Cestodes

Human infections caused by cestodes (tapeworms) may occur within the lumen of the bowel, where adult cestodes attach to the host intestine (Table 1). Alternatively, infection may result from dissemination of cestodes from the bowel to extra-intestinal sites, often via larval forms of the parasite. The cestode life cycle is determined by definitive hosts, in whom the mature adult worm lives, and intermediate hosts, which harbour the larval forms of the parasite. Identifying the host involved and determining whether disease is intestinal or extra-intestinal 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 cestode-associated disease are treatable; therefore, a careful history and physical examination to identify potential cases are warranted. Asking about travel, dietary exposures, contact with animals, and sanitation can be particularly helpful in raising suspicion of tapeworm infection.

Cestodes attach to the intestinal mucosa by means of a specialised organ called the scolex, which has a distinctive morphology for each cestode species. One to several hundred segments, called proglottids, are attached to the scolex. Each proglottid contains both male and female reproductive organs and may be classified as immature, mature, or gravid, depending on the developmental stage of the sex organs. A gravid proglottid contains a fully developed uterus filled with eggs. The uterine structure of a gravid proglottid helps differentiate cestode species. These anatomic differences are important because they allow laboratory personnel to distinguish among species when examining stool specimens.

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

Hymenolepis nana infection

Essentials of Diagnosis

• Adult worms and proglottids are rare.
• Spheroidal, thin-walled eggs (30-47 µm).
• Eggs contain two polar elements, from which 4-8 filaments project (diagnostic).
• The 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 ingestion of eggs, commonly from human stool. The eggs hatch within the stomach or small intestine, and the resulting larvae attach to the bowel wall, where adult worms develop within several weeks. Eggs are released directly from 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 alternative intermediate hosts for H nana. Eggs produced within infected humans can lead to internal autoinfection, and poor faecal-oral hygiene can allow infection to pass 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, although some patients may complain of headache, dizziness, anorexia, or abdominal pain. It is uncertain whether these symptoms are related to the infection. Children may have headache or sleep and behavioural disturbances, which resolve after successful treatment of the infection. When symptoms are present, they are usually mild and non-specific, so a high index of suspicion is needed to consider tapeworm infection.

Laboratory Findings

As in other cestode infections, blood examination in patients with H nana infection is typically normal, although mild leukocytosis with eosinophilia may be present. Microscopic stool examination frequently reveals eggs, but proglottids are uncommonly detected in H nana infection. Repeated stool examinations or concentration techniques may improve the likelihood of detecting eggs when clinical suspicion remains high.

Differential Diagnosis

Because infection with H nana is usually asymptomatic, it is most often discovered as an incidental finding on stool examination performed for another reason. In patients with non-specific gastrointestinal complaints, peptic ulcer disease and malignancy need to be ruled out. Similarly, in children with behavioural symptoms, a range of neurological disorders with organic and psychological causes should be considered. Other intestinal parasites and functional gastrointestinal disorders may also be included in the differential diagnosis, depending on the clinical context.

Complications

Seizures have been reported in H nana infection, via a mechanism that remains unclear. Although uncommon, these neurological manifestations underscore the importance of evaluating and treating confirmed infections and monitoring patients with new or unexplained neurological 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 in other cestode infections. Treatment of H nana infection consists of a single dose of either praziquantel or niclosamide (see Table 2). Follow-up stool examinations 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 professional, taking account of age, comorbidities, and potential drug interactions.

Prognosis

Because infection with H nana is usually asymptomatic and 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 through good faecal-oral hygiene and adherence to sanitation principles (eg appropriate disposal of human sewage) (Table 3). Inadvertent ingestion of arthropod hosts may also cause infection, although this route 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, thick-walled eggs (70-85 µm by 60-80 µm).
• Eggs contain no polar elements.
• The scolex has no hooklets and four suckers.

General Considerations

Hymenolepis diminuta is also distributed worldwide, but infection is much less common than H nana infection. Infection with H diminuta is acquired by ingestion of eggs produced from an obligatory arthropod intermediate host. The eggs hatch within the stomach or small intestine, and adult worms develop within several weeks. Eggs are similar in size to the eggs of H nana but can be distinguished by their ovoid shape and lack of polar filaments. In contrast to H nana, the life cycle of H diminuta requires an intermediate arthropod host, and adult worms may be passed in human stool. Human infection is usually sporadic and is 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 infection is identified.

Laboratory Findings

Microscopic stool examination frequently reveals 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 humans is commonly incidental and 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

Treatment of H diminuta infection consists of a single dose of niclosamide. Specific dosing regimens should be selected and supervised by a healthcare professional, particularly in children and in 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, for example through rat control measures (Table 3). Measures to protect stored grain and food from rodent and insect contamination also help reduce the risk of infection.