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Giardia lamblia or Giardia intestinalis is a protozoan flagellate that is among the most common disease-causing parasites in the United States and the most frequently identified agent of waterborne diarrhea.1 Cases are especially common in areas with inadequate water and sanitation facilities. Humans are the major reservoir of infection, although other mammals, such as dogs, cats, and beavers, may be colonized and excrete cysts. Massive epidemics have occurred after the contamination of reservoirs, lakes, and streams, especially when community water supplies are not adequately filtered.

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Although giardiasis affects persons of all age groups, the populations at highest risk are children ages 0 to 5 years and adults ages 31 to 40. In developing countries, Giardia infection has been reported in almost 100% of children who were followed prospectively from birth until age 2 years. In the United States and other developed countries, Giardia is prevalent in childcare centers and custodial institutions, among backpackers and others spending time in wooded areas, and among travelers to disease-endemic areas. Among children in daycare centers, Giardia cyst carriage has been documented to be as high as 50%, and many of these are asymptomatic carriers who can spread infection to household contacts. Person-to-person spread via fecal-oral route and ingestion of contaminated water are the most common modes of transmission; infection through food is less common.

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Giardia cysts, present in the stool of infected persons, are the infective form. After ingestion, they excyst in the small intestine, yielding trophozoites that subsequently multiply. The trophozoites remain limited to the mucosa, mucus, or lumen of the intestine and are rarely, if ever, invasive (Fig. 349-1). Encystation normally occurs prior to expulsion in the feces. Contamination of oneself and the environment with cysts is common. The number of cysts excreted varies, but it may reach as many as 10 million per gram. Infections are relatively frequent, because as few as 10 cysts can infect 30% of inoculated humans.

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Figure 349-1.
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Giardia lamblia trophozoite seen by scanning electron microscopy.

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The exact pathophysiology of the diarrhea is not known. The most severe cases are characterized by malabsorption and lactose intolerance, with varying degrees of inflammation and villous blunting. Host factors are also determinants of disease outcome; only 40% of humans infected with the same inoculum develop diarrhea. Patients with hypogammaglobulinemia frequently suffer from particularly severe cases of giardiasis. The observation that asymptomatic infections are more common in persons previously infected with Giardia also suggests partial immunity.

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Clinical Manifestations

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Clinical manifestations and duration of symptoms vary. Infections range from asymptomatic cases to severe, life-threatening diarrhea accompanied by malabsorption and dehydration. Infections may last from a few days to years. In naturally occurring infections, symptoms usually appear approximately 12 to 14 days after presumed exposure. Passage of cysts usually begins 7 to 10 days after inoculation ...

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