Andrew D. Badley MD
James M. Steckelberg MD
Walter R. Wilson MD
Human infections caused by cestodes, or tapeworms, may occur within the lumen of the bowel, where adult cestodes attach themselves to the host intestine (Box 87-1). Alternatively, human infection may be the result of dissemination of cestodes from the bowel to involve extraintestinal sites, often by larval forms of the parasite. The life cycle of cestodes is determined by definitive hosts, in whom the mature adult worm lives, and intermediate hosts, which harbor the larval forms of the parasite. Humans are a definitive host for six cestodes: Diphyllobothrium latum, Taenia solium, Taenia saginata, Hymenolepis diminuta, Hymenolepis nana, and Dipylidium caninum. In addition, humans may be intermediate hosts for Echinococcus granulosis and Echinococcus multilocularis. All forms of disease associated with infections caused by cestodes are treatable; therefore, a careful history and physical examination to identify potential patients is warranted.
Cestodes attach themselves to the intestinal mucosa by means of a specialized organ called the scolex, which has a distinctive morphology for each species of cestode. Attached to the scolex are one to several hundred segments called proglottids. Proglottids each contain both male and female reproductive organs and may be classified as immature, mature, or gravid, based on the state of maturation of their sex organs. A gravid proglottid contains a fully developed uterus, full of eggs. The uterine structure of a gravid proglottid helps to differentiate species of cestode.
DIPHYLLOBOTHRIUM LATUM INFECTION
Essentials of Diagnosis
General Considerations
D latum is found worldwide, and infection is acquired by ingestion of contaminated raw or improperly cooked freshwater fish. Because of enthusiasm for raw or undercooked fish, Siberia, Europe, Canada, Alaska, and Japan are endemic regions for D latum infection. Once the D latum cyst has been ingested, the worm matures within the human intestine and begins to produce eggs after 5 weeks. A mature D latum may reach lengths of several meters and contain ≤30,000 proglottids. Eggs and proglottids that are passed in stool hatch after 14 days in fresh water into ciliated coracidium larvae, which are ingested by the intermediate host, the aquatic copepod. Inside the copepod, the larvae develop into a second larval form, the procercoid. Once the copepod is ingested by a freshwater fish, the procercoid larva matures into the plerocercoid larva, which may encyst within fish tissues. Human ingestion of improperly prepared fish initiates infection by the plerocercoid larva cyst. Bears, seals, cats, mink, foxes, and wolves are alternate definitive hosts for D latum.
Clinical Findings
Differential Diagnosis
The most usual manifestation of D latum infection is asymptomatic carriage, which is incidentally discovered. If patients present with abdominal pain and diarrhea, the differential diagnosis includes a variety of infectious and noninfectious causes. Diarrhea from D latum infection will not be associated with stool leukocytes; this aids in formulating a differential diagnosis. Noninfectious etiologies to consider include osmotic (eg, lactose intolerance) and secretory (eg, villous adenoma) etiologies, malabsorption syndromes (eg, celiac sprue), and motility disorders (eg, irritable bowel syndrome). Infectious etiologies causing diarrhea without stool leukocytes include rotavirus, Norwalk virus, Giardia lamblia, Entamoeba histolytica, Cryptosporidium spp., and toxigenic diarrhea caused by Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and enterotoxigenic Escherichia coli.
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BOX 87-1 |
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Complications
The complications vary with the clinical syndrome associated with infection. Chronic diarrhea may lead to malnutrition. Megaloblastic anemia secondary to B12deficiency results when the parasite disrupts the B12-intrinsic factor complex, resulting in B12 becoming unavailable for absorption by the host. B12 deficiency may lead to neurologic sequelae including peripheral neuropathy, dementia, and possible severe combined degeneration of the posterior columns. Also, infection with D latum may rarely result in intestinal obstruction caused by a mass of entangled worms.
Treatment
Therapy for infection with D latum consists of either praziquantel or niclosamide (Box 87-2). Follow-up examinations of stool should be performed 1 and 3 months after treatment.
