Tapeworm

A tapeworm, or cestode, is a parasitic flatworm (phylum Platyhelminthes) with a long, flat, ribbon-like body. At its front end is a small head, the scolex, armed with suckers and in some species hooks that grip the lining of the host's small intestine. Behind it, a neck keeps budding off segments called proglottids, so the chain can reach several meters. Each proglottid holds a full set of male and female sex organs. Tapeworms have no mouth or gut and take in digested food straight through their outer covering, the tegument.

Most tapeworms need two hosts. Ripe proglottids full of eggs leave in the feces of the definitive host; an intermediate host swallows the eggs, and the larvae hatch, burrow into its tissues and form cysts. The cycle closes when a definitive host eats flesh containing those cysts. Humans acquire the beef tapeworm Taenia saginata and the pork tapeworm T. solium from undercooked meat, and the fish tapeworm from raw fish. An adult worm in the gut usually causes mild symptoms or none.

Swallowing T. solium eggs is more serious: larvae lodge in muscle, eyes or brain, causing cysticercosis, and in the brain this is a leading preventable cause of epilepsy. Infections are diagnosed by finding eggs or segments in stool and treated with drugs such as praziquantel.