Giardia lamblia


  • Giardia lamblia, syn. G. duodenalis or G. intestinalis
    • This flagellated bi-nucleated protozoan (see figure) is North America’s most common intestinal parasite and the third most common etiology of diarrheal disease in children under 5 years old worldwide.[2]
    • CDC image gallery accessed 2/9/2023 at
  • Life cycle forms are: 1) highly infectious cysts; and 2) disease-causing trophozoites.
    • Following excystation, trophozoites multiply, colonize the upper small intestine, and persist as extracellular, noninvasive endoparasites.[8]
    • Trophozoites have a flat ventral surface, and their adherence to the brush border of enterocytes can cause malabsorption.
  • Modes of transmission include water exposure, food contamination, person-to-person, and fecal-oral contact.[1]
    • Since the minimal infective dose is less than 10 cysts, giardiasis is highly infectious.
    • Transmission may occur in daycare centers, overcrowded areas with poor sanitation, and outdoor recreation or swimming pools.
    • Surface water is easily contaminated by cysts shed by mammalian hosts such as beaver, sheep, cattle, dogs, or cats.

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Last updated: April 15, 2023