Prognosis
Since the disease is not commonly associated with severe symptoms, the prognosis of infected individuals is excellent. One exception is with patients who manifest B12 deficiency, in whom the neurologic complications are reversible only if recognized and treated early.
Prevention & Control
Prevention of infection from D latum is achieved through adequate cooking of all freshwater fish or freezing of fish for 24–48 h at -18°C (Box 87-3). Isolation of infected persons is not required.
TAENIA SOLIUM INFECTION
Essentials of Diagnosis
General Considerations
T solium infection occurs worldwide; endemic areas include Mexico, South and Central America, Africa, Southeast Asia, India, and the Philippines. T solium infection is commonly linked to the ingestion of undercooked pork, although other animals may harbor the larval form of the parasite. Infection may be intestinal, which is typically asymptomatic, or extraintestinal (called cysticercosis, see below), which is caused by larval forms of T solium within the tissues of the human host. Ingestion of encysted T solium larvae is followed by the parasite scolex attaching to the intestinal mucosa, which allows the worm to grow into an adult within 12 weeks. There may be one or more adult worms present for ≤ 25 years, and these may reach lengths of 2 to 7 m. Each worm contains < 1000 proglottids. Identification of species is by the number of lateral branches on the side of the uterus within a gravid uterus.
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BOX 87-2 |
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Clinical Findings
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BOX 87-3 |
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Differential Diagnosis
Since infection with the adult worm of T solium is asymptomatic, the main diagnostic concern in a patient with intestinal T solium is whether the patient has cysticercosis. If the results of evaluation suggest that there is extraintestinal infection, then specific therapy for cysticercosis is required.
Complications
T solium infection of the intestine is not commonly associated with symptoms; however, in patients with high parasite loads, obstruction may occur.
Treatment
Therapy for T solium infection of the intestine consists of either praziquantel or niclosamide (see Box 87-2). Follow-up examinations of stool should be performed 1 month after treatment.
Prognosis
The prognosis for patients with intestinal T solium infection is excellent.
Prevention & Control
Prevention from infection with T solium involves adequate cooking of pork and pork products to a > 65°C core temperature (Box 87-3). Freezing, pickling, and salting do not prevent infection. Immunization of swine and provision of animal feed that is free of eggs and proglottids are other preventative measures that have been reported to be effective in controlling infection. Since infected humans are capable of transmitting cysticercus to others, enteric precautions should be used, and stool specimens should be handled with attention to decontamination. In addition, adequate facilities for disposal of human sewage should be available.
CYSTICERCOSIS (CYSTICERCUS CELLULOSEA INFECTION)
Essentials of Diagnosis
General Considerations
Cysticercosis is caused by invasion of tissue by the larval forms of T solium, which have been referred to as Cysticercus cellulosea, although the name is not taxonomically correct and introduces confusion. Within a host infected by the adult T solium, eggs or proglottids are passed in the stool. Once eggs or proglottids are ingested by either pigs (intermediate hosts) or humans (definitive and intermediate hosts), eggs hatch in the gastric juice, allowing the cysticercus to migrate from the intestine to disseminated sites, via efferent mesenteric venules. Autoinfection may also occur in humans, wherein eggs produced by the adult intestinal worm hatch and invade the intestinal epithelium.
Clinical Findings
Differential Diagnosis
Patients who seek care for symptoms of infection with T solium most commonly do so because of the neurologic symptoms associated with neurocysticercosis. The most common neurologic symptoms are mass effect and seizures (related to intraparenchymal lesions), hydrocephalus (related to intraventricular cysts), chronic meningitis (related to subarachnoid cysts), and cord compression (owing to spinal cord cysts). The differential diagnosis of each of these symptoms is beyond the scope of this discussion, but malignancy may present in identical ways, and so it must be ruled out.
Complications
Visceral cysticercosis is also commonly asymptomatic, but may obstruct local structures (eg, biliary obstruction in hepatic cysticercosis). Neurocysticercosis may lead to permanent neurologic impairment, coma, or even death.
