ENTAMOEBA HISTOLYTICA 6l 



Sellards and Baetjer believed that the atypical strains of amoebae 

 which they studied produced atypical symptoms in their patients ; and 

 that similar atypical infections with corresponding symptoms were 

 produced in the experimentally infected kittens. Careful consideration 

 of the recorded details of these experiments and the figures of their 

 findings leaves me, however, in no doubt that they mistook cells of 

 various sorts for amoebae and cysts. I can hnd no good evidence that 

 amoebae of any sort were present in their patients or transmitted to 

 their kittens, and it therefore seems superfluous to discuss whether their 

 ** atypical strains " were varieties of any particular species of Entamoeba. 

 As they stand, their recorded observations supply no evidence whatever 

 in support of the hypothesis that there are strains of E. histolytica which 

 differ in virulence : and their hypothesis that strains producing atypical 

 symptoms in man produce similar symptoms in experimentally inoculated 

 kittens, is flatly contradicted by the well established fact that strains 

 producing no symptoms in man produce the most acute dysentery in 

 kittens. Indeed, every kitten which has ever been infected per as by 

 means of the cysts of E. histolytica furnishes evidence against their view. 



Can E. histolytica live as a Commensal f — There are still a few 

 workers who find it difficult to believe that E. histolytica is a tissue- 

 parasite always. They consider — if I understand them aright — that the 

 ordinary healthy carrier of the parasite cannot have an ulcerated gut, 

 because he manifests no clinical symptoms of disease : and they believe, 

 apparently, that in healthy persons infected with E. histolytica the para- 

 sites must be living as harmless commensals — like E. coli. This view 

 appears to me to be inconsistent with nearly every fact that is definitely 

 known about E, histolytica. It is, in my opinion, unsupported by any 

 concrete evidence, and seems to rest largely upon a misunderstanding 

 of the carrier condition. 



On purely a priori grounds the hypothesis is highly improbable. It 

 is unlikely that an amoeba which is generally dependent upon living 

 tissues for its nourishment should at times completely change its habits 

 and become a feeder on bacteria, I know of no analogous instance 

 in favour of such a supposition. Moreover, if it were proved that 

 E. histolytica undergoes such radical changes in its habits from time to 

 time, then the whole question of the pathogenicity of intestinal protozoa 

 would require reconsideration. For example, there is no reason why 

 E. coli should not also be able to undergo similar drastic changes in 

 habit, and thus acquire pathogenic powers. But at present there is 

 absolutely no evidence that either species is able to perform the remark- 

 able transformations which the hypothesis demands of them.* 



The hypothesis has been advanced especially by Kuenen and Swellen- 

 grebel (1913, 1914), and adopted by Woodcock (1917), Brug (1917 6), 

 and a few other workers. Kuenen and Swellengrebel (191 3) state that 

 E. histolytica lives, during what they call its " minuta phase," as a 

 saprozoic or commensal organism. It feeds on the gut contents, and 

 not upon the tissues. Brug (19176) even asserts that the '^minuta" 

 forms are as "omnivorous" as E. coli. The only concrete evidence 



* The hypothesis appears to me to rest partly on a false analogy with bacteria, as is 

 shown by the fact that some writers speak of E. histolytica living " saprophytically " — 

 a term improperly applied to anything which is not a plant. 



