infections
Tapeworm Infection
Pork, beef, and fish tapeworm life cycle
The pork, beef, and fish tapeworms are large, flat, ribbon-like worms that live in the intestine of people and can grow 15 to 30 feet (4.5 to 9 meters) in length. People are considered the definitive host because adult tapeworms live in their intestine. Egg-bearing sections of the worm (proglottids) are passed in the stool. If untreated human waste is released into the environment, the eggs may be ingested by intermediate hosts, such as pigs, cattle, or, in the case of fish tapeworms, small freshwater crustaceans, which are in turn ingested by fish. The eggs hatch into larvae in the intermediate host. The larvae invade the intestinal wall and are carried through the bloodstream to skeletal muscle and other tissues, where they form cysts. People acquire the parasite by eating the cysts in raw or undercooked meat or certain types of freshwater fish. The cysts hatch and develop into adult worms, which latch onto the wall of the intestine. The worms then grow in length and begin producing eggs. 1. People may become infected when they eat raw or undercooked pork containing cysts of tapeworm larvae (called cysticerci). 2. In the intestine, the cysticerci mature into adult tapeworms and attach themselves to the wall of the intestine. 3. Adult tapeworms produce segments (called proglottids) that bear eggs. The proglottids may release the eggs or detach from the rest of the tapeworm and travel to the anus. 4. The eggs, proglottids, or both are passed in stool. 5. Pigs or, less often, people become infected by consuming the eggs or proglottids (for example, in food contaminated with human stool). 6. After the eggs are consumed, they hatch in the intestine and release spheres (called oncospheres) that penetrate the wall of the intestine. 7. The oncospheres then travel through the bloodstream to muscles and to the brain, liver, and other organs, where they develop into cysts. Tapeworms can grow to be 15 to 30 feet (4.5 to 9 meters) long. 