LIFE CYCLE of the trematode Ribeiroia ondatrae enables the parasite to induce deformities--including extra hind legs--in generation after generation of frogs. In its first larval form the trematode infects snails (1). After transforming into a second free-swimming form inside a snail, the parasite embeds itself near a tadpole's future hind leg (2). There it forms a cyst that disrupts normal limb development and can cause the tadpole to sprout extra legs as it grows into a frog (3). The disabled frog then becomes easy prey for the parasite's final host, often a heron or egret (4). The parasite matures and reproduces inside the bird, which releases trematode eggs into the water with its feces (5). When larvae hatch (6), they begin the cycle again. Human activities can exacerbate this process, especially where livestock manure or fertilizers enter a pond and trigger algal blooms that nourish, and thus increase, snail populations. Excess ultraviolet radiation and pesticide runoff--which might cause other types of deformities when acting alone--may facilitate the cycle by weakening a tadpole's immune system and making the animal more vulnerable to parasitic infection.
This article was originally published with the title Explaining Frog Deformities.