Hymenolepiasis, caused by the dwarf tapeworm Hymenolepis nana, is the smallest of the adult human tapeworms, can complete its entire life cycle within humans, and is the most common cestode infection in the world.
PATHOGENESIS AND EPIDEMIOLOGY
The life cycle of H nana may vary according to the epidemiologic situation. In endemic areas and among institutionalized individuals, transmission occurs mainly via fecal-oral route. The parasites’ eggs are immediately infectious after passed in the stool; thus, autoinfection and person-to-person transmission are possible. Once the eggs are ingested, they hatch in the intestine, and the released oncospheres penetrate the mucosa and form a cysticercoid larva within the villi. In a few days, the cysticercoid matures, leaves the villi, and evaginates in the intestine forming a new tapeworm. In the human host, H nana can complete its life cycle without reaching the environment, causing prolonged infections. In this case, eggs hatch in the intestine of the same host and invade the intestinal villi, forming the cysticercoids and then the tapeworms. A third cycle involves rodents as definitive hosts and beetles as intermediate hosts, but it is debatable if humans intervene in this cycle.
The adult tapeworm measures 2 to 4 cm in length. It attaches to the mucosa of the small intestine by a scolex that has 4 circular suckers and a retractable structure called a rostellum. A single host may harbor hundreds or thousands of tapeworms. Conflicting reports associate a high burden of infection with symptoms and complications, particularly in children. There is a strong immune reaction to H nana infection, which probably accounts for some of the complications and decreasing prevalence with age.
H nana was found in 1.5% of fecal specimens from subjects with gastrointestinal symptoms in the Rocky Mountain region of the United States. Infections occur more frequently in regions with temperate and tropical weather, especially where water and sanitation standards are suboptimal. H nana is especially prevalent in the southern part of the former Soviet Union, the Mediterranean, the Indian subcontinent, Africa, and South America. Children are more commonly infected than adults, with prevalence rates reaching 25%. High prevalence rates have been reported in institutionalized children because of fecal-oral transmission.
Data on the clinical manifestations of H nana are scarce, and most infections are thought to be asymptomatic or subclinical. However, several studies suggest that children with hymenolepiasis may be more likely to have diarrhea than uninfected children. Other symptoms reported are anorexia, abdominal pain, nausea, vomiting, dizziness, headache, and anal pruritus. Mild eosinophilia may be found with H nana infection but is uncommon. Hymenolepis diminuta infections are much less common than H nana infections, and the clinical differences between these parasites are not known.
Routine fecal examinations using concentration techniques for ova and parasites should reveal eggs of ...