1106 PATHOGENIC PROTOZOA 



the plasmodia are parasitic on red-blood cells. There are two 

 divisions of the life cycle; that which occurs in man, the endogenous, 

 asexual or schizogenous, and that which occurs in the mosquito, 

 the exogenous, sexual or sporogenous ; for this reason the mosquito 

 is the definitive and the man the intermediate host. 



Although the life cycle of the parasite varies in details in the 

 different forms of malaria, certain stages are common to all, and in 

 general the life cycle is usually described as follows: an infected 

 mosquito bites a warm blooded animal, often a human being, in 

 order to obtain its meal of blood. As it bites, it infects the wound 

 with its saliva which contains sporozoites coming from the salivary 

 glands. The sporozoites, in the new host, soon gain access to the 

 blood and attach themselves to the erythrocytes upon which they 

 become parasitic. In shape they are long and slender spindles, and 

 when stained with the usual eosin-methylene blue dyes, show a blue 

 cytoplasm and a compact dot of red nuclear chromatin in the center. 

 When the sporozoite escapes the phagocytes, and succeeds in es- 

 tablishing itself on a red cell it soon changes its shape to the ring 

 form and grows rapidly and during this growing stage is known 

 as a trophozoite. This last named form may develop in either of 

 two ways: in the asexual or schizogenous cycle when it is called 

 a schizont, or in the sexual or sporogenous cycle, when it is called 

 a sporont. 



The schizont goes on to full development in the human host ; the 

 sporont cannot complete its cycle until taken into the stomach of 

 a suitable anopheline mosquito, capable of conveying malaria. 



The trophozoit which ends its life as a schizont grows rapidly 

 at the expense of the red cell and develops a characteristic vacuole 

 which increases the area of the parasite in contact with the host 

 cell. When mature its nucleus undergoes mitotic changes and the 

 parasite divides into a more or less definite number of segments 

 called merozoites ; these when liberated by the disintegration of the 

 host cell, attack new erythrocytes and develop during the second 

 and subsequent generations in the same manner as the sporozoite. 



There are three well-recognized forms of the plasmodia, (1) Plas- 

 modium vivax (Grassi and Filetti), causing tertian fever (also called 

 "benign tertian"); (2) Plasmodium malarice (Laveran), causing 

 quartan fever ; (3) Plasmodium falciparum (immaculatum) (Welch), 

 causing the tropical form of malaria, the so-called aestivo-autumnal 

 or subtertian. As the details of development cannot be made out 



