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 or sporozoite-formation; 

 but the former takes place in the blood of vertebrates and the latter 

 in the alimentary canal of some blood-sucking invertebrates. Thus 

 one sees that the Haemosporidia remain always within the body of 

 one of the two hosts; hence, the sporozoites do not possess any pro- 

 tective 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. 225). An infected female anopheline moscjuito introduces sporo- 

 zoites into human blood when it feeds on it through skin {1). The 

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

 vibratory and gliding movement when seen under the microscope 

 after removal from mosquitoes. After about 7-10 days the organisms 

 are found in erythrocytes {2, 3) and are called schizonts. At the be- 

 ginning the schizonts are small rings. They grow and finally divide 

 into 12-24 or more merozoites (4, 5) which are presently set free in 

 the blood plasma {6). This schizogony requires 48 hours. The freed 

 merozoites will, if not ingested by leucocytes, enter and repeat 

 schizogony in the erythrocytes. After repeated and simultaneous 

 schizogony in geometric progression, large numbers of infected eryth- 

 rocytes will be destroyed at intervals of 48 hours, 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 mean while, .some of the merozoites develop into gameto- 

 cytes instead of undergoing schizogony (7-10). 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 (10, 11), and the macroga- 

 metocyte into a macrogamete (5, 12) in its stomach. An ookinete 

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

 {12, 13). 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 



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