Review of Medical Microbiology and Immunology, 13th Edition

54. Cestodes

CHAPTER CONTENTS

Introduction

Taenia

Diphyllobothrium

Echinococcus

Cestodes of Minor Importance

Self-assessment Questions

Summaries of Organisms

Practice Questions: USMLE & Course Examinations

INTRODUCTION

Platyhelminthes (platy means flat; helminth means worm) are divided into two classes: Cestoda (tapeworms) and Trematoda (flukes). The trematodes are described in Chapter 55.

Tapeworms consist of two main parts: a rounded head called a scolex and a flat body consisting of multiple segments. Each segment is called a proglottid. The scolex has specialized means of attaching to the intestinal wall, namely, suckers, hooks, or sucking grooves. The worm grows by adding new proglottids from its germinal center next to the scolex. The oldest proglottids at the distal end are gravid and produce many eggs, which are excreted in the feces and transmitted to various intermediate hosts such as cattle, pigs, and fish.

Humans usually acquire the infection when undercooked meat or fish containing the larvae is ingested. However, in two important human diseases, cysticercosis and hydatid disease, it is the eggs that are ingested and the resulting larvae cause the disease.

There are four medically important cestodes: Taenia solium, Taenia saginata, Diphyllobothrium latum, and Echinococcus granulosus. Their features are summarized in Table 54–1, and the medically important stages in the life cycle of these organisms are described in Table 54–2. Three cestodes of lesser importance, Echinococcus multilocularis, Hymenolepis nana, and Dipylidium caninum, are described at the end of this chapter.

TABLE 54–1 Features of Medically Important Cestodes (Tapeworms)

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TABLE 54–2 Medically Important Stages in Life Cycle of Cestodes (Tapeworms)

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TAENIA

There are two important human pathogens in the genus Taenia: T. solium (the pork tapeworm) and T. saginata (the beef tapeworm).

1. Taenia solium

Disease

The adult form of T. solium causes taeniasis. T. solium larvae cause cysticercosis.

Important Properties

The life cycle of T. solium is shown in Figure 54–1T. solium can be identified by its scolex, which has four suckers and circle of hooks, and by its gravid proglottids, which have 5 to 10 primary uterine branches (Figures 54–2A, B and 54–3). The eggs appear the same microscopically as those of T. saginata and Echinococcus species (Figure 54–4A).

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FIGURE 54–1 Taenia solium and Taenia saginata. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are infected at step 1 when they ingest undercooked pork (T. solium) or beef (T. saginata) containing cysticerci (larval stage). Adult tapeworms form in intestine and lay eggs. Pigs and cattle are infected when they ingest either the eggs or proglottids in human stool. Left side of figure describes the stages within the pigs and cattle (red arrows). (Provider: Centers for Disease Control and Prevention/Dr. Alexander J. da Silva and Melanie Moser.)

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FIGURE 54–2 A: Taenia solium scolex with suckers and hooks (10×). B: Taenia solium gravid proglottid. This has fewer uterine branches than does the proglottid of Taenia saginata (see panel D) (2×). C: T. saginata scolex with suckers (10×). D: T. saginata gravid proglottid (2×). E: Diphyllobothrium latum scolex with sucking grooves (7×). F: Entire adult worm of Echinococcus granulosus (7×). G: E. granulosus adult scolex (70×).

Images

FIGURE 54–3 Taenia solium—scolex and several proglottids. Long arrow points to one of the four suckers on the scolex of T. solium. Short arrow points to the circle of hooklets. Proglottids can be seen extending from the scolex toward the left side of the image. (Figure courtesy of Dr. M. Melvin, Public Health Image Library, Centers for Disease Control and Prevention.)

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FIGURE 54–4 A: Taenia solium egg containing oncosphere embryo. Four hooklets are visible. Taenia saginata and Echinococcus granulosus eggs are very similar to the T. solium egg but do not have hooklets. B:Diphyllobothrium latum egg with an operculum on the top (300×).

In taeniasis, the adult tapeworm is located in the human intestine (Figure 54–1). This occurs when humans are infected by eating raw or undercooked pork containing the larvae, called cysticerci. (A cysticercus consists of a pea-sized fluid-filled bladder with an invaginated scolex.) In the small intestine, the larvae attach to the gut wall and take about 3 months to grow into adult worms measuring up to 5 m. The gravid terminal proglottids containing many eggs detach daily, are passed in the feces, and are accidentally eaten by pigs. Note that pigs are infected by the worm eggs; therefore, it is the larvae (cysticerci) that are found in the pig. A six-hooked embryo (oncosphere) emerges from each egg in the pig’s intestine. The embryos burrow into a blood vessel and are carried to skeletal muscle. They develop into cysticerci in the muscle, where they remain until eaten by a human. Humans are the definitive hosts, and pigs are the intermediate hosts.

