PATHOGENICITY 819 



made up of a number of strains. Entamoeba histolytica, for example, is 

 believed to be a composite of many races differing both in cyst size (see 

 Dobell and Jepps, 1918) and virulence (see Meleney and Frye, 1935, pp. 

 431-32). The evidence for the latter is indisputable, especially since the 

 appearance of the work of Meleney and Frye (1933, 1935), although 

 Craig (1936) is still skeptical regarding the existence of avirulent 

 strains. The latter point can be conceded for the present (though it is 

 still a live issue), without impugning the significance of observations 

 on human cases and experimental infection in kittens and puppies point- 

 ing to the existence of strains of low virulence, medium virulence, and 

 high virulence. 



Meleney and Frye adopted a standardized procedure. Recognizing the 

 doubtful validity of experiments performed on too few animals and 

 conducted without due allowance for variability of individual re- 

 sponse, they made it a practice to test each strain in a large series of 

 kittens of standard size. Each strain was isolated in culture, young trans- 

 plants were used for inoculations, and inoculations were made directly 

 into the caecum after laparotomy incision. Furthermore, the history of 

 the human patient was known, and there were records regarding the 

 character of the community and the prevalence of amoebiasis in the com- 

 munity in which each patient resided. The criteria of pathogenicity were 

 success or failure in infecting, extent and intensity of lesions produced, 

 and duration of the infection. The results of the experiment showed 

 conclusively that certain strains of Entamoeba histolytica of human 

 origin exhibited more "pathogenic activity" in kittens than other strains. 



Furthermore, by correlating the experimental data with field observa- 

 tions, they were led to the following conclusion regarding the relative 

 pathogenicity of the strains for man: "The more pathogenic strains 

 (i.e., in kittens), whether they were obtained from acute cases of 

 amoebic dysentery or from so-called 'healthy carriers' were associated 

 geographically and epidemiologically with acute dysentery, whereas the 

 less pathogenic strains were associated both individually and epidemi- 

 ologically with very little evidence of acute dysentery." 



AMOEBIC DYSENTERY AND BACTERIAL COMPLICATIONS 



The problem of virulence has had its reflection in matters of specificity. 

 Brumpt (1925) described Entamoeba dispar as an amoeba of the 



