40 THE AMOEBAE LIVING IN MAN 



(f/(i^ Walker and Sellards, 1913). Of 18 men who were experimentally 

 infected with E. histolytica, only 4 (22-2 per cent.) developed symptoms of 

 amoebic dysentery— the rest (14, or 77-8 per cent.) becoming contact 

 carriers. Some of them were under observation for over two years, and 

 never showed any signs of dysentery or other amoebic disorders. 



It should be noted that 2 of the 4 subjects who developed dysentery 

 had very mild attacks, which might have been overlooked if they had 

 not been under close observation. Moreover, Wenyon and O'Connor 

 (1917) found 106 carriers among 1979 healthy men examined in Egypt ; 

 and of these 106 infected individuals they say "only 16 gave any history 

 of d3'sentery, and it is certain that the latter figure is too high, for in no 

 case can we be certain of the type of dysentery from which the case 

 suffered." Taking these points into consideration, and allowing for the 

 error due to the small number of cases studied, I think too much 

 importance should not be attached to the exact percentages recorded by 

 Walker. It is extremely difficult to obtain reliable information on this 

 subject ; but from my own experience I am persuaded that Walker's 

 percentage is too high. I do not believe that more than 10 per cent, of 

 persons who become infected with E. histolytica ever suffer to any 

 appreciable extent from their infections; and I think it very probable 

 that even this is much too high an estimate. 



Infections with E. histolytica appear to be remarkably persistent ; 

 and there is good reason to believe that, when an individual once 

 acquires an infection, it will usually — unless he is subjected to specific 

 treatment — persist for the rest of his life.* Cases are "known in which 

 infection has lasted for at least 16 years, and probably for much longer 

 periods (Dobell and Stevenson, 1918). Infected individuals may remain 

 healthy, or may show continuous or intermittent symptoms of intestinal 

 derangement, such as diarrhoea or dysentery. A man may be a 

 comparatively healthy carrier for months or even years, and then suffer 

 from an acute attack of dysentery : or he may at any time contract a 

 liver abscess or secondary infection of some other' organ from the 

 primary focus in his intestine. But the factors which determine 

 exacerbations, or the spontaneous abatement of symptoms, or the 

 origination of secondary infections of the various organs, are still too 

 obscure for discussion of them to be profitable at present. 



Multiplication. — The only process of reproduction which I have ever 

 observed in E. histolytica is equal binary fission. Dividing organisms 

 are excessively rare in the stools of human beings, even when suffering 

 from dysentery and passing large numbers of amoebae. In my ex- 

 perience they are so rare, indeed, that they may practically be said to be 

 absent. This is not surprising, when it is remembered that the organisms 

 live— and therefore probably multiply — in the tissues. Those which are 

 washed out from the intestine invariably die, and as a rule rapidly. It 

 thus seems clear that the reproductive stages must be sought in the 

 ulcers in the wall of the bowel. As this is impossible in the case of 

 human infections, I had recourse to the cat : for in this animal the 

 amoebae multiply with great rapidity, and it is therefore comparatively 

 easy to obtain ulcers in any desired stage of development and amoebae 



* Cf. especially Walker and Sellards (1913), Wenyon and O'Connor (1917), Dobell 

 and Stevenson (191 8). 



