E. histolytica: transmission 



Life-cycle. D. fragilis lives in the intestine of man, but 

 less than lOO cases have been recorded. Its process of 

 reproduction has not been described. 



III. Host 'Parasite Relations between Man and Enda- 

 moeha histolytica 



I. EPIDEMIOLOGY OF TRANSMISSION 



( I ) Infective stage. The general idea regarding the 

 infective stage of intestinal protozoa is expressed admir- 

 ably by Dobell (Dobell and O'Connor, 192 1) in the 

 following words, "Infection with any intestinal proto- 

 zoon is, in nature, always acquired through the mouth, 

 by swallowing a living cyst containing the resting form 

 of the particular organism. In ordinary circumstances the 

 free forms cannot live outside the body for more than a 

 very short time, and they die if swallowed — in other 

 words, they are non-infective" (p. 6). 



Are trophozoites capable of bringing about an infec^ 

 tionf Just how long trophozoites of E. histolytica ordi- 

 narily live in fecal material outside of the body is not 

 known. Rivas (1926) reports that active specimens re- 

 mained alive for over 24 hours in fecal material and for 

 from 32 hours to 3 days when sealed in capillary tubes 

 and kept at a temperature of 5° C. At 22° C. they lived 

 for at least several hours, at 37° C. for a shorter period 

 and at 45° C. for only 5 minutes. It seems probable that 

 trophozoites are seldom ingested by man in a living con- 

 dition although this might happen under unusual cir- 

 cumstances. That they are not quickly killed by the di- 

 gestive juices is indicated by an experiment of the writer 



65 



