HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



cent association, since most parasites live in harmony 

 with their hosts — a condition that is supposed to have 

 developed during the course of evolution. 



Types of parasitism are distinguished largely on the 

 basis of the method of attack of the parasite. Among the 

 amoebae living in man, for example, is Endamoeha coli 

 (Fig. 2a) which inhabits the lumen of the large intestine 

 where it probably lives on food taken in by the host ; this 

 type of parasite is known as a commensal and is some- 

 times termed a food-robber. It does not live in any other 

 species of host nor outside of the body (except in the cyst 

 stage) and is therefore a permanent, obligatory parasite 

 and non-pathogenic. Another species of the same genus, 

 Endamoeha histolytica (Fig. la), also lives in the large 

 intestine of man but feeds on tissue elements which it ap- 

 parently dissolves with the aid of proteolytic enzymes 

 that it secretes, or engulfs en masse. Usually the host is 

 able to repair the tissue as rapidly as it is injured, but of- 

 ten the parasites gain the upper hand and amoebic dysen- 

 tery results, sometimes ending fatally. This organism 

 is a permanent, obligatory parasite that is apparently 

 always pathogenic and sometimes lethal. Other parasites 

 penetrate tissue cells and develop within them at the 

 expense of the surrounding protoplasm. To this type 

 belong the coccidia that live on cells of the intestinal epi- 

 thelium. 



Parasitic protozoa may produce toxic substances or 

 zootoxins. Almost nothing is known about these, but 

 that they exist is certain, and that they act much like 

 bacterial toxins is probable. 



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