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Ribeiroia ondatrae is a parasite in the genus Ribeiroia, class Trematoda, Phylum Platyhelminthes, believed to be responsible for many localized outbreaks of amphibian limb malformations. Exact mechanism of deformation has not been determined, but evidence points to mechanical disruption of the cells involved in limb bud formation during the amphibian larval stage as a primary factor in limb deformation. Additional factors, such as the secretion of teratogenic chemicals by R. ondatrae may play a role in the development of deformities.

Life cycle
R. ondatrae follows an indirect life cycle and utilizes multiple intermediate hosts before reaching maturity. The eggs of R. ondatrae are passed with the definitive host's feces and hatch into miricidia. The miricidia seek out and penetrate the first intermediate host, ram's horn (Planorbidae) snails. Inside the snail R. ondatrae develops into sporocysts and redia, which give rise to many cercariae, a process referred to as embryonic amplification. The second intermediate host are fish and larval amphibians including frogs and salamanders, which the cercariae penetrate. Inside of amphibians, cercariae are attracted to region where hind limbs are formed and large numbers of metacercariae encyst near base of the hind legs. Definitive hosts are predators such as hawks, herons, ducks, and badgers, which consume the amphibian intermediate host. R. ondatrae lodges in the digestive tract and develops into sexually reproductive adults.

Pathogenicity
R. ondatrae produces a variety of pathology during its life cycle. The first intermediate hosts (Planorbidae snails) are castrated by direct consumption of gonads by the redia. In fish R. ondatrae encysts in the lateral line and may cause disorientation and erratic behavior. In amphibian intermediate hosts R. ondatrae targets the developing limb buds and can produce severe limb malformations, including: missing limbs, extra limbs, and deformed limbs. These malformations have a significant effect on the survival of the intermediate host's survival, increasing the probability of predation of the host and transmission of R. ondatrae to the definitive host. R. ondatrae produces lesions in the definitive hosts' intestinal tract, which may be fatal in heavy infections.

Population Impacts
The Planorbella individuals parasitized by R. ondatrae display a significantly reduced fitness. Sufficiently high densities of R. ondatrae can reduce populations of Planorbella to extremely low levels through top-down control. Purposeful introduction of Riberoia eggs has been attempted as a method to control Schistosoma populations, which also utilizes gastropods as an intermediate host, with some success. However, naturally occurring reduction of Planorbella populations have not been recorded.

The increase in predation of parasitized frogs has little affect on overall population as incidence rates tend to be relatively low and malformed frogs are still viable mates. However, a sufficiently large population of R. ondatrae may be able to cause a serious local population decline in species that show a high mortality upon infection.

The effect of R. ondatrae parasitization of fish has not been researched at a population level.

The pathogenicity and mortalitly of R. ondatrae infections of definitive hosts are generally very low, and no population level studies have been conducted. However, there are two cases of group mortality attributed to R. ondatrae: 42 chickens from neighboring farms in the 1930s and 5 penguins at a South African zoo in the 1980s.