Parasitic infection and deformed frog. The parasitic trematode (flatworm) Ribeiroia ondatrae causes limb deformities in amphibians. Part of the life cycle of the parasite is spent in cyst form near the base of the limbs, affecting their development. Other parts of the life cycle are spent in snails, fish and the larval stage of the amphibians. This is followed by a stage spent in the bodies of predators of the amphibians. The predators will preferentially attack the deformed amphibians.