Hymenolepis nana

Author: MBLOGSTU

Morphology

The entire worm is small, measuring up to 40 mm in length and up to 1 mm in diameter. The size of the strobila is usually inversely proportional to the worm burden in the host.

The minute scolex, measuring about 0.32 mm in diameter, is rhomboidal in shape with four suckers (each about 80 μm in cross section) and a short rostellum armed with 20–30 spines arranged in one ring. The scolex is also capable of invagination into its apex.

The proglottids begin as short and narrow (immature) and increase in width as they progress distally. Eggs are released by the gradual disintegration of the distalmost proglottids. They are spherical or subspherical, hyaline in appearance, measuring 30–47 μm in diameter. Each egg contains an oncosphere that is enclosed in an inner envelope with two polar thickenings, from which arise 4–8 polar filaments. Within the oncosphere are three pairs of lancet-shaped hooklets.

Life Cycle

Eggs of Hymenolepis nana are immediately infective upon being passed with the stool, and they can survive in the external environment for no more than 10 days. When ingested by an arthropod intermediate host—such as various species of beetles or fleas—the eggs develop into cysticercoids. These cysticercoids can infect humans or rodents when ingested and will develop into adult worms in the small intestine.

A morphologically identical variant, H. nana var. fraterna, infects rodents using arthropods as intermediate hosts. When eggs are ingested directly (either through contaminated food, water, or from hands with fecal matter), the oncospheres (hexacanth larvae) are released in the intestine. They penetrate the intestinal villus and develop into cysticercoid larvae. After rupture of the villus, the cysticercoids return to the lumen, evaginate their scoleces, attach to the intestinal mucosa, and mature into adults in the ileal portion of the small intestine producing gravid proglottids.

An alternate mode of infection is internal autoinfection, whereby the released hexacanth embryo penetrates the villus, continuing the cycle without passage through the external environment. Although the adult worms live for 4 to 6 weeks, autoinfection can allow the infection to persist for years.

Transmission of Infection

Fecal–oral route: Ingestion of eggs from contaminated hands.

Food and water: Frequently, eggs contaminate food and water supplies.

Arthropod vector: Rarely, ingestion of food contaminated with fleas harboring cysticercoid larvae can result in infection.

Pathogenesis and Clinical Manifestations

Infections with Hymenolepis nana are commonly well tolerated. Symptoms are usually produced by an allergic reaction to the parasite.

In heavy infections, a large number of worms may lead to:

• Mechanical irritation of the intestine, resulting in various clinical manifestations;
• Allergic manifestations such as anal and nasal pruritus due to the release of toxic metabolites.

Hymenolepiasis occurs more commonly in children. The incubation period varies from 15–39 days. While most infections are asymptomatic, heavy infections may present with irritability, diarrhea, abdominal pain, sleep disorders, anal pruritus, and nasal pruritus. Rare symptoms include anorexia, nausea, and vomiting.

Laboratory Diagnosis

Diagnosis of H. nana infection is based on demonstration of characteristic eggs in feces by direct microscopy. Eggs are often concentrated using techniques such as salt flotation or formalin-ether sedimentation.

Eosinophilia may also be present in some cases.

Prevention and Control

Preventing infection mainly involves:

• Improving sanitation,
• Ensuring uncontaminated food supplies, and
• Implementing rodent control in houses and adjacent areas.

The hygienic status of children and inhabitants of institutions is especially important. Because of its direct life cycle and zoonotic characteristics, H. nana is difficult to control and impossible to eradicate.

Praziquantel is the drug of choice for treatment.