Giardiasis

The Pathogen

Giardia duodenalis (synonyms: G. lamblia, G. intestinalis) is a flagellated protozoan parasite of the small intestine. It is the most common parasitic infection of the gastrointestinal tract worldwide and in the United States, affecting upward of 300 million people annually. - Goldman-Cecil Medicine, p. 3437

Two morphological forms exist:

Trophozoite - the active, disease-causing form; pear-shaped, 9-21 µm long, 5-15 µm wide, with 4 pairs of flagella, two characteristic nuclei, and a ventral sucking disc used to adhere to intestinal epithelium. The trophozoite lacks mitochondria and peroxisomes.
Cyst - the environmentally resistant, infectious form; oval, 8-12 µm long, excreted in feces.

Scanning electron micrograph of a Giardia trophozoite (Goldman-Cecil Medicine):

Giardia trophozoite - scanning electron micrograph showing pear shape, flagella, and ventral disc

Light microscopy of Giardia trophozoites (Harrison's Principles):

Flagellated, binucleate Giardia trophozoites under light microscopy

Stained Giardia cysts in stool (Goldman-Cecil Medicine):

There are eight genotypic assemblages; assemblages A and B infect humans (and are likely two separate species). Other assemblages infect animals. - Goldman-Cecil Medicine, p. 3438

Life Cycle

Ingestion of cysts from contaminated water, food, or via fecal-oral contact (as few as 10-100 cysts can establish infection).
Excystation in the small intestine - each cyst releases two trophozoites.
Trophozoites multiply by binary fission (doubling every 6 hours in vitro) and attach to mucosal epithelium via the ventral disc. They remain strictly intraluminal - no hematogenous spread.
As trophozoites encounter altered conditions in the lower small bowel, they encyst - cysts are excreted in feces.
Trophozoites predominate in watery/loose stools; cysts are the main form in formed stools and are responsible for transmission.
Cysts survive for months in cold fresh water, are resistant to routine chlorination, but are killed by boiling or filtration.

Harrison's Principles of Internal Medicine 22E, p. 1848

Epidemiology

Setting	Prevalence / Key Features
Developed countries	~0.4-6 cases/100,000/year; epidemic via waterborne or foodborne outbreaks
Developing countries	Up to 20-40%; endemic, mainly person-to-person; children infected by age 2-3
United States	~20,000 reported cases/year; actual may exceed 1 million; peaks in summer/fall
High-risk groups	Young children in daycare, travelers (esp. South Asia), backpackers, men who have sex with men, immunocompromised patients

Giardia is a nationally reportable disease in the United States. Before modern water treatment, municipal reservoir contamination caused large outbreaks; today, recreational water (pools, lakes) is a more common outbreak source. - Goldman-Cecil Medicine, p. 3437; Tietz Textbook of Laboratory Medicine, p. 3565

Unique feature: Giardia is the only bowel-dwelling parasite that undergoes antigenic variation - only one of ~250 variant-specific surface proteins (VSPs) is expressed at a time, enabling immune evasion. - Goldman-Cecil Medicine, p. 3437

Pathophysiology

The mechanisms behind symptomatic disease are not fully understood. Giardia is non-invasive but causes disease via:

Physical coating of the intestinal epithelial surface, interfering with brush-border disaccharidase activity → carbohydrate malabsorption and lactose intolerance
Cysteine proteases (cathepsin B-like) that disrupt epithelial tight junctions, cause enterocyte apoptosis, break down mucus, and degrade host immune factors
Epithelial barrier dysfunction → increased permeability
Associated small intestinal bacterial overgrowth (SIBO) with bile salt deconjugation → steatorrhea
No classic secretory enterotoxin is produced

In chronic symptomatic cases, small bowel biopsy shows flattened villi, elongated crypts, and mononuclear inflammatory infiltrate in the lamina propria - resembling tropical sprue or celiac disease. In most acute infections, bowel morphology is unaltered. - Harrison's, p. 1848; Sleisenger & Fordtran's GI and Liver Disease

Host immunity: Both cellular and humoral responses develop. Patients with hypogammaglobulinemia suffer prolonged, severe, treatment-resistant infections - indicating humoral immunity is important. Common variable immunodeficiency (CVID) is the key immunodeficiency associated with severe giardiasis (selective IgA deficiency does NOT increase severity). - Goldman-Cecil Medicine, p. 3438

Clinical Manifestations

The spectrum ranges from asymptomatic carriage to severe chronic malabsorption:

