Problem statement: Amoebic colitis and amoebic liver abscess (amoebiasis) are among the most important diseases of man. They are caused by the intestinal amoeba Entamoeba histolytica. In 1986, estimates suggested that about 480 million people are infected annually with E. histolytica of whom about 36 million developed clinical symptoms, and 40,000 died. Until 1993, the question that had vexed scientists for generations was ‘Why did such a small proportion of those infected with the parasite develop serious symptoms?’

Methods: In order to address this problem, Diamond and Clark (1993) tested the decades-old hypothesis that E. histolytica comprises two morphologically identical species, one pathogenic, the other non-pathogenic.

Three types of evidence were used to test the hypothesis:
· biochemical evidence, using electrophoretic isoenzyme analysis;
· immunological evidence, using monoclonal antibodies;
· genetic evidence, using DNA probes to analyse genomic organization of genes, comparisons of small subunit ribosomal RNA genes, etc.

Outcomes and impacts: Following an exhaustive analysis of the data, it was concluded that the overwhelming body of evidence supported the concept that E. histolytica was a complex of two species: E. histolytica Schaudinn, 1903 (emend. Walker, 1911), which is a pathogenic species displaying varying degrees of virulence and capable of invading a variety of tissues, and E. dispar Brumpt, 1925 which is not capable of tissue invasion.

Lessons: The existence of two species within what was previously called E. histolytica has profound consequences for the interpretation of epidemiological data, for clinical evaluation of carriers and for estimating the proportion of symptomatic infections.