Overview
Giardiasis
1. Clinical Overview
Summary
Giardiasis is an intestinal infection caused by the flagellated protozoan parasite Giardia duodenalis (also known as G. lamblia or G. intestinalis). It is one of the most common parasitic infections worldwide, particularly associated with contaminated water sources. Transmission is faecal-oral through ingestion of cysts in contaminated water, food, or person-to-person contact. The classic presentation is chronic diarrhoea with bloating, flatulence ("eggy burps"), steatorrhoea, and weight loss. Diagnosis is by stool antigen testing or microscopy identifying cysts or trophozoites. Treatment with metronidazole or tinidazole is highly effective. Post-infectious lactose intolerance is common.
Key Facts
- Organism: Giardia duodenalis (flagellated protozoan parasite)
- Transmission: Faecal-oral route; contaminated water (hikers, campers, travellers)
- Incubation: 1-2 weeks
- Classic Symptoms: Chronic diarrhoea, bloating, "eggy burps" (sulphurous), steatorrhoea
- Diagnosis: Stool antigen test (EIA) or microscopy (cysts)
- Treatment: Metronidazole 400mg TDS for 5 days OR Tinidazole 2g single dose
Clinical Pearls
"Eggy Burps = Think Giardia": The sulphurous, rotten egg smell of belches is nearly pathognomonic in a returning traveller with chronic diarrhoea.
"Cysts Survive Chlorination": Standard water chlorination does NOT kill Giardia cysts. Boiling or filtration is required.
"Lactose Intolerance Lingers": Post-infectious lactose intolerance can persist for weeks after successful treatment. Warn patients.
"Low Inoculum, High Infectivity": Only 10-25 cysts can cause infection. Outbreaks are common from contaminated water supplies.
2. Epidemiology
Global Burden
- 280 million symptomatic infections per year worldwide
- Most common intestinal parasite in developed countries
- Endemic worldwide; higher in developing regions
Demographics
- All ages; children and immunocompromised at higher risk
- Travellers to endemic areas
- Hikers, campers (contaminated water)
- MSM (sexual transmission)
- Childcare settings
Risk Factors
| Factor | Mechanism |
|---|---|
| Contaminated water | Swimming pools, lakes, streams, wells |
| Travel to endemic areas | Developing world, backpacking |
| Childcare/nursery | Faecal-oral spread |
| Sexual contact (MSM) | Oral-anal contact |
| Immunodeficiency | IgA deficiency, hypogammaglobulinaemia |
| Malnutrition | Increased susceptibility |
Outbreaks
- Waterborne outbreaks common
- Classically affects hikers drinking from streams ("beaver fever")
3. Pathophysiology
Life Cycle
┌──────────────────────────────────────────────────────────┐
│ GIARDIA LIFE CYCLE │
├──────────────────────────────────────────────────────────┤
│ │
│ 1. CYST INGESTION │
│ - Environmentally resistant form │
│ - Survives stomach acid │
│ - Only 10-25 cysts needed for infection │
│ │
│ 2. EXCYSTATION (Small intestine) │
│ - Each cyst releases 2 trophozoites │
│ │
│ 3. TROPHOZOITE │
│ - Active, motile form │
│ - Pear-shaped with 2 nuclei ("owl face") │
│ - 4 pairs of flagella │
│ - Adheres to duodenal/jejunal epithelium via disk │
│ - Multiplies by binary fission │
│ │
│ 4. ENCYSTATION (Colon) │
│ - Trophozoites form cysts │
│ - Cysts excreted in stool │
│ - Survive weeks in environment │
│ │
└──────────────────────────────────────────────────────────┘
Pathogenic Mechanisms
- Adherence to intestinal epithelium (ventral disk)
- Epithelial damage and villous atrophy
- Disruption of tight junctions → Increased permeability
- Malabsorption (fat, vitamins, micronutrients)
- Secondary lactase deficiency
Why Chronic Diarrhoea and Malabsorption?
- Villous atrophy reduces absorptive surface
- Brush border enzyme damage (lactase, lipase)
- Fat malabsorption → Steatorrhoea
- Osmotic diarrhoea from unabsorbed nutrients
4. Clinical Presentation
Spectrum of Disease
Symptoms
| Feature | Description |
|---|---|
| Diarrhoea | Watery initially, then fatty/greasy (steatorrhoea) |
| Bloating | Often marked |
| Flatulence | Excessive, foul-smelling |
| "Eggy burps" | Sulphurous eructations (characteristic) |
| Abdominal cramps | Upper abdominal |
| Nausea | Sometimes vomiting |
| Weight loss | If chronic |
| Fatigue | Common with malabsorption |
Classic Pattern
Differential Diagnosis
| Condition | Distinguishing Features |
|---|---|
| Traveller's diarrhoea (bacterial) | Acute, self-limiting; fever |
| IBS | Chronic; no steatorrhoea; no travel history |
| Coeliac disease | Chronic malabsorption; serology positive |
| Small bacterial overgrowth | Bloating, diarrhoea; different demographics |
50% asymptomatic carriers
Common presentation.
50% symptomatic (acute or chronic)
Common presentation.
