Diarrheal illness caused by small intestine (duodenum and jejunum) and biliary tract infection with flagellated protozoan Giardia intestinalis (also Giardia lamblia and Giardia duodenalis)


  • Giardia is the most common parasitic enteric pathogen diagnosed across the world and the United States.
  • Higher rates in developing countries
  • Affects all age groups but peaks at 1 to 4 years, followed by 5 to 9 years, and then 45 to 49 years of age
  • Transmission:
    • Acquired by ingestion of cysts directly from infected person (rarely animals) or ingestion of fecally contaminated water or food
    • Cysts infectious for as long as person excretes them (weeks to months), and infectious dose is low (10 cysts).
    • Incubation period 1 to 3 weeks
    • Person-to-person transmission accounts for the high prevalence rates in institutions, day care centers, and family contacts.
    • Waterborne transmission is an important source of endemic or epidemic spread, especially when water is supplied by surface source such as streams and reservoirs (outdoor recreation and international travel).
    • Foodborne infection is less common and generally occurs from foods (e.g., lettuce) that have been washed with a contaminated water source.


U.S. average is 5.8 to 6.4 cases per 100,000 and approximately 16,000 cases are reported each year.


  • In the U.S., nondysenteric diarrheal stool specimens range from 5% to 7%, with rates up to 15% in children.
  • Highest prevalence in early summer through early fall and among residents of the Northwestern U.S.

Risk Factors

  • Day care attendance (increases risk of exposure to inadequate fecal–oral hygiene)
  • Travel to endemic areas
  • International adoption
  • Contact with recreational fresh water during backpacking, camping, swimming (swallowing water)
  • Contact with some animal species
  • Sexual practices exposing to fecal flora
  • Hypochlorhydria (previous gastric surgery)
  • Hypogammaglobulinemia, immunodeficiency
  • Cystic fibrosis (CF)

General Prevention

  • Handwashing all surfaces with soap and water for 20 seconds after toileting, changing diapers, handling animal waste, gardening, tending to a person with diarrheal illness, and prior to preparing food
  • Exclusion from child care and pool/recreational water during acute diarrheal illnesses. Excluding asymptomatic carriers is not recommended.
  • Avoiding potentially contaminated water (recreational/drinking) and food
  • Examining water sources in endemic areas
  • Boiling (1 minute), chemical disinfection or filtration (National Sanitation Foundation [NSF] Standard 53 or 58) of suspect water sources
  • Preventing contact with feces during sex
  • Administering vitamin A to children in developing countries has been shown to improve host defenses against Giardia infection.


Giardia intestinalis has a two-form life cycle: cyst (transmission) and trophozoite (infection)

  • Gastric acid and pancreatic enzymes initiate excystation of ingested cysts.
  • Trophozoites divide asexually and adhere to brush border of proximal small bowel enterocytes.
  • Cyst formation (encystation) occurs in the colon and is passed into the environment.


Trophozoite causes direct damage to intestinal brush border and mucosa (but does not invade mucosa), which leads to the following:

  • Disruption of tight junctional zonula occludens of intestinal mucosal cells
  • Increased permeability of mucosal cells via myosin light chain kinase-dependent phosphorylation of F-actin
  • Induction of epithelial apoptosis
  • Induction of host immune response that results in secretion of fluid and damage to the gut
  • Secondary lactase deficiency

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