The Protozoa 



143 



PATHOGENIC PROTOZOA 

 Class I. Rhizopoda. 



(a) Entamoeba hystolytica (also called entamoeba dysen- 

 teriae), (Fig. 54). 



Entamoeba histolytica causes a chronic ulcerative process in 

 the large intestine, the so-called amoebic dysentery. The organisms are 

 frequently carried to the liver by the portal circulation and give rise to 

 abscesses which may attain large size and may extend to a pleural cavity 

 or to a lung. 



It is a common infection in the tropics, but occurs also more 

 or less frequently in temperate zones. 



The organism measures fifteen to twenty-five micra in diam- 



Fig. 55. 



Entamoeba gingivalis (buccalis). (After A. T. Smith in Dental Cosmos, Sept., 

 1914.) 



eter. It contains a small, round vesicular nucleus which stains but 

 poorly with the ordinary basic dyes and with alum hematoxylin. The 

 nucleus contains a minute nucleolus. The cytoplasm around the nucleus 

 is finely granular and is surrounded by an outer zone or ectosarc which 

 is transparent and refractive, and which sharply defines the outer limits 

 of the organism. 



The entamoeba hystolytica must be examined on a warm stage 

 in order to detect the characteristic movements ; but it is readily 

 identified in properly fixed tissues owing to its characteristic morphology. 



The organism is quite phagocytic and is frequently found to 

 contain red blood corpuscles, bacteria, or cellular debris. It is able to 

 penetrate fibrous and other tissues and is frequently found in the walls 

 of blood vessels as well as within the blood vessels themselves. 



It secretes a mild toxin (which may be a waste product). This 

 secretion slowly kills the cells in its neighborhood and then gradually 

 dissolves them. 



Amoebae are, however, often found in normal tissues. Some- 

 times the nuclei seem to be fading out. 



The organisms are found principally in the intestines, but some- 

 times also in the liver. 



Cultures of these amoebae have been shown to withstand drying 

 from eleven to fifteen months. 



(b) Entamoeba buccallis (also called E. gingivalis and E. den- 



