488 



PROTOZOOLOGY 



known. In addition, some authors are inclined to think that experi- 

 mental birds may have been infected not only by Plasmodium, but 

 also by other organisms such as Haemoproteus, Leucocytozoon or 

 Toxoplasma and that consequently the developmental stages of the 

 latter might have been confused with those of the former. Positive 

 proof that the reported exoerythrocytic schizogony pertains to 

 Plasmodium and further details of this change, depend upon future 

 investigations (Fig. 227). 



mosquito 



Fig. 227. Diagrammatical life-cycle of an avian Plasmodium (Several 

 authors). Well established phases are connected by solid lines, while 

 undetermined and recently suggested phases are indicated by broken 

 lines, a, sporozoite injected into host bird by a mosquito; b-e, exoerythro- 

 cytic schizonts and schizogony in monocytes; f-i, commonly seen schi- 

 zogony in erythrocytes; j, macrogametocyte; k, microgametocyte. 



The incubation period of Plasmodium infections in man varies due 

 to various factors such as the strain, vitality and number of the spo- 

 rozoites injected by the mosquitoes, the varied susceptibility on the 

 part of host, etc. Boyd and co-workers found that the incubation 

 periods for the three species of human Plasmodium which they stud- 

 ied were, as follows: In P. vivax. 8-21 days (the majority 11-14 days) 

 after the bites of infected mosquitoes, but in one case as long as 304 



