a helminthiasis of domestic and wild aquatic birds caused by parasitic worms of the phylum Acan-thocephala, which are polymorphes. It is prevalent in North America, Europe, and Asia, including the European and Asiatic parts of the USSR. The parasite’s body measures 9–16 mm long. It is orange and is divided by construction into two parts. The parasite infests the intestine and develops by means of intermediate hosts, crustaceans of the genus Gammarus. The birds become parasitized by consuming the crustaceans infested with the parasites’ larvae. Ducklings are most susceptible to the infestation, which often results in death.

Polymorphosis is treated with such anthelmintics as carbon tetrachloride and bithionol. It is prevented by raising the young in isolation and by regularly changing the water inhabited by the birds.