8/8/2019 Hymen Ole Pis

http://slidepdf.com/reader/full/hymen-ole-pis 1/5

Causal Agents:Hymenolepiasis is caused by two cestodes (tapeworm) species, Hymenolepis nana (thedwarf tapeworm, adults measuring 15 to 40 mm in length) and Hymenolepis dimnuta (rattapeworm, adults measuring 20 to 60 cm in length). Hymenolepis diminuta is a cestode of rodents infrequently seen in humans and frequently found in rodents.

Life Cycle:

Eggs of Hymenolepis nana are immediately infective when passed with the stool and cannotsurvive more than 10 days in the external environment . When eggs are ingested by an

arthropod intermediate host (various species of beetles and fleas may serve asintermediate hosts), they develop into cysticercoids, which can infect humans or rodentsupon ingestion and develop into adults in the small intestine. A morphologically identicalvariant, H. nana var. fraterna , infects rodents and uses arthropods as intermediate hosts.When eggs are ingested (in contaminated food or water or from hands contaminated withfeces), the oncospheres contained in the eggs are released. The oncospheres (hexacanthlarvae) penetrate the intestinal villus and develop into cysticercoid larvae . Upon ruptureof 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 thesmall intestine producing gravid proglottids . Eggs are passed in the stool when releasedfrom proglottids through its genital atrium or when proglottids disintegrate in the smallintestine . An alternate mode of infection consists of internal autoinfection, where the

eggs release their hexacanth embryo, which penetrates the villus continuing the infectivecycle without passage through the external environment . The life span of adult worms is4 to 6 weeks, but internal autoinfection allows the infection to persist for years.

8/8/2019 Hymen Ole Pis

http://slidepdf.com/reader/full/hymen-ole-pis 2/5

Eggs of Hymenolepis diminuta are passed out in the feces of the infected definitive host(rodents, man) . The mature eggs are ingested by an intermediate host (variousarthropod 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 thegenus Tribolium are common intermediate hosts for H. diminuta . The cysticercoid larvaepersist through the arthropod's morphogenesis to adulthood. H. diminuta infection isacquired by the mammalian host after ingestion of an intermediate host carrying thecysticercoid larvae . Humans can be accidentally infected through the ingestion of insectsin precooked cereals, or other food items, and directly from the environment (e.g., oralexploration of the environment by children). After ingestion, the tissue of the infectedarthropod is digested releasing the cysticercoid larvae in the stomach and small intestine.Eversion of the scoleces occurs shortly after the cysticercoid larvae are released. Usingthe four suckers on the scolex, the parasite attaches to the small intestine wall. Maturationof the parasites occurs within 20 days and the adult worms can reach an average of 30 cmin length . Eggs are released in the small intestine from gravid proglottids that

disintegrate after breaking off from the adult worms. The eggs are expelled to theenvironment in the mammalian host's feces .

Geographic Distribution:Hymenolepis nana is the most common cause of all cestode infections, and is encounteredworldwide. In temperate areas its incidence is higher in children and institutionalizedgroups. Hymenolepis diminuta , while less frequent, has been reported from various areasof the world.

Clinical Features:Hymenolepis nana and H. diminuta infections are most often asymptomatic. Heavyinfections with H. nana can cause weakness, headaches, anorexia, abdominal pain, and

diarrhea.

Laboratory Diagnosis:The diagnosis depends on the demonstration of eggs in stool specimens. Concentrationtechniques and repeated examinations will increase the likelihood of detecting lightinfections.

8/8/2019 Hymen Ole Pis

http://slidepdf.com/reader/full/hymen-ole-pis 3/5

Treatment:Praziquantel* is the drug of choice. For additional information, see the recommendations inThe Medical Letter (Drugs for Parasitic Infections).

* This drug is approved by the FDA, but considered investigational for this purpose.

Microscopy

A B

A, B: Eggs of Hymenolepis diminuta . These eggs are round or slightly oval, size 70 - 85

µm X 60 - 80 µm, with a striated outer membrane and a thin inner membrane. The spacebetween the membranes is smooth or faintly granular. The oncosphere has six hooks.There are no polar filaments extending into the space between the oncosphere and theouter shell. Image A contributed by the Georgia Department of Public Health.

C D

8/8/2019 Hymen Ole Pis

http://slidepdf.com/reader/full/hymen-ole-pis 4/5

E F

C- F: Eggs of Hymenolepis nana . These eggs are oval and smaller than those of H.diminuta , with a size range of 30 to 50 µm. On the inner membrane are two poles, fromwhich 4-8 polar filaments spread out between the two membranes. The oncosphere has sixhooks.

G H

G, H: Eggs of H. nana in a trichrome-stained stool specimen. Although trichrome is not thepreferred method for observing helminth eggs, they can be detected this way. The eggs aredistorted, probably due to the zinc polyvinyl alcohol (PVA) used for preserving specimens fortrichrome stain. Images courtesy of the Oregon State Public Health Laboratory.

8/8/2019 Hymen Ole Pis

http://slidepdf.com/reader/full/hymen-ole-pis 5/5

I J

I: Cross-sections of mature proglottids of H. nana stained with hematoxylin and eosin(H&E), taken at 100x. Note the craspedote (overlapping) proglottids.J: Higher magnification of the eggs in Figures I , taken at 1000x oil magnification. Hooksdo not stain with H&E but are refractile and may be visible in stained specimens with properadjustment of the microscope. Polar filaments are visible in the egg in the upper rightquadrant of the image.

Macroscopic (gross) observations

K

K: Three adult Hymenolepis nana tapeworms. Each tapeworm (length: 15 to 40 mm) hasa small, rounded scolex at the anterior end, and proglottids can be distinguished at theposterior, wider end. Image contributed by the Georgia Division of Public Health.