Acute giardiasis:

Incubation: 1-3 weeks after ingestion
Watery, foul-smelling, greasy/fatty diarrhea (no blood, no mucus - non-dysenteric)
Abdominal cramping, bloating, excessive flatulence
Nausea, anorexia
Weight loss
Patients are generally less ill than those with bacterial diarrhea (no high fever, no significant systemic toxicity)
Self-limited: typically 7-10 days in immunocompetent hosts

Chronic giardiasis:

Intermittent or persistent diarrhea lasting weeks to months
Malabsorption syndrome (similar to celiac sprue) with steatorrhea, fat-soluble vitamin deficiencies
Significant weight loss
Failure to thrive in children
Post-infectious lactose intolerance may persist after eradication (due to delayed regeneration of brush-border enzymes)

Asymptomatic carriage is common, especially in endemic regions where children are repeatedly exposed. Diarrhea is associated with trophozoites in stool; the presence of cysts alone should be interpreted with caution. - Sleisenger & Fordtran's, p. 2160; Tietz Laboratory Medicine, p. 3566

Diagnosis

Method	Notes
Stool ova & parasites (O&P) microscopy	Identifies cysts and/or trophozoites; at least 3 specimens on separate days recommended (intermittent shedding)
Direct fluorescence assay (DFA)	Sensitive and specific; uses fluorescent-labeled antibodies
Stool antigen immunoassay (ELISA or DFA)	Sensitive and specific; detects Giardia-specific antigens; widely used clinically
Fecal NAATs (nucleic acid amplification)	Increasingly standard in the US; multiplex PCR panels now standard for enteric pathogens
Duodenal/jejunal biopsy	Does NOT increase diagnostic yield over stool examination; shows trophozoites in mucus layer

The diagnostic approach per Harrison's is summarized in the table below:

Test	Giardia
Stool O-P	+
Fecal acid-fast stain	-
Fecal antigen immunoassay	+
Fecal NAATs	+
Other	DFA

Harrison's Principles of Internal Medicine 22E, p. 1848

Treatment

Drug of choice:

Drug	Dose	Duration	Notes
Tinidazole	2 g orally once	Single dose	Preferred; may be more effective than metronidazole
Metronidazole	250-500 mg TID	5-7 days	Cure rates >90%; widely used
Nitazoxanide	500 mg BID	3 days	Alternative; also active against Cryptosporidium
Paromomycin	500 mg TID	5-10 days	Preferred in pregnancy (not absorbed); less reliable
Albendazole / Mebendazole	Varies	Varies	Alternative; lower efficacy than tinidazole
Quinacrine, Ornidazole, Secnidazole, Furazolidone	Varies	Varies	Other alternatives

Refractory/Recurrent disease:

Document ongoing infection with repeat stool testing before retreating
Investigate for reinfection from family members or environmental source
Evaluate for hypogammaglobulinemia
Prolonged metronidazole (750 mg TID × 21 days) or combination therapy for multiple treatment failures

Post-treatment note: Some patients experience delayed symptom resolution after eradication due to slow regeneration of intestinal brush-border enzymes (particularly lactase). - Harrison's Principles of Internal Medicine 22E, p. 1849; Sleisenger & Fordtran's, p. 2160

Prevention

Boiling or filtration of potentially contaminated water (chlorination alone is insufficient)
Personal hygiene, hand washing - especially in daycare settings
Avoid swallowing recreational water
Travelers to endemic regions should drink only treated or bottled water

Key Summary Points

Feature	Detail
Organism	Giardia duodenalis (lamblia/intestinalis)
Type	Flagellated protozoan; assemblages A and B in humans
Infectious form	Cyst (10-100 cysts sufficient)
Disease form	Trophozoite
Location	Proximal small intestine only; never invasive
Classic stool	Foul-smelling, greasy, fatty; no blood
Unique features	No mitochondria; antigenic variation (VSPs); ventral sucking disc
DOC	Tinidazole (single dose) or Metronidazole
Pregnancy	Paromomycin (non-absorbed)
Immunodeficiency risk	CVID / hypogammaglobulinemia (NOT selective IgA deficiency)

Sources: Harrison's Principles of Internal Medicine 22E (2025), Goldman-Cecil Medicine, Sleisenger & Fordtran's Gastrointestinal and Liver Disease, Tietz Textbook of Laboratory Medicine 7th Ed., Goodman & Gilman's Pharmacological Basis of Therapeutics.

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