690 



INFECTION BY THE BLOOD PROTOZOA 



(1925), indicate that the course of infection in the blood proceeds as follows (Fig. 7): 

 After an incubation period during which no parasites are found, the organisms in- 

 crease rapidly in numbers, sometimes attaining several hundred thousand per cubic 

 millimeter of blood, then there is a crisis when most of the parasites are destroyed; 

 those that remain continue to live in the blood (from several weeks to several months) 

 until a second crisis sweeps them too from the blood. Rabinowitsch and Kempner 

 (1899) showed that thereafter the rat is immune to a second infection — an observation 

 which has been repeatedly confirmed by other investigators. 



Many years before exact enumerative studies were made on infections with T. 

 lewisi, it was noted from microscopical studies by Rabinowitsch and Kempner (1899), 

 Wasielewski and Senn (1900), and Laveran and Mesnil (1901) that when the organ- 

 isms are increasing in the blood they are actively reproducing by fission, whereas 



35(>| 



300- 



1 



S250- 



SI50H 



100- 

 50-1 



Rat 105 



2 4 6 8 10 12 14 15 18 20 Zl 24 Z6 28 30 32 34 36 

 Days after Injectioa 



Fig. 7. — Graph showing the course of an infection of T. lewisi in a rat. The changes in reproduc- 

 tive activity (represented by the coefficient of variation curve) and the type of number curve show 

 that both a reproduction-inhibiting and a parasiticidal resistance are developed. (From the author.) 



during the latter part of the infection they simply exist in the blood as non-reproduc- 

 ing adults. These facts were studied more intensively by the writer with the assist- 

 ance of L. G. Tahaferro (1922). From daily blood smears throughout the course of 

 an infection with T. lewisi the total lengths of a hundred parasites were drawn and 

 measured and the coefficients of variation computed, as outlined previously. Paren- 

 thetically, it may be recalled that the coefficient of variation is an index of the rate of 

 reproduction. From this body of data it was found that when the organisms first 

 appear in the blood (Fig. i and coefficient of variation curve [Fig. 7]), they are re- 

 producing at a very rapid rate, but that their reproduction is quickly but progressively 

 retarded until finally, about the tenth day of the infection, no reproduction is taking 

 place at all, and all of the parasites are in the adult stage. 



From this work it may be said that there are three manifestations of resistance: 

 (i) the retardation and final inhibition of reproduction by about the tenth day, (2) 

 the sudden destruction of the majority of the parasites between the eighth and twelfth 

 day, and (3) the eventual total destruction of the parasites which terminates the 

 infection in from a week to several months. 



