674 INTESTINAL PROTOZOA AND HOST-PARASITE RELATIONS 



man. It finds its optimum habitat in the duodenum where it clings to the epithelial 

 cells with its ventral sucking disk and thus is able to withstand the downward force 

 of peristalsis. Here large numbers are produced by binary division. Under certain 

 conditions not yet known, the trophozoites encyst and appear in the feces at irregular 

 intervals. 



Flagellate diarrhea, also known as "lambliasis" and "giardiasis," is frequently at- 

 tributed to this species. Most of the infected human beings do not exhibit symptoms 

 but are carriers. As in the case of Trichomonas hominis, the presence of the organisms 

 in diarrheic stools does not convict them of having caused the disturbance and hence 

 the pathogenicity of G. lamhlia is still in doubt. Recently a number of physicians have 

 accused this flagellate of causing cholecystitis, but reports are contradictory and no 

 definite conclusion has been reached. 



For many years it was supposed that the giardias that occur in human beings 

 and lower animals belonged to the same species and that human beings became in- 

 fected by swallowing food or drink contaminated with infected feces of rats, mice, etc. 

 More recent work (see Hegner [1927] for discussion) indicates that each host species 

 has its own particular species of giardia, with the exception of rats and mice which 

 are infected with the same species, and that cross-infection is rare or impossible. If 

 this is true, then human beings can be infef ted only by ingesting cysts of the human 

 species of giardia, Giardia lamblia. 



OTHER INTESTINAL FLAGELLATES 



Chilomastix mesnili. — This species is present in about 10 per cent of the general 

 population. It lives in the large intestine, has a cyst stage in its life-cycle, and is 

 probably transmitted in the same way as are other intestinal protozoa. C. mesnili is 

 also accused of causing flagellate diarrhea but has not yet been proved guilty. 



Emhadomonas intestinalis . — This species is apparently rare, having been re- 

 ported only a few times, and is hence of no practical importance. 



Tricercomonas intestinalis. — This species also is rare, having been reported in less 

 than a hundred cases. Its small size and the difficulty of studying it have resulted in 

 much confusion between this and other intestinal flagellates. 



HOST-PARASITE RELATIONS OF INTESTINAL CILIATES 



Balantidium coli is the only species of intestinal ciliate that we know with cer- 

 tainty to be a regular inhabitant of man. This large protozoon lives in the large in- 

 testine in the trophozoite stage, and produces cysts, which pass out in the feces and 

 are no doubt responsible for transmission. The trophozoite, as Rees (1927) has found, 

 will live at room temperature in material taken from the intestine of infected pigs 

 for as long as ten days. The cysts are no doubt more resistant, and hence there is 

 considerable opportunity for the contamination of food or drink with viable organ- 

 isms. 



When one considers the fact that the balantidia of man and pig are supposed to be 

 the same species, and the probability that large numbers of human beings frequently 

 ingest specimens from the pig, it seems strange that the incidence of infection should 

 be so low in the general population. Balantidiosis is very rare in temperate regions, 



