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  • Acute diarrhea may be profuse and watery.

  • Chronic diarrhea with greasy, malodorous stools.

  • Abdominal cramps, distention, flatulence, and malaise.

  • Cysts or trophozoites in stools.


Giardiasis is a protozoal infection of the upper small intestine caused by the flagellate Giardia lamblia (also called Giardia intestinalis and Giardia duodenalis). The parasite occurs worldwide, most abundantly in areas with poor sanitation. In developing countries, young children are very commonly infected. In the United States and Europe, the infection is the most common intestinal protozoal pathogen; the US estimate is 100,000 to 2.5 million new infections leading to 5000 hospital admissions yearly. Groups at special risk include travelers to Giardia-endemic areas, those who swallow contaminated water during recreation or wilderness travel, men who have sex with men, and persons with impaired immunity. Outbreaks are common in households, children’s day care centers, and residential facilities, and may occur as a result of contamination of water supplies.

The organism occurs in feces as a flagellated trophozoite and as a cyst. Only the cyst form is infectious by the oral route; trophozoites are destroyed by gastric acidity (eFigure 35–13). Humans are a reservoir for the pathogen; dogs, cats, beavers, and other mammals have been implicated but not confirmed as reservoirs. Under suitable moist, cool conditions, cysts can survive in the environment for weeks to months. Cysts are transmitted as a result of fecal contamination of water or food, by person-to-person contact, or by anal-oral sexual contact. The infectious dose is low, requiring as few as ten cysts. After the cysts are ingested, trophozoites emerge in the duodenum and jejunum. Epithelial damage and mucosal invasion are uncommon. Hypogammaglobulinemia, low secretory IgA levels in the gut, achlorhydria, and malnutrition favor the development of infection.

eFigure 35–13.

Life cycle of Giardia lamblia. Cysts are resistant forms and are responsible for transmission of giardiasis. Both cysts and trophozoites can be found in the feces (diagnostic stages) image. The cysts are hardy and can survive several months in cold water. Infection occurs by the ingestion of cysts in contaminated water, food, or by the fecal-oral route (hands or fomites) image. In the small intestine, excystation releases trophozoites (each cyst produces two trophozoites) image. Trophozoites multiply by longitudinal binary fission, remaining in the lumen of the proximal small bowel where they can be free or attached to the mucosa by a ventral sucking disk image. Encystation occurs as the parasites transit toward the colon. The cyst is the stage found most commonly in nondiarrheal feces image. Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible. While animals are infected with Giardia, their importance as a reservoir is unclear (From Global Health, Division of Parasitic Diseases and Malaria, CDC.)


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