642 Immunity and Resistance 



(Fig. 14. 5). There follows a chronic phase of a week or more, during 

 which a few parasites can be detected in blood smears. The chronic phase 

 gradually fades into a latent stage, during which the parasites cannot 

 be found in the blood. Subsequently, relapses may occur. Each relapse, 

 checked by another crisis, is followed by a new latent period. 



Taliaferro and his associates concluded that the bird acquires no 

 resistance during the incubation period and early acute phase. The 

 first crisis was attributed to a stimulated malaricidal mechanism, which 

 from an early stage of the infection, was already destroying the majority 





to 





to 



10 



30 



50 



70 



day 



Fig. 14. 5. A typical malarial infection in a canary (after Taliaferro). 



of merozoites — approximately 67 per cent at each merogony in specific 

 cases (166). Accumulation of carbon dioxide in the blood, in unfavorable 

 concentration, also has been suggested as a factor contributing to the 

 crisis (139). It was believed that the high malaricidal rate prevented 

 repopulation of the blood during latency, although reproduction con- 

 tinued at the original rate. A relapse was assumed to involve temporary 

 relaxation of the malaricidal mechanism. This concept of the malarial 

 infection in birds was modified slightly by Boyd (24) in observations on 

 P. cathemerium in canaries. During the first day, division-rate was high 

 (16.3 and 16.0 merozoites at merogony in two of the birds). During the 

 next couple of days, while the parasite density was increasing enormously, 



