Taenia solium

T. solium tapeworms consist of a head (scolex) that has suckers and hooklets that attach to the intestinal wall, a neck, and many flat segments called proglottids that contain male and female reproductive organs.

New proglottids develop behind the scolex. The most distal proglottids are mature and contain many eggs, and they can detach and be shed in the feces.

T. solium can be transmitted to humans in two ways, with distinct outcomes:

 (1) Ingestion of undercooked pork containing larval cysts, called cysticerci, leads to development of adult tapeworms in the intestine. Ingested cysticerci attach to the intestinal wall and develop into mature adult tapeworms, which can grow to many meters in length and can produce mild abdominal symptoms.

 (2) When intermediate hosts (pigs or humans) ingest eggs in food or water contaminated with human feces, the larvae hatch, penetrate the gut wall, disseminate hematogenously, and encyst in many organs. Convulsions, increased intracranial pressure, and neurologic disturbances are caused by T. solium cysts in brain tissue.

Adult tapeworms are not produced with this mode of infection. Viable T. solium cysts do not produce symptoms and can evade host immune defenses by producing taeniaestatin, a serine proteinase inhibitor that inhibits complement activation, and paramyosin, which appears to inhibit the classical pathway of complement activation.

When the cysticerci degenerate, an inflammatory response develops. Taenia saginata, the beef tapeworm, and Diphyllobothrium latum, the fish tapeworm, are acquired by eating undercooked meat or fish. In humans, these parasites live only in the gut, and they do not form cysticerci.