67 



In the second place, it jnust be remembered that it has not yefc 

 been proved that amoebic diseases are so excessively rare in 

 Britain as they are usually assumed to be. We know from the 

 observations of Dickinson, Moore, Saundby and Miller, Marshall, 

 andWorster-Drought and Rosewarne, that both amoebic dysentery 

 and liver abscess occur in persons who have never left this- 

 country.^ We can hardly doubt, from the more recent observa- 

 tions of Laidlaw (1918), that British cases of amoebic disease 

 will be found to be more frequent when they are more frequently 

 and carefully looked for. The work of Bartlett (1917), Laidlaw 

 (1918), and others, suggests that some at least of the cases hitherto 

 diagnosed as 'ulcerative colitis' by British physicians may be 

 really caused by B. histolytica. It is, indeed, almost impossible 

 to read such works as those of Gemmel (1898) or Hawkins (1909) 

 without suspecting that some of their patients suffered, though 

 they did not realize it, from amoebic dysentery. What other 

 conclusion can we°draw, for example, from G-emmel's 80 autopsie& 

 on cases of ' idiopathic ulcerative colitis ' — containing two dysen- 

 teric patients displaying ulcerated large intestines coupled with 

 abscesses in their livers ? Furthermore, intestinal disturbances, 

 such as constipation and diarrhoea, are the commonest symptoms 

 of infection with E. histolytica ; and few practitioners will be 

 prepared to prove that among their patients suffering from these 

 everyday complaints there are none whose symptoms are due to 

 amoebic infection. Again, from the recent observations of Smith 

 (1919) it appears probable that some of the ' asylum dysentery ' in 

 England may be amoebic ^ — though from the work of Settings and 

 others it seems probable that the proportion is inconsiderable. 



It thus seems reasonable to conclude that amoebic diseases are 

 rare in Britain, as they probably are everywhere else in the 

 world — rare, that is to say, in proportion to the number of 

 persons infected with the parasite which ' causes ' them — yet 

 not so extremely rare as defective observations and inadequate 

 statistics might lead one, at first sight, to suppose. 



Thirdly, there is another point which should not be overlooked. 

 Although it now appears certain that ' pathogenic ' and ' non- 

 pathogenic' strains of ^. histolytica do not exist ^— all strains of 

 the species being tissue-parasites and consequently capable of 

 being ' pathogenic ' to a susceptible host — nevertheless it does 

 not follow that all strains are equally ' harmless ' to all hosts. 

 Amoebic dysentery is the consequence of an incompatibility 

 between the amoebae and their host : it is an abnormal or 

 ' diseased ' condition for both. The carrier of the parasite is 

 a compatible host — one adapted to life with the amoebae, and 

 one to whom the amoebae themselves are adapted. Now it 

 appears probable that E. histolytica has parasitized man for 

 a very long time. This is indicated by its geographical distribu- 



' See Chapter II. 



'' The observations of Gemmel (1898) — alluded to above — also seem to point to the 

 same conclusion. 



9 Cf. Dobell (1919), p. 58 et seq. 



