caused by two cestodes (tapeworm) species, Hymenolepis nana (the
dwarf tapeworm, adults measuring 15 to 40 mm in length) and
Hymenolepis dimnuta (rat tapeworm, adults measuring 20 to 60 cm in
length). Hymenolepis diminuta is a cestode of rodents
infrequently seen in humans and frequently found in rodents.
Hymenolepis nana are immediately infective when passed with the
stool and cannot survive more than 10 days in the external environment
When eggs are ingested by an arthropod intermediate host
species of beetles and fleas may serve as intermediate hosts), they
develop into cysticercoids, which can infect humans or rodents upon
develop into adults in the small intestine. A morphologically identical
variant, H. nana var. fraterna, infects rodents and uses
arthropods as intermediate hosts. When eggs are ingested
contaminated food or water or from hands contaminated with feces), the
oncospheres contained in the eggs are released. The oncospheres
(hexacanth larvae) penetrate the intestinal villus and develop into
Upon rupture of the villus, the cysticercoids return to the intestinal
lumen, evaginate their scoleces
attach to the intestinal mucosa and develop into adults that reside in
the ileal portion of the small intestine producing gravid proglottids
Eggs are passed in the stool when released from proglottids through its
genital atrium or when proglottids disintegrate in the small intestine
An alternate mode of infection consists of internal autoinfection, where
the eggs release their hexacanth embryo, which penetrates the villus
continuing the infective cycle without passage through the external
The life span of adult worms is 4 to 6 weeks, but internal autoinfection
allows the infection to persist for years.
Hymenolepis diminuta are passed out in the feces of the infected
definitive host (rodents, man)
The mature eggs are ingested by an intermediate host (various arthropod
adults or larvae)
and oncospheres are released from the eggs and penetrate the intestinal
wall of the host
which develop into cysticercoid larvae. Species from the genus
Tribolium are common intermediate hosts for H. diminuta. The
cysticercoid larvae persist through the arthropod's morphogenesis to
adulthood. H. diminuta infection is acquired by the mammalian
host after ingestion of an intermediate host carrying the cysticercoid
Humans can be accidentally infected through the ingestion of insects in
precooked cereals, or other food items, and directly from the
environment (e.g., oral exploration of the environment by children).
After ingestion, the tissue of the infected arthropod is digested
releasing the cysticercoid larvae in the stomach and small intestine.
Eversion of the scoleces
shortly after the cysticercoid larvae are released. Using the four
suckers on the scolex, the parasite attaches to the small intestine
wall. Maturation of the parasites occurs within 20 days and the adult
worms can reach an average of 30 cm in length
Eggs are released in the small intestine from gravid proglottids
disintegrate after breaking off from the adult worms. The eggs are
expelled to the environment in the mammalian host's feces
the most common cause of all cestode infections, and is encountered
worldwide. In temperate areas its incidence is higher in children and
institutionalized groups. Hymenolepis diminuta, while less
frequent, has been reported from various areas of the world.
and H. diminuta infections are most often asymptomatic. Heavy
infections with H. nana can cause weakness, headaches, anorexia,
abdominal pain, and diarrhea.
depends on the demonstration of eggs in stool specimens. Concentration
techniques and repeated examinations will increase the likelihood of
detecting light infections.
comparison with other intestinal parasites
the drug of choice.
* This drug is
approved by the FDA, but considered investigational for this purpose.