Epidemiology & Risk Factors

Etiology: Giardia duodenalis (G. lamblia), a flagellated protozoan.
Transmission: Fecal-oral ingestion of acid-resistant cysts; extremely low infectious dose.
Risk groups:
- Hikers/campers drinking unfiltered wilderness water (streams, lakes).
- Daycare attendees and staff.
- MSM (via oral-anal contact).
- Pts w/ IgA deficiency, CVID, or X-linked agammaglobulinemia (secretory IgA is crucial for clearing Giardia).
Pathophysiology
- Specifically deactivates lactase, leading to transient/secondary lactose intolerance.
- Inhibits trypsin and lipase, causing fat malabsorption (steatorrhea) and fat-soluble vitamin deficiencies. c

Clinical Features

Incubation period: 1-2 weeks.
Acute/Chronic presentation:
- Foul-smelling, greasy, watery diarrhea (steatorrhea). c
- Significant abdominal bloating, flatulence, and crampy pain.
- Weight loss, anorexia, and fatigue.
No blood or mucus in stool (non-invasive organism that adheres to the intestinal brush border via an adhesive disk).

Initial & Preferred: Stool antigen testing (ELISA/DFA) or stool PCR (highly sensitive & specific, >95%).
Alternative: Stool microscopy for O&P.
- Requires 3 separate specimens collected on different days due to intermittent shedding.
- Identifies oval cysts (dormant) or pear-shaped, binucleated, flagellated trophozoites (active).
Gold Standard (rarely indicated): Duodenal aspiration or biopsy (reveals trophozoites adhering to enterocytes without tissue invasion).

Cryptosporidiosis: Diff by modified acid-fast stain (positive oocysts), association w/ municipal water/pools, and severe course in HIV/AIDS pts.
Celiac Disease: Diff by chronic presentation without acute exposure, presence of extraintestinal features, (+) anti-tTG IgA, and villous atrophy on duodenal biopsy.
Lactose Intolerance: Diff by symptoms localized strictly to dairy ingestion and negative infectious workup. (Note: Can co-exist as a post-infectious complication).
Amebiasis: Diff by bloody diarrhea (dysentery), systemic symptoms, colonoscopy showing flask-shaped ulcers, and trophozoites with swallowed RBCs (erythrophagocytosis).

First-line:
- Tinidazole (preferred, single-dose therapy).
- Nitazoxanide (liquid formulation, preferred in pediatric pts).
- Metronidazole (standard alternative, requires 5-7 day course; avoid alcohol due to disulfiram-like reaction).
Pregnancy: Paromomycin (non-absorbable luminal aminoglycoside; preferred in 1st trimester to avoid systemic teratogenicity).
Supportive: Hydration/IVF; temporary lactose-free diet (avoid dairy for 1 month post-treatment).

Secondary lactose intolerance (temporary, due to damage to microvilli brush border lactase enzymes).
Malabsorption (fat, lactose, Vit A/B12).
Growth failure/failure to thrive in children.
Reactive arthritis (rare).

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aliases