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, Hymenolepis, 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 either the definitive or the 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. Since 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). Ingestion by the definitive host 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. T. 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 30–40 μm, contain the oncosphere, and have a thick brown striated shell. Eggs deposited on vegetation can live for months or years until they are ingested by cattle or other herbivores (T. saginata) or pigs (T. asiatica). The embryo released after ingestion invades the intestinal wall and is carried to striated muscle or viscera, where it transforms into the cysticercus. When ingested in raw or undercooked meat, this form can infect humans. After the cysticercus is ingested, it takes ∼2 months for the mature adult worm to develop.
Patients become aware of the infection most commonly by noting passage of proglottids in their feces. The proglottids are often motile, and patients may experience perianal discomfort when proglottids are discharged. Mild abdominal pain or discomfort, nausea, change in appetite, weakness, and weight loss can occur.
The diagnosis is made by the detection of eggs or proglottids in the stool. Eggs may also be present in the perianal area; thus, if proglottids or eggs are not found in the stool, the perianal region should be examined with use of a cellophane-tape swab (as in pinworm infection; Chap. 217). Distinguishing T. saginata or T. asiatica from T. solium requires examination of mature proglottids. All three species can be distinguished by examining the scolex. Available serologic tests are not helpful diagnostically. Eosinophilia and elevated levels of serum IgE may be detected.
Treatment: Taeniasis Saginata and Taeniasis Asiatica
A single dose of praziquantel (10 mg/kg) is highly effective.
The major method of preventing infection is the adequate cooking of beef or pork viscera; exposure to temperatures as low as 56°C for 5 min will destroy cysticerci. Refrigeration or salting for long periods or freezing at –10°C for 9 days also kills cysticerci in beef. General preventive measures include inspection of beef and proper disposal of human feces.
Taeniasis Solium and Cysticercosis
The pork tapeworm T. solium can cause two distinct forms of infection in humans: adult tapeworms in the intestine or larval forms in the tissues (cysticercosis). Humans are the only definitive hosts for T. solium; pigs are the usual intermediate hosts, although other animals may harbor the larval forms.
T. solium exists worldwide but is most prevalent in Latin America, sub-Saharan Africa, China, India, and Southeast Asia. Cysticercosis occurs in industrialized nations largely as a result of the immigration of infected persons from endemic areas.
Etiology and Pathogenesis
The adult tapeworm generally resides in the upper jejunum. The scolex attaches by both sucking disks and two rows of hooklets. Often only one adult worm is present, but that worm may live for years. The tapeworm, usually ∼3 m in length, may have as many as 1000 proglottids, each of ...