MICROBIOLOGY

• 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 https://www.cdc.gov/dpdx/giardiasis/index.html.
    
• 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.