The Cestodes

Thomas Cherian

Human cestode infections may be caused either by the adult tapeworms residing in the intestinal lumen or by the larval forms that infect the tissues. Intestinal forms are acquired by ingestion of the larval forms in infected meat or fish. In this case, humans are the definitive hosts. In tissue infection with the larval forms, humans are accidental intermediate hosts.

INTESTINAL CESTODES

Adult cestodes (tapeworms) inhabit the intestinal lumen. Their bodies consist of three parts: the head, the neck, and the body. The head, or scolex, has two or more suckers or a knob of small hooks (rostellum) by which the parasite attaches itself to the intestinal wall. The scolex is attached to the body by a short neck. The ribbon-like body (the strobila) consists of segments, or proglottids. Each proglottid contains male and female reproductive systems and is responsible for the production of the parasite eggs. Proglottids begin to develop in the neck region of the parasite. As they mature, they move downward in the strobila as newer segments are formed in the neck region. The fully mature proglottids break away from the strobila and degenerate, releasing the ova in the intestine, whence they are expelled in the feces. Alternatively, the mature proglottids may migrate to the anus and pass out in the feces. The eggs are ingested by the intermediate hosts and hatch in the intestine; there they release the oncosphere that penetrates the intestinal wall to reach the circulation. The oncosphere may lodge in one of many organs, where it matures into the parasite cyst variously called cysticercus, cysticercoid, alveolar cyst, or hydatid cyst.

The adult worms produce minimal disease in the intestine. They cause nonspecific intestinal symptoms and may be associated with decreased nutrient absorption. Immune response to the adult tapeworm may produce eosinophilia and

immunoglobulin E (IgE) elevation, but the immune response does not alter the course of intraluminal infection.

TABLE 225.1. SUMMARY OF THE COMMON INTESTINAL AND TISSUE CESTODE INFECTIONS IN HUMANS

Organism	Geographic Distribution	Clinical Manifestations of Infestation	Diagnosis	Treatment
Intestinal Cestodes
Taenia saginata	Cattle breeding areas, especially central Asia, Near East, central and eastern Africa	Abdominal discomfort, hunger pains, and weight loss; intestinal obstruction may occur with multiple worms	Demonstration of ova or proglottids is feces or coproantigen by enzyme-linked immunosorbent assay	Praziquantel, 5–10 mg/kg as single dose Or Niclosamide, 50 mg/kg (2 g in adults) as single dose (not recommended for those <2 years of age)
Taenia solium	Mexico, Central and South America, Southeast Asia, India, Africa and southern Europe	Abdominal discomfort, hunger pains, and diarrhea	Same as above	Same as above
Diphyllobothrium latum	Siberia, Scandanavia, Baltic region, northern United States, Canada, South America	Abdominal discomfort, occasionally intestinal obtruction, and megaloblastic anemia	Recovery of ova or proglottids in stool	Same as above
Hymenolepsis nana	Worldwide	In those with heavy worm load anorexia, abdominal pain, nausea, vomiting, diarrhea, irritability, and dizziness	Demonstration of ova in feces	Praziquantel, 25 mg/kg as single dose
Tissue Cestodes
Cysticercosis (Taenia solium)	Mexico, central and South America, India, sub-Saharan Africa	Neurocysticercosis: seizures, headache, altered mental status, visual problems, focal deficits, and hydrocephalus Extraneural: subcutaneous nodules, and myopathy and pseudohypertrophy with heavy muscular infection	Neuroimaging; immunodiagnostics	Praziquantel, 50–60 mg/kg/day in three divided doses for 15 days Or Albendazole, 15 mg/kg/day in two to three divided doses for 8–30 days May also require antiepileptic drugs and corticosteroids (see text)
Echinococcus granulosus (hydatid cyst)	China, India, Africa, Iraq, Mediterranean basin, Uruguay, Argentina, Chile	Often asymptomatic, clinical signs when present depend on location of cysts and the pressure effects on surrounding structures	Radiographic studies, serologic tests	Surgery; albendazole, 10–15 mg/kg/day in two to three divided doses in 28 day cycles with 14 days in between for at least three cycles as an adjunct to surgery or when surgery is not possible
Echinococcus multilocularis (alveolar cyst)	Sub-Arctic regions of Alaska, Canada, northern Europe, and northern United States	Same as above	Same as above	Same as above

Taenia saginata (Beef Tapeworm)

Taenia saginata, or beef tapeworm, is found commonly in the cattle-breeding areas of the world, especially in Central Asia, the Near East, and central and eastern Africa. The parasite measures 4 to 10 m, with 1,000 to 2,000 proglottids. The scolex, which measures 1 to 2 mm, has four suckers (but no hooks) by which the parasite attaches itself to the intestinal mucosa.

