William W. Frye

The pathogenicity of Endamoeba histolytica

Abstract 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.

The pathogenicity of four strains of endamoeba histolytica from Chicago

1. Four strains ofEndamoeba histolytica received from the Department of Health of the City of Chicago shortly after the 1933 epidemic of amoebic dysentery were studied in kittens to determine their pathogenic activity.

Infection of Kittens with Endamoeba histolytica by Direct Injection of Cultures into the Ileum.

In the course of experiments to test the relative pathogenicity for kittens of a number of strains of Endamoeba histolytica in culture, rectal injection proved unsatisfactory because of the small percentage of kittens which became infected. The technique of rectal injection was simply to withhold food on the day of injection, and inject the sediment from a rich culture by means of a rubber-tipped pipette which was inserted gently through the anus to a distance of about 3 inches. The kitten was then held head downward for a few minutes before being released. In one series the anus was sutured for 24 hours, without producing any better results. A preliminary enema, as used by Kessel 1 was not given because of the desire to avoid even microscopic trauma as far as possible and to avoid early evacuation of the injected material. Feeding cysts by mouth produced no better results with the one strain with which the method was employed. This method was also impracticable because some strains did not form cysts in culture. In considering a better route of infection, injection directly into the cecum after laparotomy, as practiced by Rees 2 was not adopted because it was not desirable to pass a needle directly through the wall of the cecum. It was felt that this might create a small lesion in which a few amoebae or bacteria might be left in withdrawing the needle. Fausts method 3 of inoculating dogs directly into the ileum through the anus was not feasible because the colon of the cat is acutely flexed in its upper third, and because there was too much danger of trauma.