Hymenolepis nana and Hymenolepis diminuta
A. Classification
Phylum: Platyhelminthes
Class: Cestoda
Order: Cyclophyllidea
Genus Species:
Hymenolepis nana - "The Dwarf Tapeworm"
Hymenolepis diminuta - "The Rodent Tapeworm"
B. Morphology
Hymenolepis nana:
The adult tapeworm occurs in humans and rodents and has a typical length of 1.5 to 4 cm,
comprised of about 150-200 proglottids. The proglottids are shaped like trapezoids and
are four times as wide as they are long. Each proglottid contains three round testes, a bilobed ovary, a vitelline gland and a large uterus opening to a lateral genital pore. The
adult tapeworm has a small, rounded scolex with a four suckers and a retractile rostellum
with a single row of 20 to 30 hooks.
The eggs are usually oval or spherical, measuring 30-47 µm in diameter, have a shell, and
contain an oncosphere that has 6 hooklets. Polar filaments are present in the area
between the membrane and the colorless shell, unlike those of H. diminuta that do not
possess any filaments.
Hymenolepis diminuta:
The adult tapeworm occurs in rodents and has a typical length of 20 to 60 cm comprised
of about 1000 proglottids. Each adult has a scolex with four suckers but lacks a
rostellum. The proglottids of the H. diminuta are similarly shaped and contain the same
number of organs as the H. nana.
The eggs are round or slightly oval, and larger than H. nana, measuring 70 to 86 µm by
60 to 80 µm, with a striated outer membrane and a thin inner membrane. The space
between the membranes is smooth, thus lacking the polar filaments found in H. nana
eggs. The oncosphere has six hooks, similar to H. nana.
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Figure 1. H. nana scolex – Note the 4 suckers
and retractable rostellum.
Figure 2. H. nana adult
Figures 3 & 4. H. nana egg – Note the polar filaments and the 6 hooklets.
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Figures 5. H. diminuta scolex – Note the 4 suckers and the absence of a
rostellum
Figure 6. H. diminuta mature proglottid
Figures 7 & 8. H. diminuta eggs – Note the 6 hooks and the smoothness between the
membranes.
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H. diminuta egg
H. nana egg
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C.
Life Cycle and Epidemiology
Hymenolepis nana Life Cycle
Humans or rodents, usually mice, act as the definitive host for the Hymenolepis nana.
The eggs of H. nana are infective when they are passed with the stool, but cannot survive
more than 10 days in the external environment. The intermediate host, usually beetles or
fleas, ingest the eggs. Inside the insect, the eggs develop into cysticercoids. The
definitive hosts can become infected if they ingest of the beetles or fleas. The adults
develop and reside in the ileal portion of the in the small intestine of the definitive host.
The proglottids release the eggs through its genital atrium or when proglottids
disintegrate, egges are released into the small intestine.
Autoinfection of the definitive host can occur if the eggs remain in the intestine. The
eggs would release the oncospheres, penetrate the intestinal villus and develop into
cysticercoid larvae. The cysticercoids return to the intestinal lumen and evaginate their
scoleces after the rupture of the villus. They attach to the intestinal mucosa where they
then develop into adults. The adult worm may live between 4 to 6 weeks, but internal
autoinfection allows the infection to persist for years.
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Hymenolepis diminuta Lifecycle
The infected definitive host, a human or rodent, passes the eggs of Hymenolepis diminuta
in its feces. The intermediate host, typically a grain or flour beetle, Tribolium sp. or
Tenibrio sp., ingest the mature eggs, which then release oncospheres. These oncospheres
will then penetrate the intestinal wall of the host and develop into cysticercoid larvae in
the body caviety. These larvae will persist through the arthropod’s morphogenesis to
adulthood. The definitive host will ingest the arthropod carrying the cysticercoid larvae
directly or through precooked cereals or other food items. The cysticercoid larvae are
released into the small intestine and then eversion of the scoleces occurs. The H.
diminuta will then attach to the small intestine wall using its suckers. The tapeworm will
mature in about 20 days and then the eggs are released in the small intestine from gravid
proglottids and expelled to the environment in the host’s feces.
D.
Geographic Distribution
Hymenolepis nana is found worldwide, but is more frequent in countries with warmer
climates. Hymenolepis diminuta is also found worldwide, but is not as prevalent as H.
nana.
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E. Pathology and Symptoms
Infections of Hymenolepis nana and H. diminuta are generally asymptomatic in humans.
When symptoms do occur, their frequency correlates with the increasing number of
parasites within the host. These symptoms include restlessness, irritability, diarrhea,
abdominal pain and anal and nasal itching. In even more rare cases, symptoms range
from anorexia, vomiting, nausea, bloody diarrhea, to hives, weakness, headaches, and
increased appetite.
F. Diagnosis
Diagnosis of both H. nana and H. diminuta is made by examination of a stool sample for
eggs. The egg membrane of H. nana has 2 polar thickenings from which arise threadlike
filaments extending into the space between the membrane and the colorless hyaline shell.
H. diminuta lack these filaments in the egg. Concentration techniques and repeated
examinations of stool increase the likelihood of detecting light infections.
G. Treatment
The most common form of treatment for these two species of Hymenolepsis is a single
dose of Praziquantel given at 25 mg/kg body weight. Alternate drugs include
Niclosamide and Paromycin, but both of these have a seven-day course of treatment.
H. Public Health and Eradication
The major method of control is to eradicate rodent feces and to keep insects out of human
food. This obviously involves control of rodent populations. Disposal of human feces
and frequent hand washing is suggested.
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Works Cited
Alves De Fonesca, Rodrigo. University of Brazil, Laboratory of Parasitology. Atlas of
Medical Parasitology, “Intestinal Parasites (Helminths)”,
<http://www.cdfound.to.it/html/hym1.htm>. (02/15/05)
Bates, Kim. Hymenolepis nana. January 26, 2004.
<http://bio.winona.msus.edu/bates/Parasitology/Hymenolepis%20nana.htm.>
(2/15/05)
Centers for Disease Control. Laboratory Identification of Parasites of Public Health
Concern.
<http://www.dpd.cdc.gov/dpdx/HTML/Hymenolepiasis.asp?body=Frames/GL/Hymenolepiasis/body_Hymenolepiasis_page1.htm.> (2/15/05)
Chiodini, P L. Hymenolepis nana. UKNEQAS Parasitology.
<http://www.btinternet.com/~ukneqas.parasitologyscheme/Faecal_Scheme/Teaching_Inf
ormation/Helminths/Cestodes/Hymenolepis_nana/hymenolepis_nana.html.>
(2/15/05)
Corriss, Michael Patrick. 15 August 2001, …My barbaric YAWP…, “Hymenolepis
diminuta”, <http://www.gate.net/~mcorriss/PH1.html.> (02/15/05)
Ghaffar, Abdul. University of South Carolina, Microbiology and Immunology Online, 1
November 2004, “Parasitology Chapter 5, Cestodes (Tape Worms)”,
<http://www.med.sc.edu:85/parasitology/cestodes.htm.> (02/15/05)
“Parasite Images.” Parasites and Parasitological Resources. <http://www.biosci.ohiostate.edu/~parasite/hymenolepis_nana.html.> (02/16/05)
Stewart, Terry. University of Cambridge, Department of Pathology, Schistosome
Research Group, 5 October 1998, “Tapeworms of the Genus Hymenolepis”,
Helminthology and General Parasitology Pages,
<http://www.path.cam.ac.uk/~schisto/Tapes/Hymenolepis.html.> (02/15/05)
Jenny Costanzo
Michele Alfred
Angela Chung
February 18, 2005
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