Synopsis
Differentiate protozoal malabsorptive diarrhoea from invasive amoebic disease, select organism-specific tests and complete both tissue and luminal treatment where Entamoeba histolytica is confirmed.
- Giardia usually causes foul-smelling watery or greasy diarrhoea, bloating, flatulence, nausea and weight loss after contaminated water, travel, childcare or close contact.
- Entamoeba histolytica causes variable disease from asymptomatic luminal infection to dysentery and liver abscess after exposure in endemic settings.
- Send stool antigen or PCR for Giardia; intermittent shedding can require additional specimens when probability remains high.
Key red flags
Bloody diarrhoea with fever, marked tenderness or systemic toxicity suggests invasive amoebic colitis rather than uncomplicated Giardia.
Gradual abdominal pain, tenesmus and stool containing blood and mucus after endemic exposure indicates invasive colonic disease.
Investigation priorities
Detect Giardia with greater sensitivity and specificity than a single routine microscopy specimen.
Management branches
Prolonged watery or greasy diarrhoea follows water, travel, childcare or close-contact exposure.
- Assess hydration and send stool Giardia antigen or PCR with parasite testing appropriate to travel and duration.
- Give an exact locally approved metronidazole or tinidazole course after checking pregnancy, liver disease and interactions.
Key medicines
Metronidazole for giardiasisGive 400 mg orally three times daily for five days, or use another exact local first-choice regimen if specified by the current formulary.
Tinidazole for giardiasisGive 2 g orally as a single dose with food when the local formulary selects tinidazole for an adult.