PROBLEMS IN HOST-PARASITE SPECIFICITY 



The differences in the percentages noted above and the 

 small numbers of infections that have been recorded for 

 the other species of intestinal protozoa that occur in man 

 are probably due principally to two factors: first, the 

 success of the species in gaining entrance to the digestive 

 tract, and second, the infectivity of the species in the 

 human host. Trichomonas hominis, for example, does 

 not possess a cyst stage in its life-cycle and hence must 

 pass from man to man in the trophozoite stage (Hegner, 

 1924a). The trophozoite stage is not as resistant as the 

 cyst stage; hence it is more often destroyed before in- 

 gested by man than are cysts. This may account for the 

 fact that less than 10 per cent of the general population 

 seems to be infected with this species. But great differ- 

 ences exist between species that are spread by cysts. 

 Endamoeha coli, with infections in about 50 per cent of 

 its possible hosts, seems much more successful than 

 Endamceha histolytica with an incidence of infection of 

 only about 10 per cent. This is true in spite of the fact 

 that E. histolytica cysts are apparently more abundant 

 in fecal material from a host than those of E. coli. The 

 chances of reaching new, susceptible hosts seems about 

 the same for the two species. There may be a difference 

 in the resistance of the cysts of the two species while out- 

 side of the body, especially since the degree of resistance 

 depends somewhat on the thickness of the cyst wall. 

 Perhaps E. coli is more successful because its ripe cysts 

 normally contain eight nuclei and presumably give rise 

 to eight offspring within the intestine, whereas the cysts 

 of E. histolytica possess only four nuclei. This would 

 give E. coli a better chance of starting an infection. The 



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