Treatment
Therapy for cysticercosis includes surgery when feasible and necessary, in combination with praziquantel or albendazole (see Box 87-2). Therapy of cysticercosis commonly increases the local inflammation, thereby transiently causing a paradoxical worsening of the patient's symptoms. Whether corticosteroids should be used as adjunctive therapy for patients being treated for neurocysticercosis remains in debate.
Prognosis
The mortality rate for untreated neurocysticercosis approaches 50%, but treatment decreases this rate to 5–15%.
Prevention & Control
Guidelines for the prevention of cysticercosis are the same as for the prevention of T solium infection (Box 87-3).
TAENIA SAGINATA INFECTION
Essentials of Diagnosis
General Considerations
T saginata infection is commonly associated with the ingestion of undercooked beef. This is distinguished from infection with T solium because human infection with the larval form (as in cysticercosis) is extremely rare with T saginata infection. T saginata infection is common in areas of the world with intensive cattle breeding, such as central Asia and central and eastern Africa. Alternative intermediate hosts for T saginata include llamas, buffalo, and giraffes. The life cycle for T saginata is similar to that of T solium; larvae are ingested in infected meat, and the tapeworm attaches to the intestinal epithelium and matures in 12 weeks. Mature tapeworms produce gravid proglottids with characteristic 15–20 lateral branches, which contain numerous eggs. Ingestion of eggs or proglottids by cows leads to hatching of eggs, and larvae that migrate into striated muscle. Case reports exist about T saginata cysticercosis in humans, although the incidence is exceedingly uncommon.
Clinical Findings
Differential Diagnosis
Infection with T saginata is usually not associated with clinical symptoms. Patients most often seek medical attention after finding T saginata proglottids in stools or on clothing. The main differential diagnosis is to differentiate T saginata proglottids from T solium proglottids. If no gravid proglottids are present, then differentiation may not be possible, in which case patients should be treated as though they have infection with T solium.
Complications
Usually no complications are associated with T saginata; however, regurgitation and aspiration of proglottids may occur.
Treatment
Therapy for infection with T saginata is similar to treatment of intestinal T solium, a single dose of either praziquantel or niclosamide (see Box 87-2). Follow-up examinations of stool should be performed 1 month after treatment.
Prognosis
The prognosis for patients with intestinal T saginata infection is excellent.
Prevention & Control
Prevention of infection with T saginata involves thorough cooking of beef and beef products to > 65°C core temperature (Box 87-3). Beef should also be inspected for the presence of cysts, and infected carcasses destroyed.
HYMENOLEPIS NANA INFECTION
Essentials of Diagnosis
General Considerations
H nana is distributed worldwide and is called the dwarf tapeworm because of its small size (2–4 cm). Endemic areas include Asia, Africa, South and Central America, and southern and eastern Europe. Infection with H nana is acquired by the ingestion of eggs, commonly from human stool. The eggs hatch within the stomach or small intestine, and the resultant larvae attach to the bowel wall, where adult worms develop in several weeks. Eggs are released directly from the gravid proglottids while these proglottids are still attached to the adult worm; therefore proglottids are rarely seen on stool examination. Various arthropods such as fleas can serve as alternate intermediate hosts for H nana. Eggs produced within infected humans can lead to internal autoinfection, and poor fecal-oral hygiene can cause infection to be passed from one person to another.
Clinical Findings
Differential Diagnosis
Since infection with H nana is usually asymptomatic, patients most often discover H nana infection as an incidental finding on stool examination done for another reason. In patients with nonspecific gastrointestinal complaints, peptic ulcer disease and malignancy need to be ruled out. Similarly, in children with behavioral symptoms, a variety of neurologic disorders of organic and psychologic origins need to be considered.
Complications
Through a mechanism that is still unclear, seizures have been reported with H nana infections.
Treatment
Cysts of H nana are more resistant to therapy than adult worms. Therefore higher doses or longer courses of therapy are required to eradicate cysts than with other cestode infections. Therapy for infection by H nana consists of a single dose of either praziquantel or niclosamide (see Box 87-2). Follow-up examinations of stool should be performed at 2 weeks and 3 months after therapy.
Prognosis
Since infection with H nana is usually asymptomatic and infection responds to therapy, the prognosis is excellent.
