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Epidemiology and Pathology

The trophozoites are tiny pear-shaped protozoa containing a sucker disc and four pairs of symmetrical flagella. Two prominent nuclei are located anteriorly. These flagellated forms are usually found in the mucus overlying the intestinal villi and crypts. They attach themselves to the mucosal epithelium of the duodenum and jejunum, and rarely the ileum, by their sucking discs but do not actually invade the mucosa; on occasion they produce a mild inflammatory reaction in the lamina propria.

Study of intestinal biopsy material, especially using the electron microscope, has provided insight into the pathological changes of giardiasis. In a number of patients, small bowel biopsies have shown acute and chronic inflammatory changes in addition to villous atrophy and crypt hyperplasia The microvilli are short and thick with increased cellular infiltrate in the lamina propria. There is a characteristic focal acute inflammation of the epithelium in the crypts and occasionally in the villi. In addition to inflammatory infiltrates of leukocytes, lymphocytes and eosinophils, epithelial changes, including vacuolization, compression and increased mitotic activity are often present, and are most prominent when the inflammation is most severe.
Malabsorption is frequently associated with giardiasis and is thought to result from irritation, inflammatory reaction and altered epithelial function, resulting from cellular damage and disturbed epithelial cell maturation. Other theories suggest that there is a mechanical barrier to the absorption of nutrients caused by overwhelming numbers of parasites attached to the mucosal surface, or that massive numbers of G. lamblia compete with the host for nutrients.

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