224 A TICK-RESISTANT CONDITION IN CATTLE. 



In some cases extremely heavy losses have been exper- 

 ienced owing to the use of an unsuitable " bleeder " (as 

 the animal from which the inoculating blood is periodically 

 drawn, is termed) ; to conditions exciting the cattle iust 

 before or after inoculation ; to carelessness which brings- 

 about contamination of inoculating blood ; and to low con- 

 dition of the inoculated cattle. 



Tidswell (1900, p. IS) found that the amount of resist- 

 ance to tick fever did not depend on controllable factors 

 since " in certain animals the resistance was equally perfect 

 though inoculation treatment had been different in each 

 case ; and on the other hand in certain other animals the 

 resistance exhibited was very different although the inocu- 

 lation treatment was the same in each case." His obser- 

 vations did not reveal correspondence between any special 

 feature of inoculation and the amount of resistance obtained, 

 the issue appearing to depend more on the individual 

 peculiarities of the animals than upon anything under 

 human control. He went on to say that " It is known that 

 the infectivity of the blood of inoculated animals does not 

 depend on controllable factors. The duration of infectivity 

 does not depend necessarily u}Jon the source of inoculating 

 blood nor on the amount used at one time nor on the total 

 amount used on several occasions nor on the number of 

 inoculations, nor the severity of the reaction, nor the kind 

 of blood used (whether " recovered blood " or " virulent 

 blood," i.e., blood taken from an animal during the height 

 of the fever). Loss of infectivity does not imply loss of 

 protection and the protection of the mother does not 

 imply protection of the offspring. . . From these 

 various observations it appeared that the retention or 

 Joss of infectivity had no dependence upon any of the 

 controllable factors of inoculation. One is foiced to con- 

 clude that this feature like the protection is a matter of 

 idiosyncrasy." (Tidswell, 1900, p. 14, 15). 



The same ideas are expressed by Stewart and others 

 (1917, pp. 12-13) who state that the infectivity of recovered 

 blood varies both in degree and duration in individual 

 beasts ; and that the protection produced is not absolute 

 but is more of the nature of a tolerance than of an 

 immunity, its duration and degree being subject to variation 



