ENTAMOEBA HISTOLYTICA 37 



of equilibrium.* This is the " normal " or most usual condition, and 

 an infected individual in such a state of equilibrium is called — following 

 Walker (1911, 1913) — ^ "carrier" of the parasite. He can only be 

 distinguished from uninfected individuals by the presence of the cysts 

 of E. histolytica in his stools. His large bowel is ulcerated, more or less, 

 but this is not visible externally and gives rise to no clinical symptoms. 



When the parasites and their host do not live in harmony with 

 one another — as happens in a certain proportion of cases — pathological 

 conditions result. These affect both the host and the parasite. In the 

 case of the former, they are manifested as various diseases, which are 

 of three main types: (i) Irritation of the intestine, producing most 

 commonly diarrhoea and intestinal irregularities of divers sorts, and 

 leading, in severe cases, to a typical form of dysentery (Amoebic 

 Dysentery). (2) Generalized effects resulting from the destruction of 

 the lining of the bowel, but not manifested as local intestinal diseases 

 (General Amoebiasis). (3) Secondary disorders consequent upon the 

 wandering of the parasites from the gut into other organs, such as the 

 liver, where they give rise to inflammatory and suppurative conditions 

 (Amoebic Hepatitis, Hepatic and Cerebral Abscess, etc.). All these 

 diseased conditions of man are harmful to the parasite also, for they 

 disturb its food supply, interrupt its normal life-history, and lead to 

 a great wastage and mortality among the amoebae concerned. In 

 amoebic dysentery, for example, the amoebae are cast out of the body 

 in large numbers before they can encyst ; and they consequently perish 

 and are unable to propagate their species. Similarly, in the case of 

 secondary infections of the organs, the parasites may enjoy a brief spell 

 of reproductive activity ; but they do not encyst in any situation save 

 the gut, and are, in the organs, cut off from the outside world with no 

 means of continuing their race. The various amoebic diseases are thus 

 " diseases " for the parasites as much as they are for their hosts. And 

 it is clear that infection with is. histolytica cannot invariably, or even 

 usually, be accompanied by acute dysentery ; for if it were the parasite 

 would soon be exterminated. 



Entamoeba histolytica is thus a pathogenic parasite in a restricted 

 sense. It is always a destroyer of tissue, but by no means always 

 productive of disease. The usual type of human infection is that 

 exemplified by the carrier of the parasite. The carrier state will there- 

 fore be considered here rather more fully, as it involves several matters 

 of importance. 



Carriers. — The carrier of E. histolytica was first defined by Walker 

 (1911, 1913). It is true that others had previously spoken of amoebic 

 carriers, but they used the term in a different sense, and without com- 

 prehending the facts upon which Walker's conception rests. For 

 example Martini (1908) and Vincent (1909) both described and discussed 

 " carriers " of E. histolytica ; but they were ignorant of the life-history 

 of the amoeba, and of the part played by its cysts. They apparently 

 regarded "carriers" merely from the clinical point of view, and on 

 analogy with the carriers of various bacillary infections. Walker's 

 conception of the carrier of E. histolytica is, however, in many ways 

 different : his " carrier " is, in fact, something siii generis — by no means 



*I have already elaborated this conception somewhat in an earlier publication 

 Dobell, 1918 a). 



