IX] LIFE-CYCLE OF MALARIAL PARASITE 



129 



products is so considerable as to produce an attack of fever 

 in the patient. The febrile attacks only occur when schizogony 

 is taking place and the merozoites are being liberated into the 

 blood stream. The interval between the attacks indicates, 

 therefore, the time taken by the parasite to develop from the 

 merozoite through the trophozoite stages up to the schizont, 

 and to subdivide into a number of merozoites, or, in other 

 words, to pass through the complete asexual cycle. It has been 

 found that the time taken for the completion of this cycle 

 varies in different species of Plasmodium, being 72 hours in the 

 case of P. malaricB, 48 hours in P. vivax, and from 24 to 48 hours 

 in P. falciparum. Hence the fevers caused by each of these 

 parasites differ from one another by the intervals elapsing 

 between the febrile attacks. The fuU description of these 

 species, together with their distinguishing characteristics, is 

 described below [vide p. 155). 



In these few words we have briefly described the asexual 

 cycle of development that takes place in the vertebrate host, 

 but, as will be obvious, the only method by which the merozoites 

 could infect another host would be by the inoculation of blood 

 containing them. This method of transmission is successful, 

 for if blood containing malarial parasites is inoculated into a 

 normal human being, the latter becomes infected with malaria. 

 However, such a mode of transmission probably never occurs 

 in nature, for the merozoites are incapable of withstanding 

 desiccation, or any other of the vicissitudes to which they 

 would be exposed on the proboscis of a biting arthropod, the 

 only conceivable agent for such direct transmission. Con- 

 sequently, in addition to its asexual multiphcation, which 

 merely serves to increase its numbers in the infected individual, 

 it is necessary for the parasite to possess some means of being 

 carried from one host to another. We shall see that provision 

 is made for this transmission, but first it is necessary to describe 

 another development that the young trophozoite may undergo 

 in the red cell. 



Development of the sexual forms in the blood. About a week 

 after a patient becomes infected with malaria certain large 

 intracorpuscular forms may be observed that do not go through 



