Giardia lamblia

**Organism:** Flagellated protozoan with two life stages. The cyst is the infectious, environmentally hardy form. It resists chlorination, so filtration or boiling is needed. The trophozoite is pear-shaped ("tennis racquet" or "face-like") with 2 nuclei (the "eyes"), 4 pairs of flagella, and a ventral sucking disk. It is binucleate and lacks mitochondria (it has mitosomes). It is an anaerobic, microaerophilic organism.

**Transmission:** Fecal-oral. Sources include contaminated water (streams, lakes, wells; "beaver fever" in campers/hikers), daycare centers, men who have sex with men (MSM), international travelers, and people with poor sanitation. Infection requires few cysts (about 10–100). Cysts are ingested, excyst in the duodenum, and trophozoites attach to the proximal small bowel (duodenum/jejunum) without invading the mucosa.

**Pathophysiology:** Trophozoites attach via the ventral disk. They cause microvillus blunting and brush-border damage, which impairs disaccharidase activity (secondary lactose intolerance) and fat absorption. The result is malabsorptive/osmotic diarrhea. There is no tissue invasion, so there is no blood, no fecal leukocytes, and little or no fever. Host defense depends on secretory IgA, so IgA deficiency and common variable immunodeficiency (CVID) predispose to chronic or severe giardiasis. Hypogammaglobulinemia and achlorhydria (e.g., PPI use) are also risk factors.

**Clinical features:** The incubation period is 1–2 weeks. Symptoms include foul-smelling, greasy, fatty (steatorrhea), non-bloody watery diarrhea, flatulence, bloating, cramping, and nausea. It can be self-limited or chronic, with weight loss, failure to thrive, and fat-soluble vitamin and B12 deficiency. Lactose intolerance can persist after clearance. Many infections are asymptomatic carriage.

**Diagnosis:** Stool antigen (ELISA/DFA) or PCR is the most sensitive and specific test. Stool ova and parasites (O&P) shows cysts or trophozoites, but several samples may be needed because shedding is intermittent. The string test or duodenal aspirate/biopsy is rarely needed. A biopsy shows trophozoites on the mucosal surface and blunted villi.

**Treatment:** Metronidazole is the classic answer for boards. Alternatives are tinidazole (single dose), nitazoxanide, and (in pregnancy) paromomycin. Prevention includes water filtration or boiling and hand hygiene.

**High-yield comparisons:** - *Entamoeba histolytica*: bloody diarrhea (dysentery), flask-shaped ulcers, liver abscess ("anchovy paste"), trophozoites with engulfed RBCs. Also treated with metronidazole, then a luminal agent (paromomycin). - *Cryptosporidium*: watery diarrhea in HIV patients, acid-fast oocysts, treated with nitazoxanide. - *Tropical sprue / celiac disease*: other causes of malabsorption with villous blunting. - Mnemonic: Giardia = "Giant (greasy) stools, hikers/campers, Gastro-IgA deficiency, Goes to metronidazole." Ingested cyst, no invasion, "fat" malabsorption.