Pseudophyllidean

Pseudophyllidean refers to members of the order Pseudophyllidea, a group of tapeworms (cestodes) that parasitize vertebrate hosts. They are characterized by a scolex with two shallow bothria (sucking grooves) rather than a rostellum with hooks, and by an operculate egg stage that is released into aquatic environments. The body consists of a scolex, a strobila of proglottids, and reproductive organs that mature as they move toward the posterior.

The life cycle of pseudophyllideans is typically aquatic and involves two intermediate hosts. Eggs released in the feces reach water and hatch into free-swimming coracidia. Coracidia are consumed by small aquatic crustaceans (first intermediate host), where they develop into procercoid larvae. The crustacean is then eaten by a second intermediate host, usually a fish or amphibian, in which the larva develops into a plerocercoid (sparganum). Humans and other mammals become definitive hosts when they ingest raw or undercooked tissue containing the plerocercoid, which then develops into an adult tapeworm in the small intestine.

Clinically, infections may be asymptomatic or cause nonspecific gastrointestinal symptoms such as abdominal pain, diarrhea, or weight loss. Some species, notably the broad fish tapeworm Diphyllobothrium latum, can cause vitamin B12 deficiency and megaloblastic anemia. Diagnosis is typically by stool examination for eggs or gravid proglottids, or by molecular methods. Treatment options include praziquantel and, in some cases, niclosamide, along with measures to prevent reinfection through proper cooking of fish.

Taxonomically, pseudophyllideans include several genera, with Diphyllobothrium being among the most well known. They are distributed worldwide in freshwater or cool coastal environments where their intermediate hosts occur.