By Sean Henahan, Access Excellence

Oneonta, NY (Nov. 21, 1996)
Reports of an unusually high number of deformed frogs from all parts of the country first led to suspicions that the mutations were the result of pollution, pesticides, or UV-induced genetic changes. However, it is more than likely that these changes can be attributed to parasitic worm infection, according to experts.

Caption: Frog with extra Leg. (c)J. Helgen and the Minnesota Pollution Control Agency

Stan Sessions, professor of biology at Hartwick College, did an analysis of similar deformities discovered in populations of frogs and salamanders from northern California [Sessions and Ruth, 1990: "Explanation for naturally occurring supernumerary limbs in amphibians," The Journal of Experimental Zoology 254: 38-47]. The study showed that the limb deformities in these amphibians were induced by cysts of a parasitic flatworm called a digenetic trematode.

Sessions spoke about his research on limb deformities in natural populations of amphibians at the recent "Central North American Amphibian Deformities" conference hosted by the U.S. Environmental Protection Agency. Deformed frogs have now been found at more than 100 sites in 54 of Minnesota's 87 counties, and they have also turned up in Wisconsin, Quebec, and Delaware. The EPA is concerned that the widespread nature of this phenomenon suggests a major environmental problem that could have negative impacts on amphibians, with potential implications for human health.

Trematodes have a complex life cycle that involves several hosts, in this case including snakes, pond snails, and amphibians. The trematodes increase dramatically in number while in the snail hosts by a process called "polyembrionic amplification." During a second larval stage, free-swimming flatworm larvae are released from the snails and attack frog tadpoles in which they form cysts called metacercariae. Once they find an amphibian host, the cercariae burrow into tissues around the cloaca and form metacercarial cysts.

The developing hind legs of frog tadpoles are located right next to the cloaca and are almost always invaded by the cysts. Sessions hypothesized that the cysts caused massive mechanical damage to the developing limb, causing it to reorganize into numerous smaller limb buds and resulting in the outgrowth of up to 10 extra hind limbs. He confirmed this idea by implanting tiny plastic beads into the developing limb buds of captive tadpoles; this caused extra limbs to develop. These results showed that the mechanical effects of the parasite cysts are sufficient to cause the limb deformities in frogs, although they do not rule out other factors.

These results strongly suggest that parasites account for many of the recent reports of deformed frogs from the Midwest and elsewhere. But why the sudden increase? One possibility, suggested by Sessions at the conference, is that there are simply more people looking for deformed frogs and this happened to be a good year for trematodes (or especially pond snails). On the other hand, it could indicate a new and more serious environmental problem.

Other possible causes or contributing factors suggested at the Minnesota conference include various kinds of environmental pollution, pesticides, and UV-induced genetic mutation. Some commonly used pesticides, for example, may contain residues that can potentially interfere with basic biochemical pathways in developing cells. One outcome of the conference was the recognition of the need for a realistic program of scientific research that is broad enough to take into account both the role of parasites and the possibility of chemical pollutants or other factors.

Sessions' future research plans include an analysis of the kinds of environmental factors, such as fertilizer run-off in agricultural areas, that can encourage large increases in pond snail populations, which in turn would generate enormous population explosions of trematode cercariae through the process of polyembryonic amplification.

"Basic natural history research has been neglected in recent years, but it is this kind of information that is required in order to make realistic assessments of possible environmental threats to humans," he notes.