Cestodes, or tapeworms, are segmented worms. The adults reside in the gastrointestinal tract, but the larvae can be found in almost any organ. Human tapeworm infections can be divided into two major clinical groups. In one group, humans are the definitive hosts, with the adult tapeworms living in the gastrointestinal tract (Taenia saginata, Diphyllobothrium, and Dipylidium caninum). In the other, humans are intermediate hosts, with larval-stage parasites present in the tissues; diseases in this category include echinococcosis, sparganosis, and coenurosis. Humans may be the definitive and/or intermediate hosts for Taenia solium; both stages of Hymenolepis nana are found simultaneously in the human intestines.
The ribbon-shaped tapeworm attaches to the intestinal mucosa by means of sucking cups or hooks located on the scolex. Behind the scolex is a short, narrow neck from which proglottids (segments) form. As each proglottid matures, it is displaced further back from the neck by the formation of new, less mature segments. The progressively elongating chain of attached proglottids, called the strobila, constitutes the bulk of the tapeworm. The length varies among species. In some, the tapeworm may consist of more than 1000 proglottids and may be several meters long. The mature proglottids are hermaphroditic and produce eggs, which are subsequently released. Because eggs of the different Taenia species are morphologically identical, differences in the morphology of the scolex or proglottids provide the basis for diagnostic identification to the species level.
Most human tapeworms require at least one intermediate host for complete larval development. After ingestion of the eggs or proglottids by an intermediate host, the larval oncospheres are activated, escape the egg, and penetrate the intestinal mucosa. The oncosphere migrates to tissues and develops into an encysted form known as a cysticercus (single scolex), a coenurus (multiple scolices), or a hydatid (cyst with daughter cysts, each containing several protoscolices). The definitive host’s ingestion of tissues containing a cyst enables a scolex to develop into a tapeworm.
TAENIASIS SAGINATA AND TAENIASIS ASIATICA
The beef tapeworm T. saginata occurs in all countries where raw or undercooked beef is eaten. It is most prevalent in sub-Saharan African and Middle Eastern countries. Taenia asiatica is closely related to T. saginata and is found in Asia, with pigs as intermediate hosts. The clinical manifestations and morphology of these two species are very similar and are therefore discussed together.
Etiology and Pathogenesis
Humans are the only definitive host for the adult stage of T. saginata and T. asiatica. The tapeworms, which can reach 8 m in length with 1000–2000 proglottids, inhabit the upper jejunum. The scolex of T. saginata has four prominent suckers, whereas T. asiatica has an unarmed rostellum. Each gravid segment has 15–30 uterine branches (in contrast to 8–12 for T. solium). The eggs are indistinguishable from those of T. solium; they measure ...