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Contributors: James J. Douglas MD, FRCPC, Ricardo M. La Hoz MD, James H. Willig MD, MSPH
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Giardia lamblia, also known as Giardia duodenalis or Giardia intestinalis, is a flagellated enteric protozoan and a common cause of diarrhea (endemic and epidemic) throughout the world. In the United States, G lamblia has been demonstrated in 4%-7% of stool specimens, making it the most commonly identified intestinal parasite. There is a bimodal distribution, with the illness being reported most frequently in children aged 1-9 years and adults aged 35-45. Disease prevalence is highest during late summer and fall.

The parasite is spread via the feces of an infected person or animal. Acquisition occurs when cysts are ingested via contaminated water or food, or via person-to-person contact. After excystation, trophozoites colonize and multiply in the small bowel and may disrupt epithelial brush border, mucosally invade, or elaborate an enterotoxin. The incubation is usually 7-14 days, with symptoms often lasting more than 2-4 weeks; however, patients may continue to shed cysts for 6 months or longer.

  • Prolonged diarrhea – more than 7-10 days
  • Malaise
  • Flatulence
  • Foul-smelling, greasy stools
  • Sulfuric belching
  • Abdominal cramps
  • Bloating
  • Nausea
  • Vomiting
  • Malabsorption
  • Weight loss
Risk factors:
  • Travel to endemic regions
  • Children in daycare
  • Men who have sex with men


A07.1 – Giardiasis [lambliasis]

58265007 – Giardiasis

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Differential Diagnosis & Pitfalls

  • Viral gastroenteritis – Usually shorter duration.
  • Amebic colitis – Patient has traveled to endemic region.
  • Dysentery – Presence of blood and WBCs in stool.
  • Noninvasive bacteria (eg, Staphylococcal food poisoning, Clostridium perfringens food poisoning) – Usually shorter duration, possibly extra-intestinal manifestations.
  • Cryptosporidiosis – Public swimming pools.
  • Tropical sprue – Patient has lived more than 1 month in tropical region.
  • Celiac disease – Gluten relationship.
  • Acquired immune deficiency syndrome – With or without intestinal opportunistic infections.
  • Inflammatory bowel disease (Ulcerative colitis, Crohn disease) – Abdominal pain and alternating diarrhea and constipation.
  • Neoplasm – Weight loss and fecal occult blood test positive.
  • Other Malabsorption syndrome – Risk factor and temporal relationship.
  • Clostridioides difficile colitis – Previous antibiotic exposure.

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Last Updated:10/18/2022
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Patient Information for Giardiasis
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A medical illustration showing key findings of Giardiasis (Acute) : Diarrhea, Fever, Vomiting, Abdominal cramp, Contaminated drinking water exposure, Anorexia, Steatorrhea, Flatulence
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