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Infection with Hymenolepis
nana, the dwarf tapeworm, is the most common tapeworm infection
in the world. H nana is found in 0.4% of
fecal specimens submitted to state laboratories in the United States.
Infections occur most frequently in warm countries. It is especially
prevalent in the southern part of the former Soviet Union, the Mediterranean, the
Indian subcontinent, and South America. Children are more commonly
infected than adults, and high prevalence rates have been reported
in institutionalized children because of fecal-oral transmission.1
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In the usual cycle, H nana is passed between
rodents as definitive host and beetles as intermediate hosts. H
nana is unique among tapeworms, because humans can serve
as both intermediate and definitive hosts and can close the cycle
without the need for an animal intermediate host. This leads to
human infection being directly acquired (from another human definitive
host), thus contributing to its high prevalence.
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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. Eggs
passed in the feces are immediately infectious for another human
or for the original host (autoinfection). Ingested eggs hatch in
the small intestine. The embryos penetrate the villi and transform into
larval cysticercoids. After 4 or 5 days, the new adult tapeworms emerge
from the tissue and attach to the intestinal mucosa. Egg production
by the new worms begins about 2 to 4 weeks after infection. Eggs
released from gravid proglottids in the intestine may hatch and cause
internal autoinfection, producing hundreds or thousands of adult
tapeworms in a single host.1,2
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Clinical Manifestations,
Diagnosis, and Treatment
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Although well-controlled studies of clinical manifestations of H
nana infections are scarce, most H nana infections
are most probably asymptomatic or unnoticed.3 Symptoms
reported from several series of H nana infections
are anorexia or increased appetite, nausea, vomiting, pains in the
extremities, dizziness, and headache. Other reported symptoms are
abdominal pain, diarrhea, restlessness, restless sleep, irritability,
and nasal and anal pruritus. There are conflicting reports about
correlation between the numbers of parasites and the presence of
symptoms. Although a mild eosinophilia is a common finding in H
nana infections, it is often absent. Rarely, H
diminuta may be diagnosed, mostly in asymptomatic cases
in stool surveys.4 The few reported cases do not
allow examination of differences in clinical manifestations or response to
treatment between H nana and H
diminuta.
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Routine fecal examinations using concentration techniques for
ova and parasites should reveal eggs of H nana (Fig. 339-1) or more rarely H diminuta (Fig. 339-2). However, a single examination may
not be adequate to rule out infection. Proglottids are rarely found
in stools since they disintegrate after breaking from the tapeworm.
ELISA tests for serum antibodies to H nana have
been ...