The Organism
G. lamblia (also referred to as Giardia duodenalis or Giardia intestinalis) is the causative agent of giardiasis and is the only common pathogenic protozoan found in the duodenum and jejunum of humans. Giardia exists in two forms: the trophozoite and the cyst forms. The trophozoite of G. lamblia is a heart-shaped organism, has four pairs of flagella, and is approximately 15 μm in length (Figure 1A). A large con cave sucking disk on the ventral surface helps the organism to adhere to intestinal villi. As the parasites pass into the colon, they typically encyst, and the cysts are passed in the stool (Figure 1B). They are ellipsoid, thick-walled, highly resistant, and 8–14 μm in length; they contain two nuclei as immature forms and four as mature cysts.
Fig1. Giardia lamblia. A: Trophozoite (12–15 μm). (Used with permission from Sullivan J: A Color Atlas of Parasitology, 8th ed. 2009.) B: Cyst (11–14 μm). (Courtesy of D. Petrovic, Microbiology Section, Clinical Laboratories, UCSF.)
Pathology and Pathogenesis
G. lamblia is usually only weakly pathogenic for humans. Cysts may be found in large numbers in the stools of entirely asymptomatic persons. In some persons, however, large numbers of parasites attached to the bowel wall may cause irritation and low-grade inflammation of the duodenal or jejunal mucosa, with consequent acute or chronic diarrhea associated with crypt hypertrophy, villous atrophy or flattening, and epithelial cell damage. Stools may be watery, semisolid, greasy, bulky, and foul smelling at various times during the course of the infection. Symptoms of malaise, weakness, weight loss, abdominal cramps, distention, and flatulence may continue for long periods. Collecting multiple stool samples over several days is recommended to increase the likelihood of microscopically detecting cysts in smears.
Epidemiology
G. lamblia occurs worldwide. Humans are infected by ingestion of fecally contaminated water or food containing Giardia cysts or by direct fecal contamination, as may occur in day-care centers, refugee camps, and institutions, or during oral–anal sex. Epidemic outbreaks have been reported at ski resorts in the United States, where overloading of sewage facilities or contamination of the water supply has resulted in sudden outbreaks of giardiasis. Cysts can survive in water for up to 3 months. Outbreaks among campers in wilderness areas suggest that humans may be infected with various ani mal giardia harbored by rodents, deer, cattle, sheep, horses, or household pets.
