Ecologist Pieter Johnson holds a deformed Pacific chorus frog and a ramshead snail in a freshwater pond near San Francisco in 2009. Johnson, of the University of Colorado at Boulder, and colleagues have been studying amphibians with "sick and twisted" deformities in the western United States. (Full story: Parasites Creating Deformed Frogs in U.S. West.)
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.
(See "Weed Killer Makes Male Frogs Lay Eggs.")
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.)
Pictures: U.S. Frogs Deformed by Parasite Infections
See how a parasite passed from snail to frog in the western U.S. can cause "grotesque" malformations such as extra hind limbs.