HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 

 4. THE PRIMARY SITE OF INFECTION 



The large intestine is the primary site of infection 

 with E. histolytica. This is an excellent location for the 

 parasite since escape from the body of the host, which 

 is of great importance for the maintenance of the race, 

 is easy from this habitat. Access to the tissues of the 

 wall was for many years considered necessary since the 

 amoebae were supposed to live only on tissue elements, 

 but the discovery that they feed on bacteria in cultures 

 suggests that they are able to exist in the lumen of the 

 intestine or on the outside of its wall. Those trophozoites 

 or cysts that become embedded in the fecal matter are 

 carried out of the body, hence it seems probable that 

 only those excysted amoebae that are able to reach the 

 intestinal wall to which they can attach themselves are 

 able to withstand the peristaltic movements of the bowel. 

 These, therefore, are the specimens responsible for bring- 

 ing about an infection. 



Experiments with E. histolytica on cats indicate that 

 the primary site of infection is located in that part of 

 the large intestine where stasis first occurs. Stasis evi- 

 dently prevents the organisms from being carried down 

 the intestine and gives them sufficient time to reach and 

 anchor themselves to the wall of the intestine. Data 

 regarding the location of ulcers in human cases of amoe- 

 biasis favor this hypothesis. Thus a resume of records 

 of 6800 post-mortem examinations made in the Panama 

 Canal Zone from 1905 to 1923 principally by Darling 

 and Clark has been published by the latter (Clark, 1924). 

 Of these, 186 died of amoebiasis or had amoebic ulcers 



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