TRANSMISSION OF INTESTINAL PROTOZOA 83 



render them safe for human consumption in an uncooked con- 

 dition." 



Both infective cysts and infective trophozoites of intestinal 

 protozoa are passive during transmission and man is responsible 

 for his own infection. Many opinions have been expressed regard- 

 ing the method of entrance of cysts into the mouth of man. It 

 seems probable that cysts reach the mouth in contaminated food 

 and drink but they might also find their way there on soiled hands. 

 Among the most important factors that bring about the dissemina- 

 tion of cysts, and especially their presence in food and drink, are 

 probably the handling of food in homes, restaurants, hotels and 

 markets by infected persons who are passing cysts ; the use of 

 night soil as fertilizer in vegetable gardens ; the common use of 

 toilet, washbowl, and towel ; and the presence of insects, such as 

 flies, ants and cockroaches, and of domestic animals, such as rats, 

 mice, dogs and cats. 



No one has yet determined how efifective these various factors 

 are in the dissemination of protozoan cysts. Perhaps the work 

 with flies has been carried out more carefully than that with any 

 other factor. The data obtained by Wenyon and O'Connor (1917), 

 Buxton (1920), Root (1921) and others seem to prove conclu- 

 sively that flies both in the laboratory and in nature ingest fecal 

 material containing protozoan cysts ; that the cysts are not quickly 

 killed in the flies' intestine ; that living cysts may be deposited in 

 the feces or vomita of the fly from five minutes to forty-nine hours 

 after feeding; and that these cysts may be deposited in the food 

 or drink of man where they may remain viable until ingested by 

 a human host. The conclusion seems inevitable that flies play an 

 important role in the dissemination of histolytica cysts and that 

 food and drink should be protected from their visits, especially in 

 localities where amoebic dysentery occurs and sanitary conditions 

 are such as to allow flies access to infected fecal matter. It may 

 be pointed out here that the fly does not become infected but is a 

 "passive" carrier, in contrast to the rat described below which is 

 an "active" carrier. 



Among the animals that have been found to be infected with 

 E. histolytica in nature are cats, dogs and rats. Apparently cysts 

 are not developed in either the cat or the dog, hence there is little 

 danger of transmission as a result of infections in these animals. 

 The experiments of Kessel (1923) and Chiang (1925), however, 