In cysticercosis, a more dangerous sequence occurs when a person ingests the worm eggs in food or water that has been contaminated with human feces (Figure 54–5). Note that in cysticercosis, humans are infected by eggs excreted in human feces, not by ingesting undercooked pork. Also, pigs do not have the adult worm in their intestine, so they are not the source of the eggs that cause human cysticercosis. The eggs hatch in the small intestine, and the oncospheres burrow through the wall into a blood vessel. They can disseminate to many organs, especially the eyes and brain, where they encyst to form cysticerci (Figure 54–6). Each cysticercus contains a larva.

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FIGURE 54–5 Taenia solium. Life cycle including cysticercosis stage. Center and left side of figure describes the cycle of T. solium within the human and the pig similar to Figure 54–1. Note, however, that there are now blue arrows between the egg at the bottom that go up the left side of the figure to the person at the top right. In cysticercosis, humans are infected when they ingest the eggs of T. solium in food contaminated with human feces. The eggs differentiate into cysticerci primarily in brain, eyes, and skin. (Provider: Centers for Disease Control and Prevention/Dr. Alexander J. da Silva and Melanie Moser.)

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FIGURE 54–6 Cysticercus of Taenia solium in brain—long arrow points to a larva of T. solium. Short arrow points to the wall of the cysticercus (sac) that surrounds the larva. (Figure courtesy of Rhodes B. Holliman, PhD, Professor Emeritus, Virgina Tech.)

Pathogenesis & Epidemiology

The adult tapeworm attached to the intestinal wall causes little damage. The cysticerci, on the other hand, can become very large, especially in the brain, where they manifest as a space-occupying lesion (Figure 54–6). Living cysticerci do not cause inflammation, but when they die, they can release substances that provoke an inflammatory response. Eventually, the cysticerci calcify.

The epidemiology of taeniasis and cysticercosis is related to the access of pigs to human feces and to consumption of raw or undercooked pork. The disease occurs worldwide but is endemic in areas of Asia, South America, and Eastern Europe. Most cases in the United States are imported.

Clinical Findings

Most patients with adult tapeworms are asymptomatic, but anorexia and diarrhea can occur. Some may notice proglottids in the stools. Cysticercosis in the brain causes headache, vomiting, and seizures. Cysticercosis in the eyes can appear as uveitis or retinitis, or the larvae can be visualized floating in the vitreous. Subcutaneous nodules containing cysticerci commonly occur. Cysts also are commonly found in skeletal muscle.

Laboratory Diagnosis

Identification of T. solium consists of finding gravid proglottids with 5 to 10 primary uterine branches in the stools. In contrast, T. saginata proglottids have 15 to 20 primary uterine branches. Eggs are found in the stools less often than are proglottids. Diagnosis of cysticercosis depends on demonstrating the presence of the cyst in tissue, usually by surgical removal or computed tomography (CT) scan. Serologic tests (e.g., enzyme-linked immunosorbent assay [ELISA]) that detect antibodies to T. solium antigens are available, but they may be negative in neurocysticercosis.

Treatment

The treatment of choice for the intestinal worms is praziquantel. The treatment for cysticercosis is either praziquantel or albendazole, but surgical excision may be necessary.

Prevention

Prevention of taeniasis involves cooking pork adequately and disposing waste properly so that pigs cannot ingest human feces. Prevention of cysticercosis consists of treatment of patients to prevent autoinfection plus observation of proper hygiene, including handwashing, to prevent contamination of food with the eggs.

2. Taenia saginata

Disease

T. saginata causes taeniasis. T. saginata larvae do not cause cysticercosis.

Important Properties

T. saginata has a scolex with four suckers but, in contrast to T. soliumno hooklets. Its gravid proglottids have 15 to 25 primary uterine branches, in contrast to T. solium proglottids, which have 5 to 10 (Figure 54–1C and D). The eggs are morphologically indistinguishable from those of T. solium.

