The pathogenicity of Endamoeba histolytica

The variations in the clinical picture of infections with E. histolytica appear to depend first upon variations in the resistance of individual hosts and secondly upon variations in the pathogenic activity of different strains of the amoeba. Both of these factors can be demonstrated experimentally in kittens if a large enough number of animals is used and if uniform conditions of experiment are employed. There is evidence that strains of a similar degree of pathogenic activity predominate in certain population groups where conditions favour the limitation of these strains to the groups concerned and the exclusion of other strains. Travel and the development of cosmopolitan populations tend to introduce other strains which may possess a different degree of pathogenicity. The pathogenic index of a given strain seems to be a fairly stable condition in the strains we have studied from this point of view. Prolonged cultivation in artificial media has not decreased the pathogenic index. Attempts to raise the pathogenic index have so far not been successful, although further attempts are necessary before conclusions can be drawn. In medical practice, however, any strain of E. histolytica should be considered dangerous and should be eradicated by treatment.