HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



activities of the two species are also different within the 

 intestine. E. coli lives in the lumen on bacteria and food 

 particles, whereas E. histolytica apparently depends on 

 tissue elements from the intestinal wall. E. histolytica 

 must therefore gain access to this tissue and successfully 

 attack it against the resistance of the host, whereas 

 E, coli is continually bathed in a favorable nutrient me- 

 dium. Furthermore, E. histolytica often brings about a 

 diarrheic or dysenteric condition during which no infec- 

 tive (cyst) stages are passed by the host, and sometimes 

 this species actually brings about the death of the host, 

 thus destroying its own chances of further distribu- 

 tion. 



The conclusion is reached that the behavior of the host 

 or intermediate host plays an important role in host- 

 parasite specificity since the infective stage of a parasite 

 can reach its specific host only by being passively trans- 

 ferred by the latter; and this must occur regularly in 

 nature in order that the race of parasites may continue 

 to exist. 



(2) Do SPECIES OF PROTOZOAN PARASITES THAT ARE 

 RESTRICTED TO ONE SPECIES OF HOST GAIN ACCESS TO 



OTHER SPECIES OF HOSTS? The answcr to this question 

 differs for the different species of parasites and depends, 

 as above, on the behavior of the hosts and intermediate 

 hosts. Man's food and drink, for example, are no doubt 

 frequently contaminated by the feces of rats, mice, cats, 

 dogs, and other domestic animals that contain living, in- 

 fective cysts of various intestinal protozoa, such as 

 Endamoeha muris of the rat, Giardia canis of the dog, 

 and Isospora felis of the cat. No human beings, however, 



48 



