HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



I. EPIDEMIOLOGY OF TRANSMISSION 



(i) Infective stage. Perhaps the most satisfactory 

 point at which to begin the study of the biology of host- 

 parasite relations is the infective stage of the parasite. 

 The organisms during the period of the infective stage 

 are usually subjected to various environmental factors 

 from the time they escape from the body until they gain 

 entrance to a new host. The idea maintained by many of 

 the older authorities that disease-producing organisms 

 may multiply outside of the host and bring about foci of 

 infection in soil or water has been abandoned since it 

 has been abundantly proved that very little if any increase 

 in numbers occurs under these conditions. 



Most of the disease-producing protozoa of man escape 

 from the body in a cyst-like condition and spend part of 

 their life-cycle outside of the host. The problems en- 

 countered during this period are extremely serious and 

 very few of the organisms survive the vicissitudes of a 

 free-living existence. The two principal problems en- 

 countered by the parasites are ( i ) that of withstanding 

 the factors in their new environment and (2) that of 

 developing to the infective stage. The first problem is 

 particularly difficult for protozoa such as the intestinal 

 flagellate, Trichomonas hominis (Fig. 9), that do not 

 form resistant cysts but pass from one host to another 

 in the active, trophozoite stage. These trophozoites can 

 exist only in a liquid medium and are presumably very 

 susceptible to modifications of temperature, to chemical 

 changes in the environment and to mechanical injury. 

 Most protozoa, however, are protected by one or more 



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