The life cycle of Taenia saginata is shown in Figure 54–1. Humans are infected by eating raw or undercooked beef containing larvae (cysticerci). In the small intestine, the larvae attach to the gut wall and take about 3 months to grow into adult worms measuring up to 10 m (Figure 54–7). The gravid proglottids detach, are passed in the feces, and are eaten by cattle. The embryos (oncospheres) emerge from the eggs in the cow’s intestine and burrow into a blood vessel, where they are carried to skeletal muscle. In the muscle, they develop into cysticerci. The cycle is completed when the cysticerci are ingested. Humans are the definitive hosts and cattle the intermediate hosts. Unlike T. solium, T. saginata does not cause cysticercosis in humans.

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FIGURE 54–7 Taenia saginata—adult tapeworm. Note the tiny scolex on the right side of the image and the gravid proglottids on the left side of the image. White arrow points to the scolex. Ruler is 12 inches long. (Figure courtesy of Public Health Image Library, Centers for Disease Control and Prevention.)

Pathogenesis & Epidemiology

Little damage results from the presence of the adult worm in the small intestine. The epidemiology of taeniasis caused by T. saginata is related to the access of cattle to human feces and to the consumption of raw or undercooked beef. The disease occurs worldwide but is endemic in Africa, South America, and Eastern Europe. In the United States, most cases are imported.

Clinical Findings

Most patients with adult tapeworms are asymptomatic, but malaise and mild cramps can occur. In some, proglottids appear in the stools and may even protrude from the anus. The proglottids are motile and may cause pruritus ani as they move on the skin adjacent to the anus.

Laboratory Diagnosis

Identification of T. saginata consists of finding gravid proglottids with 15 to 20 uterine branches in the stools. Eggs are found in the stools less often than are the proglottids.

Treatment

The treatment of choice is praziquantel.

Prevention

Prevention involves cooking beef adequately and disposing waste properly so that cattle cannot consume human feces.

DIPHYLLOBOTHRIUM

Disease

Diphyllobothrium latum, the fish tapeworm, causes diphyllobothriasis.

Important Properties

In contrast to the other cestodes, which have suckers, the scolex of D. latum has two elongated sucking grooves by which the worm attaches to the intestinal wall (Figure 54–2E). The scolex has no hooks, unlike T. solium and Echinococcus. The proglottids are wider than they are long, and the gravid uterus is in the form of a rosette. Unlike other tapeworm eggs, which are round, D. latum eggs are oval and have a lidlike opening (operculum) at one end (Figure 54–4B). D. latum is the longest of the tapeworms, measuring up to 13 m.

Humans are infected by ingesting raw or undercooked fish containing larvae (called plerocercoid or sparganum larvae). In the small intestine, the larvae attach to the gut wall and develop into adult worms. Gravid proglottids release fertilized eggs through a genital pore, and the eggs are then passed in the stools. The immature eggs must be deposited in fresh water for the life cycle to continue. The embryos emerge from the eggs and are eaten by tiny copepod crustacea (first intermediate hosts). There, the embryos differentiate and form procercoid larvae in the body cavity. When the copepod is eaten by freshwater fish (e.g., pike, trout, and perch), the larvae differentiate into plerocercoids in the muscle of the fish (second intermediate host). The cycle is completed when raw or undercooked fish is eaten by humans (definitive hosts).

Pathogenesis & Epidemiology

Infection by D. latum causes little damage in the small intestine. In some individuals, megaloblastic anemia occurs as a result of vitamin B12 deficiency caused by preferential uptake of the vitamin by the worm.

The epidemiology of D. latum infection is related to the ingestion of raw or inadequately cooked fish and to contamination of bodies of fresh water with human feces. The disease is found worldwide but is endemic in areas where eating raw fish is the custom, such as Scandinavia, northern Russia, Japan, Canada, and certain north-central states of the United States.

Clinical Findings

Most patients are asymptomatic, but abdominal discomfort and diarrhea can occur.

Laboratory Diagnosis

Diagnosis depends on finding the typical eggs (i.e., oval, yellow-brown eggs with an operculum at one end, in the stools). There is no serologic test.

Treatment

The treatment of choice is praziquantel.

Prevention

Prevention involves adequate cooking of fish and proper disposal of human feces.

ECHINOCOCCUS

Disease

Echinococcus granulosus (dog tapeworm) causes echinococcosis. The larva of E. granulosus causes unilocular hydatid cyst disease. Multilocular hydatid disease is caused by Echinococcus multilocularis, which is a minor pathogen and is discussed later.

Important Properties

E. granulosus is composed of a scolex and only three proglottids, making it one of the smallest tapeworms (Figure 54–2F and G). The scolex has a circle of hooks and four suckers similar to T. soliumDogs are the most important definitive hosts. The intermediate hosts are usually sheep. Humans are almost always dead-end intermediate hosts.

