Infectious Disease · PANCE / PANRE

Giardiasis

Protozoan small bowel infection (Giardia duodenalis) causing prolonged greasy diarrhea, bloating, and malabsorption after exposure to contaminated water.

Also known as: giardiasis, Giardia lamblia, Giardia intestinalis, Giardia duodenalis, beaver fever

Overview

Diarrheal illness caused by Giardia duodenalis (also called G. lamblia or G. intestinalis), a flagellated binucleate protozoan that colonizes the proximal small intestine without invading tissue. Most commonly reported intestinal parasite in the US.

Epidemiology

Roughly 1.2 million US cases annually (CDC estimates). Peaks in summer/fall. Outbreaks linked to recreational water exposure (lakes, pools, hot tubs) and untreated drinking water; daycare and MSM populations also at risk.

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Risk factors

Drinking untreated surface water (camping, backpacking, hiking)
International travel to endemic regions
Daycare attendance, household contact with infected child
Men who have sex with men (oral-anal contact)
Immunocompromise (HIV, CVID, IgA deficiency) — chronic infection
Inadequate water sanitation

Pathophysiology

Cysts are ingested via contaminated water/food and excyst in the duodenum to release trophozoites. Trophozoites attach to small bowel mucosa via a ventral adhesive disc, causing villous blunting, brush border enzyme reduction, and malabsorption. Some trophozoites encyst and pass in stool as the infectious form. No invasive disease.

Clinical presentation

Symptoms

Incubation 1-2 weeks
Foul-smelling, greasy, malodorous diarrhea (steatorrhea)
Bloating, abdominal cramping, flatulence
Weight loss, malaise, anorexia
Symptoms may persist for weeks; some develop chronic infection with continued symptoms over months

Signs / physical exam

No fever or blood in stool typically
Diffuse abdominal tenderness without peritoneal signs
Weight loss; growth failure in chronic pediatric infection
Lactose intolerance may persist for weeks after clearing infection

Classic findings

Camper or backpacker returning with 2 weeks of foul-smelling, greasy, non-bloody diarrhea, bloating, and weight loss — classic giardiasis.

Differential diagnosis

Cryptosporidiosis — Watery diarrhea, similar exposures; acid-fast oocysts in stool
Bacterial gastroenteritis (Campylobacter, Salmonella, Shigella) — Acute onset, often bloody diarrhea, fever; stool culture
Viral gastroenteritis (norovirus) — Shorter duration <72 h, vomiting prominent
Tropical sprue — Steatorrhea, weight loss in tropics; small bowel biopsy
Celiac disease — Chronic malabsorption, IgA-tTG positive; biopsy with villous atrophy
Lactose intolerance — Bloating after dairy; hydrogen breath test
IBS — Chronic intermittent symptoms without infectious exposure
Cyclospora — Imported berries/produce; acid-fast oocysts; TMP-SMX responsive

Diagnostic workup

Diagnostic criteria

Detection of Giardia antigen, DNA, or cysts/trophozoites in stool from a patient with consistent illness.

Labs

Stool antigen (EIA or rapid immunoassay) — preferred test, sensitivity 85-100%
Stool nucleic acid amplification (PCR) — increasingly available, multiplex GI panels
Ova and parasite exam — multiple specimens required (3 over 3-5 days) due to intermittent shedding
String test (Entero-Test) or duodenal aspirate — historical, rarely needed
Consider testing for other parasites in returning travelers

Imaging

Not required; imaging may be obtained for atypical presentation
Duodenal biopsy shows trophozoites adherent to villi and villous blunting (only if endoscopy performed for other indication)

Diagnostic algorithm

Drug	Adult Dose	Pediatric Dose	Comments
Tinidazole	2 g PO x 1	50 mg/kg (max 2 g) x 1	Preferred; single dose
Metronidazole	250-500 mg TID x 5-7 d	15 mg/kg/day TID x 5-7 d	Most familiar; disulfiram-like reaction with alcohol
Nitazoxanide	500 mg BID x 3 d	Age-based dosing x 3 d	FDA-approved peds suspension
Paromomycin	500 mg TID x 5-10 d	25-35 mg/kg/day TID	Pregnancy alternative; not absorbed

Antimicrobial options for giardiasis (CDC/IDSA-aligned).

Treatment

First-line

Tinidazole 2 g PO × 1 dose — preferred (single-dose, excellent cure rate)
Metronidazole 250-500 mg PO TID × 5-7 days — widely available alternative
Nitazoxanide 500 mg PO BID × 3 days — also effective; FDA-approved for giardia in adults and children (suspension for kids)

Second-line / adjunct

Paromomycin — preferred in pregnancy (poorly absorbed; less data on efficacy but safer)
Albendazole or quinacrine — alternatives for resistant infections
Combination therapy (tinidazole + albendazole) for refractory cases

Complications

Persistent lactose intolerance after clearance (treatment-resistant lactase deficiency)
Chronic malabsorption with weight loss, anemia, vitamin deficiencies (B12, fat-soluble)
Growth failure in children
Reactive arthritis
Post-infectious IBS

PANCE pearls

Suspect giardia in a returning hiker/camper with greasy, foul-smelling diarrhea and bloating without fever.
Single-dose tinidazole has better adherence and cure rates than 7-day metronidazole — first-line where available.
Daycare outbreaks: treat symptomatic children; asymptomatic carriers usually do not require treatment.
Recurrent or treatment-refractory giardia → suspect immunoglobulin deficiency (CVID, IgA deficiency).
Lactose intolerance can persist for weeks after Giardia eradication — counsel patients.

References

CDC — Giardia: Diagnosis and Treatment (Parasites)
Red Book — AAP Red Book — Giardiasis chapter
Cochrane 2010 — Granados et al., Drugs for treating giardiasis

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