WILLIAM H. TALIAFERRO 



683 



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chronic, relapse) shows whether the rate of reproduction is constant or varying. In 

 one strain of bird malaria, for example, the cycle took twenty-four hours throughout 

 the infection, and therefore the same number of 

 parasites were being produced throughout. This 

 measure of the rate of reproduction presupposes 

 that the average number of young produced by each 

 full-grown parasite does not vary at any time. This 

 assumption has been shown to be valid (L. G. 

 Taliaferro [1925]). Furthermore, this method has the 

 same advantage as the coefificient of variation 

 method used in the trypanosome infections in that 

 it is independent of the other two terms of the 

 equation and can be used as long as sufficient para- 

 sites remain in the blood to obtain a statistically 

 valid sample. 



CONTINUOUS PROGRESSIVE INFECTIONS OF PATHO- 

 GENIC TRYPANOSOMES IN THE MOUSE 



The simplest type of blood infection is seen when 

 the pathogenic trypanosomes are grown in the mouse. 

 Here, as has been noted by earlier writers on tr3q3ano- 

 somiasis, the parasites appear in the blood after a 

 short incubation period and increase in numbers 

 steadily and uniformly until the death of the host. 

 Sometimes, but not invariably, the same type of 

 infection is seen in the rat. 



This type of infection is illustrated in Figure 4 

 when T. rhodesiense, the causative agent of one type 

 of human sleeping sickness, is grown in the mouse. 

 The mouse was injected intraperitoneally with some 

 infected blood. Its blood contained trypanosomes 

 four days later, and from then until the animal died, 

 blood smears were made every twelve hours. The 

 data obtained are represented graphically in the 

 figure. In the first place, the variability of the para- 

 sites remained fairly high (C.V. = 8.87-10.46 per 

 cent) throughout the infection with no indication 

 of a progressive change. From this we may conclude 

 that reproduction was occurring at an approximately 

 constant rate. The occurrence of dividing forms in 

 the blood films corroborated these findings. In the 

 second place, the organisms steadily accumulated in 

 the blood, and increased according to a geometrical 

 progression. From the constant rates of reproduc- 

 tion and accumulation in the blood we may conclude 

 that no resistance whatever is acquired by this host, either affecting the rate of 

 reproduction of the parasites or destroying them after they are formed. 



- ^ 5 



Coefficimt 

 ofWidlion 





2 4 6 8 



Daijsafler Injection 



Fig. 4. — Graph showing the 

 course of an infection of T. rhode- 

 siense in a mouse. The high repro- 

 ductive activity (represented by 

 the coefficient of variation curve) 

 and the tj^e of number curve in- 

 dicate that neither a reproduction- 

 inhibit ing nor a parasiticidal resist- 

 ance is developed. (From data by 

 the author and L. G. Tahaferro.) 



