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        1    Mild to moderate colitis: Recurrent diarrohea and abdominal cramps, sometimes alternating with constipation; mucus may be present; blood is usually absent.

        2     Severe colitis: Semiformed to liquid stools streaked with blood and mucus, feer, colic, prostration, ileus, perforation with peritonitis and haemorrhage occur.

        3     Hepatic amebiasis: Fever, hepatomegaly, pain, localised tenderness.

        4      Labaratory findings: Amebas in stools or in abscess aspirate; serologic tests positive with severe colitis or hepatic abscess, which is readily imaged by ultra sonography or CT scan.

        General considerations

        Though the causative protozoan parasite Entamoeba histolytica was once considered a single species with varying virulence, it is now recognised that the entamoeba complex contains two morphologically identical species: (1) E-dispar (about 90 per cent of the complex), which remains in the colon as a stable commensal that is avirulent and produces an asymptomatic carrier state: and (2) E-histolytica, which shows varying degrees of virulence ranging from a commensal state in the colon - in which it does not cause disease, yet is potentially invasive - to being invasive of the intestinal wall, resulting in aute diarrhea or dysentery or chronic diarrhea. E-histolytica may also be carried by the blood to the liver, where it may produce hepatic abscess. Rarely, the lungs, brain other organs, or perianal skin may be infected. About 10 per cent of asymptomatic carrier of E-histolytica develop invasive disease: the others clear the infection within one year.

        Both E-histolytica and E-dispar exist as two forms in the lumen and mucosal crypts of the large bowel: identical-appearing cysts and motile trophozoites. In the absence of diarrhea, trophozoites encyst in the large bowel.

        Of 500 million persons worldwide infected with entamoeba, most are infected with E-dispar and an estimated 10 per cent (50 million) with E-histolytica. Invasive E-histolytica may constitute 5 million cases, with mortality in the range of one lakh per year. In the United States, infections are most common in immigrants from - and travellers to - developing countries.

        Humans are the only established hosts and are universally susceptible. Only cysts are infectious, since after ingestion they survive gastric acidity which destroys trophozoites. Transmission occurs through ingestion of cysts from fecally contaminated food or water. Flies and other arthropods also serve as mechanical vectors: to an undetermined degree, transmission results from contamination of food by the hands of food handlers. Where human excrement is used as fertiliser, it is often a source of food and water contamination. Person-to -person contact is also important in transmission: therefore , all household members as well as an infected person's sexual partner should have their stools examined. In communal settings such as mental hospitals, prevalence rates as high as 50 per cent have been reported. Amebiasis is rarely epidemic, but urban outbreaks have occured because of common-source water contamination. Although entamoeba infections are usually due to the E-dispar and do not require treatment. In AIDS, E-histolytica infection does not become an opportunistic infection.

        Fulminant infections may occur in pregnancy and in young children. Corticosteroids and other immunosuppressive drugs given in error for infammatory bowel disease may convert a commensal infection into an invasive one.

        The characteristic intestinal lesion is the amebic ulcer, which can occur anywhere in the large bowel (including the appendix) and sometimes in the terminal ileum but predomnates in the cecum, descending colon, and the rectosigmoid colon - areas of greatest fecal stasis.