XI] CAUSAL AGENT 1 97 



species of birds, but there is no justification for this course, as 

 hitherto all attempts to find any specific differences, apart 

 from the kind of host, have failed. Therefore, for the present, 

 all the plasmodial parasites occurring in birds may be regarded 

 as belonging to one species, Plasmodium prczcox (Grassi and 

 Feletti, 1890). 



The parasites have a decidedly pathogenic action upon the 

 birds they infest. They grow at the expense of the red cor- 

 puscles, which lose their haemoglobin, and gradually degenerate. 

 As a result infected birds become anaemic and, in addition, the 

 substances produced by the parasites seem to have a general 

 toxic effect upon the organs of the body. As in the case 

 of human malaria, the body temperature rises, but rarely 

 more than i-i'5 C. Birds not infrequently succumb to the 

 infection, especially during the early part of the year, when 

 climatic conditions are somewhat severe. 



Causal agent. Plasmodium pracox in its general form 

 somewhat resembles P. vivax. The small amoeboid sporozoites 

 penetrate into the red cells and there generally assume a some- 

 what triangular form, with a round vesicular nucleus containing 

 a karyosome. This young trophozoite grows at the expense of 

 the contents of the red cell and deposits waste materials in 

 the form of pigment granules, which are scattered through its 

 protoplasm. Danilewsky refers to these young forms as 

 " pseudo-vacuolae." The protoplasm is finely granular and 

 contains numerous vacuoles. 



According to Labbe there are two kinds of schizogony in 

 the circulating blood. Sometimes the parasite divides up 

 into six or seven merozoites arranged in the form of a rosette, 

 whilst at other times the parasite divides up into very numerous 

 merozoites scattered irregularly throughout the infected blood 

 corpuscle. In both cases the pigment and waste materials 

 are left behind in the form of a residual body. The merozoites 

 escape and penetrate other blood corpuscles, where they either 

 develop and repeat the schizogonic cycle, or give rise to the 

 gametocytes. The development of the latter closely resembles 

 that of the corresponding forms in other species of Plasmodia 

 (e.g.' P. vivax}. 



