HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



chemical experiments on free-living protozoa, are par- 

 ticularly sensitive, since harmful substances may easily 

 reach the interior of the body by way of the cytostome. 

 The fact, however, that B. coli is able to live in the large 

 intestine of man, monkey, and pig proves that it can 

 withstand the different conditions encountered in these 

 different species of hosts, and indicates an adaptability 

 or resistance greater than that of other human species. 



We have a fairly good idea regarding the incidence of 

 infection with B. coli in pigs and man but not in monkeys. 

 The pig seems to be most susceptible to infection and the 

 usual absence of pathological conditions indicates that 

 the association is an old one. Perhaps the high incidence 

 of infection in pigs may be accounted for by the produc- 

 tion in this animal of numerous infective cysts and by its 

 uncleanliness which probably results in the ingestion of 

 large numbers of cysts daily. In man, on the contrary, 

 cysts are rare, hence the transfer of infective cysts from 

 man to man is no doubt exceptional and man probably 

 receives his infection in almost every case as a result of 

 the ingestion of cysts from pigs. This type of infection, 

 however, is also exceptional since only a very small per- 

 centage of those who work with pigs and who fre- 

 quently swallow cysts from pigs become infected, thus 

 indicating high powers of resistance in the human host. 



Both man and monkeys may be infected with balan- 

 tidia without exhibiting symptoms but both may also 

 suffer from more or less severe diarrhea or dysentery 

 which sometimes terminates fatally. The adjustments be- 

 tween the balantidia and man and monkey are therefore 

 less perfect than between these ciliates and the pig. 



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