• Clinical science



Giardiasis is a common parasitic infection caused by the protozoan Giardia lamblia. Transmission usually occurs via the fecal-oral route (e.g., from contaminated drinking water) when traveling or living in an endemic region. Giardia live in two states: as active trophozoites in the human body and as infectious cysts surviving in various environments. Following the ingestion of the cyst, individuals may experience abdominal cramps and frothy, greasy diarrhea. Diagnosis of giardiasis involves analyzing stool for microscopic confirmation of cysts or trophozoites, and possibly immunoassays to detect antigens. Treatment consists of antibiotic administration with metronidazole.


  • Giardia lamblia is widespread throughout the world and affects ∼ 200 million people per year worldwide.
  • Incidence: estimated 5–8/100,000 per year in the US
  • In the US, giardiasis is the most common intestinal disease caused by parasites.


Epidemiological data refers to the US, unless otherwise specified.


  • Pathogen: Giardia lamblia; (also known as Giardia intestinalis or duodenalis), a protozoan
  • Transmission
    • Waterborne: from drinking recreational water (e.g., lakes, rivers, ponds, swimming pools)
    • Fecal-oral; (e.g., through food handlers, people in daycare and nurseries, oral-anal sexual contact): Giardia cysts are passed into the environment from the feces of infected people and animals. [2][4]
    • Infection is more likely to occur after traveling to endemic regions such as the tropics, subtropics, and North-American mountain regions.



  • Life cycle
    • Giardia have 2 stages in the life cycle.
      1. Trophozoite: active form of the pathogen that multiplies, lives within the host's body
        • Morphology: long oval shape with two nuclei and four pairs of flagella that resemble a kite
      2. Cysts: excreted, infectious form of the pathogen, able to survive in moist environments
        • Morphology: oval, four nuclei
    • Ingestion of cysts → excystation and conversion to trophozoite form → rapid multiplication, adhesion to intestinal walls → encystation in large bowel → excretion of cysts → possible reinfection
  • Mechanism:


Clinical features

Infection with Giardia lamblia must be considered as a differential diagnosis for persistent diarrhea.

  • Diarrhea: foul-smelling, voluminous, frothy, and fatty stools (stools tend to float and do not appear bloody)
  • Excessive gas (flatulence, bloating), abdominal pain, and cramps
  • Fatigue, nausea/vomiting, anorexia
  • Can be asymptomatic

To remember that GiARDia causes fatty DIarrhea, think “Gee, Arty has fatty stools.”






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  • 2. Centers for Disease Control and Prevention. Parasites - Giardia - Sources of Infection & Risk Factors. https://www.cdc.gov/parasites/giardia/infection-sources.html. Updated July 21, 2015. Accessed May 9, 2017.
  • 3. Feng Y, Xiao L. Zoonotic potential and molecular epidemiology of Giardia species and giardiasis. Clin Microbiol Rev. 2011; 24(1): pp. 110–140. doi: 10.1128/cmr.00033-10.
  • 4. Shelton AA. Sexually Transmitted Parasitic Diseases. Clinics in Colon and Rectal Surgery. 2004; 17(04): pp. 231–234. doi: 10.1055/s-2004-836943.
  • 5. Yel L . Selective IgA deficiency. J Clin Immunol. 2010; 30(1): pp. 1–16. doi: 10.1007/s10875-009-9357-x.
  • 6. Chapel H, Cunningham-Rundles C. Update in understanding common variable immunodeficiency disorders (CVIDs) and the management of patients with these conditions. Br J Haematol. 2009; 145(6): pp. 709–727. doi: 10.1111/j.1365-2141.2009.07669.x.
  • 7. Justiz Vaillant AA, Qurie A. Immunodeficiency. https://www.ncbi.nlm.nih.gov/pubmed/29763203. Updated January 1, 2020. Accessed July 13, 2020.
  • 8. Centers for Disease Control and Prevention. Parasites - Giardia - Pathogen & Environment. https://www.cdc.gov/parasites/giardia/pathogen.html. Updated February 16, 2017. Accessed January 4, 2018.
  • 9. Adam RD. Biology of Giardia lamblia. Clin Microbiol Rev. 2001; 14(3): pp. 447–475. doi: 10.1128/cmr.14.3.447-475.2001.
  • 10. Troeger H, Epple H-J, Schneider T, et al. Effect of chronic Giardia lamblia infection on epithelial transport and barrier function in human duodenum. Gut. 2007; 56(3): pp. 328–335. doi: 10.1136/gut.2006.100198.
  • 11. Centers for Disease Control and Prevention. Parasites - Giardia - Illness & Symptoms. https://www.cdc.gov/parasites/giardia/illness.html. Updated July 21, 2015. Accessed May 9, 2017.
  • 12. Kasper DL, Fauci AS, Hauser S, Longo D, Jameson LJ, Loscalzo J . Harrisons Principles of Internal Medicine . New York, NY: McGraw-Hill Medical Publishing Division; 2016.
  • 13. Centers for Disease Control and Prevention. Parasites - Giardia - Diagnosis & Detection. https://www.cdc.gov/parasites/giardia/diagnosis.html. Updated July 21, 2015. Accessed January 4, 2018.
  • 14. Shane AL, Mody RK, Crump JA, et al. 2017 Infectious Diseases Society of America Clinical Practice Guidelines for the Diagnosis and Management of Infectious Diarrhea. Clin Infect Dis. 2017; 65(12): pp. e45–e80. doi: 10.1093/cid/cix669.
  • Nazer H. Giardiasis. In: Cagir B. Giardiasis. New York, NY: WebMD. http://emedicine.medscape.com/article/176718. Updated February 15, 2016. Accessed May 9, 2017.
last updated 10/01/2020
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