Giardia lamblia and Other Protozoan Infections

Giardia lamblia, also called Giardia intestinalis (or G. duodenalis ), is a flagellated intestinal protozoan ( Fig. 116.1 ). In the United States an estimated 1.2 million individuals are reported to develop gastrointestinal symptoms annually because of giardial infection. G. lamblia is the most frequently identified intestinal parasite in the United States.

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Giardia lamblia and Other Protozoans.

The life cycle of G. lamblia includes a trophozoite (active) phase in the intestine, which encysts to a cystic phase. The cysts are transmitted easily in water or through contamination from numerous hosts, including domestic and wild animals as, for example, the beaver has gained attention as a potential source of Giardia contamination of lakes, reservoirs, and streams. The infection occurs through ingestion of the cysts via waterborne, foodborne, and person-to-person transmission. Infants, children, elderly persons, and immunocompromised patients are at high risk for infection. Other risk factors include day-care centers, immunodeficiency disorders, and men having sex with men (MSM). Hikers, backpackers, and campers who drink untreated water from lakes, streams, or wells also have a high chance of developing the infection. As few as 10 cysts may result in infection. Once the cysts pass through the stomach, acid stimulates them to form trophozoites, which enter the duodenum and attach to the mucosa. Some persons in areas with high infection rates develop immunity. Giardia attaches to the intestinal cells by virtue of its ventral disc. Once attached, it can cause a pathologic response, resulting in the clinical disease spectrum. Trophozoites multiply by binary fission, and when exposed to a hostile environment in the intestine, they can encyst. A heavily infected host may pass thousands of cysts into the environment.