234 A TICK-RESISTANT CONDITION IN CATTLE. 



permanent resistance to tick attack.* However, when once 

 such resistance is firmly established, it is hardly likely that 

 dipping woidd destroy or suppress it (provided the aiiimare 

 condition be maintained) iiiiless the arsenic which- — as 

 stated later in this paper- — becomes to some extent 

 accumulated in the subcutaneous tissues, actually interferes 

 with the action of, or destroys, the antitick substances. 



Tick Resistance. 

 In speaking of natural resistance to bacterial invasion, 

 Zinsserf referred to the Very different results arising from 

 invasion of different individuals by a given species of micro- 

 organism whereas the same individual may be highly 

 resistant to certain organisms but very susceptible to others ; 

 while " even in reactions with one and the same micro- 

 organism, the susceptibility or resistance of the individual 

 may be determined by variations in the physiological 

 state, or by environmental conditions under which the two 

 factors — invader and invaded — are brought together. . . 

 Within the same race or species, an epidemic sweeping 

 through n. community will kill many individuals, and leave 

 others xmscathed. Such differences point to variations 

 in the defensive mechanism since the invader in these cases 

 is the same." There may be specific {i.e., belonging to the 

 species), racial and individual natural immunity. He 

 points outj that in the case of human beings it is probable 

 that individual differences in resistance are due, ' not to 

 any fundamental individual variations, but rather to such 

 fortuitous facti>rs as nutrition, metabolic fluctuations, 

 temporary physical depression, fatigue, or chilling." These 

 remarks would apply equally to tick invasion, since we have 

 already shown that cattle may become habituated to tick 

 and we know that in many herds there are to be found 

 one or more indiA^duals which are markedly resistant to 

 tick infestation. 



*Tliis statement must not bo construed to mean that we advocate the 

 abolition of dipping. We behe\e that, in the present state of our 

 knowledge regarding tick control, systematic dipping, combined with a 

 satisfactory rotation of paddocks, is the most efficient method of attacking, 

 on a large scale, the problem of tick eradication. 



fZinsser, Z.c, p. 49, 50. 



JZirisser, I.e., p. 59. 



