Giardia lamblia
Binucleate flagellated protozoan and one of the earliest-diverging eukaryotic lineages. Trophozoites colonise the duodenum and jejunum and attach exclusively via the ventral adhesive disc — a rigid cytoskeletal organelle generating hydrodynamic suction via flagellar beating. Infection is strictly extracellular: tight junctions are disrupted by cysteine protease cleavage of ZO-1 (via PAR-2/PKC/claudin-2), brush border disaccharidases are displaced, and bile salts are deconjugated — producing a combined osmotic and secretory diarrhoea with steatorrhoea. VSP antigenic switching (epigenetically regulated, ~190 variants) continuously subverts mucosal IgA. In hypogammaglobulinaemia (CVID), absence of IgA allows chronic, treatment-refractory infection. Single-dose tinidazole (2 g) achieves 90–95% cure.
Classification & Structure
| Kingdom / Supergroup | Eukaryota; Excavata; Metamonada; Fornicata; Diplomonadida — one of the earliest-diverging eukaryotic lineages; sister group to Retortamonadida |
| Trophozoite | 9–21 µm × 5–15 µm, pear/tear-drop; 2 nuclei (symmetrically paired); 4 pairs of flagella (anterior, posterior-lateral, caudal, ventral); 1 ventral adhesive disc; 2 L-shaped median bodies (tubulin-based, function uncertain) |
| Cyst | 8–12 µm oval; 4 nuclei; β-1,3-GalNAc polymer wall (CWP1/2/3); extremely hardy — viable months in cold water; resistant to standard chlorination; killed by boiling or UV |
| Ventral adhesive disc | Rigid concave cytoskeletal plate; spiral coil of α/β/γ-tubulin with contractile microribbons (giardins α, β, δ, ε, ζ — unique to Giardia); hydrodynamic low-pressure suction created by flagellar current; mannose-binding lectins supplement mechanical adhesion |
| VSP coat | ~190 Variant Surface Protein genes; only one expressed per trophozoite at a time; highly cysteine-rich; C-terminal CRGKA transmembrane anchor; forms dense protease-resistant coat; switched by IgA-driven epigenetic derepression (H3K4me3 / RNAi mechanism) |
| Mitosomes | Rudimentary mitochondria-derived organelles retaining only Fe-S cluster assembly pathway; no oxidative phosphorylation; no conventional Golgi or peroxisomes — reflects deep evolutionary divergence and anaerobic lifestyle |
| Fermentation end-products | Ethanol and acetate; cannot synthesise long-chain fatty acids de novo; relies on host bile for lipid acquisition |
Infection Mechanism & Pathogenesis
1 · Transmission and excystation
Ingestion of as few as 10 cysts from contaminated water or food (the lowest infectious dose of any intestinal protozoan). Gastric acid (pH 2) triggers excystation; pancreatic trypsin in the duodenum completes the process — two trophozoites emerge per cyst within minutes. Cysts resist standard chlorination (require UV or filtration); person-to-person transmission in childcare centres and institutions; zoonotic transmission from beavers and dogs (assemblage A strains). VSPs are expressed immediately on emerging trophozoites.
2 · Ventral disc adhesion
Trophozoites colonise the duodenum and proximal jejunum — where bile (paradoxically) stimulates their growth. The ventral disc generates hydrodynamic suction via caudal flagellar current creating a low-pressure zone beneath the disc; α/β-giardin contractile microribbons adjust disc curvature to maximise contact with enterocyte microvilli. Mannose-binding lectins on the parasite surface provide additional adhesion to the enterocyte glycocalyx. Giardia is strictly extracellular and never invades the lamina propria.
3 · Tight junction disruption and barrier failure
Secreted cysteine proteases (CP2, CP14, CP49) cleave ZO-1 (zonula occludens-1) from tight junctions; trophozoite signalling via PAR-2 (protease-activated receptor 2) activates PKC, upregulating claudin-2 (paracellular cation channels). Claudin-1 and occludin are redistributed away from junctions. Simultaneously, disc adhesion physically strips microvilli from the brush border, reducing mucosal surface area. The result is a "leaky gut" phenotype with paracellular leak of luminal antigens and water.
