HOST-PARASITE RELATIONS! INTESTINAL PROTOZOA 



case of many species of protozoa. Surveys of intestinal 

 protozoa in various parts of the world have established 

 the fact that children, as a rule, are more highly infected 

 than adults. Perhaps hosts that are infected while young 

 acquire immunity before the adult stage is reached; but 

 a protozoon that is able to live in a host for only a brief 

 period cannot be considered entirely successful as a spe- 

 cific parasite. Similar results have been obtained in lab- 

 oratory experiments; for example, kittens become in- 

 fected with Endamocha histolytica much more readily 

 than adult cats. There are no essential differences among 

 the cysts used in such experiments; hence the factors 

 involved must reside in the hosts. Some type of resist- 

 ance develops with age. Are the cysts of intestinal pro- 

 tozoa unable to excyst? Are the trophozoites prevented 

 in some way from entering the tissues? Does the medium 

 (intestinal content) become unfavorable as the host 

 grows older? 



Finally the point may be emphasized that the subject 

 of host-parasite specificity is one that needs and is 

 worthy of careful investigation, and that this section 

 is intended merely to indicate some of the interesting 

 problems involved. 



IV. Problems in Host-Parasite Relations among 



vm Intestinal Protozoa 



The succeeding chapters in this book are devoted to a 

 discussion of the Intestinal Amoebae, Intestinal Flagel- 

 lates, Intestinal Coccidia, and Intestinal Infusoria of 

 man. An attempt is made to follow the plan indicated 



52 



