HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



ceptible and the parasite infective. This can be done in 

 the laboratory with hosts and parasites that do not ordi- 

 narily encounter each other in nature. A study of proto- 

 zoan transmission in nature, however, reveals the fact 

 that the parasite is passive during its passage from one 

 host to another and that it is the behavior of the host or 

 intermediate host that is responsible for transmission. 

 "Intestinal" protozoa are transmitted in the active (tro- 

 phozoite) stage or in the form of cysts. Those inhabiting 

 the mouth and vagina are probably transferred by con- 

 tact, entirely by the host, during kissing or coitus. These 

 appear to be present in from one-third to one-half of the 

 general population. Those that live in the intestine are 

 transmitted by the contamination of food or drink with 

 feces containing cysts or trophozoites. The host, man, is 

 responsible for the proper disposal of his own feces so 

 that food or drink may not become contaminated. By 

 certain methods of control, such as the elimination of 

 infected food handlers, of the common towel, of soil 

 pollution and house flies, he can to a considerable degree 

 prevent the spread of infection. That insanitary condi- 

 tions are prevalent is indicated by the high incidence of 

 infection among the general population, which is esti- 

 mated approximately as follows : 50 per cent with Enda- 

 }}i(rba coli, 25 per cent with Endolimax nana, 10 per cent 

 wdth Endamceha histolytica, 10 per cent with lodamocha 

 zmlliamsi, 15 per cent with Giardia lamhlia, and 10 per 

 cent with Chilomastix mesnili. Frequently one individual 

 is infected with two or more of these species at the same 

 time. Fortunately Endamoeha histolytica is the only 

 pathogenic species of great importance in this list. 



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