Immunity and Resistance 



639 



few flagellates are present during the first three weeks. A week or so later, 

 the population reaches a moderate density and then undergoes a crisis 

 in which most of the flagellates are destroyed. During the succeeding 

 chronic phase the survivors, as the relapse strain, multiply at the normal 

 rate in the presence of a trypanolysin to which they seem to be no longer 

 susceptible. The resulting increase in trypanosomes produces the first 

 relapse, which is followed by a second crisis. This crisis presumably 

 involves a new lysin, the appearance of which is induced by the anti- 

 genically modified relapse strain. The action of the second lysin on 



300 



thousands/mm-^ 



200 



100 



% 



\ — ' "-1 



'*"'■' *"* * divding forms 



10 



20 



30 



days 



Fig. 14. 3. A non-lethal infection: Trypanosoma leiuisi in a rat (after 

 Taliaferro and Pavlinova). Early cessation of reproduction is indicated by 

 marked decreases in the coefficient of variation (C. V.) and the percentage of 

 dividing forms. 



the relapse strain is comparable to that of the first on the original, or 

 passage strain. The typical infection shows only a few crises and relapses, 

 and death of the host usually occurs during the second month. Since 

 fission-rate remains practically constant, as indicated by the coefficient of 

 variation, the acquired immunity is expressed primarily through the 

 action of lysins which produce the crises. 



Trypanosoma lewisi in the rat (178) produces a non-lethal infection. 

 After a short incubation period, the parasite density rises rapidly (Fig. 

 14. 3), usually without killing the adult rat. During the second week, the 

 trypanosomes begin to decrease in number. This decrease may be quite 

 rapid in some rats (178), somewhat more gradual in others (Fig. 14. 3). 



