Ribeiroia ondatrae (Trematoda : Digenea) infection induces severe limb malformations in western toads (Bufo boreas)

Widespread reports of malformed amphibians in North America have prompted i nvestigations into the cause(s) and implications of the phenomenon. Recentl y, a trematode parasite (Ribeiroia ondatrae) was identified as the probable cause of hind-limb malformations in Pacific treefrogs (Hyla regilla) from California. We exposed a second anuran species, the western toad (Bufo bore as), to specific levels of R. ondatrae infection. In a dose-dependent manne r, R. ondatrae infection induced high frequencies (40-85%) of severe limb m alformations in surviving toads. Survivorship declined significantly with i ncreasing parasite exposure, falling to 42% in the heaviest treatment. Larv ae in control treatments exhibited normal development and low mortality lev els. In contrast to previous experiments with R. ondatrae infection in tree frogs, cutaneous fusion was the predominant malformation among infected toa ds in all treatments. Infection also caused polymely (extra limbs; fore and hind), ectromely (missing limbs), polydactyly (extra digits), and a variet y of additional limb malformations. Taken together, these results demonstra te that (i) the teratogenic effects of R. ondatrae are not limited to treef rogs, (ii) the spectrum of R. ondatrae-induced malformations is not confine d to the hind limbs, and (iii) the frequency and composition of malformatio ns resulting from infection may vary among amphibian species. Finally, we r eview historical reports of limb abnormalities in the genus Bufo and discus s established and proposed causative agents, with emphasis on trematode inf ection and predation.