Phylum Protozoa 365 



The Haemosporidians. — Another group of sporozoans are the 

 blood parasites. These are members of the order Haemosporidia. 

 The best known of these haemosporidians are the members of the 

 genus Plasmodium which are the causative organisms for human and 

 avian malaria. In human malaria, the asexual cycle with the tro- 

 phozoite occurs in man; the sexual cycle occurs in mosquitoes of the 

 genus Anopheles. 



The trophozoite occurs within the red blood cell. Each develops 

 into a typical schizont which divides by fission into a number of 

 merozoites. Rupture of the red blood corpuscle liberates the merozoites 

 into the blood stream. They then are able to penetrate new red blood 

 cells and develop through a ring stage into a typical trophozoite. Some 

 of the merozoites at times develop into gametocytes. These develop 

 no further unless they are picked up by the proper kind of mosquito. 

 In the Anopheles, the female gametocyte develops into a macrogamete 

 and the male gametocyte into several microgametes. Fusion of macro- 

 and microgametes produces zygotes which develop into ookinetes. Each 

 ookinete passes through the wall of the mosquito's gut and comes to lie 

 on the outside of the wall. Here it rounds up and develops into a heavy 

 walled oocyst. Each cyst produces by sporogony thousands of sporo- 

 zoites. By rupture of the cyst wall, the sporozoites are liberated and 

 migrate to the salivary gland. When the infected mosquito bites its 

 next victim it injects a quantity of its saliva containing the sporozoites. 

 The sporozoites do not immediately invade the red blood cells; rather 

 they appear to undergo a series of developmental stages in the liver 

 and perhaps in other tissues. Gradually they are released from these 

 sites and invade the red blood cells. The liver may act as a reservoir 

 for an infection and thus be responsible for malarial attacks occurring 

 months or even years after the person has left a malarial area. 



Depending upon environmental temperatures, the cycle in the 

 mosquito takes from 10 to 20 days. It takes from 10 to 12 days for 

 the first appearance of symptoms in human beings after the initial in- 

 fection. Typically, the symptoms of malaria in man are chills followed 

 by fevers which occur when the merozoites are released from the red 

 blood cells. Depending upon the infecting species, this time varies. 

 With P. vivax and P. ovale, fevers occur about every 48 hours; with P. 

 malariae every 72 hours; and with P. falciparum an irregularity of 

 fevers occurs. Of these four species, P. falciparum is the most virulent 

 form. P. vivax is not so virulent but may persist for about five years. 



