Hymenolepsis

The Platyhelminthes (Flatworms): Tapeworms Hymenolepis nana – The Dwarf Tapeworm Frequency - most common tapeworm infection in humans. Requires no intermediate host - can use insects such as fleas or beetles, but these are not required. Definitive hosts - man, rodents. Autoinfection - infected humans can ingest eggs from their own feces. Mode of Infection - Ingestion of eggs or insects containing larvae. Diagnosis - most often by finding eggs in a feces specimen.

The Platyhelminthes (Flatworms): Tapeworms Hymenolepis nana - The Dwarf Tapeworm Morphology - small, 2 to 4 cm in length. Scolex - 4 sucking disks & short rostellum with hooks. Proglottid - are broader than long; rarely seen in feces specimens (usually disintegrate in intestine). Eggs - most often seen stage in specimens, measuring 45 to 50 microns in diameter and exhibiting polar filaments lying between the egg shell and the hexacanth embryo.

The Platyhelminthes (Flatworms): Tapeworms Hymenolepsis diminuta – The Rat Tapeworm Must be able to differentiate from H. nana. A common parasite of rats, mice and other rodents. Mode of infection - ingestion of intermediate host containing larval stage(s). Grain weevils are the most important intermediate host (other intermediate hosts include earwigs, larval fleas, various beetles). Infection is often due to ingesting bugs in precooked cereals.

Grain weevil

The Platyhelminthes (Flatworms): Tapeworms Hymenolepsis diminuta – The Rat Tapeworm Morphology - considerably larger than H. nana (adult as well as egg). Size - adults measure 20 to 60 cm in length. Scolex - exhibits no hooks; is larger than that of H. nana. Eggs - measure 60 to 80 microns in diameter and do not exhibit polar filaments.

Hymenolepiasis(1): Causal Agents: Hymenolepiasis is caused by two cestodes (tapeworm) species: - Hymenolepis nana (the dwarf tapeworm, adults measuring 15 to 40 mm in length) - 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 Life Cycle: Eggs of 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 (various species of beetles and fleas may serve as intermediate hosts), they develop into cysticercoids, which can infect humans or rodents upon ingestion and 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 (in 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 cysticercoid larvae .  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 environment .  The life span of adult worms is 4 to 6 weeks, but internal autoinfection allows the infection to persist for years.

Hymenolepiasis(2): Geographic Distribution: Hymenolepis nana is 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.

Clinical Features: Hymenolepis nana and H Clinical Features: Hymenolepis nana and H. diminuta infections are most often asymptomatic.  Heavy infections 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.  Concentration techniques and repeated examinations will increase the likelihood of detecting light infections. Hymenolepiasis(3):

Egg of Hymenolepis diminuta,round or slightly oval, size 70 to 86 µm X 60 to 80 µm, with a striated outer membrane and a thin inner membrane.  The space between the membranes is smooth or faintly granular.  The oncosphere has six hooks.   Egg of Hymenolepis nana,oval or subspherical and smaller than those of H. diminuta, their size being 40 to 60 µm X 30 to 50 µm.  On the inner membrane are two poles, from which 4 to 8 polar filaments spread out between the two membranes.  The oncosphere has six hooks.

Hymenolepiasis(4): Treatment: - Praziquantel (25mg/kg once, F.C. Tab. 600mg ) is the drug of choice. - NICLOSAMIDE (4 tab daily for 5-7 days,Chewable Tab 500 mg)