BY T. HARVEY JOHNSTON AND M. J. BANCROFT. 23S 



blood was strongly antitoxic to ticks, as a " bleeder " for 

 providing blood to inoculate tick-infested stock and confer 

 some degree of resistance — a passive immunity. The 

 production of such antibod}'' in the case of most ticky cattle 

 is probably slow. 



After mentioning that we may justly assume that 

 natural resistance may be largely a matter of inheritance, 

 Zinsser* goes on to sa}' that natural immunity, unlike 

 acquired iramunit3% cannot be passively transferred from 

 one animal to another and implies therefore a fundamental 

 cellular difference rather than a condition depending, 

 merely upon antibodies circulating in the blood. 



Some graziers and dairy farmers maintain that if animals 

 in fair condition be allowed to remain in tick}- pastures 

 without l^eing li-eated in any way for tick infestation, sucK 

 animals would become not mci'ely habituated to ticks^ 

 but actually resistant. The cattle tick apparently reached 

 Australia along with the Brahmin cattle introduced into 

 the Northern Territory, and yet that jjarticular breed is 

 admitted to possess a marked resistance to BoopMlus. 

 Perhaps this resistance arose naturally in the manner just 

 indicated. Marshall f jioints out that antibodies may be 

 transferred from the mother to the young before birth, but 

 only after foetal circulation has become established, being 

 carried from the maternal blood through the placenta into 

 the offspring. 



It does not seem to be an unreasonable suggestion 

 that, as a residt of continued light or moderate infection, 

 animals w hich manage to maintain condition, may eventually 

 develop more or less resistance to tick infestation. It may 

 be urged that the tick is an ectoparasite and not an entozoon, 

 but the distinction between it and a haematozoon in regard 

 to food supply is really very slight, since the mouth parts 

 of the tick are as truly bathed in plasma as is the absorptive 

 surface of an internal blood parasite. If this point of view 

 be correct then the use of dips to free cattle from tick 

 infestation, though admittedly valuable as a temporary 

 measure, really prevents the establishment of more or less 



*H. Zinsser, Infection and Resistance, 1918, p. 56, 58. 



fMarshall, l.r., p. 090. 



