Background
Hymenolepis nana is the cestode that most commonly infects humans, especially school-aged children. In contrast, only a few hundred human infections with the rodent tapeworm, Hymenolepis diminuta, a tapeworm for which the human is an incidental host, have been reported.
Hymenolepiasis most frequently occurs in warm, dry regions of the developing world, where exposure to human feces results in hand-to-mouth infection. Direct person-to-person spread of H nana may occur.
Pathophysiology
Humans become infected with H nana when they ingest infective eggs, most commonly by direct fecal-oral exposure. The eggs pass into the ileum and hatch into oncospheres (the larval form), which penetrate the lamina propria of the villus. Within 3-4 days, the larvae mature into the preadult cysticercoid, which then enters the gut lumen to attach to the mucosae of the villus. Mature adults, measuring 35-45 mm in length and comprising 150-200 proglottids, result within about 3 weeks. Self-mating between adjacent proglottids generates hundreds of eggs, some of which penetrate intestinal villa and some of which pass into the feces. Occasionally, rodents may ingest the eggs in feces and serve as incidental hosts and reservoirs for spread of infection. Although infection usually does not produce infection, autoinfection (which is common) or intense exposure may result in a symptomatic infection caused by a heavy parasite burden.
Human infection with H diminuta results from accidental ingestion of insects (immature fleas, flour beetles, meal worms, cockroaches) that carry the parasite in their body cavities. Infective eggs are ingested by insects and hatch in their guts. After hatching, they invade into the body cavity and become cysticercoid larvae, which are infectious for humans. After the insects are consumed and digested, the larvae are released in the small intestine and mature within 25 days into 50-cm adults. When the adult tapeworm begins to pass eggs, insect hosts can become infected again. Most infections produce no symptoms.[1]
Epidemiology
Frequency
United States
Infection is most common in the Southeast (1% of school children in one study) and among institutionalized children. Among more than 200,000 stool specimens submitted to the state laboratories in 1987 for ova and parasite analysis, 0.4% were positive for H nana. Because most infections do not produce symptoms, the true incidence is likely considerably higher.
International
Infection is most common in children aged 4-10 years, in dry, warm regions of the developing world. H nana infection affects millions of people, primarily children, worldwide. Estimated rates of infection in various regions range from 0.1-58%. Regions with high reported infection rates include Sicily (46%), Argentina (34% of school children), and southern areas of the former Soviet Union (26%). In contrast, only 0.1% of stools examined at a children's hospital in Calgary were positive for H nana. Most cases with associated neurologic symptoms have been reported from the former Soviet Union.
Mortality/Morbidity
Morbidity is uncommon, only occurring when parasite burden is very high. Death has not been reported in association with this infection.
Race
No racial predisposition is known for hymenolepiasis.
Sex
No gender predilection is known for hymenolepiasis.
Age
Infection can occur in persons of any age; however, because of the increased likelihood of exposure to human feces, school-aged children have the highest risk of hymenolepiasis. Infection in adolescents tends to clear spontaneously, and hymenolepiasis is uncommon among adults.
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