Several members of the cestode (tapeworm) family Diphyllobothriidae are known to infect humans. These pseudophyllidean cestodes have a scolex bearing bothria (grooves), instead of suckers as in the cyclophyllidean cestodes (the group including nearly all human-infecting species). All species associated with human diphyllobothriid infections have marine or aquatic life cycles and transmission occurs via ingestion of undercooked fish.
Recent research incorporating morphologic and molecular data has led to the re-classification and re-naming of most of the human-infecting diphyllobothriids. Dibothriocephalus latus (=Diphyllobothrium latum), the “broad fish tapeworm”, is usually assumed to be the most common agent of human diphyllobothriasis. However, it is possible that many historical cases were falsely attributed to this species. Dibothriocephalus nihonkaiense (=Diphyllobothrium nihonkaiense), Dibothriocephalus dendriticus (=Diphyllobothrium dendriticum), Diphyllobothrium stemmacephalum (=Diphyllobothrium stemmacephalum; =Diphyllobothrium yonagoense), Diphyllobothrium balaenopterae (=Diplogonoporus grandis; =Diplogonoporus balaenoptera), and Adenocephalus pacificus (=Diphyllobothrium pacificum) are also known to infect humans. Sporadic case reports exist involving several other diphyllobothriid species, although some of the species identifications in these reports are of questionable validity.
Eggs are passed unembryonated in feces . Under appropriate conditions, the eggs mature (approximately 18 to 20 days) and yield oncospheres which develop into a coracidia . After ingestion by a suitable crustacean (first intermediate host) the coracidia develop into procercoid larvae . Procercoid larvae are released from the crustacean upon predation by the second intermediate host (usually a small fish) and migrate into the deeper tissues where they develop into a plerocercoid larvae (spargana), which is the infectious stage for the definitive host . Because humans do not generally eat these small fish species raw, the second intermediate host probably does not represent an important source of human infection. However, these small second intermediate hosts can be eaten by larger predator species that then serve as paratenic hosts . In this case, the plerocercoid migrates to the musculature of the larger predator fish; humans (and other definitive host species) acquire the parasite via consumption of undercooked paratenic host fish . In the definitive host, the plerocercoid develops into adult tapeworms in the small intestine. Adult diphyllobothriids attach to the intestinal mucosa by means of two bilateral groves (bothria) of their scolex . The adults can reach more than 10 m in length, with more than 3,000 proglottids. Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm) and are passed in the feces. Eggs appear in the feces 5 to 6 weeks after infection.
Life cycle image and information courtesy of DPDx.