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.
- Use species-specific PCR or antigen testing to distinguish pathogenic E histolytica from morphologically similar non-pathogenic Entamoeba species.
- Treat symptomatic giardiasis with a defined nitroimidazole regimen and investigate adherence, reinfection or immune deficiency if symptoms persist.
- Invasive amoebiasis needs a tissue-active nitroimidazole followed by a luminal amoebicide; metronidazole alone does not eradicate bowel cyst carriage.
- Amoebic liver abscess is often diagnosed through imaging plus serology and exposure because stool microscopy may be negative.
- Manage household, childcare, food-handling and sexual transmission through hygiene and health-protection advice rather than treating untested contacts automatically.
Key red flags
Bloody diarrhoea with fever, marked tenderness or systemic toxicity suggests invasive amoebic colitis rather than uncomplicated Giardia.
Right-upper-quadrant pain, fever and liver lesion after endemic exposure may be amoebic abscess even when stool tests are negative.
Left-lobe hepatic abscess risks rupture into pericardium and needs urgent specialist imaging and drainage consideration.
Peritonism, megacolon, shock or sudden pleural symptoms may indicate colonic or abscess rupture.
Weight loss, persistent malabsorption, pregnancy, immunosuppression or inability to maintain hydration lowers the threshold for specialist care.
Apparent inflammatory bowel disease after travel must be tested for E histolytica before corticosteroids because immunosuppression can precipitate fulminant colitis.
Gradual abdominal pain, tenesmus and stool containing blood and mucus after endemic exposure indicates invasive colonic disease.
Fever, right-upper-quadrant pain, tender hepatomegaly and a solitary liver lesion may occur months after travel and without current diarrhoea.
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.
- Reinforce hand, toilet, laundry, food and sexual hygiene and seek health-protection advice for high-risk occupation or clusters.
- If symptoms continue, confirm adherence and repeat testing before changing treatment; assess reinfection, lactose intolerance and immune deficiency.