HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



(2) Avenues of infection. Reaching and invading a 

 new host is perhaps the most serious problem in the entire 

 life-cycle of a parasitic protozoon so far as the mainte- 

 nance of the species is concerned. Only the smallest frac- 

 tion of the total number of infective organisms can pos- 

 sibly reach a susceptible animal in which to live, and only 

 the almost inconceivable fecundity of the parasites pre- 

 vents the various species from dying out. In the most sim- 

 ple cases the infective stage of the parasite is ingested 

 with the food or drink of the proper host. The parasite 

 is passively carried in the medium by which it is sur- 

 rounded and it is the behavior of the host that leads 

 to invasion. This is the contaminative method of para- 

 site transmission. Laboratory experiments, mostly with 

 lower animals, have established this as an effective 

 method and there is no other obvious way in which infec- 

 tive cysts in nature can obtain entrance to the host. 



The problem of the parasite is to reach the mouth of 

 the host before death results from drying, bacterial ac- 

 tion, or starvation within the cyst. A moist, warm climate 

 is therefore favorable for transmission. Insanitary con- 

 ditions due to neglect on the part of the host are also 

 favorable since this leads to the pollution of drinking 

 water, milk and other food substances. Flies probably 

 play an important role in the distribution of the cysts. 

 In general it may be said that transmission by the con- 

 taminative method is more easily effected in rural than 

 in urban communities, and in the tropics than in the tem- 

 perate regions. 



There are three other principal methods of reaching 

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