Chapter 26 

 Order 3 Haemosporidia Danilewsky 



THE development of the Haemosporidia is, on the whole, similar 

 to that of the Coccidia in that they undergo asexual reproduction 

 or schizogony, and also sexual reproduction resulting in sporozoite- 

 f ormation ; but the former takes place in the blood of vertebrates and 

 the latter in the alimentary canal of some blood-sucking inverte- 

 brates. Thus one sees that the Haemosporidia remain always within 

 the body of one of the two hosts; hence, the sporozoites do not pos- 

 sess any protective envelope. 



The Haemosporidia are minute intracorpuscular parasites of ver- 

 tebrates. The malarial parasites of man are typical members of this 

 order. The development of Plasmodium vivax is briefly as follows 

 (Fig. 256). An infected female anopheline mosquito introduces sporo- 

 zoites into human blood when it feeds on it through skin (a). The 

 sporozoites are fusiform and 6-15ju long. They are capable of slight 

 vibratory and gliding movement when seen under the microscope 

 after removal from mosquitoes. After about 7-10 days of exo-eryth- 

 rocytic development (p. 602), the organisms are found in erythro- 

 cytes (c, d) and are called schizonts. At the beginning the schizonts 

 are small rings. They grow and finally divide into 12-24 or more 

 merozoites (e,f) which are presently set free in the blood plasma (g). 

 This schizogony requires 48 hours. The freed merozoites will, if not 

 ingested by leucocytes, enter and repeat schizogony in the erythro- 

 cytes. After repeated and simultaneous schizogony in geometric pro- 

 gression, large numbers of infected erythrocytes will be destroyed at 

 intervals of 48 hours, apparently setting free ever-increasing amounts 

 of toxic substances into the blood. This is the cause of the regular 

 occurrence of a characteristic paroxysm on every third day. 



In the meanwhile, some of the merozoites develop into gameto- 

 cytes instead of undergoing schizogony (h-k). When fully formed 

 they are differentiated into macro- and micro-gametocytes, but re- 

 main as such while in the human blood. When a female anopheline 

 mosquito takes in the blood containing gametocytes, the microgame- 

 tocyte develops into 4-8 microgametes (k, I), and the macroga- 

 metocyte into a macrogamete (i, m) in its stomach. An ookinete 

 (zygote) is formed when a microgamete fuses with a macrogamete 

 (m, n). The ookinetes are motile. As they come in contact with the 

 stomach epithelium, they enter it and become rounded into oocysts 

 which lie between the base of the epithelium and the outer membrane 

 of the stomach (o). Within the oocysts, repeated nuclear division 



599 



