Fig. 1

Life cycle of pseudophyllidean cestodes. (A) Diphyllobothrium latum inhabits the intestine of humans, cats, dogs, and other fish-eating animals (final hosts), being attached to the intestinal wall with two longitudinal bothria. (B) Schistocephalus solidus occurs in the intestines of a wide range of fish-eating birds. Ligula intestinalis has a very similar life cycle. 1 Final hosts. 2 Adults. D. latum reaches a maximum size of 25 m; its mature proglottids are broader than long; coils of the gravid uterus form a centrally located rosette. S. solidus is lanceolate shaped with a size of 5–8 × 1 cm, bothria-like apical indentations are of poor adhesive power. 3 The operculated eggs are excreted unembryonated; completion of development to coracidium larvae (3.1) takes one to several weeks depending on the water temperature. 4 Free coracidium larva containing the oncosphaera which is endowed with 6 hooks. 5–6 Having ingested free coracidia several species of copepods are suitable intermediate hosts within which development of second-stage larvae (Procercoid; 6) occurs. 7–8 As second intermediate hosts, brackish and freshwater fish become infected by ingesting infected copepods. Inside the intestine the procercoid is released, and eventually bores its way into the body cavity and muscles where it grows rapidly into a plerocercoid (Sparganum). In D. latum the plerocercoids remain mainly undifferentiated, whereas in S. solidus the plerocercoids show the main features of the adults (i.e., division into 62–92 proglottids and the presence of genital anlagen; however, they are not yet fertile). Unlike D. latum, the progenetic plerocercoids of S. solidus are extremely specific in their host, developing only in the body cavity of the marine and freshwater forms of the 3-spined stickleback ( Gasterosteus aculeatus ). 8.1 In D. latum plerocercoids may become accumulated without further development in the muscles (not encysted) of carnivorous fish (paratenic hosts). 9 Infections of final hosts occur by ingestion of raw meat of fish containing plerocercoids. Having reached the intestine the plerocercoids of D. latum grow rapidly and become adult worms in 5–6 weeks, whereas S. solidus plerocercoids mature rapidly (within 36–48 h) and release eggs. Humans, who accidentally eat meat of fish containing plerocercoids of other nonhuman pseudophyllidea may also become infected; however, plerocercoids do not mature, but creep around inside the human body, leading to a disease called sparganosis . CI cilia, GP genital pore, HK hooks of ON, ON oncosphaera, OP operculum, OU opening of the uterus, UT uterus, VI vitellarium