• Giardia lamblia (also known as Giardia duodenalis or Giardia intestinalis) is a protozoan pathogen that leads to intestinal infection and is one of the most common causes of diarrhea worldwide.
  • The life cycle consists of cysts and trophozoite stages. Transmission occurs via ingestion of infective cysts, followed by excystation in the duodenum with the release of trophozoites, leading to symptoms.
    • Trophozoites pass to the large intestine, where they revert to the infectious cyst form, which are excreted in the stool back into the environment.
  • Most infections result from ingestion of unfiltered surface water (e.g., contaminated swimming areas) or fecal-oral transmission; less commonly acquired through contaminated food


  • Age
    • Most commonly children <5 years old and adults aged 25 to 44 years (1)
  • Gender
    • More common in males
  • Minimal seasonal variability: slight increase in cases reported in summer and early fall
  • The highest risk areas include South and Southeast Asia, North Africa, the Caribbean, and South America (1).

Pediatric Considerations

  • Most common in early childhood
  • Chronic infection in children can lead to intestinal malabsorption (may also be associated with growth restriction/failure to thrive).


  • Estimated to cause 280 million diarrheal infections annually (2)
  • In 2019, 14,887 cases were reported in the United States (~6 cases per 100,000) (3).
  • In the United States, giardiasis affects 1.2 million individuals annually, resulting in 3,584 hospitalizations per year (4).
  • The actual rate of giardiasis infection is likely much higher due to underreporting.

Occurs in about 2% of adults and 6–8% of children in developed countries, whereas 33% of people in developing countries have had giardiasis (4)

Etiology and Pathophysiology

Giardia trophozoites colonize the surface of the proximal small intestine.

  • Cellular attachment to host enterocytes via the ventral suction disc and the excretion of parasite products results in structural damage compromising intestinal epithelial cells and inhibiting the function of brush border enzymes. This causes electrolyte imbalances and increased intestinal permeability, leading to the production of diarrhea (2).

G. lamblia has eight defined genotypes (referred to as assemblages A–H), but only A and B are known to infect humans (2).

Risk Factors

  • Daycare centers
  • Anal intercourse
  • Wilderness camping
  • Travel to developing countries
  • Children adopted from developing countries
  • Drinking untreated water from lakes, streams, or wells
  • Swimming in contaminated recreational water (pools, lakes, streams, rivers)
  • Pets with Giardia infection/diarrhea
  • Eating raw produce

General Prevention

  • Hand hygiene
  • Water purification when camping and when traveling to developing countries; boiling is the most effective.
  • Properly cook all foods.
  • Protect public water supply from fecal contamination.
  • Sanitary disposal of feces
  • Barrier protection during anal intercourse

Commonly Associated Conditions

Hypogammaglobulinemia, common variable immunodeficiency, IgA deficiency, and immunosuppression are associated with prolonged course of the disease and treatment failures.

There's more to see -- the rest of this topic is available only to subscribers.