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Cestodes
Definition
The Pathogens
Cestode parasites are members of the animal kingdom, subphylum Cestoda. The organisms are characterized by several life cycle stages, which typically develop in distinct hosts. The adult stage is the tapeworm, which is acquired by ingestion of uncooked tissues that harbor larval forms. After ingestion, the larvae excyst, and the scolex attaches to the intestines. Segments, termed proglottids , develop at the base of the scolex and are displaced from the scolex by new proglottids to form a chain or tapeworm. The host in which the tapeworm develops is termed the definitive host . The proglottids, which contain male and female sexual organs, produce large numbers of ova. The proglottids or their ova are shed in stools. Humans are the definitive hosts for a number of different tapeworms, including the Taenia species, Dibothriocephalus species, and Hymenolepis nana . Humans also can be an accidental host for the dog and cat tapeworms of the genus Dipylidium ( Table 325-1 ).
TABLE 325-1
COMMON HUMAN TAPEWORM INFECTIONS
| ORGANISM | INTERMEDIATE HOST | COMMON NAME | CLINICAL PRESENTATION | TREATMENT |
|---|---|---|---|---|
| Dibothriocephalus species | Fish | Fish tapeworm | Passing segments, pernicious anemia | Praziquantel, niclosamide |
| Hymenolepis nana | Humans | Dwarf tapeworm | Asymptomatic, diarrhea | Praziquantel, niclosamide |
| Taenia saginata | Cattle | Beef tapeworm | Asymptomatic, passing segments | Praziquantel, niclosamide |
| Taenia asiatica | Pigs | Asian tapeworm | Asymptomatic, passing worms | Praziquantel, niclosamide |
| Taenia solium | Pigs | Pork tapeworm | Asymptomatic, passing segments | Praziquantel, niclosamide |
| Dipylidium caninum | Fleas | Dog tapeworm | Passing segments | Praziquantel, niclosamide |
The intermediate hosts harbor the larval form of the parasite. Infection follows ingestion of the ova. Under the influence of gastric and intestinal fluids, the ova hatch and release the invasive larvae (oncospheres), which migrate to tissues, forming tissue forms. The forms in tissue vary among organisms and may include the cysticercus (a bladder containing a single invaginated scolex), the coenurus (a bladder with multiple scolices), the hydatid (a cystic structure with a germinal layer, which forms numerous protoscolices), or the plerocercoid (a solid form seen in Spirometra species). Humans can harbor the intermediate forms of Taenia solium (cysticercosis), Echinococcus granulosus group (cystic hydatid disease), Echinococcus multilocularis (alveolar hydatid disease), and rarely other organisms ( Table 325-2 ). Humans can serve as both the definitive host and an intermediate host for two species, T. solium and H. nana . In the case of T. solium , humans are the obligate host for the tapeworm stage (pork tapeworm) but can also harbor the cystic form (cysticercosis). In the case of H. nana , both stages typically develop in a single person, with the cysticercoid form in the intestinal wall and the tapeworm in the lumen.
TABLE 325-2
HUMAN LARVAL CESTODE INFECTIONS
| ORGANISM | COMMON NAME | ORGANS INVOLVED |
|---|---|---|
| Taenia solium | Cysticercosis | Brain, spinal fluid, eye, muscle |
| Echinococcus granulosus group | Cystic hydatid disease | Liver, lung, other |
| Echinococcus multilocularis | Alveolar hydatid disease | Liver |
| Taenia multiceps, Taenia species | Coenurosis | Brain, eyes |
| Spirometra species | Sparganosis | Subcutaneous tissue, viscera |
Intestinal Tapeworm Infections
Dibothriocephalus Species (Fish Tapeworms)
Diphyllobothriid tapeworms are large segmented parasites that are acquired by ingestion of undercooked or pickled fish dishes (sushi, sashimi, ceviche, carpaccio, gefilte fish). The tapeworms develop within a few weeks and can live for more than 10 years. These organisms were formerly termed Diphyllobotrium species, but recent changes in taxonomy have now reclassified the organisms into several genera and multiple species, including Dibothriocephalus latus (formerly Diphyllobothrium latum ), Dibothriocephalus nihonkaiensis , and Adenocephalus pacificum .
