The Case of the Three-legged Frog

December 2008

A great blue heron wades through the shallows at the edge of the lake. She pauses, staring at the weeds. A leopard frog sits, camouflaged by its black spots on a green body. The frog sports only a stump in place of one of its back legs. The frog senses the bird’s presence. The heron coils its neck and strikes. The frog leaps but it can not escape quickly enough. The heron has an easy meal.

Back in the late 1990s, reports of deformed frogs around Lake Champlain received a great deal of media attention. These reports followed closely on the heels of similar reports from Minnesota and other areas. Frogs were found with missing or extra limbs. Immediately some human-cause was suspected. Scientists have tried to track down different possible causes over the last decade.

A fascinating organism known as a trematode soon emerged as a leading suspect. Trematodes are parasitic flatworms that depend on anywhere from two to four hosts to complete their lifecycle. There are over 6,000 species of trematodes including a number that parasitize humans. 

The life-cycle of the trematode implicated has four stages. Adults are found within birds like the heron. There, two trematodes may get together to produce eggs which pass through the bird’s digestive track and are deposited in water where they may be ingested by certain snails. Once within the snail the egg hatches and a trematodes grows that can produce a different type of egg even in the absence of a mate. This egg develops into a cercariae, a free-swimming tadpole-shaped larva which seeks out a real tadpole. Cercariae of some trematode species cause swimmer’s itch. The larva enters the tadpole and migrates to developing limb buds where it can form a cyst. When the cysts form on cells that are supposed to develop into legs, they cause growth disruptions that lead to missing limbs, split limbs, or multiple limbs. The adult frog is then more vulnerable to predation by birds, thus assisting the trematode in completing its life-cycle.

The presence of a natural parasite has not completely absolved humans from a role in causing deformities. Researchers at Penn State University have shown that trematodes and pesticides together lead to more deformed frogs than either alone. In pond studies, they showed that only frogs exposed to parasites developed deformities. Then in lab studies with trematodes plus one of three pesticides – Atrazine, Malathion, and Esfenvalerate – they found a higher incidence of deformities than in a control group without pesticides. They believe the presence of pesticides hampers the immune response of the frogs, reducing their ability to prevent cyst formation.

Additionally, a team of researchers from the University of Colorado – Boulder, Washington University in St. Louis, and the University of Wisconsin have demonstrated a link between nutrient pollution and frog deformities. They artificially fertilized some lakes in Wisconsin, stocked them with a known number of tadpoles and snails, and added some trematode eggs. Over a period of time, as algae bloomed in response to the added nutrients, the number of snails also increased. The fertilized ponds contained more cercariae and more infected frogs.

The evidence against the trematodes in Lake Champlain is weaker than in other places. The Vermont Agency of Natural Resources reports that according to a California study between 30 and 50% of malformations caused by trematodes result in extra limbs. However, of 7,000 deformed leopard frogs examined locally, only one had an extra limb; most of the rest were missing part or all of their limbs. Additionally, the particular trematode implicated in other areas was not found at many of the local sites where frog deformities were documented. No one has a good explanation for the discrepancy. 

It is not clear whether the attention that deformed frogs received in the late 1990s was due to an increase in deformations or an increase in public attention leading to more reporting of deformities. Thus it is difficult to evaluate whether there has been any actual change in the incidence of deformities over time.

The case file for the three-legged frog on Lake Champlain remains open. The leading suspect, the trematode has an alibi, and no direct links have been discovered between the accomplice, humans, and the particular situation here. The case has resulted in a wealth of intriguing research and exposed hidden ecological connections, but assessing cause and effect in the natural world is rarely straight forward.