AMOEBINA 449 



While the description of Entamoeba histolytica given above applies 

 in general, diversities in dimensions of trophozoites and cysts, and 

 in pathogenicity in human host as well as in experimental animals 

 have been reported. A number of observers are inclined to think 

 that there are several varieties or races of this amoeba, as has 

 already been mentioned (p. 226). 



Entamoeba histolytica, commonly known as "the dysentery 

 amoeba," was first definitely recognized by Losch in Russia in 1875. 

 It is now known to be widely distributed in tropical, subtropical 

 and temperate regions alike, although it is more prevalent in warmer 

 regions. The incidence of infection depends mainly on the sanitary 

 conditions of the community, since the cysts of the organism are 

 voided from host in faeces. Faecal examinations which have been 

 carried on by numerous investigators in different countries of the 

 world, reveal that the incidence of infection is as high as over 50 per 

 cent in some areas. According to Craig (1934), 49,336 examinations 

 made by many observers in various parts of the United States show 

 that the infection rate varied from 0.2 to 53 per cent, averaging 11.6 

 per cent, which justifies Craig's (1926) earlier estimate that about 10 

 per cent of the general population harbor this protozoan. An acute 

 infection by E. histolytica is accompanied by dysentery, while in 

 chronic cases or in convalescence, the host may void infectious cysts 

 without suffering from the infection himself. Such a person is known 

 as a cyst-carrier or -passer. 



The trophozoite if voided in faeces perish in a comparatively short 

 time. The dissemination of infection is thus exclusively carried on by 

 the cyst. Viable cysts may be transmitted (1) by contamination of 

 food through contact with contaminated water or through unsani- 

 tary habit of food handlers who are cyst-carriers; (2) by droppings of 

 flies and cockroaches which, as noted below, contain viable cysts for 

 a comparatively long time after feeding on faeces containing cysts 

 and by soiled appendages of these insects which may directly trans- 

 fer the cysts to food by walking on it; and (3) by contaminated wa- 

 ter in which the cysts live considerably longer than in faeces (p. 450). 



The seriousness of water-borne infection in crowded areas is easily 

 realized when one recalls the outbreak (some 1400 cases) of amoebic 

 dysentery and amoebiasis which originated in Chicago in 1933, where 

 defective plumbing in certain establishments contaminated the wa- 

 ter system with the cysts of Entamoeba histolytica (Bundesen et al., 

 1936) and the development of some 100 cases of amoebic dysentery 

 among firemen who drank contaminated water in connection with 

 the 1934 fire of the Union Stockyards in Chicago (Hardy and Spec- 

 tor), although in the latter instance, some workers believe that se- 



