HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



factors of the environment. Those that are able to Hve 

 and reproduce may be said to have successfully invaded 

 this particular habitat. Each species of animal has an 

 optimum habitat ; this for a parasitic protozoon is a fa- 

 vorable host and for a free-living protozoon a body of 

 water with certain physical, chemical and biological char- 

 acteristics. 



Acquired resistance. One of the effects of the infec- 

 tion of animals with parasitic organisms is the produc- 

 tion by the host of an active (acquired) resistance which 

 may result in the destruction of many and often all of 

 the parasites and the immunity of the host to subsequent 

 infection. There is no type of resistance similar to this 

 that may be acquired by the environment of free-living 

 protozoa ; only living organisms are capable of this type 

 of reaction. However, both parasitic and free-living pro- 

 tozoa may ''foul their own nests" by their secretions and 

 excretions to such an extent as to make their environ- 

 ments unfit for further life activities. In this way cul- 

 tures of free-living species may die out and infections 

 with parasitic species may come to an end. 



Latency and relapse. An interesting phenomenon char- 

 acteristic of many parasitic infections is the cessation of 

 symptoms for a time, followed by the appearance of 

 symptoms again, that is, a relapse. Every one who has 

 collected protozoa from any particular pond at various 

 times from year to year knows that a condition resem- 

 bling relapse exists in such an environment. Sometimes 

 a certain species is very abundant ; at other times speci- 

 mens can be found only by patient search. 



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