Just a few months ago, some school children in Minnesota were on a field trip (just as many of our children visit the Louisiana Nature Center) and found a pond that had many small frogs which had extra legs - 4-6 hind legs instead of the normal two. Locals were shocked and scared, the press picked it up immediately, and word spread throughout the world on the internet. Suddely, people from across the continents began to share stores of this deformed frog and that strange tadpole. Theories abounded, mostly focusing on chemically contaminated water.
Fortunately, word rapidly reached a scientist, Dr. Stanley Sessions of Hartwick College in Oneonta, New York, who had experience with just such phenomena. Dr. Sessions explained that frogs with multiple legs are not especially rare, and that the general rule is that frogs having multiple legs are normally infested with trematode (a parasitic flatworm called a fluke) cysts. The story of how the cysts got there is full of intrigue and marvel.
Adult flukes live in the intestines of garter snakes. When the snakes defecate in water, zillions of fluke eggs are released. These are eaten by certain aquatic snails. In the snail, each egg hatches into hundreds of thousands of tiny larvae, which, when released back into the water, swim about and seek out tadpoles. They burrow into the tadpoles, seeming to prefer the soft tissue where the legs will form. After they settle in, the larvae encyst (become inactive and encapsulated in a hardened covering, technically called metacercariae). These cysts interfere with the tadpoles’ leg development and usually result in many legs being formed. To complete the flukes’ life cycle, a garter snake comes along and eats the tadpoles or frogs and it all starts over again.
So, why is this happening at such an alarming rate? Dr. Sessions suggests that, because of the publicity, there are more people out looking for malformed frogs, it may be a very good year for trematodes, and/or there might be many more pond snails this season. Nevertheless, he is quick to say that there could be other factors at work and he urges the scientific community to further research the issues by pursuing the impact of such environmental components as pesticides, heavy metals, radiation, and ultraviolet-induced mutations.