684 



INFECTION BY THE BLOOD PROTOZOA 



PATHOGENIC TRYPANOSOME INFECTIONS PROCEEDING BY 

 CRISES AND RELAPSES 



When the same pathogenic trypanosomes as were considered in the preceding 

 section are inoculated into guinea pigs and certain other animals, they exhibit, instead 

 of a continuous and progressive infection, one that is marked by irregular increases 

 and decreases in numbers. The periodic decreases, when the parasites more or less 

 suddenly disappear from the blood, are generally spoken of as "crises," while the 

 subsequent repopulations are termed "relapses." This type of infection has been 

 recognized for many years and is probably very widespread. It is characteristic of 

 T. rhodesiense, T. gambiense, and T. hrucei in guinea pigs, according to the exact enu- 

 merative studies of Fantham and Thomson (191 1), and Thomson (1911); and of T. 

 rhodesiense in man, according to Ross and Thomson (1910). A number of authors have 

 observed it in the dog, and it generally occurs when T. equinum is grown in rats. 



25Ch 



1200- 



i. 



(^150- 



-3 



§1 

 3 



f2 



20 II 24 26 



Daijs after InjGctioa 



40 4Z 



Fig. 5. — ^Graph showing the course of an infection of T. rhodesiense in a guinea pig. The high 

 reproductive activity (represented by the coefficient of variation curve) indicates that no reproduc- 

 tion-inhibiting resistance is developed, but the type of number curve indicates that a parasitical 

 resistance is developed. (From data by the author and L. G. Taliaferro.) 



The sudden disappearance of the parasites from the blood, coupled with the in 

 vitro serological studies to be reviewed later, early convinced investigators that the 

 crises were actually periods of wholesale destruction of the parasites. An examination 

 of the number curve in Figure 5, showing T. rhodesiense in the guinea pig, and a com- 

 parison with Figure 4 emphasizes how marked this destruction is. Instead of increas- 

 ing steadily and rapidly, the parasites were very scarce for twenty-two days and 

 subsequently only showed several comparatively small rises and falls in numbers until 

 the death of the host on the forty-second day. Therefore, it may be concluded that a 

 resistance is acquired which destroys the parasites after they are formed. The rate of 

 reproduction, on the other hand, as expressed by the coefficient of variation curve, is 

 strikingly similar in the two figures. Also, daily microscopical examination of the 

 blood films showed cell division of the parasites to be taking place. Essentially 

 analogous results were obtained in the dog. Therefore, this type of infection may be 

 summed up as follows: Once the parasites are introduced into the host, they probably 

 reproduce at a uniform rate during the entire infection. At intervals, however, most 

 of the parasites that have accumulated in the blood are destroyed, but the few which 

 escape destruction, since their rate of reproduction is unchanged, repopulate the blood 

 again and again until the host dies. Exj)ressed in terms of resistance, in this type of 



