This August, University of Colorado-Boulder disease ecologist Pieter Johnson made a ghoulish discovery in an Oregon pond: an "octo frog," with eight hind legs. It was a particularly disturbing example of the kind of amphibian malformations Johnson has recorded in 17 states, six of them Western, since 1996.

A common, period-sized flatworm, Ribeiroia ondatrae, plays a key role in making frogs grow extra or misshapen limbs and skin flaps between joints. It breeds inside the common freshwater ramshorn snail, burrows into tadpoles' limbs, and creates cysts that interfere with development, explains Johnson. The adult frogs have trouble feeding themselves and become easy prey.

This parasite is especially worrisome because amphibians are "the most threatened group of vertebrates on the planet," says Johnson. Environmental changes are making frogs more vulnerable to infection. Nutrient run-off from agricultural fertilizers creates algae blooms that can boost snail populations, providing more parasite hosts. Biodiversity loss also contributes: Those frog species that resist infection help cull parasites in the water bodies where they live, since the flatworm dies inside them. Meanwhile, if there are fewer fish and other animals, such as larval dragonflies, to eat the flatworms the parasite can proliferate.