Makindo Medical Notes

Entamoeba dispar is identical in appearance to Entamoeba histolytica but does not cause disease.

About

• Amoebiasis is caused by the protozoan Entamoeba histolytica.
• Infection occurs through the ingestion of food or water contaminated with cysts of the parasite.

Transmission

• Direct faecal-oral contamination of food or water.
• Oral-anal sexual practices can also spread the infection.
• Insects may contaminate food with cysts.

Pathology

• The cyst stage is the infective form, while the trophozoite stage is responsible for invasive disease.
• Cysts are ingested through contaminated food or water, and they release trophozoites in the small intestine, which pass to the colon, causing "flask-shaped" ulcers.
• Trophozoites can travel via the portal vein to the liver, forming a liver abscess.
• Amoebic liver abscess: Occurs when trophozoites reach the liver via the portal circulation.

Epidemiology

• Amoebiasis is prevalent in tropical and subtropical regions.
• Entamoeba histolytica is a notifiable disease in many regions due to its public health significance.

Clinical Features

• Amoebic dysentery: Invasion of the colonic epithelium leads to colitis, characterised by diarrhoea with blood and pus, abdominal pain, fever, and weight loss. Complications include toxic megacolon, strictures, and severe lower GI bleeding.
• Amoeboma: An inflammatory mass in the colonic wall, often around the sigmoid or caecum, which can be mistaken for a tumour.
• Amoebic liver abscess: Presents with fever, right upper quadrant pain, tender hepatomegaly, and potential complications like rupture, leading to empyema, peritonitis, or pericarditis.
• Pericardial amoebiasis: Usually due to the rupture of a liver abscess into the pericardium, causing chest pain, dyspnoea, tachycardia, and hypotension.
• Brain abscess: Resembles pyogenic abscess, presenting with headache, fever, and neurological signs.
• Cutaneous amoebiasis: Typically seen near the anus or genitals, presenting as painful skin ulcerations.

Investigations

• FBC: Anaemia and elevated white cell count (WCC).
• Serology: Amoebic fluorescent antibody test (FAT) is positive in 90% of liver disease cases and 70% of colitis cases.
• Stool microscopy: Several samples may be needed. Shows motile trophozoites containing red blood cells and cysts.
• Colonic biopsy: May reveal characteristic flask-shaped ulcers and strictures.
• Chest X-ray: Can show an elevated right diaphragm and fluid in the right chest due to liver abscess complications.
• Ultrasound (USS) or abdominal CT: Used to assess liver abscesses, usually located in the right lobe. Raised ALP may be seen.
• CT/MRI head: For suspected brain involvement.

Differential Diagnosis

• Inflammatory bowel disease.
• Bacillary dysentery.
• Salmonella infection.
• Pseudomembranous colitis.

Differential Diagnosis of Liver Abscess

• Pyogenic abscess.
• Hydatid cyst.
• Primary or secondary tumour.

Management

• Metronidazole 800 mg PO three times a day for 5 days for amoebic colitis.
• Metronidazole 400 mg PO three times a day for 10-14 days for liver abscess. This should be followed by Diloxanide 500 mg three times a day for 10 days to clear intestinal parasites. A follow-up scan may be required.
• Liver aspiration may be needed if the abscess is at risk of rupture or if medical therapy fails. Aspiration produces "anchovy paste" or chocolate-coloured fluid.

Prevention

• Drink bottled water and practice good personal hygiene in endemic areas.
• There is currently no effective vaccine for amoebiasis.