134 



The nuclei have exactly the same structure as in the free amebse 

 but are smaller. The majority of freshly passed cysts contain 

 blocks or masses of a highly refractile substance, called chroma- 

 toid bodies by Dobell (Fig. 36B, Chr. b.). These gradually dis- 

 appear in the course of a few days. The normal cysts of the 

 dysentery ameba are distinguishable from those of E. coli by 

 the number of nuclei, and also by the central position of the 

 karyosomes. The cysts pass out with the faeces of the infected 

 individual, and live outside the body for a number of weeks if 

 kept cool and moist. They are destroyed by desiccation and 

 degenerate rapidly at high temperatures. 



As cysts the amebse are dispersed by " night soil " when used 

 as fertilizer, by seepage into water, by flies and other insects, 

 or by other means. If by any of these means they reach human 

 food or water and thus secure entrance to the digestive tract, 

 the cyst wall is dissolved by the pancreatic juice and four little 

 amebse, each containing one of the daughter nuclei which were 

 formed when the cyst first developed, are set free in the intestine 

 and begin to grow and multiply. The active vegetative amebse 

 from an acute case of dysentery are destroyed in the stomach if 

 swallowed, and only the parasites in the encysted stage, with an 

 enclosing capsule to protect them from being digested, can reach 

 the intestine and cause disease. 



The dysentery amebse inhabit the tissues of the walls of the 

 large intestine, where they cause more or less extensive ulcer- 

 ation. From this point they are carried by the lymph or blood 

 stream to various other parts of the body, where they form 

 local abscesses. Such abscesses are commonest in the liver but 

 occasionally occur in other organs, such as the lungs, brain and 

 spleen. The amebse probably live entirely on the living tissues 

 and blood corpuscles, the tissues being destroyed by means of a 

 powerful cell-destroying substance produced by them. 



Until recently the dysentery ameba was thought to be almost 

 exclusively a parasite of tropical and subtropical countries, but 

 in recent years it has been shown to be widespread throughout 

 temperate climates as well, having been found endemic in nearly 

 all parts of the United States and in the British Isles and Central 

 Europe. The amount of endemic infection in temperate parts 

 of both Europe and America has been greatly increased since the 

 European war. Kofoid Kornhauser and Plate found 10.8 per 



