Summary

Amoebiasis is infection with the protozoan parasite Entamoeba histolytica, transmitted via the faecal-oral route through ingestion of cysts in contaminated food or water. Infection ranges from asymptomatic carriage to invasive disease causing amoebic colitis (bloody diarrhoea) and extraintestinal spread, most commonly amoebic liver abscess (ALA). E. histolytica is responsible for approximately 100,000 deaths annually worldwide, primarily in developing countries with poor sanitation. Treatment requires both a tissue amoebicide (metronidazole) for invasive disease and a luminal amoebicide (paromomycin or diloxanide furoate) to eradicate intestinal cysts and prevent relapse or transmission.

Key Facts

• Prevalence: 500 million infected globally; 50 million symptomatic cases/year
• Transmission: Faecal-oral (contaminated water/food); cysts are infectious
• Invasive disease: 10% of infected individuals develop invasive disease
• ALA: 80% occur in young adult males; typically right lobe
• Diagnosis: Stool microscopy (trophozoites with ingested RBCs), serology, PCR
• Key treatment: Metronidazole (tissue) + Paromomycin (luminal) — both needed

Clinical Pearls

Anchovy Sauce Pus: Aspirate from amoebic liver abscess classically has a reddish-brown "anchovy sauce" appearance and is sterile on bacterial culture (no bacteria, only amoebae).

Two Drugs, Two Jobs: Metronidazole kills trophozoites in tissues but does NOT eliminate cysts in the gut lumen. Always follow with a luminal agent (paromomycin or diloxanide) to eradicate cysts and prevent relapse.

Erythrophagocytosis: Trophozoites containing ingested red blood cells are pathognomonic for E. histolytica and distinguish it from non-pathogenic Entamoeba dispar.

Why This Matters Clinically

Amoebiasis is a major cause of diarrhoeal illness in travellers and immigrants from endemic areas. Failure to complete treatment with a luminal agent leads to relapse and continued transmission. Amoebic liver abscess can be life-threatening if it ruptures but has excellent outcomes with appropriate treatment.

Structured Approach

General:

• Vital signs (fever, tachycardia, hypotension)
• Hydration status
• Signs of weight loss

Abdominal:

• Tenderness (RIF in colitis; RUQ in ALA)
• Hepatomegaly (ALA)
• Percussion tenderness over liver
• Signs of peritonitis

Respiratory:

• Reduced breath sounds right base (reactive effusion or rupture)

Special Tests

Natural History

• Asymptomatic carriage: May persist for years
• Untreated ALA: High mortality from rupture or sepsis
• With treatment: Excellent outcomes

Outcomes with Treatment

Prognostic Factors

Good Prognosis:

• Early diagnosis
• Right lobe abscess
• Prompt treatment
• No complications

Poor Prognosis:

• Large abscess (greater than 10cm)
• Left lobe abscess (pericardial rupture risk)
• Multiple abscesses
• Delayed treatment
• Rupture

Key Guidelines

1. CDC Guidelines — Diagnosis and treatment of amoebiasis. cdc.gov/parasites/amebiasis
2. WHO Guidelines — Endemic parasitic diseases management.
3. UpToDate/Sanford Guide — Current treatment recommendations.

Key Studies

Stanley SL (2003) — Pathogenesis of amoebiasis

• Review of invasion mechanisms and tissue damage
• Clinical Impact: Foundation for understanding pathophysiology

Blessmann et al. (2002) — PCR for E. histolytica

• Demonstrated importance of distinguishing E. histolytica from E. dispar
• Clinical Impact: PCR becomes diagnostic standard

Evidence Strength

What is Amoebiasis?

Amoebiasis is an infection caused by a tiny parasite called Entamoeba histolytica. You catch it by swallowing contaminated water or food, usually in parts of the world with poor sanitation. The parasite can cause bloody diarrhoea by infecting your gut, or it can spread to your liver and form an abscess (a collection of pus).

Why does it matter?

Most people who catch the parasite have no symptoms, but about 1 in 10 develop illness. Amoebic dysentery causes painful bloody diarrhoea. A liver abscess can be very serious if it bursts. The good news is that with the right antibiotics, the infection is completely curable.

How is it treated?

1. Two medications are needed:
   • First, metronidazole (an antibiotic) kills the parasites causing damage
   • Then, a second medicine (paromomycin) clears the remaining parasites from your gut
2. If you have a liver abscess, you may need drainage with a needle if it's very large.
3. Rest and fluids are important if you have diarrhoea.

What to expect

• Symptoms usually improve within 48-72 hours of starting treatment
• You must complete both courses of medicine to prevent the infection coming back
• Liver abscesses typically resolve over weeks to months on imaging

When to seek help

See a doctor urgently if you have:

• Bloody diarrhoea, especially after travel to developing countries
• Severe abdominal pain, especially on the right side under your ribs
• High fever with abdominal pain
• Signs of dehydration (dizziness, not passing urine)