72 



COLLEGE ZOOLOGY 



ber of related species may occur. Entamoeba 

 histolytica (Fig. 36), the causative agent of 

 amoebic dysentery, is the only serious dis- 

 ease-producing parasite of this group. About 

 10 per cent of the general population is in- 

 fected, but fortunately most of them are 

 merely carriers; that is, the entamoebas are 

 present but do no damage, and hence no 

 symptoms appear. Occasionally, however, en- 

 tamoebas invade the intestinal wall and form 

 abscesses, which later rupture and become 

 persistent ulcers, resulting in diarrhea and 

 dysentery. Occasionally they are carried to 

 other parts of the body, such as the brain or 

 liver, where abscess formation may cause 

 death. Infected persons pass the cysts in 

 their feces. These cysts gain access to new 

 hosts by contaminated food or water, soiled 



hands, or the activities of flies. Though more 

 prevalent in the tropics, amoebiasis is fairly 

 common in the temperate zones, and out- 

 breaks have even been reported in arctic 

 regions. 



Several drugs have been found effective 

 in treating amoebic dysentery, the choice of 

 which depends on the location of the infec- 

 tion and the condition of the patient. 



There are other intestinal amoebas of less 

 importance such as Entamoeba coli, which 

 is a harmless form often found associated 

 with histolytica. Dientamoeba fragilis is 

 characterized by the presence of two nuclei 

 in the active state; it does not form cvsts. 

 Mild diarrhea appears to have been traced 

 to an infection with Dientamoeba. 



There are four flagellates which live in 



Ciardia lamblia Trichomonas bominis 



Figure 37. Intestinal flagellates of man. Highly magnified. 



