PLASMODIUM, HAEMOPROTEUS AND LEUCOCYTOZCX5N 



277 



have budded (Cowan) or perhaps a hyper- 

 trophied host cell nucleus (Huff). 



According to Cowan, spherical pri- 

 mary cytomeres are first formed. Their 

 chromatin first diffuses and then prolifer- 

 ates to form peripheral clusters, which 

 separate to form secondary cytomeres, 

 which in turn multiply in the same manner. 

 The multiplying cytomeres become smaller 

 and more granular, their chromatin be- 

 comes more concentrated, and finally 

 merozoite-like bodies are formed. These 

 reproduce until the central body is greatly 

 compressed and the megaloschizont mem- 

 brane is ruptured, releasing the mero- 

 zoites into the blood. Many thousands of 

 bipolar merozoites are produced by each 

 megaloschizont. 



In addition to the hepatic schizonts 

 and megaloschizonts, small structures 

 thought to be schizonts were found by 

 R. C. Ritchie (cited by Fallis, Anderson 

 and Bennett, 1956) in the Kupffer cells 

 of the liver of a duck killed 3 days after 

 exposure. 



On the basis of these observations, 

 Fallis, Anderson and Bennett postulated 

 the following life cycle: The first asexual 

 generation occurs in the Kupffer cells of 

 the liver. Some of the merozoites from 

 these schizonts may develop into gameto- 

 cytes; this explains the presence of a few 

 large parasites in the blood 5 to 6 days 

 after infection. Other merozoites from 

 the first generation schizonts develop into 

 hepatic schizonts, megaloschizonts and 

 perhaps other Kupffer cell schizonts. 

 Merozoites arising from megaloschizonts 

 and hepatic schizonts develop into gameto- 

 cytes which flood into the peripheral circu- 

 lation beginning 6 to 7 days after infection. 

 Some of these merozoites presumably de- 

 velop into another asexual generation. 



The development of the gametocytes 

 in the blood cells has already been men- 

 tioned in the section on morphology. 

 According to Chernin (1952), the gameto- 

 cytes may disappear from the blood about 

 30 days after they first appear. Following 

 this primary parasitemia, which begins in 

 mid-summer in northern Michigan, only 



an occasional parasite is seen in the blood 

 during the fall and winter (O'Roke, 1934; 

 Huff, 1942; Chernin, 1952a). With the 

 development of sexual activity in the spring, 

 gametocytes reappear in the blood and in 

 some cases continue to be present thruout 

 the summer. 



It is clear from this account that schi- 

 zogony continues in the internal organs for 

 an indefinite, long time, altho at a much 

 reduced rate. There are about 1000 times 

 fewer gametocytes in the relapse phase 

 than in the primary infection, and these 

 adult birds are not seriously affected. 

 However, they serve as the source of in- 

 fection for the new crop of ducklings. 



According to Chernin (1952a), the 

 early season infections in ducklings are 

 comparatively light, but the heavier pool 

 of gametocytes provided by these primary 

 infections in the first crop ducklings en- 

 sures the heavier and highly fatal infec- 

 tions which occur during midsummer. 



The vectors of L. simondi are various 

 species of blackflies {ShnuUmn). O'Roke 

 (1934) showed that S. venustum is the vec- 

 tor in Michigan. Fallis, Anderson and 

 Bennett (1956) found that S. croxtoni and 

 S. euryacbniniculHm are the important 

 vectors during the early part of the black- 

 fly season (May to June) in Ontario, while 

 S. rugglesi is the important vector in late 

 June and July. 



In the blackfly's stomach (O'Roke, 

 1934; Fallis, Davies and Vickers, 1951; 

 Rawley, 1953), 4 to 8 microgametes are 

 formed within a few minutes by exflagella- 

 tion from the microgametocytes. These 

 fertilize the macrogametes to form a mo- 

 tile zygote or ookinete about 33 /i long and 

 5|Lt wide. Ookinetes are present in the 

 blackfly stomach 2 to 6 hours after inges- 

 tion of infected blood. They develop into 

 oocysts both in the stomach wall and in 

 the stomach itself. 



The oocysts are 10 to 13 |i in diameter. 

 They can be found 2 to 3 days after infection, 

 and complete their development 2. 5 to 4 

 days after infection. They produce rela- 

 tively few sporozoites compared with 