The life cycle of E. granulosus is shown in Figure 54–8. In the typical life cycle, worms in the dog’s intestine liberate thousands of eggs, which are ingested by sheep (or humans) (Figure 54–4). The oncosphere embryos emerge in the small intestine and migrate primarily to the liver but also to the lungs, bones, and brain. The embryos develop into large fluid-filled hydatid cysts, the inner germinal layer of which generates many protoscoleces within “brood capsules.” The life cycle is completed when the entrails (e.g., liver containing hydatid cysts) of slaughtered sheep are eaten by dogs.

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FIGURE 54–8 Echinococcus granulosus. Life cycle. Center and left side of figure describes the natural cycle of E. granulosus within dogs (top half) and sheep (bottom half). Dogs are the definitive hosts and contain the adult tapeworm in the intestines. Sheep are an important intermediate host and ingest the eggs in dog feces. Hydatid cysts containing larvae form in the sheep. Humans are accidental intermediate hosts when they ingest food contaminated with dog feces containing the eggs (#2 at blue arrow at right). Eggs hatch oncosphere embryos in human intestine (#3 in human figure) Hydatid cysts form primarily in the liver, lung, brain, and bone (#4 in human figure). (Provider: Centers for Disease Control and Prevention/Dr. Alexander J. da Silva and Melanie Moser.)

Pathogenesis & Epidemiology

E. granulosus usually forms one large fluid-filled cyst (unilocular) that contains thousands of individual scoleces as well as many daughter cysts within the large cyst. Individual scoleces lying at the bottom of the large cyst are called “hydatid sand.” The cyst acts as a space-occupying lesion, putting pressure on adjacent tissue. The outer layer of the cyst is thick, fibrous tissue produced by the host. The cyst fluid contains parasite antigens, which can sensitize the host. Later, if the cyst ruptures spontaneously or during trauma or surgical removal, life-threatening anaphylactic shock can occur. Rupture of a cyst can also spread protoscoleces widely.

The disease is found primarily in shepherds living in the Mediterranean region, the Middle East, and Australia. In the United States, the western states report the largest number of cases.

Clinical Findings

Many individuals with hydatid cysts are asymptomatic, but liver cysts may cause hepatic dysfunction. Cysts in the lungs can erode into a bronchus, causing bloody sputum, and cerebral cysts can cause headache and focal neurologic signs. Rupture of the cyst can cause fatal anaphylactic shock.

Laboratory Diagnosis

Diagnosis is based either on microscopic examination demonstrating the presence of brood capsules containing multiple protoscoleces or on serologic tests (e.g., the indirect hemagglutination test).

Treatment

Treatment involves albendazole with or without surgical removal of the cyst. Extreme care must be exercised to prevent release of the protoscoleces during surgery. A protoscolicidal agent (e.g., hypertonic saline) should be injected into the cyst to kill the organisms and prevent accidental dissemination.

Prevention

Prevention of human disease involves not feeding the entrails of slaughtered sheep to dogs.

CESTODES OF MINOR IMPORTANCE

1. Echinococcus multilocularis

Many of the features of this organism are the same as those of E. granulosus, but the definitive hosts are mainly foxes and the intermediate hosts are various rodents. Humans are infected by accidental ingestion of food contaminated with fox feces. The disease occurs primarily in hunters and trappers and is endemic in northern Europe, Siberia, and the western provinces of Canada. In the United States, it occurs in North and South Dakota, Minnesota, and Alaska.

Within the human liver, the larvae form multiloculated cysts with few protoscoleces. No outer fibrous capsule forms, so the cysts continue to proliferate, producing a honey-comb effect of hundreds of small vesicles. The clinical picture usually involves jaundice and weight loss. The prognosis is poor. Albendazole treatment may be successful in some cases. Surgical removal may be feasible.

2. Hymenolepis nana

H. nana (dwarf tapeworm) is the most frequently found tapeworm in the United States. It is only 3 to 5 cm long and is different from other tapeworms because its eggs are directly infectious for humans (i.e., ingested eggs can develop into adult worms without an intermediate host). Within the duodenum, the eggs hatch and differentiate into cysticercoid larvae and then into adult worms. Gravid proglottids detach, disintegrate, and release fertilized eggs. The eggs either pass in the stool or can reinfect the small intestine (autoinfection). In contrast to infection by other tapeworms, where only one adult worm is present, many H. nana worms (sometimes hundreds) are found.

