ROBERT HEGNER 667 



The Balantidium coli that lives in man is supposed to be the same species as that 

 which occurs in pigs and monkeys. Other intestinal protozoa, however, seem to be 

 more strictly limited to man. On the other hand, man undoubtedly ingests large num- 

 bers of the infective stages of intestinal protozoa peculiar to lower animals but does not 

 become infected by them. The factors that make it possible for a certain species of 

 parasite to live in a certain host species and not in others are little known but may be 

 studied experimentally. 



HOST-PARASITE RELATIONS OF INTESTINAL AMEBAE 



Very little is known regarding three of the species of human intestinal amebae, 

 namely, Endolimax nana, lodamoeba williamsi, and Dientamoeha fragilis. Of the 

 other three, Endamoeha histolytica undoubtedly is pathogenic, whereas Endamoeha 

 coli and Endamoeha gingivalis are of doubtful pathogenicity. 



ENDAMOEBA HISTOLYTICA 



Epidemiology.— A considerable amount of experimental work has been done to 

 determine the reactions of parasitic amebae to factors in the external environment. 

 Most of this work has been done with Endamoeha histolytica but the information 

 obtained is probably for the most part applicable to Endamoeha coli as well. The 

 trophozoites of these amebas die rapidly after their escape from the body and proba- 

 bly seldom reach a new host in a viable condition. The cyst stage, therefore, represents 

 the form capable of resisting exogenous conditions and is responsible for the trans- 

 mission of the species. Experiments indicate that these cysts are incapable of with- 

 standing desiccation and hence cannot be distributed in dust. Dilution of fecal ma- 

 terial is supposed to increase the length of life of the cysts, hence cysts that find their 

 way into contaminated milk or drinking water remain infective longer than in the 

 raw feces. Efforts have been made to determine the resistance of cysts to tempera- 

 tures with the result that cysts have been found to live at icebox temperatures for 

 many days and still be infective. High temperatures of from 60° to 70° C. will kill 

 cysts of Endamoeha histolytica in a few minutes. These temperatures, however, are 

 not ordinarily encountered in nature. The effects of many different kinds of disin- 

 fectants on ameba cysts have also been determined. Apparently the best agent for 

 the destruction of cysts is cresol in the strength of i : 20. No practical method has 

 yet been discovered of sterilizing drinking water, but the temperature employed in 

 the pasteurization of milk is sufficient to destroy any cysts present. 



The only conceivable method of infection with ameba cysts is the ingestion of 

 them by the host, principally, probably, in contaminated food or drink. Contamina- 

 tion is brought about by insanitary conditions or by the carriage of the cysts by 

 flies or other animals. A number of investigations have been carried on to determine 

 the role played by flies in the transmission of intestinal protozoa. These studies have 

 demonstrated that house flies ingest fecal material in nature, that ameba cysts may 

 remain alive for many hours in the intestine of the fly, and that these cysts may be 

 deposited in the feces of the fly in a viable condition after passing through the fly's 

 digestive tract. It is obvious from these results that flies may carry ameba cysts in 

 their bodies and deposit them in a living condition in drinking water or food and 



