66 



it obviously merits very careful attention. There is as yet no 

 conclusive evidence to prove that any of the other intestinal 

 protozoa of man are pathogenic ; while there is much evidence, 

 on the other hand, to show that they are, in the majority of 

 cases, harmless. As possible causes of human disease they may, 

 therefore, all be left out of account for the present. ; 



As regards E. histolytica, the evidence clearly shows that this 

 parasite is not responsible for any considerable proportion of the 

 disease of this country, though it shows equally clearly that it is 

 responsible for a small number of cases. This statement is, how- 

 ever, in all probability applicable to all other countries where the 

 parasite is known to occur. Britain is not in a peculiar position 

 in this respect. 



The fact that the majority of indigenous infections with 

 E. histolytica have been found in comparatively healthy indi- 

 viduals, with no past or present symptoms of dysentery or liver 

 abscess, has led some people to wonder whether the 'E. histolytica ' 

 of natives of Great Britain is, after all, the real E. histolytica of 

 the tropics — the amoeba which causes amoebic dysentery and 

 other diseases. There is, however, no longer any room for doubt 

 on this subject. It can be confidently asserted that the British 

 parasite is of the same species as the amoeba of tropical amoebic 

 dysentery. It is morphologically indistinguishable ; and it is 

 similarly a facultatively pathogenic parasite — as the indigenous 

 human cases of amoebic dysentery and liver abscess indicate, and 

 as its transmission from healthy human carriers to the cat 

 confirms. In this animal the organism causes acute amoebic 

 dysentery indistinguishable from that produced by tropical 

 strains of the parasite. 



Although there can be no doubt as to the identity of the 

 British parasite, there is possibly still room for wonder at the 

 fact that, although some 7 to 10 per cent, of the population is 

 infected, yet amoebic dysentery and liver abscess are rarities in 

 this country. But in contemplating this apparent discrepancy 

 the following points have been all too often overlooked. First, 

 infection with E. histolytica does not invariably, or even usually, 

 result in dysentery — even in the tropics or other places where 

 ' amoebiasis ' is reputed to be ' endemic '. The belief that it does 

 was based upon faulty knowledge of the life-history of the parasite 

 and its relations to its host,^ and it is now certain that it is the 

 exception, and not the rule, for E. histolytica to cause dysentery 

 or liver abscess in human beings. (This does not mean, of course, 

 that it does not usually cause ulceration or erosion of its host's 

 intestine. It must always do this, as it is an obligatory tissue- 

 parasite : but the injury of the intestine is not usually sufiicient 

 to give rise to any clinically recognizable symptoms.) The con- 

 ditions observed in England are thus not really peculiar. They 

 merely illustrate a special instance of a general phenomenon. 



' I need not labour the point here. The facts now are, or should be, well known. 

 The reader who seeks further information on the subject will find it in my book on 

 the amoebae of man (1919). 