Infection causes little damage, and most patients are asymptomatic. The organism is found worldwide, commonly in the tropics. In the United States, it is most prevalent in the southeastern states, usually in children. Diagnosis is based on finding eggs in stools. The characteristic feature of H. nana eggs is the 8 to 10 polar filaments lying between the membrane of the six-hooked larva and the outer shell. The treatment is praziquantel. Prevention consists of good personal hygiene and avoidance of fecal contamination of food and water.

3. Dipylidium caninum

D. caninum is the most common tapeworm of dogs and cats. It occasionally infects humans, usually young children, while playing with their pets. Human infection occurs when dog or cat fleas carrying cysticerci are ingested. The cysticerci develop into adult tapeworms in the small intestine. Most human infections are asymptomatic, but diarrhea and pruritus ani can occur. The diagnosis in animals and humans is made by observing the typical “barrel-shaped” proglottids in the stool or diapers. Niclosamide is the drug of choice.

SELF-ASSESSMENT QUESTIONS

1. Regarding Taenia solium, which one of the following is most accurate?

(A) The scolex of T. solium has four suckers and a circle of hooklets.

(B) The drug of choice for the adult worm in humans is metronidazole.

(C) The cysticercus of T. solium contains the mature eggs of the organism.

(D) In the laboratory, identification of adult worms is based on finding the typical scolex in the stool.

(E) Ingestion of the terminal proglottids of T. solium by pigs results in mature tapeworms in the pig’s intestine.

2. Cysticercosis is most likely to be acquired by:

(A) Drinking water contaminated with feces of an infected pig

(B) Drinking water contaminated with feces of an infected cow

(C) Drinking water contaminated with feces of an infected human

(D) Ingestion of undercooked pork from an infected pig

(E) Ingestion of undercooked beef from an infected cow

3. Regarding D. latum, which one of the following is most accurate?

(A) Cattle are the most important intermediate hosts.

(B) Megaloblastic anemia may occur as a result of vitamin B12 deficiency.

(C) The laboratory diagnosis depends on finding a scolex with hooklets in the stool.

(D) Infection is acquired by the ingestion of eggs in food or water contaminated with human feces.

(E) Larvae migrate from the gastrointestinal tract via the portal circulation to the liver, where abscesses can occur.

4. Regarding E. granulosus, which one of the following is most accurate?

(A) The drug of choice for E. granulosus infection is metronidazole.

(B) Dogs are a required part of the life cycle of the causative organism.

(C) E. granulosus is one of the longest tapeworms, sometimes measuring 10 ft in length.

(D) E. granulosus larvae typically migrate to skeletal muscle, where they cause an abscess.

(E) The main mode of transmission to humans is ingestion of eggs in food or water contaminated with human feces.

5. Your patient is a 15-year-old girl with a 2-week history of headache and vomiting and a 3-day history of confusion and incoherent speech. She was born in Ecuador but moved to this country 5 years ago. MRI of the brain reveals multiple lesions bilaterally. The following day, she has a seizure and dies. On autopsy, the brain lesions consist of a cyst-like sac containing a larva. Of the following, which one is the most likely cause?

(A) D. latum

(B) E. granulosus

(C) T. saginata

(D) T. solium

6. Your patient is a 40-year-old man with occasional mild right upper abdominal discomfort but is otherwise well. On examination, his liver is enlarged. An MRI reveals a cystic mass in the liver. On questioning, he says that he was born and raised in rural Argentina on a sheep ranch and came to this country 10 years ago. Of the following, which one is the most likely cause?

(A) D. latum

(B) E. granulosus

(C) T. saginata

(D) T. solium

7. Your patient is a 20-year-old woman who is a recent immigrant from Central America. On routine exam, a stool ova and parasite test reveal eggs resembling those of T. solium. Which one of the following is the best choice of drug to treat this patient?

(A) Ivermectin

(B) Pentamidine

(C) Praziquantel

(D) Pyrimethamine and sulfadiazine

(E) Stibogluconate

ANSWERS

1. (A)

2. (C)

3. (B)

4. (B)

5. (D)

6. (B)

7. (C)

SUMMARIES OF ORGANISMS

Brief summaries of the organisms described in this chapter begin on page 664. Please consult these summaries for a rapid review of the essential material.

PRACTICE QUESTIONS: USMLE & COURSE EXAMINATIONS

Questions on the topics discussed in this chapter can be found in the Parasitology section of PART XIII: USMLE (National Board) Practice Questions starting on page 710. Also see PART XIV: USMLE (National Board) Practice Examination starting on page 731.