BY T. HARVEY JOHNSTON AND M. J. BANCROFT. 225 



and apparently largely depending on tlie idiosyncrasy of 

 tJio animal. Hunt (1898, p. 452) stated tliat probably 

 indiv'dnal animals differ somewhat in natural suscepti- 

 bility to tick fever and jierliaps various breeds of animals 

 differ also, just as iiidividuals and races of men differ in 

 natural susceptibility to any given disease or noxious 

 influence ; but that he was not aAvare that constitution, 

 breed or colour had been proved to have anj' protective 

 influence or that there was any special , protective virtue 

 in one class of coujitry over another except in so far as it 

 might be unfavourable or otherwise to the life of the cattle 

 tick. Cattle from all classes of country were attacked 

 alike, when removed to places where the ticks were fe\'er 

 carriers. 



As already mentioned, resistance to tick feVer is known 

 to be most satisfactorily attained during early life. Young 

 calves have great powers of resistance and may pass through 

 an attack so mild in form as to be unnoticed by an ordinary 

 observer. Dodd (1908, p. 0) suggests that this nnxy be 

 partly due to the great acti\'ity of young animals in produc- 

 ing blood cells counterbalancing the destruction of 

 erythrocytes ])}' the piroplasm. 



Practically all cattle in permanently tick-infested 

 districts of Qixeensland are now ■"immune "' (Pound, 1899, 

 p. 99 ; Dodd, 1904) and the losses that are occasionally 

 reported are generally due to the break-down of 

 " immunity " by some means such as over-driving, injury, 

 starvation, rough handling, want of water, etc., or the 

 introduction of susceptible animals (Dodd, 1908, p.l, p. 6). 



Pound (1899, p. 103) has given an instance of the 

 occurrence of what he terms hereditary immunity, but the 

 context shows that he really means to state that calves 

 are occasionally born " immune," that is, that they have 

 actuall}' suffered from an attack of tick fever while in 

 utero, due to the passage of piroplasms through the 

 maternal placenta. 



To sum up : There exists in manj^ cattle a "" resistance" 

 more or less marked, to tick fever, such resistance depend- 

 ing on age, sex, sexual development, food supply, con- 

 tentment, general health and apparently on some individual 

 ■characteristic. 



