EPIDEMIOLOGY OF AMCEBIASIS 255 



concluded that Entamosba coli is an obligate parasite, nonpathogenic, and cannot be 

 cultured. 



The third series of 20 feedings, carried on by Walker alone, was with Entamceba 

 histolytica. The material was mixed with powdered starch or magnesium oxide and 

 given in gelatin capsules. In these experiments they obtained tetragena cysts in 

 the stools of men fed only motile Entamceba histolytica, and motile Entamosba histolyt- 

 ica in the stools of men who were fed only tetragena cysts and, finally, an alterna- 

 tion of motile E. histolytica and tetragena cysts in the stools of a man having a 

 recurrent attack of amoebic dysentery. 



Seventeen of the men became parasitized after the first feeding; one required three 

 feedings, and two, who did not become parasitized at the first feeding, were held as 

 controls. The average time for parasitization was nine days. Only four of the 18 

 parasitized men developed dysentery, which came on after 20, 57, 87, and 95 days, 

 respectively, after the ingestion of the infecting material. 



In four cases fed with material from acute dysenteric stools or from amoebae con- 

 taining pus from liver abscess, and containing motile amcebae, there was no resulting 

 dysentery, the four cases of experimental dysentery resulting from feeding of material 

 from normal stools of carriers. 



As regards the cases which became parasitized, but did not develop dysentery, 

 it is suggested that the amoebae live as commensals in the intestine of the host and 

 only penetrate the intestinal mucosa and become tissue parasites when there occurs 

 depression of the natural resistance of the host or as the result of some lesion of the 

 intestine. That the pathogenic amcebae are more than harmless commensals, how- 

 ever, is shown by the fact that they alone, and not the nonpathogenic Entamosba 

 coli, are capable of penetrating a possibly damaged intestinal mucosa. 



Epidemiology. The old idea that water, fruit or vegetables, from 

 which one can isolate amcebae upon culture, are sources of infection 

 must be abandoned, as such cultural amcebae are known to have no 

 pathogenic relation to man. 



The chief factor in the spread of amoebic dysentery would seem to be the encysted 

 amcebse in the stools of convalescents or healthy carriers rather than the motile ones 

 in dysenteric stools. This probably explains the endemic rather than epidemic 

 characteristics of the spread of amoebic dysentery because if the innumerable vege- 

 tative amcebse in dysenteric stools were equally operative with the more sparsely 

 eliminated cysts there would be epidemics of amoebic dysentery similar to those of 

 bacillary dysentery. 



Our present view is that the carrier is the chief factor in the spread 

 of amoebic dysentery and when such an individual has to do with the 

 preparation of food he becomes a particular source of danger. 



Vegetative amcebze undergo disintegration in a short time after the stool is passed 

 so that they are probably rarely concerned in amoebic infections but the resisting 

 cysts may be washed from a dried stool into a water supply or even be transported in 

 dust to lodge on unprotected food stuffs. 