Prevention & Control
Infection with H nana can be prevented with good fecal-oral hygiene and adherence to the principles of sanitation (eg, appropriate disposal of human sewage) (Box 87-3). Incidental ingestion of arthropod hosts may also produce infection, although this mechanism of infection is uncommon.
HYMENOLEPIS DIMINUTA
Essentials of Diagnosis
General Considerations
H diminuta is also distributed worldwide, but the incidence of infection is much less common than with H nana. Infection with H diminuta is acquired by the ingestion of eggs, produced from an obligatory arthropod intermediate host. The eggs hatch within the stomach or small intestine, and the adult worms develop in several weeks. Eggs are similar in size to the eggs of H nana but may be distinguished by their lack of polar filaments and ovoid shape. In contrast to H nana, the life cycle of H diminuta requires an intermediate arthropod host, and adult worms may be passed in the stool of humans.
Clinical Findings
Differential Diagnosis
The finding of H diminuta in human infection is commonly an incidental finding that is asymptomatic.
Complications
No complications have been reported.
Treatment
Therapy for infection for H diminuta consists of niclosamide in a one-time dose (see Box 87-2).
Prognosis
H diminuta responds promptly to therapy, so the prognosis is excellent.
Prevention & Control
Infection with H diminuta can be reduced by decreasing exposure to arthropod vectors, such as by rat control measures (Box 87-3).
DIPYLIDIUM CANINUM INFECTION
Essentials of Diagnosis
General Considerations
D caninum is distributed worldwide and is associated with wild and domesticated cats and dogs. The life cycle is similar to that of H diminuta, with an obligatory arthropod intermediate host. The adult worm lives in dogs, cats, or humans, and gravid proglottids are released from the adult worm either singly or in short chains. Eggs are passed in the stool, and ingestion of eggs by the intermediate host results in the development of the larval form within the arthropod host. Ingestion of the arthropod that contains larvae results in the development of an adult worm in dogs, cats, or humans. Adult worms may reach 10–80 cm in length.
Clinical Findings
Differential Diagnosis
Since D caninum infection in humans is often asymptomatic, most patients do not seek medical care. Once eggs are found in the stool, the appearance of egg clusters is so characteristic that no other parasite could be confused with D caninum. When a patient does seek the care of a physician for indigestion or anorexia, a variety of gastric pathologies need to be ruled out (eg, peptic ulcer disease and gastric outlet obstruction).
Complications
No complications are associated with D caninum infection.
Treatment
Therapy for D caninum infection is niclosamide in a one-time dose (see Box 87-2).
Prognosis
The prognosis for D caninum infections is excellent.
Prevention & Control
Infection with D caninum can be reduced by screening domestic dog and cat stools and treating pets found to be infected (Box 87-3).
ECHINOCOCCAL INFECTION
Essentials of Diagnosis
General Considerations
The normal life cycle of Echinococcus species does not involve humans. Human disease occurs when humans become an accidental intermediate host for the parasite, and tissue invasion is followed by the formation of cysts (hydatid cysts). The definitive hosts for echinococcal species are canines (usually dogs), in whom the adult worms live. There may be several hundred worms within a host, and the worms are small, usually 3–6 mm long.
The scolex is attached to the dog intestine, and to each scolex is attached a single proglottid. As the proglottids mature and become gravid proglottids containing several eggs, they detach and are passed in the stool. The eggs are resistant to desiccation and may remain viable for weeks. Eggs are ingested by intermediate hosts, commonly sheep and cattle but occasionally humans. The eggs hatch in the duodenum, and the larval forms penetrate the intestinal mucosa and disseminate through the blood stream to distant sites, most commonly the liver. Within tissues, the larva develops an encasing cyst composed of an outer layer and an inner fluid-filled layer. Within 1 year, cysts may reach 5–10 cm in diameter.
