AMOEBINA 357 



tion 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 dysenterj^, 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 exclusively carried on by the 

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

 through contact with contaminated water or through unsanitary 

 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; (3) by contaminated Avater in 

 which the cysts live considerably longer than in faeces (p. 358) ; and 

 (4) by cysts originating in faeces voided by animals such as dogs (in 

 which spontaneous infections by apparently this amoeba have been 

 reported to occur), rats, monkeys, etc. 



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 the water contaminated bj^ cyst-contain- 

 ing faeces in connection wdth the 1934 fire of the Union Stockyards in 

 Chicago (Hardy and Spector). 



The cysts remain viable for a considerable length of time outside 

 the human intestine, if environmental conditions are favorable. Since 

 information regarding the viability and longevity of the cyst is 

 highly important from the epidemiological standpoint, many papers 

 have dealt with it. In testing the viability of the cyst, the following 

 two tests have been used by the majority of investigators. 



(a) Eo sin-staining test. Kuenen and Swellengrebel (1913) first 

 used a dilute solution of eosin (1:1000). It has since been used by 

 Wenyon and O'Connor, Root, Boeck, and many others. Solutions 

 used vary from 1:10,000 (Root) to 1:100 (Boeck). A small amount 

 of fresh cyst-containing material and a drop of eosin solution are 

 mixed on a slide, then dead cysts will appear stained reddish under 

 the microscope, while living cysts remain unstained. Whether or not 



