82 RESEARCH IN PROTOZOOLOGY 



left in fecal material at 2° C, but lose their infectiveness after 

 two weeks. Walker and Sellards (1913) were able to infect man 

 with cysts of E. histolytica after they had been outside the body 

 in fecal material for two days at tropical temperature and with 

 cysts of E. coll after ten days under similar conditions. It seems 

 certain from this that cysts of these two species are able to live 

 outside of the body long enough to become disseminated and to 

 find their way into the food or drink of human beings. 



Cysts of E. histolytica are not only able to live at room tempera- 

 ture (21° C.) but at lower temperatures. Whether they could 

 survive winter conditions in temperate regions has not been deter- 

 mined. Boeck (1921) found that all cysts of E. histolytica were 

 killed at 68° C. when held at that temperature for five minutes. 

 Yorke and Adams (1926) put 50° C. for five minutes as the 

 lethal temperature for these cysts but found that they would with- 

 stand a temperature of 45° C. for thirty minutes. It seems prob- 

 able that the organisms in the cyst stage are able to withstand 

 ordinary temperature conditions in tropical and sub-tropical 

 regions and all but the most severe winter conditions in the colder 

 parts of the world so long as they are kept moist. Further experi- 

 ments with temperatures under various conditions are desirable 

 both from scientific and practical standpoints. 



Various investigators have attempted to determine the effects 

 of chemicals on protozoan cysts with a view to the disinfection 

 of fecal material. Among these are Kuenen and Swellengrebel 

 (1913), Wenyon and O'Connor (1917) and Bercovitz (1924). A 

 simple method seems to be the addition of cresol in the strength 

 of 1 :20 to the fecal material. It is obvious that a thorough mix- 

 ture must be made in order to reach all of the cysts. The practical 

 value of this method is still to be determined. We are in need also 

 of a practical method for sterilizing drinking water. It is probable 

 that the pasteurization of milk raises the temperature to a high 

 enough point (60° C.) for a sufficient time to destroy any proto- 

 zoan cysts present and of course the boiling of water quickly kills 

 all cysts. Mills, Bartlett and Kessel (1925) have concluded from 

 their experiments that "Dipping fruits and vegetables for ten sec- 

 onds in boiling water, or water which remains above 80° C. during 

 immersion, is the onfy method thus far discovered which will uni- 

 formly kill all pathogenic bacteria, protozoan cysts and helminth 

 eggs which might be found contaminating such food products, and 



