Organism Amoebiasis is caused by the protozoan Entamoeba histolytica, which exists in an amoeboid form in the human large intestine and as a cyst in the environment.

The pathogenic amoeba enters a muco-sal fold and feeds on red blood cells (RBCs). Penetrating through the muscularis mucosae, an abscess is formed with vascular necrosis taking place at its base. This leads to tissue disintegration and the development of an ulcer (the so-called flask-shaped ulcer). Active amoebae can be found in the base of an ulcer.

Clinical features Illness presents as acute diarrhoea with frank blood, chronic diarrhoea or as an abscess with no apparent transitional period of diarrhoea. If the amoebic ulcer penetrates a blood vessel, fresh blood is passed in the stool, which is a characteristic feature. Amoebae from the breached circulatory system are carried to various parts of the body, the liver being the commonest. In the liver, an abscess is formed, with the right lobe being the predominant site. Liver damage is a predisposing cause with liver abscess more common in males than females. The expanding abscess can track outwards through the peritoneum, abdominal wall and on to the skin or upwards to form a sub-phrenic abscess or enter the pleural cavity. The most serious site of amoebic abscess development is in the brain. All these features are illustrated on Fig. 8.3.

Symptoms of an abscess are fever, weight loss and localized tenderness. Amoebic pus is characteristically a pale reddish brown colour (without odour) and can be discharged on to the skin from a penetrating ulcer, or coughed up from the lung. In a chronic infection, an amoeboma can be formed, which may be confused with carcinoma.

Diagnosis is made by examining fresh stool specimens within half an hour of their production for motile amoebae with ingested RBCs. Amoebae are occasionally found in amoebic pus, which similarly must be examined as soon as possible as the active forms rapidly die off. The finding of cysts indicates infection, but search must be made of fresh stool or pus for motile amoebae. Liver abscess is diagnosed by X-ray (raised diaphragm) or by ultra sound. The abscess is usually not tapped, unless in differential diagnosis from a bacterial abscess or is about to burst.

Transmission Cysts of E. histolytica are formed in the large intestines and passed into the environment in the faeces. They survive in faeces for only a few days, but if they enter water, they remain viable for considerably longer periods. Infection occurs through drinking contaminated water or eating irrigated salad vegetables. Flies can carry cysts for some 5 h. In circumstances of poor hygiene, direct faecal-oral transfer via food, or by utensils, can take place.

Cysts can survive in the cold for considerable periods, but they are killed by a temperature over 43°C, which must be obtained in any composting system where human faeces are used. Non-survival of amoebic cysts is a useful indicator of effective decomposition (see Fig. 1.2).

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