The pathogenicity of 37 strains of E. histolytica, isolated from human patients and carriers, was studied in white rabbits. Five of these strains (Group 1) consistently produced severe ulceration in 88.4 per cent of the rabbits inoculated, with crater-like lesions, intense diarrhea, marked loss of weight and huge numbers of trophozoites containing red blood cells. Four of the 5 patients from whom these strains were isolated suffered from amebic dysentery. Group 2, comprising 14 strains, was mildly pathogenic producing only small lesions in 45.2 per cent of the rabbits inoculated, which lost little weight. Two of the patients furnishing these strains had amebic liver abscesses with few or no intestinal symptoms; the rest were asymptomatic. The 18 strains of Group 3, secured from asymptomatic carriers, were also nonpathogenic to rabbits.
The trophozoites of all the highly virulent strains of Group 1 were larger than 10 microns, but there were also large amebae in Group 3; none of the strains measuring less than 10 microns (size of trophozoites in culture) produced macroscopic evidence of lesions in rabbits. Amebae multiplying in ulcers were invariably larger than those in the cultures from which they were derived, but reverted to their former size when cultures were made from the lesions.
The lesions in infected rabbits showed only occasional infiltration by lymphocytes and plasma cells, but in all but 4 of the 36 animals examined there was a characteristic invasion of eosinophils which are believed to correspond to the neutrophils of man.