In the 1950s, famous French writer and biologist Jean Rostand (1894-1977) discovered morphological anomalies of an unknown etiology in water frogs of the genus Pelophylax, which he named “the anomaly P”. The anomaly P is the morphological deformations of water frogs that have light and severe forms of manifestation.

The light form of the anomaly is symmetrical polydactyly. Specimens suffering from severe forms of the anomaly have symmetrical brachymely, polydactyly, hind limb edema, bone outgrowths, spikes, flexions, and additional limbs in the inguinal region.
The Rostand’s anomaly, in comparison with deformities caused by trematode Ribeiroia ondatrae (see Johnson's Lab), had no additional full limbs and can have small autopod elements in inguinal regions only. The most important difference is symmetry of bilateral traits: number of structures and their shapes usually same on both sides of the body.

The study of anomaly P was discontinued after Jean Rostand and did not resume for almost 50 years. In 2016, we found first severe cases of anomaly P in Central Russia (Ostrovtsovskaya Lesosteppe About our studies in Russian). And two years after we described three additional new localities from Russia where severe cases of anomaly P was observed in two species of western Palearctic water frogs.

The frequency of anomaly P occurrence in the studied population reached 24.7% (n = 384). Moreover, the severe forms of the anomaly P were noted in 4.7% of cases, and the light (polydactyly) form were noted in 20.0%.

Individuals of other syntopic amphibian species had no such anomalies. Studied specimens of the moor frog Rana arvalis were without polydactyly. No limb anomalies were observed in individuals of the firebellied toad Bombina bombina except one specimen with missing forelimb (amely). However, some tadpoles of the Pallas’s spadefoot toad Pelobates vespertinus had red “swellings” in eyes and abnormal behavior.
Jean Rostand suggested that the anomaly P could be caused by some infectious agent transmitted by fish, which he hypothesized to be a teratogenic virus with a temporary effect on limb development.

We found that the mollusk Planorbarius corneus could be an intermediate host of the infectious agent of the anomaly P: symmetrical cases of polydactyly and severe cases of the anomaly P were obtained in laboratory when tadpoles reared together with mollusk. As the most probable cause, we assume a species of trematodes, for which the first intermediate hosts are planorbid snails.

The experiments with direct cercariae exposure provide compelling evidence that Strigea robusta (Trematoda: Strigeidae) leads to anomaly P in tadpoles of water frogs. The manifestation of anomaly P turned out to be dependent on the stage of development, cercariae dose, and the location of the cysts.