Giardia lamblia


  • Flagellated protozoan
  • Most common cause of parasitic diarrhea worldwide
  • Transmitted by water contaminated with feces (human, beaver, muskrat, dogs, raccoons, etc)
  • Common among campers and is also known as “backpacker’s diarrhea”
  • Common in travelers to former Soviet Union, Caribbean, Latin America, India, Africa
  • Infection rate is twice as high during summer months
  • Also may be transmitted by contaminated food or close physical contact (sexual activity, daycare centers, etc)
  • Patients with decreased gastric acidity, immunoglobulin deficiency, or immunocompromise are more susceptible
  • Other names: “beaver fever”, “the Trotskys” (common in travelers to Leningrad) [1] [2] [3]


  • Trophozoites infect duodenum, jejunum, and ileum where they form cysts
  • Cysts are passed in feces; viable for long periods of time
  • A single diarrheal stool may contain hundreds of millions of cysts or parasites

Clinical Features

  • Often asymptomatic
  • Most common symptoms include:
  • No blood or mucus in stool
  • Sudden onset after incubation period of 1-3 weeks
  • Symptoms usually resolve in 7-10 days
  • 85% of the time infection resolves spontaneously within 6 weeks
  • May cause chronic malabsorption-like illness, especially in those with immunoglobulin deficiency
  • Chronic infections cause weight loss, anemia, lactose intolerance

Differential Diagnosis

Acute diarrhea



Watery Diarrhea

Traveler's Diarrhea


  • Normal WBC, no eosinophilia
  • Stool O&P
    • Motile trophozoites or cysts
    • Able to diagnose infection readily in acute illness
    • More difficult to diagnose in chronic or asymptomatic infection
  • Antigen testing with ELISA, DFA, etc starting to replace microscopic examination with similar cost [5]
  • Suspect protozoan illness in patients with diarrhea > 2 weeks


Treatment is not always successful [6]

  • Metronidazole
    • Adult: 250mg TID x 7-10 days
    • Children: 5mg/kg TID x 7 days (max dose 500mg TID)
  • Albendazole[7]
    • 400mg PO daily x 5-10 days
  • Tinidazole
    • Adult: 2 grams PO x 1 dose
    • Children: 50mg/kg PO x 1 dose
  • Quinacrine
    • > 8 years old: 100mg TID x 7 days
    • < 8 years old: 2mg/kg TID x 7 days
  • Nitazoxanide
    • > 12 years old: 500mg BID x 3 days
    • 4-11 years old: 200mg BID x 3 days
    • 12-47 months old: 100mg BID x 3 days

Public Health Measures

  • Strict adherence to handwashing (toileting, diaper changes, playing with pets, etc)
  • Treat household members and/or sexual contacts if infected
  • Treat asymptomatic infections in those at high-risk of transmitting to others (children in daycare, food handlers, etc) or those at risk of chronic symptoms
  • Reinfection universal within 3 months in heavily infected endemic areas; treatment is not cost-effective in this setting


  • Disease is usually self-limited
  • Admit those with systemic symptoms, severe dehydration, inability to tolerate PO fluids, or those with significant co-morbidities
  • Supplementation with zinc and probiotics reduce severity/duration of diarrhea [8]


  1. Marx, John A., Robert S. Hockberger, Ron M. Walls, James Adams, and Peter Rosen. "Chapter 94 -- Gastroenteritis." Rosen's Emergency Medicine Concepts and Clinical Practice. Philadelphia: Mosby/Elsevier, 2010. Print.
  2. Tintinalli, Judith E., and J. Stephan. Stapczynski. "Chapter 156 -- World Traveler." Tintinalli's Emergency Medicine: A Comprehensive Study Guide. New York: McGraw-Hill, 2011. Print.
  3. Tintinalli, Judith E., and J. Stephan. Stapczynski. "Chapter 154 -- Foodborne and Waterborne Diseases." Tintinalli's Emergency Medicine: A Comprehensive Study Guide. New York: McGraw-Hill, 2011. Print.
  4. Marx et al. “Cholera and Gastroenteritis caused by Noncholera Vibrio Species”. Rosen’s Emergency Medicine 8th edition vol 1 pg 1245-1246.