IMMUNOLOGY 855 



percentage of human beings recovering from kala azar, it is impossible 

 to decide from the available data whether suppression of the infection 

 is predominantly due to a destruction of the parasites or to an inhibition 

 of their reproduction. The fact that immunity is more or less generalized 

 indicates, however, that some humoral principle is involved. 



NONLETHAL INFECTION WITH THE Trypanosoma lewisi group of 



TRYPANOSOMES 



The trypanosomes differ from the plasmodia in that they live in the 

 blood stream and do not infect the red blood cells. Some are pathogenic. 

 Others are nonpathogenic. Among the latter is a large group of trypano- 

 somes which produce nonlethal infections in rodents, are morphologi- 

 cally identical or similar to T. lewisi of the rat, and are differentiated 

 almost entirely by their specificity for their rodent hosts. Of these, T. 

 lewisi of the rat and T. duttoni of the mouse have been extensively 

 studied and furnish the basis for the conclusions in the following discus- 

 sion. 



The number curve of T. lewisi in the rat when a few parasites are 

 injected, as shown by Steffan (1921), W. H. Taliaferro and L. G. 

 Taliaferro (1922), W. H. Taliaferro (1924), and Coventry (1925), 

 starts, as does a malarial infection, with an incubation period and an 

 acute rise, until the trypanosomes may reach 300,000 or more per cu. 

 mm. Then there is a crisis between the eighth and the fourteenth days, 

 during which most of the parasites are destroyed. Those that remain 

 continue to live in the blood for some time (varying from several weeks 

 to several months), until they are removed either gradually or suddenly. 

 Thereafter they are not found in the blood, and relapses seldom occur, 

 but a few may persist, as ascertained by relapses which sometimes ensue 

 after splenectomy and blockade with India ink, or after other conditions 

 which lower the immunity of the host. Whether a few always persist 

 cannot be determined from the available data. The rat, however, is 

 immune to reinfection for long periods, as was first shown by Kanthack 

 et al. (1898). Fatal infections of T. lewisi in young rats were first 

 reported by Jiirgens in 1902 (see also W. H. Brown, 1914; Herrick and 

 Cross, 1936; Duca, 1939; Culbertson and Wotton, 1939). They are 

 often complicated by a concomitant occurrence of either or both of the 