Epidemiology and Pathobiology
Diphyllobothriid species are found worldwide, and perhaps 20 million people are thought to be infected. Major foci include Russia, Japan, and South America. The disease was formerly highly endemic in Scandinavia, where it is now rarely diagnosed.
In most cases, infection has little impact on the host. However, one species, D. latus , contains vitamin B 12 receptors on the surface of the tapeworm and can out-compete the host, thereby leading to vitamin B 12 deficiency ( Chapter 150 ). This manifestation has been described only in Scandinavia.
Clinical Manifestations and Diagnosis
In most infected persons, Diphyllobothriid species produce few or no symptoms. Some individuals may complain of gastrointestinal symptoms (abdominal discomfort, nausea, weight loss). The main clinical manifestation is the observation of proglottids being passed in stool. Pernicious anemia ( Chapter 150 ) with symptoms of anemia or peripheral neuropathy may develop with D. latus infection. The diagnosis depends on observation of the characteristic operculated eggs in stool.
Treatment and Prevention
A single oral dose of praziquantel (5 to 10 mg/kg) is usually adequate for therapy ( Table 325-3 ). Niclosamide can be used as an alternative (2 g [adults] or 50 mg/kg [children] in a single dose chewed and swallowed), but it is not available in the United States. Parasites in fish can be killed by cooking (>56° C, >5 minutes) or freezing (−20° C, 24 hours). Infected fish also may be identified by inspection.
TABLE 325-3
ORAL THERAPY FOR INTESTINAL TAPEWORM INFECTIONS
| PRAZIQUANTEL | NICLOSAMIDE | NITAZOXANIDE | |
|---|---|---|---|
| Dosage Adults |
5-10 mg/kg for all age groups (15-25 mg/kg for Hymenolepis nana ) | 2 g (4 tablets) | 500 mg |
| Children >34 kg | 1.5 g (3 tablets) | 200 mg | |
| Children 11-34 kg | 1 g (2 tablets) | 100 mg | |
| Administration | Taken as a single dose | Taken as a single dose; tablets must be chewed and swallowed | Taken twice a day for 3 days |
| Side effects | Mild but frequent, including dizziness, myalgias, nausea, vomiting, diarrhea, abdominal pain | Nausea, vomiting, abdominal pain, diarrhea, drowsiness, dizziness, headache, pruritus | |
| Pregnancy | No known mutagenic effects; considered safe if indicated |
Hymenolepis nana
Hymenolepis nana is the human dwarf tapeworm. Hymenolepis diminuta , a rat tapeworm, can also cause human infection.
Epidemiology and Pathobiology
H. nana is prevalent worldwide, with estimates of at least 50 to 75 million people infected. Infection follows ingestion of ova. The larvae are released, invade, and develop into cysticercoid forms in the intestinal villi. After a few days, the cysticercoids mature, invade the lumen, are transformed into a scolex, and form small tapeworms (up to 5 cm long), which begin producing eggs within 2 to 3 weeks. Autoinfection either in the intestines or by the fecal-oral route can lead to heavy infection.
Clinical Manifestations and Diagnosis
Most infections are asymptomatic. However, some children may be infected by hundreds or thousands of worms, which can cause abdominal pain, loose stools, diarrhea, and malabsorption. Diagnosis depends on observation of the characteristic eggs in stool. More than one specimen may be required.
