58 PROTOZOAN PARASITISM 



HABITAT AND EFFECTS 



When the cysts of Endamoeba histolytica are swal- 

 lowed by man they pass unharmed to the intestine, 

 where the cyst is dissolved or ruptured and the tropho- 

 zoite liberated. The organism is probably liber- 

 ated as a four nucleated amoeba and then completes 

 the division. The daughter amoebae pass down 

 to the large intestine, the natural habitat. Growing 

 to mature size they become seated largely in the por- 

 tions of this tract in which the content accumulates 

 and remains longest, notably the caecum and the 

 descending colon, although the whole of the colon 

 may be involved in the infection, especially in am- 

 oebic dysentery, and there may be ulcers on and 

 above the ileo-caecal valve. 



Depending upon unknown factors, involving the 

 capabilities of the parasite for invasion of the tissue 

 as balanced against the resistance of the host to it, 

 the results of the infection may be chronic intestinal 

 amoebiasis or clinical amoebic dysentery, the for- 

 mer more commonly. 



The manner of penetration and invasion of the 

 wall of the intestine is subject to some debate, as to 

 whether the amoebae pass through an intact mucosa 

 between living epithelial cells or produce destruction 

 of epithelium and a gateway for their admission. 

 For many years it was believed by some that they 

 gained entrance through lesions of other primary 

 production. This is evidently not essential and they 



