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Update on Deformed Frogs

By Donald L. Blanchard
Cold Blooded News Editor

Following in the wake of reports of amphibian declines worldwide, and a growing concern over ecological issues, habitat loss, and environmental pollution, the deformed frog issue has received considerable press, and deformed amphibians have been popping up all over. Various causes for the deformities have been hypothesized and tested, and while not all deformities can be traced to a single cause (a frog might have lost a limb to a predator, for example), a culprit has been identified for at least the majority of cases, and it is a naturally occurring parasitic infection.

Much of the material from this article is derived from the web site of Dr. Stanley K. Sessions, at Hartwick College in Oneonta, New York <http://www.hartwick.edu/biology/def_frogs/>, which was brought to my attention by an article by Steve Shockney in Herp Beat, the newsletter of the Upstate (New York) Herpetological Society.

Dr. Sessions started his research on deformed amphibians in 1987, when he began investigating some deformed amphibians (Pacific Treefrog, Hyla regilla, and Long-toed Salamanders, Ambystoma macrodactylum) from Northern California. Because the deformities were found initially in two unrelated amphibians, Dr. Sessions immediately deduced that the source was environmental rather than genetic. After finding that the water tested negative for heavy metals and chlorinated hydrocarbons, he turned to an examination of the deformed specimens. There he found heavy infections of a trematode (genus Riberoria), a kind of parasitic flatworm. These trematodes have a complex life cycle. The adult worm lives as a parasite in Garter snakes (genus Thamnophis) or other amphibian predators such as the Blue Heron (Ardea herodias). Trematode eggs are passed in the feces, and hatch into larvae which infect a pond snail. Inside the snail, the larvae reproduce prodigiously, one egg yielding thousands or tens of thousands of larvae. On exiting the snail, this later larval stage attacks and burrows into the soft skin of tadpoles, where they form cysts. The cysts remain dormant until the amphibian is eaten by a predator, whereupon the encysted worm hatches into an adult, thereby completing the cycle.

The hind leg buds of developing tadpoles are a favored site for trematode larvae to infest. The cysts can disrupt the alignment of those cells destined to become hind legs, causing multiple legs, deformed legs, or, if enough cysts form, no leg at all to grow. Dr. Sessions has demonstrated experimentally that trematode infestations can produce all the various kinds of deformities reported in amphibians.

References:
de Superville. 1740. Some reflections on generation, and on monsters, with a description of some particular monsters. Phil. Trans. Roy. Soc. London 41:294-307.

Dumeril. 1865. Observations sur la monstruosite dite polymelie ou augmentation du nombre des membres chez les batraciens anoures. Nouv. Arch. Mus. Hist. Nat. [Paris] 1:311-319.

Sessions, S. K. 1999. Deformed Amphibian Research at Hartwick College http://www.hartwick.edu/biology/def_frogs/.

Shockney, S. 1999. Trematodes and the Five Legged Frog. Herp Beat, Vol.10, No.1, January, 1999.