PARASITE · Flagellated protozoan
Giardia lamblia (G. duodenalis / intestinalis)
Giardia lamblia — Trophozoite microscopy
Stain: Light microscopy (trichrome/Giemsa) of stoolMorphology: Pear-shaped trophozoite with two symmetric nuclei resembling an 'owl's face' / 'old man face,' 4 pairs of flagella, and ventral sucking discYield: HIGHDifficulty: EASY
Image: Wikimedia Commons · The Other 95% (CC BY-SA 4.0)
Key facts
**Pathogenesis**: Fecal-oral cysts ingested → excyst in duodenum → trophozoites adhere to (but don't invade) small bowel villi via ventral sucking disc → villous flattening → fat malabsorption. **Diagnostic clue**: Foul-smelling, fatty (steatorrhea), non-bloody diarrhea in hikers/campers/daycares; stool antigen (ELISA) or PCR most sensitive; trophozoites or cysts on stool O&P (low yield — string test if needed). Particularly severe in IgA deficiency. **Virulence**: Ventral sucking disc, variant-specific surface protein antigenic variation.
Boards buzzwords
- owl-face trophozoite
- fatty foul-smelling diarrhea
- campers/hikers
- beavers
- IgA deficiency
- duodenal aspirate
Associated diseases
- Giardiasis (chronic / relapsing fatty diarrhea)
- Malabsorption / failure to thrive in children
- Severe disease in IgA deficiency / CVID
Treatment
Tinidazole (single dose) OR metronidazole × 5–7 days; nitazoxanide for pediatrics; albendazole alternative
