PLASMODIUM, HAEMOPROTEUS AND LEUCOCYTOZOON 



261 



customarily called macrogametocytes, 

 but this name is incorrect since they are 

 haploid from the start (see below). They 

 remain in this stage until the blood is in- 

 gested by a mosquito. 



In the stomach of the mosquito, micro- 

 gametes are produced. The changes in the 

 microgametocytes are striking. Within 10 

 to 15 minutes the nucleus divides, and 6 

 to 8 long, heavy flagellum-like micro- 

 gametes are extruded. This process is 

 known as exflagellation. The microgam- 

 etes break off and swim freely until they 

 find a macrogamete. Fertilization takes 

 place, and a motile zygote (ookinete) is 

 formed. 



The ookinete penetrates into the stom- 

 ach wall and grows into an oocyst, which 

 forms a ball 50 to 60 /i in diameter on the 

 outer surface of the stomach. The oocyst 

 nucleus divides repeatedly and a number 

 of sporoblasts are formed. The nucleus 

 of each sporoblast then divides repeatedly, 

 and eventually each oocyst comes to con- 

 tain 10,000 or more slender, spindle- 

 shaped sporozoites about 15/i long with a 

 nucleus in the center. These break out of 

 the oocyst into the body cavity and migrate 

 to the salivary glands. They are then in- 

 jected into a new host when the mosquito 

 bites again. The process of sporozoite 

 development takes 10 days to 3 weeks or 

 longer, depending on the species of Plas- 

 modin))i, the species of mosquito and the 

 temperature. 



Once infected, a mosquito remains 

 infected for life, and can transmit the 

 parasites every time it bites. There is a 

 case on record (James, 1927) of a mos- 

 quito which lived from August 5 to Novem- 

 ber 16 and infected more than 40 general 

 paresis patients as part of their therapy. 



In vivax and malariae malaria, re- 

 lapses are common and may occur for a 

 number of years after the individual has 

 had his first attack. Between attacks the 

 parasites are ordinarily not found in the 

 blood. What apparently happens is that 

 all the parasites do not leave the liver 

 when the metacryptozoites emerge into the 

 blood stream, but a few remain there and 



continue to multiply in secret until such 

 time as the body's defenses have decreased 

 sufficiently so that the parasites can again 

 invade the blood. 



There are several variations of the 

 above general pattern. In P. falciparimi 

 of man, there is only a single generation 

 of metacryptozoites in the liver, and re- 

 lapses rarely occur. In addition, the 

 schizonts and merozoites of this species 

 are rarely seen in the peripheral blood. 

 Instead, the infected red cells become 

 viscid and clump together in the internal 

 organs. 



In the avian species, exoerythrocytic 

 schizogony does not take place in the liver 

 parenchyma, but either in the endothelial 

 cells (P. gallinaceiiiii . P. relictum, P. 

 catlienieriuDi, P. lophnrae. P. fallax, P. 

 circitiii/lexiiiii, P. diirae, P. juxtaimcleare, 

 P. hexameriitm) or largely in the haenio- 

 poietic cells (P. elo)2gatiau , P. vanghaiii 

 and probably P. Imffi and P. rouxi). 



In bird malaria also, but not in mam- 

 malian malaria, some of the merozoites 

 which have been formed in the erythrocytes 

 are able to enter the tissue cells and de- 

 velop exoerythrocytically. They are known 

 as phanerozoites, but they do not differ 

 morphologically from the forms derived 

 from sporozoites. 



Plas))todiuiii is haploid thruout its life 

 cycle except for a brief period following 

 fertilization and zygote formation. In a 

 cytologic study of the early oocysts of 7 

 species of Plasmodium in mosquitoes, 

 Bano (1959) found that the oocysts undergo 

 meiosis 2 to 3 days after the infective blood 

 meal, the time depending on the species. 

 For P. vivax it was 48 hours, for P. gal- 

 linaceiDii 53 to 55 hours, and for P. inui 

 72 to 79 hours. After that, division is by 

 mitosis. 



The haploid number of chromosomes 

 is 2 for P. falciparum, P. malariae, P. 

 ovale, P. lophurae, P. relictum, P. 

 floridense (Wolcott, 1955, 1957), P. vivax, 

 P. kiioivlesi, P. berghei (Wolcott, 1955, 

 1957; Bano, 1959), and P. gallinaceum 

 (Bano, 1959); it is 3 for P. gonderi and 



