694 INFECTION BY THE BLOOD PROTOZOA 



MALARIAL INFECTIONS 



Investigators have been very much handicapped until recently in the study of the 

 life-cycle of uninfluenced malarial infections in man. With the advent of the use of 

 malaria in the treatment of paresis many advances have been made; but even so, 

 most of our knowledge has been gained from the study of bird malarial parasites, in 

 particular Plasmodium praecox. 



In general, the course of infection in the canary is as follows (Fig. 9) : After an 

 incubation period (/) during which no organisms can be found, the parasites rapidh^ 

 increase in the blood (2, acute period) until sometimes every other cell is parasitized, 

 then if the bird does not die, there is a crisis (j) when most of the parasites are killed, 

 but some may remain for a week or more (4, chronic period), and in time all the para- 

 sites apparently disappear from the blood (5, latent period). Thereafter, if the bird's 

 resistance is lowered in any way, they may reappear for a week or more in a kind of 

 miniature acute rise and crisis (6, relapse). 



Exact enumerative studies of this form have been made by the Sergents (see 

 especially 1918), Ben Harel (1923), L. G. Taliaferro (1925), Boyd (1925), and Hart- 

 man (1926). L. G. Taliaferro (1925) found that although the parasites increased at 

 a constant rate according to a geometrical progression during the acute period, out 

 of an average of 15.5 merozoites produced by each mature schizont approximately 

 10 die. The constant rate of increase during the acute period has been verified by 

 Hartman (1927), who finds, however, that the actual rate may vary considerably 

 among different birds. Furthermore, he has made a careful study of the rate of death 

 of the merozoites which perish between each asexual generation and finds that the 

 rate of death is a constant for twenty-one hours of the twenty-four hour period. As 

 pointed out by Taliaferro, this non-viability of the majority of merozoites produced 

 by each sporulation probably represents the suitability of the bird as a culture medi- 

 um for the parasites and is not in any sense an acquired resistance. In line with the 

 constant rate of increase during the acute period Boyd (1925) has found a definite 

 correlation between the number of parasites injected and the number of parasites at 

 the peak of the infection (.340 + .074). 



Leaving the consideration of the acute period in which there is no evidence of an 

 acquired resistance and considering the remainder of the infection, it is evident from 

 the wholesale death of the parasites at the crises which terminate the acute, chronic, 

 and relapse periods (Fig. 9) that some type of resistance is acquired which destroys 

 large numbers of the parasites after they are formed. Unlike T. lewisi, however, there 

 is no retardation of the rate of reproduction of the parasites, according to L. G. 

 Taliaferro (1925). She found that the asexual cycle which is a measurement of the 

 rate of reproduction, as explained previously, takes the same time (twenty-four hours 

 in one strain) throughout the acute, chronic, and relapse periods (this is shown for the 

 acute period and crisis in Fig. 10), and that furthermore each stage in the cycle takes 

 place at exactly the same time in the relapse as it had in the acute and chronic 

 periods — a fact which indirectly indicates that the cycle had continued undisturbed 

 throughout the latent period. Quoted from her jiaper, the probable picture of an 

 infection with bird malaria is somewhat as follows: 



