A 4-year-old child presents with acute watery diarrhea and abdominal cramps. Stool microscopy reveals trophozoites with falling leaf motility. The etiological agent is -

Giardia lamblia. (A) Trophozoite--ventral view (B) Trophozoite--lateral view (C) Cyst Infection is acquired by the ingestion of cysts in contaminated food and water. Infectivity is high, as few as 10 cysts being capable of initiating infection. Within half an hour of ingestion, the cyst hatches out into two trophozoites which multiply successively by binary fission and colonise the duodenum. The trophozoites as they pass down the colon develop into cysts. G. lamblia is seen typically within the crypts in the duodenum. It does not invade tissues, but remains tightly attached by means of the sucking disc to the epithelial surface in the duodenum and jejunum. The diarrhoea in some cases may be steatorrhoeic with excess mucus and fat, but no blood. Children may develop chronic diarrhoea, malabsorption, weight loss and a sprue-like syndrome. The incubation period is variable, but is usually about 2 weeks. TEXTBOOK OF MEDICAL PARASITOLOGY,CKJ PANIKER,6TH EDITION,PAGE NO 39