Life Cycle

Infection is acquired by the ingestion of raw or undercooked beef. In the intestine, the scolex evaginates from the cysticercus, attaches itself to the intestinal mucosa, and develops into the adult worm. Gravid proglottids appear 90 to 120 days after infection. Eggs may be liberated in the intestinal lumen by detached proglottids. Alternately, single proglottids may migrate actively through the anus or from the fecal mass after it has been passed. The eggs remain viable in the soil for days to weeks. After ingestion by cattle, the eggs hatch to release oncospheres that penetrate the intestinal mucosa and reach the bloodstream, where they are filtered out in the striated muscle. They develop into mature cysticerci within approximately 70 days. Reindeer and certain herbivorous wild animals are also known to harbor cysticerci of T. saginata.

Clinical Manifestations

Commonly, adult worms in the intestine are asymptomatic, although nonspecific abdominal discomfort, hunger pains, and weight loss have been reported. Infection with multiple worms may cause intestinal obstruction. The most common complaint is the discomfort and embarrassment caused by mature proglottids migrating out through the anus. Significant eosinophilia may occur in a small proportion of patients.

Diagnosis

Specific diagnosis is established by the recovery of the parasite proglottid. The proglottids of T. saginata may be distinguished from those of T. solium by the number of main lateral uterine branches. The eggs of T. saginata may be indistinguishable from those of T. solium.

Immunodiagnostic tests for Taenia-specific faecal antigen based on polyclonal rabbit antisera against T. saginata or T. solium proglottid extracts in capture-type enzyme-linked immunosorbent assay (ELISA) have been developed. These tests are more sensitive than microscopy and are highly specific.

Treatment

The treatment of choice is praziquantel. The drug is administered in a single dose of 5 to 10 mg/kg. It is not recommended in pregnancy, and its safety in children younger than 4 years of age has not been established.

The alternative drug is niclosamide. This drug is available as chewable 500-mg tablets. It is administered as a single dose that either should be chewed thoroughly or crushed and made into a paste before administration. The dose is 2 g for adults and 50 mg/kg in children. It is not recommended in pregnancy or in children younger than 2 years because of lack of safety data.

Taenia solium (Pork Tapeworm)

T. solium infection is endemic in Mexico, Central and South America, Southeast Asia, India, Africa, and southern Europe. The adult worm has a scolex with four large suckers and a rostellum with a double row of hooks by which it anchors itself to the intestinal mucosa. A narrow neck connects the scolex to a strobila consisting of some 1,000 proglottids. Humans can be either the definitive host or the intermediate host for T. solium. Ingestion of eggs by humans results in cysticercosis (discussed later under tissue cestodes).

Life Cycle

The T. solium life cycle is similar to that of T. saginata. Infection is acquired by ingestion of infected pork that is raw or undercooked. The scolex evaginates and develops into the adult worm in the intestine. The eggs or proglottids are passed out in the feces. Unlike the proglottids of T. saginata, those of T. solium do not migrate actively. On ingestion by hogs or humans, the eggs hatch in the duodenum or jejunum. The released oncospheres penetrate the intestinal wall and are carried throughout the body via the bloodstream. They are filtered out in the subcutaneous tissues, muscle, eye, brain, and other body sites, where they develop into cysticerci, completing the cycle.

Clinical Manifestations

Usually, the adult worms do not cause any symptoms; however, they may cause occasional vague abdominal discomfort, hunger pains, and diarrhea. Cysticerci in human beings may cause symptoms, depending on the site of infection (see the later discussion of tissue cestodes).

Diagnosis

Diagnosis is established by demonstration of ova in the stool or by coproantigen detection by ELISA. However, differentiation from T. saginata may be difficult. DNA-based assays have been developed and used to differentiate between the two parasites.

Treatment

Treatment of T. solium is the same as for T. saginata. Because praziquantel is absorbed systemically, there is a small risk that patients who may have concomitant asymptomatic neurocysticercosis may develop neurologic complications (headache, seizures) as a result of the action of the drug on the cysts.

Diphyllobothrium latum (Fish Tapeworm)

Diphyllobothrium latum, or “fish tapeworm,” belongs to the pseudophyllidean cestode group. This group has a scolex with two bothria (sucking organs) rather than the typical four seen in the Taenia group. The worms are large, measuring up to 25 m long, and consist of 3,000 to 4,000 proglottids that have a characteristic rosette-shaped uterus. An individual may be infected with more than one worm at a time. Areas of high endemicity include the lake and delta areas in Siberia, Scandinavia, the Baltic regions, and adjacent areas of the former Soviet Union, northern United States, Canada, and South America. Infection can be maintained in the absence of humans by fish-eating mammals.

Life Cycle

Infection is acquired by ingestion of raw, poorly cooked, or pickled freshwater fish. The worm matures 3 to 6 weeks after ingestion of eggs. After maturation, both eggs and proglottids are passed in the stool; often a partial chain of proglottids a few inches to several feet long may be passed. The eggs develop in fresh water, after which they hatch and release ciliated coracidium larvae that are ingested by the first intermediate hosts, the copepod. The copepods containing the second-stage
larvae (procercoid) are ingested by fish and develop into more advanced stages in the muscles of the fish. If smaller fish are ingested by larger fish, the muscles of the larger fish are parasitized. The cycle is completed when infected fish are ingested by humans or fish-eating mammals.

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