BALANTIDIUM COLI : ATTACK ON HOST 



phozoites that succeed in reaching the large intestine or 

 in escaping from the cyst walls may find the intestinal 

 environment lacking in certain elements essential for 

 their metabolic processes, or find these elements in un- 

 suitable quantities, or encounter harmful substances. 

 These factors might not produce death immediately but 

 might hinder or prevent growth and reproduction. Ap- 

 parently B. coli does not exercise any selection as regards 

 food (de Leon, 19 19), but no doubt certain types of food 

 particles are more satisfactory than others and thus the 

 diet of the host becomes of importance in the successful 

 colonization of the parasite. Even after a successful in- 

 fection is established changes in the diet may affect the 

 intestinal environment so adversely as to destroy the 

 parasites (Greene and Scully, 1923). Our knowledge of 

 free-living ciliates leads to the conclusion that optimum 

 conditions are not necessary for the growth and repro- 

 duction of B. coli, but too wide a departure from these 

 conditions would obviously result in the prevention of 

 both these processes and constitute passive host resist- 

 ance too great for the existence of the parasite. 



Attack on the host. Most of the earlier cases of human 

 infection with balantidia involved intestinal symptoms 

 that were evidently due to the presence of the protozoa 

 (Strong, 1904). The examination of the feces of healthy 

 persons later revealed the fact that infection with B. coli 

 does not necessarily result in symptoms. Apparently this 

 organism is able to live and reproduce within the intes- 

 tinal lumen without access to the tissues of its host, and 

 may therefore be non-pathogenic. In this environment 

 it probably feeds on digested or undigested food taken 



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