HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



Localization among protozoa that invade the tissues 

 of the host offer many problems that are still unsolved. 

 What factors influence the coccidia to attack the epithe- 

 lial cells of the intestine ? What causes Endamoeha histo- 

 lytica (Fig. la) 2ind Balantidium coli (Fig. 15) to pene- 

 trate the intestinal wall, whereas Endamceha coli (Fig. 

 2a) and Endolimax nana (Fig. 3a), which are also in- 

 habitants of the intestine do not? It may safely be said, 

 that we really know almost nothing about the mechanism 

 of localization of any human protozoon. 



(3) Secondary sites of infection. Many protozoa 

 become localized in a particular organ or tissue and do 

 not spread to other parts of the body. A few, however, 

 bring about secondary foci of infection in other regions. 

 This requires in the first place either movement on the 

 part of the parasite from one place to another or activi- 

 ties within the host that passively transport the parasite 

 to new situations. The circulatory system of the host is 

 the most important agency. For example, the dysentery 

 amoeba, Endamoeha histolytica (Fig. la), sets up a pri- 

 mary infection in the large intestine ; specimens may en- 

 ter the blood stream in the capillaries of the intestinal 

 wall and are carried to all parts of the body; frequently 

 the liver becomes a secondary site of infection and an 

 amoebic liver abscess results. Less often amoebic abscesses 

 occur in the lungs and brain, and amoebae have been dis- 

 covered in many other parts of the body. Recently Kof oid 

 (1923) has described dysentery amoebae from bone and 

 believes that they are responsible for Ely's second type 

 of arthritis; he believes also that these amoebae may be 

 the etiological factor in Hodgkin's disease and in some 



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