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The pork tapeworm Taenia solium and the beef tapeworm Taenia saginata are the most common tapeworms of humans. The diseases associated with infection by these organisms have been known since ancient times, being found wherever insufficiently cooked pork or beef is eaten. Human infection with the larval stage of T solium (Cysticercus cellulosae), or cysticercosis, is found in places where adult T solium infection is common. T saginata infection occurs among those who eat raw or insufficiently cooked beef. As opposed to T solium, human infection with larval T saginata (Cysticercus bovis) almost never occurs. Both parasites are responsible for a tremendous burden of disease globally, with neurocysticercosis due to T solium being one of the most important causes of seizures worldwide.
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PATHOGENESIS AND EPIDEMIOLOGY
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Humans are the mandatory definitive hosts who disseminate the organism to porcine or bovine intermediate hosts. Transmission to swine usually occurs through contaminated soil, where gravid proglottids are deposited with human feces. Eggs can survive for weeks in moist soil. In cattle, grazing lands, water, or cattle feed that is contaminated with infected human feces are sources of infection. Intrauterine infection of calves has been reported.
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Adult worms live in the upper small intestine, with T solium measuring 2 to 8 m and T saginata measuring 3 to 10 m. The scolex of the pork tapeworm is distinguished by a crown or rostellum with a double row of hooklets. The scolex of T saginata is without hooks. The gravid uterus holds thousands of eggs, each with a mature 6-hooked (hexacanth) embryo. Eggs are 30 to 40 μm in diameter and similar in both human Taenia species. If the eggs are ingested by a suitable intermediate host such as swine (T solium) or cattle (T saginata), the embryo is liberated, penetrating the intestinal wall and disseminating via the bloodstream. The embryo of T solium may invade all tissues of the body and develop into a cysticercus or bladder worm. Cysticerci are ellipsoidal, white, translucent cysts into which the scolex is inverted. When infected meat is eaten, the cysticercus is activated by gastric juices and bile, which stimulate evagination of the scolex. The scolex attaches to the jejunal wall, and the embryo becomes a mature tapeworm in 10 to 12 weeks for T saginata and 5 to 12 weeks for T solium. In humans, eggs produced by this mature tapeworm are passed in feces and may be ingested by intermediate hosts (beginning the cycle anew) or ingested by other humans. Human disease is not always contracted through consumption of contaminated meat; transmission from close contacts who are tapeworm carriers that harbor eggs on their hands and fingernails may actually be a more common route. If ingested by humans, the larvae (termed oncospheres) escape from the egg and penetrate the duodenum, enter the lymphatic and vascular systems, ...