Treatment and Prevention
Praziquantel (15 to 25 mg/kg as a single oral dose) is usually effective in treating H. nana infection, but it may need to be repeated in heavy infection (see Table 325-3 ). Nitazoxanide (100 mg by mouth twice daily for 3 days for children 1 to 3 years of age, 200 mg by mouth twice daily for 3 days for children 4 to 11 years of age, and 500 mg by mouth twice daily for 3 days for older children and adults) is a reasonable alternative therapy; efficacy is about 75 to 82%. Niclosamide also can be used as an alternative, but it must be continued for 7 days. Transmission is by the fecal-oral route and can be prevented by improved hygiene. Mass chemotherapy has been used to control infection in some populations.
Dipylidium caninum
Dipylidium caninum is a common tapeworm of dogs and cats. Dogs are infected by ingestion of fleas, which carry the cysticercoid form in their body cavities. The tapeworms can also develop in children who have ingested the fleas. It is widespread worldwide, but human infections are unusual.
Clinical Manifestations and Diagnosis
Infection may be asymptomatic. In some cases, the motile proglottids may be noted in stool. The proglottids are similar in size and shape to rice grains. Diagnosis depends on identification of the ova in stool or identification of the proglottids.
Treatment and Prevention
There are no controlled trials of treatment for Dipylidium infection, but infection is likely to respond to regimens used for other tapeworms (see Table 325-3 ). The main measure for prevention is treatment of pets for fleas and tapeworms.
Taenia saginata
Taeniasis refers to infection with the tapeworm form of one of three Taenia species. T. solium and Taenia asiatica are acquired from ingestion of undercooked pork. Taenia saginata , called the beef tapeworm, is a common intestinal infection worldwide; cattle, which are the intermediate hosts, harbor the tissue cysticerci in their muscle. Humans, which are the obligate definitive host, harbor the tapeworm form.
Epidemiology and Pathobiology
T. saginata is common worldwide in areas where cattle are raised and human fecal material contaminates the pastures. Approximately 45 to 60 million people are thought to be infected. Infections are found on most continents. Very high rates (>20% of the population) have been noted in east Africa, Bali, and Tibet, but infection is also endemic in the Middle East, the Americas, and Europe. T. saginata is also common in other parts of Asia.
T. saginata tapeworms are acquired by ingestion of undercooked beef. The scolex attaches to the intestinal wall, and proglottids form at the base of the scolex. The proglottids gradually enlarge as they are displaced from the scolex by newer proglottids. The chain of proglottids can reach a length of up to 30 feet. The terminal proglottids are shed periodically in the stool. Terminal proglottids are typically off-white, 2 to 3 cm long, 0.5 to 1 cm wide, and 1 to 2 mm thick.
Clinical Manifestations and Diagnosis
Mild symptoms (e.g., nausea, abdominal discomfort, anorexia, and pruritus) may be noted. The motile proglottids may cause discomfort as they exit the anus or may be noted in stool.
Ova, which may be noted in stool, are 40 µm in diameter and are surrounded by brown radial striations; the embryos have six hooks. However, the ova of the three Taenia species are morphologically indistinguishable. The proglottids can be distinguished from those of T. solium by counting the number of uterine branches (≥14 branches suggests T. saginata ). However, the proglottids of T. saginata cannot be readily distinguished from T. asiatica .
Treatment, Prevention, and Prognosis
Taeniasis can be treated with praziquantel in a single dose (see Table 325-3 ). Single doses of niclosamide are effective, and nitazoxanide has also been used for T. saginata . Taeniasis can be prevented by inspection of beef. Also, cooking to 56° C for 5 minutes or freezing at −20° C for 7 to 10 days destroys the infective larvae. Symptoms are self-limited and resolve with or without treatment.
Taenia asiatica
T. asiatica is a cause of taeniasis in Asia, termed Asian taeniasis . Infection is acquired by ingestion of undercooked viscera from pigs. Pigs are infected by ingestion of the ova from tapeworm carriers. T. asiatica has been widely described in China, Taiwan, Korea, Indonesia, and Southeast Asia. The clinical manifestations, diagnosis, treatment, and prevention of T. asiatica infection are similar to those noted for T. saginata infection.
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