68 



COLLEGE ZOOLOGY 



to the combined efforts of zoologists, physi- 

 cians, and engineers. 



There are three important kinds of hu- 

 man malaria. Benign tertian malaria, caused 

 by Plasmodium vivax, is characterized by an 

 attack of fever every 48 hours. Quartan ma- 

 laria, caused by Plasmodium malariae, usu- 

 ally produces fever every 72 hours; while 

 pernicious malaria, caused by Plasmodium 

 falciparum, manifests an irregular tempera- 

 ture curve; sometimes the fever is practically 

 continuous. Plasmodium falciparum is the 

 greatest killer of man, sometimes striking in 

 an unusual fashion, with symptoms resem- 

 bling typhoid or apoplexy. 



The life cycles of all three species are es- 

 sentially similar and may be summarized as 

 follows : 



Malaria is transmitted through the bite 

 of a female mosquito; the male mosquito 

 cannot suck blood because he lacks piercing 

 mouth parts. Animal malarias are carried by 

 various species, but only mosquitoes of the 

 genus Anopheles can transmit the parasite 

 to man. 



When the insect's mouth parts (Fig. 146, 

 p. 247) pierce the skin, a certain amount 

 of saliva containing anticoagulants leaves 

 the salivary glands and passes into the 

 wound (Fig. 33). If the mosquito is har- 

 boring malarial parasites in its glands, this 

 gives an opportunity for some of them to 

 pass into the human blood stream. The in- 

 fective stage is called a sporozoite. The 

 sporozoites do not invade the blood corpus- 

 cles directly, but penetrate certain tissue 

 cells, where they grow, multiply, and go 

 through at least two cycles. The persistence 

 of the tissue-cell forms provides the reservoir 

 from which relapses occur. Sooner or later 

 some of the parasites are released into the 

 blood stream, where they proceed to enter 

 the red blood cells. Each parasitized ery- 

 throcyte contains as a rule a single Plasmo- 

 dium. The latter assumes at first a ring 

 shape, then an irregular form that soon fills 

 the cell. In both stages the organism is 

 termed a trophozoite. The time required for 

 its development will depend upon the spe- 



cies concerned. The mature trophozoite 

 eventually divides into a number of daugh- 

 ter cells called merozoites, which are then 

 released into the blood stream by rupture 

 of the red blood cell. Each merozoite in- 

 vades a new erythrocyte, where it becomes a 

 trophozoite. Each time the cycle is repeated, 

 a larger number of erythrocytes is involved; 

 when these burst, chills and fever occur. 



Eventually, a few merozoites, instead of 

 becoming trophozoites, develop into game- 

 tocytes. These are potential gametes, but as 

 long as they remain in the human host they 

 undergo no further development. They are 

 of no significance to the human host as they 

 float harmlessly in the circulating blood; 

 their survival depends upon their being 

 sucked up by a mosquito of the appropriate 

 type. Gametocytes which pass into a mos- 

 quito's stomach become active at once. 

 There are two kinds: the female gametocyte, 

 which develops into a single spherical egg 

 or female gamete; and the male, which, on 

 the other hand, undergoes exflagellation, a 

 process by which a number of slender male 

 gametes (sperms) grow out radially from 

 the surface of the cell and finally float free. 

 Union of a male gamete with an egg pro- 

 duces a zygote, which, because it has the 

 ability to move about, is called an ookinete. 

 The ookinete (fertilized egg) migrates 

 through the stomach epithelium and takes 

 up a position in the stomach wall. There its 

 nucleus divides and redivides, resulting in 

 the formation of a great number of sporo- 

 zoites, which, for a time, are contained 

 within a swelling called an oocyst. The 

 stomach of an infected mosquito often bears 

 a considerable number of these oocysts 

 clearly visible on its exterior surface. 



The oocysts finally rupture, and the sporo- 

 zoites are released into the insect's body 

 cavity. Sporozoites are motile, and many 



Figure 33. Facing page, life cycle of Plasmodium 

 vivax, one of the protozoans causing malaria in 

 man. Diagram of a mosquito's body and a human 

 blood vessel showing asexual stages in the blood 

 vessel and sexual stages in the mosquito. 