5. Clinical Examination
General
- Usually well-appearing
- May have weight loss if chronic
- Signs of dehydration if acute
Abdominal Examination
- Mild epigastric/periumbilical tenderness
- Distension (bloating)
- Active, increased bowel sounds
Signs of Malabsorption (Chronic)
- Weight loss
- Pallor (anaemia)
- Steatorrhoea (pale, greasy, floating stools)
6. Investigations
First-Line Tests
| Test | Notes |
|---|---|
| Stool antigen (EIA) | Most sensitive (>5%); First-line |
| Stool PCR | High sensitivity; increasingly available |
| Stool microscopy | Cysts or trophozoites; less sensitive (50-70%); may need 3 samples |
Microscopy Features
- Cyst: Oval, 8-14μm, 4 nuclei
- Trophozoite: Pear-shaped, 10-20μm, 2 nuclei ("owl face"), 4 pairs of flagella
Additional Investigations
- Stool culture (to exclude bacterial causes)
- Duodenal aspirate/biopsy (rarely needed; if diagnosis unclear)
When to Investigate Further
- Failure to respond to treatment
- Immunocompromised patient
- Recurrent infection
7. Management
First-Line Treatment
┌──────────────────────────────────────────────────────────┐
│ GIARDIASIS TREATMENT │
├──────────────────────────────────────────────────────────┤
│ │
│ FIRST-LINE: │
│ • Metronidazole 400mg TDS for 5 days (or 2g OD 3 days) │
│ • OR Tinidazole 2g single dose (preferred convenience) │
│ │
│ ALTERNATIVE (if failure or contraindication): │
│ • Nitazoxanide 500mg BD for 3 days │
│ • Albendazole 400mg OD for 5 days │
│ │
│ PREGNANCY: │
│ • Paromomycin 25-35mg/kg/day in 3 doses for 5-10 days │
│ • (Metronidazole can be used if necessary, 2nd/3rd tri) │
│ │
│ REFRACTORY CASES: │
│ • Combination therapy (metronidazole + quinacrine) │
│ • Investigate for immunodeficiency │
│ │
└──────────────────────────────────────────────────────────┘
Supportive Care
- Oral rehydration
- Lactose-free diet (temporarily, due to secondary lactase deficiency)
- Nutrition support if malabsorption
Infection Control
- Hand hygiene
- Avoid contaminated water
- Treatment of symptomatic household contacts (consider)
8. Complications
Of Giardiasis
- Chronic malabsorption
- Weight loss and failure to thrive (children)
- Iron, vitamin B12, fat-soluble vitamin deficiency
- Post-infectious IBS
- Secondary lactose intolerance (common, may persist weeks)
- Growth retardation (children in endemic areas)
Of Treatment
- Metronidazole: Metallic taste, nausea, Antabuse effect (avoid alcohol)
- Tinidazole: Similar (generally better tolerated)
9. Prognosis & Outcomes
With Treatment
- Cure rate: 85-95% with first-line therapy
- Relapse: 5-15% (may need repeat or alternative treatment)
Without Treatment
- Can persist for months
- Chronic malabsorption and weight loss
- Eventually may self-resolve in immunocompetent
Post-Infectious Lactose Intolerance
- Common (40-70%)
- Usually resolves within weeks
- Advise temporary avoidance of dairy
10. Evidence & Guidelines
Key Guidelines
- CDC: Parasitic Diseases - Giardia
- WHO: Guidelines on Drinking-water Quality
- Public Health England: Giardiasis Guidance
Key Evidence
Treatment Efficacy
- Cochrane Review: Metronidazole and tinidazole equally effective
- Tinidazole has better tolerability and compliance (single dose)
Prevention
- Boiling water kills cysts
- Chlorination alone is often insufficient
11. Patient/Layperson Explanation
What is Giardiasis?
Giardiasis is an infection of the gut caused by a tiny parasite called Giardia. It's commonly caught from drinking contaminated water, particularly when travelling or camping.
How Do You Catch It?
- Drinking contaminated water (rivers, streams, swimming pools in some countries)
- Swallowing water while swimming
- Eating contaminated food
- Close contact with an infected person
What Are the Symptoms?
- Diarrhoea (often greasy or fatty-looking)
- Bloating and excessive wind
- Foul-smelling burps (sometimes described as "eggy")
- Stomach cramps
- Weight loss if it goes on a long time
How is it Diagnosed?
By testing a stool (poo) sample for the Giardia parasite.
How is it Treated?
With antibiotics such as metronidazole or tinidazole. Most people feel better within a few days of starting treatment.
After Treatment
You may have temporary difficulty digesting milk (lactose intolerance). This usually gets better within a few weeks. You can use lactose-free alternatives in the meantime.
How Can You Prevent It?
- Only drink bottled or boiled water when travelling
- Avoid ice cubes in drinks
- Don't swallow water when swimming in lakes or rivers
- Wash hands thoroughly
12. References
Primary Guidelines
- Centers for Disease Control and Prevention. Parasites - Giardia. cdc.gov/parasites/giardia
- Public Health England. Giardiasis: Guidance, Data and Analysis.
Key Studies
- Solaymani-Mohammadi S, Singer SM. Giardia duodenalis: the double-edged sword of immune responses in giardiasis. Exp Parasitol. 2010;126(3):292-297. PMID: 20452344
- Escobedo AA, Cimerman S. Giardiasis: a pharmacotherapy review. Expert Opin Pharmacother. 2007;8(12):1885-1902. PMID: 17696791