HOST- PARASITE RELATIONS I INTESTINAL PROTOZOA 



amoebae is also of interest. Amoeba proteus seems to be 

 very widespread, having been found in bodies of fresh 

 water in many countries. Endamocha colt is likewise cos- 

 mopolitan in its distribution, occurring in man wherever 

 it has been looked for. The factors that control the dis- 

 tribution of the two species are different in certain re- 

 spects, but the end result is the same. Both species are 

 spread by running water, A. proteus mostly in the active 

 stage and E. coli in the cyst stage whenever water is pol- 

 luted with cyst-containing feces. There is evidence that 

 E. coli is also carried to the food or drink of man by flies 

 and we know it to be transported to all parts of the world 

 by its human host. A. proteus is no doubt carried from 

 one pond to another by aquatic birds and by other or- 

 ganisms and may also be transported by man on aquatic 

 animals or plants. 



Similar comparisons could be made with similar results 

 between other species of free-living and parasitic pro- 

 tozoa. It seems unnecessary, however, to describe in de- 

 tail the similarities and differences between these types 

 of protozoa, but certain characteristics in the lives of 

 these organisms may be referred to with profit. 



Tissue invasion. Balantidium coli is an occasional in- 

 habitant of the large intestine of man, especially in trop- 

 ical and subtropical countries. It is very similar to the 

 common free-living ciliate, Paramoecium caudatum, and 

 could be compared with this species just as Endamocba 

 coli has been compared above with Amoeba proteus. One 

 very interesting activity in the life-cycle of B. coli is its 

 invasion of the tissues of the intestinal wall and the pro- 

 duction of ulcers and dysenteric symptoms. The evidence 



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