Microbiology · Intestinal and urogenital protozoa

Giardia duodenalis !

Parasite
Exam priority: high

A small-intestine parasite caught from contaminated water or food. It causes greasy diarrhea, gas and malabsorption, especially in children.

Updated 2026-11-01

Also called: Giardia lamblia, Giardia intestinalis

At a glance

Shape: Pear-shaped trophozoite with ventral disk and two nuclei; oval cyst
Family: Hexamitidae
Type: Intestinal flagellated protozoan (trophozoite and cyst)
Stain: Direct exam, iodine and trichrome; stool antigen
Size: Trophozoite about 10 to 20 µm; cyst about 8 to 12 µm

Morphology

Pear-shaped trophozoite with two large nuclei that give it a «face» look.
Ventral (sucking) disk for attaching to the mucosa and four pairs of flagella.
Oval cyst with four nuclei when mature (two when immature).

Transmission

Fecal-oral: untreated water (rivers, wells), contaminated food and hands.
Person to person in daycare centers, care homes and institutions.
Oral-anal sexual contact.
Contact precautions for diapered or incontinent patients.

Reservoir: Humans; Dogs, cats, cattle and beavers (some zoonotic genotypes)

Isolation precautions

What the care team adds to standard precautions.

Standard

Life cycle

Cysts are swallowed; a small number is enough to infect.
In the duodenum each cyst releases two trophozoites.
Trophozoites attach to the mucosa with the ventral disk and divide by longitudinal binary fission.
As they move toward the colon they encyst.
Cysts leave in feces and survive usual chlorine levels and cold water for weeks.

Virulence factors

Ventral disk: adhesion to the mucosa without invasion.
Brush-border damage, villus blunting and disaccharidase (lactase) deficiency.
Antigenic variation of surface proteins.
More disease in IgA deficiency and hypogammaglobulinemia.

Diseases it causes

Giardiasis: watery or greasy diarrhea (steatorrhea), bloating, flatulence and malabsorption. Giardiasis →
Acute gastroenteritis in children and travelers, sometimes turning chronic. Acute (viral) gastroenteritis →

Clinical picture

Non-bloody, foul-smelling, greasy diarrhea; stools that float.
Bloating, cramps, nausea, sulfur-tasting burps.
Chronic form: weight loss, poor growth in children, lactose intolerance that can outlast the infection.
Many infected people have no symptoms.

Diagnosis

Serial stool exams with concentration: cysts on direct exam, trophozoites on permanent stains.
Stool antigen (immunoassay or immunofluorescence), more sensitive than a single microscopy sample.
PCR in gastrointestinal panels.
Duodenal aspirate or biopsy, or the string test, in difficult cases.

Treatment

Nitroimidazoles (metronidazole, tinidazole) or nitazoxanide; alternatives include albendazole or paromomycin (the latter in pregnancy, in selected cases). If symptoms persist, reinfection, lactose intolerance or treatment failure are considered before retreating.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

Boiled, filtered or treated water; usual chlorine levels do not always kill it.
Handwashing, especially in daycare and after changing diapers.
Washing raw fruit and vegetables.
Avoiding swallowing water from rivers, lakes or pools.

Vaccine: There is no human vaccine.

Test yourself

Think of the answer, then tap the question to see it.

Which structure lets Giardia attach to the duodenal mucosa?

The ventral (sucking) disk, without invading the intestinal wall.

Which immunodeficiency predisposes to chronic giardiasis?

IgA deficiency and other hypogammaglobulinemias, such as common variable immunodeficiency.

What kind of diarrhea does Giardia cause?

Non-bloody watery or greasy diarrhea (steatorrhea), with bloating and flatulence, from malabsorption.

Why might diarrhea continue after giardiasis is cured?

Often because of secondary lactose intolerance from brush-border damage, which takes weeks to recover.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Giardiasis (CDC, 2024); Patient Care for Giardia Infection | Giardia | CDC (CDC, 2024); Giardia infection: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024).

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Representative model, for educational purposes only.