections trigger an assortment of defects such as no legs or multiple legs that jut out at weird angles from the frogs' bodies, the scientists say. The parasite has a complex life cycle that includes the ramshead snail as its first host.
Scientists already knew that the parasite was the culprit in the frog malformations, but the researchers wanted to find out whether known hot spots of Ribeiroia populations in four western states had changed since they were last surveyed in 1999. So Johnson and colleagues gathered data on frogs and parasites in 48 wetlands in California, Oregon, Washington, and Montana.
"These severe malformations—even though it's not in the headline news—these continue to occur in a lot of amphibian populations in the western U.S.," said Johnson, who received funding from the National Geographic Society's Committee for Research and Exploration. (The Society owns National Geographic News.)
A Pacific chorus frog with an extra hind leg is seen in a 2009 picture.
During their investigations, Johnson and colleagues found that hot spots of frog parasite infections had shifted since the previous surveys—but overall the number of infected frogs is about the same.
"We found that although the distribution of Ribeiroia across wetlands changed, there was little net effect on overall parasite prevalence, with 31 percent of wetlands gaining the parasite and 27 percent losing the parasite," according to the study.
Specimens of Pacific chorus frogs and larger red-legged frogs are shown with limb deformities caused by Ribeiroia parasite infections. In the wild, affected animals are often unable to move and either die or quickly get eaten by predators.
This raises "additional concerns over the long-term consequences for amphibians, which are now the most threatened group of vertebrates worldwide," according to the study.
A northern leopard frog with an underdeveloped hind leg is seen in a 2010 picture.
When scientists began observing amphibian deformities in the 1990s, they proposed various causes, including excess ultraviolet radiation, chemical pollution, predation attempts, and parasite infection.
In the western U.S., where such limb malformations are prevalent, Johnson and colleagues have found increasing evidence that the parasitic flatworm is a "major culprit," according to the study.
Graduate researcher Dan Preston holds a bullfrog tadpole in a pond near San Francisco, California, in 2010.
The new results showed that "the locations of [parasite] hot spots had changed substantially over the last decade," Johnson said.
Ponds where scientists had found few "grotesque" frogs in 1999 now had 30 percent or more frogs with deformed limbs, he said. Likewise, former hot spots now had fewer of the diseased amphibians, according to the results, which are not yet published in a journal.
The frog-deforming parasite has a complex, multihost life cycle that begins with the ramshorn snail, a creature common to many western U.S wetlands (above, a ramshorn snail in 2009).
The flatworm asexually clones itself inside the snail, stripping the mollusk of its gonads and converting it into a "parasite machine," Johnson said. Each night the snail releases hundreds of free-swimming Ribeiroia larvae, which seek out their next hosts—tadpoles—with "remarkable precision."
The parasite larvae penetrate the tadpoles' tissue and zero in on the developing limb buds, so that when a tadpole begins to metamorphose into a frog, its "primary system of locomotion doesn't work—it can't jump, can't swim," he said.
"That's when the birds"—the parasite's final host—"zoom in and eat them up like popcorn."
For the 2010 surveys, the scientists did not specifically measure whether frog populations at the study sites had declined (pictured, a deformed bullfrog in 2009)—"though it's high on our priority list," Johnson noted.
Overall, amphibian populations tend to fluctuate widely from year to year, said Erin Muths, a zoologist at the U.S. Geological Survey in Fort Collins, Colorado. Those fluxes are usually based on an unpredictable resource: water. For instance, sometimes ponds dry up, killing any frog eggs.
"This variability makes it difficult to tell whether we are looking at a real decline" in frogs due to parasites, for example, "or just a blip in a crazy up-and-down pattern that is normal," she said.
Once a bird eats an infected frog, the flatworm parasite reproduces sexually inside the bird. When those birds defecate, their waste contains parasite eggs that eventually make their way back into the snails—starting the cycle anew.