Inside the inner layer, daughter cysts form which may detach and float within the fluid, and daughter scolices may form. Ingestion of meat containing hydatid cysts leads to infection of definitive hosts, and adult worms develop. Infections with Echinococcus granulosus cause the formation of unilocular cysts. In contrast, infections with Echinococcus multilocularis are associated with multilocular cysts. E multilocularis also differs from E granulosus in that definitive hosts include foxes, wolves, cats, and dogs, and intermediate hosts include small rodents. Endemic areas for E granulosus include Africa, the Middle East, southern Europe, Latin America, and the southwestern United States. For E multilocularis, forested areas of Europe, Asia, and North America are endemic.
Clinical Findings
Although rare, hydatid cysts in the brain are potentially the most serious and may cause obstructive hydrocephalus with ataxia and dementia or mass effect with seizures, headache, or focal neurologic deficits. Bony hydatid cysts most commonly involve vertebrae and present with bone pain. Other presentations include soft tissue swelling and bone pain or pathologic fractures secondary to cysts weakening cortical bone. In ≤ 20% of infected patients, cysts will be multiple, so a thorough evaluation is required of all patients in whom the diagnosis is suspected.
Differential Diagnosis
Since the usual presentation of hydatid cysts is either liver pain or mass, lung mass with irritative symptoms of obstruction, or seizures with focal neurologic symptoms, the primary differential diagnosis is to rule out primary or metastatic malignancy of each of these organs. In a unilocular hydatid cyst, the radiographic appearance is often sufficient to exclude malignancy. In contrast, multilocular cysts caused by E multilocularis are slow growing, often with little or no fluid, and frequently have central necrosis, all suggestive of malignancy. Therefore biopsy and histologic examination are necessary for definite differentiation from malignancy.
Complications
On occasion, hydatid cysts may leak fluid into the systemic circulation of the host, causing sensitization of the host. Subsequent fluid leaks may then be responsible for the induction of an allergic response or even anaphylaxis. In addition, release of cyst tissue may be associated with embolization and the development of additional cysts at distant alternate sites. Cysts also may become secondarily infected, producing abscesses. Mechanical complications of the cyst are also possible, most commonly leading to portal hypertension, ascites, and portosystemic shunting.
Treatment
Therapy for hydatid cysts caused by infection with E granulosus combines surgical and pharmacological interventions (see Box 87-2). Solitary unilocular cysts at operable sites are generally treated with surgical excision, percutaneous drainage, or both. Extreme care must be taken to avoid spillage of cyst contents and subsequent seeding of other sites. One approach has been to remove a portion of the cyst fluid and instill a cystocidal agent such as 95% ethanol before removing the entire cyst. Some authorities also recommend pre- and postoperative therapy with either albendazole or mebendazole.
Recent experience with albendazole is a promising alternative. Mebendazole is an alternate therapy but may be less effective that albendazole. Complicated or multiloculated cysts (caused by E multilocularis) require surgery, often in association with albendazole as above. An experimental approach to inoperable cysts involves oral therapy with albendazole or mebendazole, combined with percutaneous aspiration and instillation of 95% ethanol. Although promising, this approach has not been validated in large trials.
Prognosis
The prognosis of hydatid disease is variable. With early diagnosis and treatment of simple unilocular cysts, the outlook is excellent; in contrast, advanced multilocular disease at multiple sites with advanced portal hypertension is a potentially lethal condition. Medical therapy of inoperable E granulosus cysts is associated with cure in 30% of cases and improvement in 50%. Inoperable E multilocularis is associated with a 10-year mortality rate in 90% of cases. In such cases, indefinite treatment with albendazole or mebendazole is recommended by some authorities.
Prevention & Control
In areas endemic for hydatid disease, the disease is commonly transmitted incidentally during activities such as camping and picking berries. Education is the best prevention for transmission in these settings (Box 87-3). Routine screening of household pets and appropriate treatment of animals found to carry Echinococcus spp. is another important control measure. Pet owners should be educated as to techniques of good hygiene to prevent accidental inoculation of eggs from dog stools. Lastly, carcasses of infected hosts must be disposed of in ways that prevent transmission to canines.
REFERENCES
Garcia LS, Bruckner DA (editors): Diagnostic Medical Parasitology, 3rd ed. ASM Press, 1997.
Strickland GT (editor): Hunters Tropical Medicine, 7th ed. WB Saunders, 1991.