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Cestode Zoonoses - HYMENOLEPIS NANA

AGENT:

Dwarf tapeworm, Hymenolepis nana - measures 5-90 mm long.

RESERVOIRS AND INCIDENCE:

The animal reservoir is the house mouse, but humans can be both definitive and intermediate hosts. Worldwide occurrence in warm climates.

TRANSMISSION:

Gravid proglottids disintegrate and eggs pass in the feces and may be ingested by another human. Larvae then develop in the intestinal villi and pass to the lumen of the gut to become the adult forms. Dogs, cats and their fleas can be infected as well as grain beetles which can serve as intermediate hosts.

DISEASE IN ANIMALS:

Mild catarrhal enteritis with diarrhea occurs if the infection is heavy.

DISEASE IN MAN:

Light infections are generally asymptomatic. Heavy infections may cause diarrhea, abdominal pain, anorexia, vomiting, weight loss, and irritability, particularly in young children.

DIAGNOSIS:

Infections are diagnosed by finding characteristic eggs in feces; proglottids are usually not seen.

TREATMENT:

Niclosamide or praziquantel.

PREVENTION/CONTROL:

Personal hygiene, protective clothing and gloves Vermin control. Protect stored grains and feeds from grain beetles.

LIFE CYCLE:

Life cycle of Hymenolepis nana

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. (Source CDC)