TRANSMISSION OF INTESTINAL PROTOZOA 8i 



tive but ordinarily are not infective in nature. Another problem 

 involves the infectivity of immature cysts. When cysts of E. his- 

 tolytica escape from the body they possess one, two, three or four 

 nuclei. Those with less than four nuclei are immature. Do these 

 continue their development and become infective? Yorke and 

 Adams (1926) have shown that immature cysts continue their de- 

 velopment in artificial culture media. This suggests that all cysts 

 may be infective regardless of the number of nuclei they contain. 



There is no question but that a sufficient number of cysts are 

 formed in E. histolytica carriers to account for the incidence of 

 infection in the general population ; for example, it has been esti- 

 mated that a single person may pass several hundred million cysts 

 in a single day. These cysts, however, must be able to withstand 

 various environmental factors outside of the host until they are 

 ingested by new hosts. One of these factors concerning which 

 there can be little doubt is that of desiccation. Cysts when dried 

 are killed and hence distribution of viable cysts in a dried condi- 

 tion in dust appears to be impossible. 



\'arious methods of determining whether cysts are alive or dead 

 have been devised. One of these is the eosin test. Eosin in a con- 

 centration of I : 1000 is added to a smear and is supposed to pene- 

 trate and stain dead cysts but not those that are aHve. This test 

 is not entirely satisfactory since dead cysts are not always stained. 

 Neutral red and Congo red have also been employed but are both 

 open to the same objections as is eosin. The best method of deter- 

 mining whether cysts are alive or dead is to test them in susceptible 

 animals or in artificial culture. The culture method perfected by 

 Boeck and Drbohlav (1925) and modified by Dobell (1928) and 

 others enables one to inoculate media with material and to deter- 

 mine with certainty whether living specimens of E. histolytica 

 are present. If positive cultures result there can be no question 

 but that living specimens were present. However, living specimens 

 may have been present even if they do not appear in the cultures 

 since not all trophozoites or cysts continue to live and reproduce 

 in culture media. 



The viability of cysts can also be determined by injecting them 

 into laboratory animals as described in Chapter XXI. Kittens are 

 the most susceptible of all available animals for this purpose. The 

 experiments of Sellards and Theiler (1924) with kittens indicate 

 that cysts of £. histolytica are still infective after six days when 



