864 IMMUNOLOGY 



uniform increases in the length of each stage. Furthermore, Krijgs- 

 man's inability to find degenerating stages in the blood early in the 

 infection and his finding them during the terminal phases is not con- 

 clusive evidence that trypanosomes are not dying during the early low 

 rates of increase. It is very likely that the macrophages remove such 

 forms less quickly during the latter part than during the early part of 

 the infection, because they have become partially blockaded. The inter- 

 pretation of the varying rates of increase of T. evanst in the mouse, as 

 found by Krijgsman, will therefore have to await further analysis. From 

 an enormous mass of work on the mouse as a carrier of so-called pas- 

 sage strains, however, it appears that no trypanolysin usually develops 

 in the mouse which kills most of them and toward which the residue 

 become antigenically resistant, as has been demonstrated in the infec- 

 tions to be described in the following section. 



We may accordingly conclude that the course of the infection in the 

 mouse and sometimes in the rat most closely approximates the simplest 

 type of infection, which increases as a geometrical progression and in 

 which little, if any, immunity is acquired of either an ablastic or trypano- 

 cidal type. 



Intermittent Fatal Trypanosomiasis in Various Laboratory 

 Animals 



When the same pathogenic trypanosomes considered in the preced- 

 ing section are grown in the guinea pig, the infection is typically char- 

 acterized by an incubation period, followed by alternate increases (the 

 first is an acute rise and the succeeding ones are relapses) and decreases 

 (crises) in the parasite population until the animal dies. Besides the 

 guinea pig, this kind of infection is observed in rabbits, dogs, cats, and 

 occasionally in rats infected with these same trypanosomes, in man 

 infected with T . rhodesiense and sometimes in mice infected with T. 

 congolense. Sometimes the initial acute rise and crisis do not develop, 

 as is shown in Figure 192. In other animals, such as sheep, the entire 

 infection is of such low grade that trypanosomes are rarely found and 

 then only in thick film. These statements are based on work by Ross 

 and D. Thompson (1910, 1911), J. G. Thomson (1912), W. H. 

 Taliaferro and L. G. Taliaferro (1922), Knowles and Das Gupta 

 (1928), Davis (1931), Krijgsman (1933), Browning et al (1934), 



