ROBERT HEGNER 669 



occur less frequently in the lungs, brain, and other organs, Amebae have been re- 

 corded from almost all the organs and tissues of the body, the most recent addition 

 to the list being the bones, by Kofoid and his colleagues. Several cases of infection 

 of the ileum have been reported. Why the ileum is not more frequently infected is an 

 interesting but unsolved problem. 



Symptoms. — The symptoms due to infection with Endamoeba histolytica are well 

 known. They are diarrhea and dysentery. Most of the human hosts do not exhibit 

 symptoms; presumably the parasites in these cases do not interfere sufficiently with 

 the normal functions of the body to bring about obvious changes. A question of con- 

 siderable current interest is whether or not symptoms are exhibited by carriers. 

 Kofoid and his colleagues believe that they are and that they are able to distinguish a 

 definite disease entity which they call "chronic amebiasis." Acton and Knowles 

 (1924) and Craig (1927) also believe that the so-called "healthy" carrier may exhibit 

 clinical symptoms. 



Pathogenesis. — The pathology of amebiasis is also well known, and descriptions 

 of the lesions are available in a number of textbooks and reference-books on proto- 

 zoology and tropical medicine.' The way in which the organism produces these lesions, 

 however, is not entirely clear. The amebae are supposed to secrete enzymes which 

 dissolve away the tissues of the host. Until quite recently it was supposed that they 

 were unable to obtain nutrition except from the hosts' tissues, but it was found that 

 they ingested bacteria in cultures and hence it is possible that some of them may live 

 in the lumen of the intestine and be non-pathogenic. Those that invade the tissues 

 are large and do not appear to ingest red blood cells. 



Resistance and susceptibility of the host. — The resistance of the host to infection 

 with Endamoeba histolytica, as noted above, may be natural or acquired. There is 

 some evidence that racial differences exist with respect to resistance and susceptibil- 

 ity. For example, the Chinese seem to be more frequently infected than foreigners 

 in China but their infections are not so severe. 



Age likewise seems to have an effect on resistance and susceptibility of the host. 

 It is well known that kittens can be experimentally infected with Endamoeba his- 

 tolytica more easily than adult cats. The nature of this apparent increase in resistance 

 with age is, however, unknown. Certain experiments reported by Kessel (1923) indi- 

 cate that infections with amebae of the same or of a different species add to the 

 resistance of the host. 



One of the most interesting problems in the study of amebiasis is the apparently 

 greater number of severe infections in the Tropics than in the temperate regions. 

 There may be more aggressive strains in the Tropics but there is no definite evidence 

 of this. The facts observed can better be explained by the favorable conditions re- 

 sulting from the warm, moist climate and by frequent mass infections due to the 

 insanitary habits of many natives of the Tropics. The largely carbohydrate diet of 

 the inhabitants of many tropical countries may also be favorable for the growth and 

 multiplication of Endamoeba histolytica in the intestine. 



Immunological reactions. — Practically nothing is known regarding immunity re- 

 actions in amebiasis. Several investigators have attempted to obtain complement- 

 fixation reactions; the experiments of Craig (1927) are the only ones that have given 



