• 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): Giardia cysts are passed into the environment from the feces of infected people and animals.
    • 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.”





  • Metronidazole (commonly used in the US)
  • Tinidazole (single oral dose)
  • Alternatives: nitazoxanide, albendazole, or mebendazole