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Epidemiology and Pathology

Man acquires giardiasis by ingesting food or water which has been fecally contaminated with viable cysts. There are two stages in the life cycle of Giardia lamblia: the trophozoite (an actively moving flagellated form) and the cyst, which may be passed in the stool (Fig. 9.1). After ingestion, the cysts pass unharmed through the gastric juices and undergo excystation in the duodenum, with each cyst becoming two trophozoites. The trophozoites reproduce by binary fission in the upper small bowel. They can be identified in stool specimens, material from duodenal drainage, and histologic sections of biopsies of the proximal small bowel (Figs. 9.2, 9.3, 9.4). They have rarely been noted in the gallbladder. The cysts may be found in the lower small bowel and colon and also in formed stools (Fig. 9.5). Both the flagellated and cystic stages reproduce.

Fig. 9.1 Giardia lamblia trophozoites (1, 2) and cysts (3, 4).

Fig . 9.2 Giardia lamblia trophozoite with the characteristic two nuclei (the "Old Man of Parasitology") and curved median bodies in a stool specimen. Xl440. AFIP 56-3350.

Fig. 9.3 Giardia lamblia. Protargol-stained trophozoites in stool, showing characteristic flagellae. (Courtesy of Dr. Herman Zaiman).

Fig. 9.4 Giardia lamblia. Trichrome-stained biopsy (peroral) of duodenum showing trophozoites in close proximity to the mucosa (Coutesy of Dr. Herman Zaiman).

Fig. 9.5 Giardia lamblia cysts. The left cyst is mature with four nuclei. (Courtesy of Dr. Herman Zaiman)

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