384 RESEARCH IN PROTOZOOLOGY 



These figures were obtained in every case from observation of 

 canaries which had been infected by blood inoculation ; there is 

 however great individual variation in different birds and the dura- 

 tion of the prepatent period is said to be greater when mosquito 

 inoculation is used. 



5. Acute stage of infection. In the case of Plasmodium cathe- 

 merium the crisis of the infection is generally reached three or 

 four days after parasites first appear in the peripheral blood, and 

 four or five days later they have nearly or quite disappeared. The 

 acute stage of the infection produced by Plasmodium inconstans 

 is usually considerably more prolonged, frequently lasting for two 

 weeks or more, and it is seldom as severe. Plasmodium prcecox 

 is the most benign of the three. Parasites are often so few that, 

 even at the crisis of the infection, it may be necessary to examine 

 several oil-immersion fields to find any, and in this case it is the 

 gametocytes which are most frequently seen in the peripheral 

 blood. But although the infections are usually light they are very 

 prolonged, and in many cases the crescent-shaped gametocytes of 

 Plasmodium prcecox can be found in the peripheral blood months 

 after the acute stage of the infection has passed. 



6. Pathogenicity. The three species also differ in pathogenicity. 

 Plasmodium cathemerium is the most pathogenic of them all, the 

 mortality averaging about thirty-three per cent during the acute 

 stage. The death rate from infection with either of the other two 

 parasites is almost negligible if the birds are healthy to begin 

 with, and kept under favorable environmental conditions. 



7. Pathology. The pathology of avian malaria is very similar to 

 that of human malaria. The most marked changes are seen in the 

 spleen which undergoes an enormous increase in size, and in the 

 blood. The number of red cells drops very greatly during the 

 course of the infection, and Ben Harcel (1923) found that the 

 mononuclears increased in number, and that the bone marrow 

 showed a marked hyperplasia, with an accompanying decrease in 

 the amount of adipose tissue present. 



8. Relapses. When death does not occur during the acute stage 

 the birds appear to recover completely, but except in rare cases 

 the disease simply passes into a long period of latency, with occa- 

 sional slight relapses which seem to be most frequent during the 

 fall and winter, in the case of Plasmodium cathemerium at least 

 (Manwell, 1929). Relapse is apparently less frequent about two 



