Amoebiasis

Definition

Epidemiology

• Worldwide distribution; major health problem in China, South East and West Asia, Latin America (especially Mexico), the Indian subcontinent, Africa, and parts of Central and South America
• Globally, ~50 million people carry E. histolytica; approximately 1 in 10 infected individuals develop invasive disease; ~50,000 deaths per year estimated
• Prevalence in stool samples in high-endemic zones averages 10%; incidence of amoebic liver abscesses can reach 21 per 100,000 population
• India: approximately 15% of the population is affected (ranging 3.6-47.4% in different areas)
• More prevalent in lower socioeconomic groups due to poor sanitation
• Bailey and Love's Short Practice of Surgery 28th Ed., p. 88; Park's, p. 283

Aetiology and Agent Factors

• E. histolytica can be differentiated into at least 18 zymodemes; 7 are pathogenic, 11 are non-pathogenic
• Man is the only reservoir of infection
• Healthy carriers can discharge up to 1.5 × 10⁷ cysts daily
• Period of communicability: as long as cysts are excreted (potentially years if untreated)

Mode of Transmission

1. Faecal-oral route (primary) - contaminated water or food; raw vegetables from sewage-irrigated fields; direct hand-to-mouth via contaminated fingers
2. Sexual transmission - oral-rectal contact, especially in men who have sex with men
3. Vectors - flies, cockroaches, and rodents can carry cysts and contaminate food

Pathogenesis

• Ingested cysts hatch in the small bowel releasing trophozoites, which are carried to the colon
• Trophozoites invade the submucosa producing characteristic "flask-shaped" (bottleneck) ulcers - these have considerably undermined edges and a yellow necrotic floor, with blood and pus
• Typical sites: caecum and ascending colon most commonly; then rectum and sigmoid

• Trophozoites enter the portal circulation and are trapped in the interlobular veins of the liver
• They cause focal infarction and liquefactive necrosis via proteolytic enzymes
• Areas of necrosis coalesce to form an abscess cavity
• Right lobe involved in 80% of cases, left lobe 10%, remainder are multiple - the right lobe is more often affected because blood from the superior mesenteric vein takes a straighter course through the portal vein into the larger lobe
• Abscesses most common on the high diaphragmatic surface of the right lobe, potentially causing pulmonary symptoms
• Abscess cavity contains chocolate-coloured, odourless "anchovy sauce"-like fluid (necrotic liver tissue + blood)
• "Amoebic hepatitis" refers to the microscopic picture before macroscopic abscess forms
• Bailey and Love's, p. 88

Clinical Features

Intestinal Amoebiasis

• Ranges from mild abdominal discomfort and diarrhoea to acute fulminating dysentery (bloody mucoid diarrhoea)
• Can clinically mimic ulcerative colitis
• Complications: haemorrhage, stricture, perforation, pericolitis with adhesions causing intestinal obstruction

Amoebic Liver Abscess

• Non-specific: abdominal pain, anorexia, fever, night sweats, malaise, weight loss
• More specific: right upper abdominal pain, right shoulder tip pain, hiccoughs, non-productive cough
• Past history of bloody diarrhoea or travel to endemic area raises suspicion

Amoeboma

• A chronic granulomatous lesion in the large bowel, most commonly the caecum
• Occurs in long-standing infection treated incompletely (intermittent/self-medication)
• Appears as a mass in the right iliac fossa with generalised ill health, pyrexia, and blood-stained mucoid diarrhoea
• Easily mistaken for carcinoma - always biopsy

Histological Appearance

Diagnosis

Laboratory

• Stool microscopy: Demonstration of trophozoites containing ingested red cells is diagnostic - must be performed on fresh, warm specimens (cooling destroys motility); mucus from rectal swabs is ideal
• Absence of pus cells in stool helps distinguish from shigellosis
• Endoscopic biopsy or stool examination for amoebae (note: presence of parasite alone doesn't confirm pathogenicity)

Serology

• Complement fixation, indirect haemagglutination (IHA), indirect immunofluorescence, counter-immunoelectrophoresis (CIE), ELISA
• IHA is the most sensitive serological test for acute amoebic liver abscess in non-endemic regions
• In endemic areas, persistent antibodies in healthy population limit serological utility
• Antigen detection or PCR combined with serology improves specificity

Imaging

Treatment

Medical (First-line)

Aspiration and Drainage

• Aspiration is carried out when imminent rupture is expected, especially left lobe abscesses
• Pigtail catheter drainage if no response to IV metronidazole within 48-72 hours
• Low threshold for draining left lobe abscesses (propensity to rupture into peritoneal, pleural, or pericardial cavity)
• Diagnostic aspiration: limited value except to confirm typical "anchovy sauce" appearance and sterility (unless secondarily infected)

Surgery (Reserved for Complications)

• Rupture into pleural, peritoneal, or pericardial cavity: resuscitation, drainage, lavage + vigorous medical treatment
• Acute toxic megacolon or severe haemorrhage: subtotal colectomy with terminal ileostomy and closure of rectal stump
• Amoeboma: first confirm cancer excluded by imaging and biopsy; these masses can resolve with vigorous medical treatment (re-colonoscopy in 3-4 weeks)
• Bailey and Love's, p. 89-90

Prevention and Control

• Sanitation: safe disposal of human excreta; hand washing after defecation and before eating
• Water supply: filtration and boiling are more effective than chemical disinfection (cysts survive routine chlorination); sand filters effectively remove cysts
• Food hygiene: aqueous acetic acid (5-10%) or full-strength vinegar can disinfect uncooked vegetables and fruits; thorough washing with detergents removes cysts; food handlers should be periodically examined and treated
• Health education

• Symptomatic cases: metronidazole orally; response within 48 hours confirms suspected diagnosis
• Asymptomatic infections in endemic areas: generally not treated (consensus)
• Park's, p. 284

Key Differentiating Points

Important: Always exclude amoebic infection before starting steroids in a patient suspected of having ulcerative colitis - steroids in unrecognized amoebiasis can precipitate fulminant colitis.