4 · Malabsorption mechanisms
Brush border disaccharidases (lactase, sucrase, maltase) are displaced from the epithelial membrane → carbohydrate malabsorption → osmotic diarrhoea. Giardia deconjugates bile salts via its own hydrolases → deconjugated bile salts are ineffective emulsifiers → fat malabsorption → steatorrhoea. Cysteine protease activation of PAR-2 on enterocytes → increased Cl⁻ secretion via CFTR → secretory diarrhoea. The dual (osmotic + secretory) mechanism explains the classic foul-smelling, greasy, non-bloody stools of giardiasis.
5 · VSP antigenic variation and immune escape
One of ~190 VSP genes is expressed per trophozoite; silent VSPs are epigenetically suppressed (H3K4me3 marks the active locus; RNAi suppresses inactive loci). When mucosal IgA targeting the surface VSP accumulates, parasites switch expression to an alternative VSP — escaping antibody-mediated killing. This cycle allows chronic infection to persist despite IgA responses. In hypogammaglobulinaemia (CVID, agammaglobulinaemia), absence of secretory IgA leads to severe, treatment-refractory chronic giardiasis that requires repeated or long-term courses of antiparasitic therapy.
Host Immune Response
Disease Spectrum
| Presentation | Proportion | Key Features |
|---|---|---|
| Asymptomatic carriage | ~50–60% | Cyst excretion without illness; common in endemic areas from partial acquired immunity; major reservoir for transmission |
| Acute giardiasis | ~40% | 1–3-week incubation; watery, foul-smelling, greasy, non-bloody diarrhoea; bloating, flatulence, abdominal cramps, nausea, weight loss; fever absent or low-grade; self-limited in 2–6 weeks in immunocompetent hosts |
| Chronic giardiasis | ~5–10% | Intermittent diarrhoea, steatorrhoea, malabsorption syndrome; lactose intolerance (may persist post-clearance); children: growth stunting, cognitive impairment; persists months to years without treatment |
| CVID / hypogammaglobulinaemia | Near universal in untreated | Chronic severe infection; profound malabsorption; repeated treatment courses required; can be life-threatening from nutritional failure |
| Post-infectious IBS | ~10–15% of acute cases | Persistent GI symptoms after eradication; altered gut microbiome and epithelial function; lactase deficiency often persists >6 months post-infection |
Treatment & Prophylaxis
Tinidazole — First-line (single dose)
2 g orally, single dose; 90–95% cure rate; nitroimidazole class; fewer GI side effects than metronidazole; preferred for compliance simplicity. Avoid in first trimester of pregnancy.
Metronidazole — Effective alternative
250 mg TID × 5–7 days orally; metallic taste and nausea common; alcohol interaction (disulfiram-like); avoid in first trimester. Widely available globally; cure rate ~85–90%.
Nitazoxanide — Broad-spectrum alternative
500 mg BID × 3 days; also active against Cryptosporidium; paediatric liquid formulation (100 mg/5 mL) available; useful for mixed infections and when metronidazole-resistant strains suspected.
Paromomycin — Pregnancy-safe
500 mg TID × 5–10 days; poorly absorbed aminoglycoside; preferred in pregnancy (minimal systemic fetal exposure); efficacy ~60–70%, lower than systemic agents; suitable when other drugs contraindicated.
Prevention
Water filtration or UV treatment (chlorination alone is insufficient — cysts are chlorine-resistant); hand hygiene in childcare and institutional settings; no licensed vaccine available as of 2026.
Cross-Atlas Connections
References
- Ankarklev J, Jerlström-Hultqvist J, Ringqvist E, Troell K, Svärd SG. Behind the smile: cell biology and disease mechanisms of Giardia species. Nat Rev Microbiol. 2010;8(6):413-22. doi:10.1038/nrmicro2317 · PubMed 20400969
- Buret AG. Pathophysiology of enteric infections with Giardia duodenalis. Parasite. 2008;15(3):261-5. doi:10.1051/parasite/2008153261 · PubMed 18814694
- Bennett JE, Dolin R, Blaser MJ. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Elsevier; 2020.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
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This entry covers Giardia lamblia biology, pathogenesis, and treatment. Post-infectious IBS mechanisms, proteomics of the ventral disc, and VSP regulation are planned expansions. Every entry follows the same schema: structured frontmatter, peer-reviewed citations, and cross-atlas links.