• Noninvasive diarrhea
  • Found worldwide:
    • 2–15% prevalence in developed nations
    • 20–40% prevalence in developing nations
  • 5% of all travelers’ diarrhea
  • Most common intestinal parasite in the U.S.:
    • Highest incidence in early summer months through fall
    • Highest incidence in children aged 1–9 yr and adults aged 30–39 yr
    • In 2010, 19,888 cases reported in the U.S. (mostly from Northern States)
  • Fecal–oral transmission:
    • Humans are major reservoir
    • Waterborne and foodborne
    • Zoonotic reservoir in both domestic and wild mammals
    • Reservoir in contaminated surface water
  • Populations at risk:
    • Travelers to endemic areas (developing countries, wilderness areas of the U.S.)
    • Children in day care centers and their close contacts
    • Institutionalized persons
    • Practitioners of anal sexual activity


  • Giardia lamblia:
    • A protozoan flagellate
  • Also called Giardia intestinalis or Giardia duodenalis
  • Ingested Giardia attach to intestinal villi
  • Alters the intestinal brush-border enzymes, impairing digestion of lactose, and other saccharides
  • No toxin produced

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