HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



in feces that are kept moist and for weeks in feces diluted 

 with drinking water. Wenyon and O'Connor (1917) 

 record cysts still alive in diluted fecal material at the end 

 of a month; and Dobell (1919a) states that they will 

 survive for several weeks in feces that are kept moist 

 and cool and for five weeks if placed in water. 



A comprehensive experimental study of this subject 

 was carried out by Boeck (1921b). Washed cysts of E. 

 histolytica when kept in bottles in distilled water at 12° 

 to 22° C. were still viable at the end of 153 days, and 

 those of E. coll at the end of 244 days ; cysts of E. histo- 

 lytica in eosin-stained wet preparations under a cover 

 glass sealed with vaseline were still unstained after 211 

 days and those of E. coli after 124 days. Yorke and 

 Adams (1926b) were able to obtain cultures from cysts 

 that had remained in raw feces at room temperature for 9 

 days, in washed suspensions at room temperature for 10 

 days, and in saline or water at 0° C. for 17 days. 



Conclusions from viability tests. These results indicate 

 that when kept moist the cysts of these amoebae remain 

 alive for long periods outside of the body. They live 

 longer in water than in fecal material. It may be assumed, 

 therefore, that dilution of the infected feces is favorable 

 for the continued existence of the cysts. Cysts that reach 

 drinking water or water used for washing vegetables, 

 etc., or milk or moist food are thus in a comparatively 

 advantageous position both as regards length of life and 

 the chances of being ingested by a susceptible human 

 being. 



Resistance of cysts to temperature. Very few of the 

 factors encountered by cysts outside of the body have 



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