12 MALARIA 



and is separate from the spomlation stage. The members 

 of the order Haemosporidia are intracellular in the tropho- 

 zoite stage, have no resistant spores and undergo an alter- 

 nation of schizogony in a vertebrate and sporogony in a 

 blood sucking invertebrate (e.g. mosquito). To the family 

 Plasmodidae belong the genera Haemocystidium which occurs 

 in reptiles, and Plasmodium which includes the malarial 

 organisms. Three species of Plasmodium are known from 

 man, (1) P. vivax, which causes tertian malaria, (2) P. 

 malariae, which causes quartan malaria, and (3) P. fal- 

 ciparum, which causes aestivo-autumnal malaria. Malarial 

 organisms also inhabit lower animals, such as P. danilewskyi 

 in birds, P. kochi in chimpanzees, P. bovis in cattle, P. canis 

 in dogs, P. equi in horses and P. diploglossi in lizards. In 

 the accompanying table the distinguishing features of the 

 three species of Plasmodium occurring in man are contrasted 

 as an aid in identification. 



2. Life history (Plate I). Certain species of mosquitoes 

 of the x genus Anopheles are the transmitting agents of the 

 malarial organisms of man. Infective mosquitoes carry the 

 organisms in their salivary glands; from here they pass into 

 the blood of any animal the mosquito bites. The stages in 

 the life history of the tertian parasite, Plasmodium vivax 

 are briefly as follows : 



a. Sporozoite: a falciform body about 14 microns long, 

 present in infective mosquitoes; the stage that is inoculated 

 into man. The sporozoites enter red blood cells, usually 

 only one to a cell, where they become amoeboid in shape. 



b. Trophozoite: the first stage in the red blood cell, at 

 first ring shaped, later amoeboid, and finally circular or 

 oval in outline. The trophozoites develop into either 

 schizonts or gametocytes. 



c. Schizont: a stage that develops from a trophozoite, 

 and undergoes segmentation into from 15 to 20 spores known 

 as merozoites. 



