diphyllobothriid

Diphyllobothriids are a group of tapeworms (cestodes) that parasitize the small intestine of vertebrate hosts, including fish-eating mammals and birds. The best known member is Diphyllobothrium latum, the fish tapeworm. Morphology: adult worms are long and flat, with a scolex that bears two shallow grooves called bothria for attachment rather than suckers. Proglottids are produced continuously and many become gravid, releasing eggs into the host’s feces. Eggs are operculated and released into water where they hatch into free-swimming coracidia under suitable conditions.

Life cycle: in freshwater environments, eggs hatch to coracidia that are eaten by copepods (the first intermediate host). When small fish eat the copepods, coracidia develop into plerocercoid larvae in muscle tissue (the second intermediate host). Humans and other mammals acquire infection by consuming raw or undercooked freshwater fish containing plerocercoids. In the small intestine, the plerocercoids mature into adult tapeworms that can reach meters in length and attach to the mucosa with their scolex.

Disease: most infections are asymptomatic. When symptoms occur, they may include abdominal discomfort, nausea, diarrhea, and weight loss. A notable complication is vitamin B12 deficiency leading to megaloblastic anemia in some cases.

Diagnosis and treatment: diagnosis relies on detecting eggs or gravid proglottids in stool, or recognizing a tapeworm on examination. Praziquantel is the preferred treatment; niclosamide is an alternative. Post-treatment follow-up stool testing helps verify clearance.

Prevention and epidemiology: prevention centers on thorough cooking or freezing of freshwater fish before consumption. Diphyllobothriids occur in regions where raw or undercooked fish is eaten, with distribution in parts of Europe, North America, and Asia.