820 PATHOGENICITY 



hJstolytka-ty^Q dwelling in man, incapable of producing symptoms of 

 dysentery in its human host, and producing no definite macroscopic 

 ulcerations in the cat, but capable of penetrating the intestinal wall of the 

 latter animal so far as the muscularis mucosae. Simic, in several papers, 

 has corroborated Brumpts's claim for the validity of E. dispar, but his last 

 (1935) paper presents the strong argument that E. dispar infection in 

 dogs lasts only from 6 to 8 days, while E. histolytica infection lasts 

 from 60 to 120 days. E. dispar infection in dogs is quite benign, while 

 E. histolytica produces characteristic amoebic ulcerations, and amoebae 

 with ingested red cells may be found in the stools of the infected dog. 

 Wenyon (1936), however, in commenting on the strong case built 

 up by Simic (1935) states: 



It still seems futile to attempt to separate E. dispar from E. histolytica on 

 the grounds of pathogenicity. It seems that all the differences described can be 

 more reasonably accounted for by the supposition that races of E. histolytica 

 of varying virulence occur and that hosts vary in their susceptibility to the 

 one species. 



Meleney and Frye (1935) likewise prefer to consider E. dispar as a 

 strain of E. histolytica, possessing a low degree of pathogenicity. Even 

 the least virulent strains encountered by the latter authors, though not 

 producing clinical symptoms in the persons in whom they had their 

 origin, were capable of producing lesions in some kittens. Hence the 

 skepticism of these authors regarding totally avirulent strains of E. his- 

 tolytica. 



Is virulence-level retained by a strain of E. histolytica, or is it sub- 

 ject to modifying factors, such as attenuating effects of artificial culture 

 media or exaltation by animal passage? Meleney and Frye (1933) first 

 noted the contrast in pathogenic activity in kittens between "A" strains 

 of low virulence from the hill country, where symptomless carriers and 

 persons with mild symptoms were the rule, and "B" strains of high 

 virulence from severe cases of amoebic dysentery in the bottom lands, 

 where acute cases were much more common than in the hills. Later 

 (1935), they were able to report, after adequate testing in kittens, that 

 two "A" strains and two "B" strains had retained their respective patho- 

 genic indices after a period of three years of artificial cultivation. The 

 highly virulent strains did not decline in pathogenicity. But what about 

 the effect of animal passage.'' Meleney and Frye (1936) state that their 



