Cestodes

Human infections caused by cestodes, or tapeworms, may occur within the lumen of the bowel, where adult cestodes attach themselves to the host intestine (Table 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. Understanding which host is involved and whether disease is intestinal or extraintestinal helps clinicians anticipate potential complications and select appropriate diagnostic tests.

Humans are definitive hosts 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 granulosus 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. Asking about travel, dietary exposures, contact with animals, and sanitation can be particularly helpful in raising suspicion for tapeworm infection.

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. These anatomic differences are important because they allow laboratory personnel to distinguish among species when examining stool specimens.

This classification figure summarizes the major human cestode species and illustrates how intestinal and extraintestinal infections are related across different hosts and life-cycle stages.

Hymenolepis nana infection

Essentials of Diagnosis

• Adult worms and proglottids are rare.
• Spheroidal and thin-walled eggs (30-47 µm).
• Eggs contain two polar elements from which 4-8 filaments project (diagnostic).
• Scolex has hooklets and four suckers.

General Considerations

Hymenolepis nana (H nana) is distributed worldwide and is called the dwarf tapeworm because of its small size, measuring about 0.8-1.6 inches (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. Crowding, limited access to sanitation, and close contact among household members may all facilitate transmission.

Clinical Findings

Signs and Symptoms

Infection with H nana is most often asymptomatic, yet some patients may complain of headache, dizziness, anorexia, or abdominal pain. Whether these symptoms are related to the infection is uncertain. Children may have headache or sleep and behavioral disturbances, which resolve after successful treatment of the infection. When symptoms are present, they are usually mild and nonspecific, so a high index of suspicion is needed to consider a tapeworm infection.

Laboratory Findings

As for patients with other cestode infections, examination of blood from patients with H nana infection is typically normal, although a mild leukocytosis with eosinophilia may be present. Microscopic stool examination will frequently reveal eggs, but finding proglottids is uncommon with H nana infection. Repeated stool examinations or concentration techniques may improve the likelihood of detecting eggs when clinical suspicion remains high.

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. Other intestinal parasites and functional gastrointestinal disorders may also enter into the differential diagnosis, depending on the clinical context.

Complications

Through a mechanism that is still unclear, seizures have been reported with H nana infections. Although uncommon, such neurologic manifestations underscore the importance of evaluating and treating confirmed infections and monitoring patients with new or unexplained neurologic symptoms.

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 Table 2). Follow-up examinations of stool should be performed at 2 weeks and 3 months after therapy. Treatment decisions, including drug choice and dosing, should be made in consultation with a healthcare provider, taking into account age, comorbidities, and potential drug interactions.

Prognosis

Since infection with H nana is usually asymptomatic and infection responds to therapy, the prognosis is excellent. Relapse or reinfection is more likely in settings with ongoing exposure, so attention to household contacts and environmental conditions is helpful.

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) (Table 3). Incidental ingestion of arthropod hosts may also produce infection, although this mechanism of infection is uncommon. Handwashing with soap and water, safe food handling, and control of household pests all contribute to lowering the risk of infection and reinfection.

Hymenolepis diminuta

Essentials of Diagnosis

• Proglottids are rare in stool, but adult worms may be present.
• Ovoid and thick-walled eggs (70-85 µm by 60-80 µm).
• Eggs contain no polar elements.
• Scolex has no hooklets and four suckers.

General Considerations

Hymenolepis 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. Human infection is usually sporadic and often associated with inadvertent ingestion of infected insects in food or the environment.

This illustration highlights the general structure of cestodes, including the scolex and chains of proglottids, which are shared features across many tapeworm species.

Clinical Findings

Signs and Symptoms

Infection with H diminuta is not associated with clinical symptoms. Most cases are discovered incidentally, and patients usually remain well, even when the infection is identified.

Laboratory Findings

Microscopic stool examination will frequently reveal eggs and adult worms. Blood examination may demonstrate mild leukocytosis with eosinophilia. As with other helminth infections, the degree of eosinophilia does not always correlate with symptom severity.

Differential Diagnosis

The finding of H diminuta in human infection is commonly an incidental finding that is asymptomatic. Other causes of eosinophilia, intestinal discomfort, or abnormal stool tests should be evaluated when the clinical picture is not fully explained by this tapeworm infection.

Complications

No complications have been reported. Nevertheless, consultation with a healthcare professional is appropriate to confirm the diagnosis and determine whether treatment is indicated.

Treatment

Therapy for infection with H diminuta consists of niclosamide in a one-time dose. Specific dosing regimens should be selected and supervised by a healthcare provider, particularly in children and individuals with underlying medical conditions.

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 (Table 3). Measures to protect stored grains and food from rodent and insect contamination also help reduce the risk of infection.