BALANTIDIUM COLI : RESISTANCE OF HOST 



maintaining itself in the small intestine against the action 

 of peristalsis. In the large intestine, movement of the 

 contents is so slow that the organism, which probably 

 reacts positively to currents, is able to progress suffi- 

 ciently against these to escape being carried out of the 

 body in the feces. Many specimens do not succeed, hence 

 trophozoites are frequent in the feces, especially when the 

 stools are loose. Evidently movement of the intestinal 

 contents is so rapid when the stools are loose that many 

 of the ciliates are unable to swim against it and are thus 

 extruded. The location of B. coli within the large intes- 

 tine may have an influence on successful colonization. If 

 the organisms reside in the mucus of the walls they are 

 not in as much danger of being carried down as they are 

 if they live within the mass of fecal material. 



Under certain conditions B. coli may live in the ileum 

 of man; for example, Reis (1923) records 4 cases in 

 which the ciliates occurred both in the large intestine and 

 in the ileum. 



Passive resistance of the host. The conditions encoun- 

 tered by the balantidia within the digestive tract, which 

 constitute the passive resistance of the host, are the same 

 as those met by other protozoa that are ingested by man 

 (see p. 30). The walls of the cysts no doubt protect the 

 organisms during their passage to the large intestine. It 

 is generally supposed that trophozoites are not infective.. 

 Thus Fantham, Stephens and Theobald ( 1916) state that 

 "As they are killed by acids even when much diluted, they 

 cannot pass through the normal stomach alive except 

 under the most unusual circumstances." Experiments 

 with B. coli from the pig, however, indicate that tro- 



17s 



