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For further information, see CMDT Part 35-10: Giardiasis

Key Features

Essentials of Diagnosis

  • Acute diarrhea; may be profuse and watery

  • Chronic diarrhea with greasy, malodorous stools

  • Abdominal cramps, distention, and flatulence

  • Cysts or trophozoites in stools

General Considerations

  • This upper small intestine infection is caused by the flagellate Giardia lamblia (also called G intestinalis and G duodenalis)

  • The organism occurs in feces as a flagellated trophozoite and as a cyst

    • Only the cyst form is infectious by the oral route

    • Trophozoites are destroyed by gastric acidity

  • Reservoirs for infection

    • Humans

    • Dogs, cats, beavers, and other mammals are implicated but not confirmed

  • Under moist, cool conditions, cysts can survive in the environment for weeks to months

  • Transmission occurs as a result of

    • Fecal contamination of water or food

    • Person-to-person contact

    • Anal-oral sexual content

  • Infectious dose is low, requiring as few as 10 cysts

  • Hypogammaglobulinemia, low secretory IgA levels in the gut, achlorhydria, and malnutrition favor development of infection


  • The parasite occurs worldwide, especially in areas with poor sanitation

  • In the United States and Europe, the infection is the most common intestinal protozoal pathogen

  • US estimate is 100,000 to 2.5 million new infections leading to 5000 hospital admissions yearly

  • Groups at special risk include

    • Travelers to endemic areas

    • Persons who swallow contaminated water during recreation or wilderness travel

    • Men who have sex with men

    • Persons with impaired immunity

  • Multiple cases are common in households, children's day care centers, and residential facilities

  • Outbreaks occur from contamination of water supplies

Clinical Findings

Symptoms and Signs

  • About 50% of infected persons have no discernable infection and about 10% become asymptomatic cyst passers

  • Acute diarrheal syndrome develops in 25–50% of persons

  • Incubation period is usually 1–3 weeks but may be longer

Acute infection

  • May resolve spontaneously but is commonly followed by chronic diarrhea

  • May begin gradually or suddenly

  • May last days or weeks and is usually self-limited

  • The initial illness may include profuse watery diarrhea

  • Hospitalization may be required due to dehydration, particularly in young children

Chronic infection

  • Abdominal cramps, bloating, flatulence, nausea, malaise, and anorexia are typical

  • Malabsorption may also be present

  • Fever and vomiting are uncommon

  • Diarrhea

    • Usually not severe

    • May be daily or recurrent

    • Intervening periods may include constipation

  • Stools are greasy or frothy and foul smelling, without blood, pus, or mucus

  • Weight loss is frequent

Differential Diagnosis

  • Viral or bacterial gastroenteritis

  • Amebiasis

  • Lactase deficiency

  • Irritable bowel syndrome

  • Malabsorption due to other causes, eg, celiac sprue

  • Laxative abuse

  • Crohn disease

  • Cryptosporidiosis


Laboratory Tests

  • Stool is generally without blood ...

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