More frogs are growing extra limbs and other deformities now than 50 years ago, according to research published by UW-Madison scientists in the December issue of the journal \Conservation Biology.""
Deformed frogs often cannot survive long enough to reproduce, according to Pieter Johnson, UW-Madison graduate student and lead author of the study. The increase in deformities could endanger affected frog populations.
Johnson, who discovered a non-native species of spiny water flea in northern Wisconsin last year, said he wants to study how humans affect the spread of diseases in aquatic environments. He started researching deformed frogs as part of his undergraduate honors thesis at Stanford.
""A local landowner was finding six-legged frogs and wanted to know why,"" he said. ""I thought it sounded like a fascinating problem.""
Six legs is a relatively small number. Johnson and his colleagues, who studied cases of deformed frogs from the first recorded findings in the 1940s to the 1980s, found some frogs with 40 extra hind limbs.
The cause appears to be parasitic worms called trematodes. They can kill some cells and change the way other cells grow, according to Rebecca Cole, who studies parasites at the National Wildlife Health Center. The exact process is unknown but may involve blocking communication between cells.
The parasites are not just found in frogs, she said. They can inhabit small fish as well as tadpoles, and they go through several different host animals before returning to frogs or fish.
Birds and mammals that eat infected tadpoles or fish carry the parasites over long distances. Their droppings transmit the infection to snails, and the parasites then multiply in a burst of asexual reproduction and swarm to new hosts.
Any parasite that does not find a fish or tadpole within 48 hours will die, Cole said.
That means the parasites have more opportunities when there is less cover, such as vegetation that provides hiding places for snails and keeps them away from fish and frogs. Runoff sites, for example, may provide less cover and lead to more infections because of human influence, she said.
The trematode thought to cause deformities, , is specific to ram's horn snails, according to Timothy Yoshino, chair of the department of animal health and biomedical sciences at the UW-Madison School of Veterinary Medicine. It is found in a wide variety of amphibians, fish, birds and mammals, he said.
The next question is why the number of deformed frogs seems to have increased, Johnson said. This may mean tracking down influences, human and otherwise, on a chain of events that involves a different species at every step.
