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  • Intestinal infection caused by the protozoan parasite Giardia lamblia:
    • G. lamblia is also called Giardia duodenalis and Giardia intestinalis.
  • Infection results from ingestion of cysts, which transform into trophozoites and colonize the small intestine to cause symptoms.
    • The infectious cycle is continued when trophozoites encyst in the small intestine to be subsequently transmitted through water, food, or hands contaminated by feces of an infected person.
  • Most infections result from fecal–oral transmission or ingestion of contaminated water (e.g., swimming).
  • Less commonly acquired through contaminated food


  • Predominant age:
    • All ages but most common in early childhood (ages 1 to 9 years) and adults 35 to 44 years
  • Predominant gender:
    • Male > female (slightly)
  • Minimal seasonal variability; slight increase in summer and early fall
Pediatric Considerations
  • Most common in early childhood
  • Chronic infection in children can lead to intestinal malabsorption (may also be associated with growth restriction).
  • 10% of cases of traveler’s diarrhea are caused by parasites, most commonly Giardia (1).
  • >19,000 cases per year from U.S. states where Giardia is reportable:
    • Giardia is currently not reportable in Indiana, Kentucky, Mississippi, North Carolina, and Texas.

Etiology and Pathophysiology

Giardia trophozoites colonize the surface of the proximal small intestine: The mechanism of diarrhea is unknown.

No known genetic risk factors

Risk Factors

  • Daycare centers
  • Anal intercourse
  • Wilderness camping
  • Travel to developing countries
  • Children adopted from developing countries
  • Public swimming pools
  • Pets with Giardia infection/diarrhea

General Prevention

  • Hand hygiene
  • Water purification when camping and when traveling to developing countries
  • Properly cook all foods.

Commonly Associated Conditions

Hypogammaglobulinemia, IgA deficiency, and immunosuppression are associated with prolonged course of the disease and treatment failures.

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