CHAPTER VII 

 INTESTINAL FLAGELLATES AND CILIATES 



The human intestine furnishes a habitat for a considerable 

 number of animals belonging to all four classes of Protozoa, 

 though it is not so subject to such infections as are the digestive 

 tracts of many lower animals, especially the ruminants. By 

 far the most important intestinal protozoan of man is an ameba, 

 Endamoeba histolytica, discussed in connection with other para- 

 sitic amebse in a subsequent chapter. Probably next to the 

 amebse from a pathogenic point of view should stand the ciliate, 

 Balantidium coli, which is, however, not common in most parts 

 of the world. The various flagellates of the intestine, from the 

 simple bi-flagellate forms, such as Bodo, Cercomonas and Embado- 

 monas, some of which are probably only accidentally parasitic, to 

 the highly organized multi-flagellate forms, such as Trichomonas 

 and Giardia, which are very common human parasites, differ 

 greatly as regards their pathogenic importance, and opinions 

 do not agree concerning the importance of particular ones. 



General Characteristics of Intestinal Protozoa. In some 

 respects nearly all the Protozoa which make their home in the 

 digestive tracts of animals resemble one another. Nearly all of 

 them secrete for themselves resistant transparent cysts which 

 protect them from drying up or from the presence of an unfa- 

 vorable medium. In the encysted state intestinal protozoans are 

 able to exist under the unfavorable conditions found outside the 

 body of the host, and are capable of remaining in this state in a 

 sort of torpid condition for long periods of time until they gain 

 access to a new host. The cysts of intestinal protozoans are 

 analogous to the resistant eggs of intestinal worms, and like 

 worm eggs their presence in the faeces of infected persons serves as 

 a convenient means of diagnosis. The unencysted protozoans 

 which may be carried out of the intestine die quickly and 

 probably could not produce a new infection even if swallowed 

 immediately, since in some species at least they are unable to 



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