HOST-PARASITE RELATIONS! INTESTINAL PROTOZOA 



human "intestinal" protozoa on the basis of specimens 

 found in the feces which were not active within the intes- 

 tine but had passed through in the cyst stage and had 

 emerged only after the feces were passed; these are 

 "coprozoic" protoza, a considerable number of which 

 have been described. 



5. PASSIVE (natural) RESISTANCE OF THE PARASITE 



If some parasitic protozoa were not able to overcome 

 the obstacles presented by the host, animals would be to- 

 tally free from them. Thus far no such animal has been 

 discovered. This passive resistance of the parasite, as in 

 the case of that of the host, may have no relation to an- 

 cestral infections or may be due in part to evolutionary 

 processes. Whether the resistant coverings of protozoan 

 cysts serve to enable the parasite to reach a certain loca- 

 tion within the host or are more important as a protec- 

 tion against injury during their life outside of the host it 

 is not possible to decide with certainty. It is known that 

 such cysts will survive when subjected to strong solutions 

 of various chemicals (see p. 71) and might therefore 

 easily withstand the body juices of the host. It is also 

 known, as pointed out below in the case of Trichomonas 

 hominis (see p. 142), that active trophozoites may pass 

 unharmed through the digestive tract and set up an in- 

 fection in the cecum. The value of the cyst wall as a 

 means of resisting conditions within the host is there- 

 fore very doubtful. 



Every animal is able to exist in an environment in 

 which the various factors cover a considerable range, 



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