312 A TICK-KESISTANT CONDITION IN CATTLE. 



Only one definite reference to such exudate is to be found 

 in literature. In the conclusion of a paper on the " Biology 

 of some North American ticks of the genus Dermacentor," 

 Bishopp and Wood (1914, p. 179) made the following 

 statement : "" Three of our bovine hosts used in experiments 

 shewed such a tendency to immunity. As stated in Table 

 VI. (Infestation No. 8), no females became engorged on a 

 Jersey calf used as a host in comparison with a bull (Infest- 

 ation No. 9). This calf as well as two other animals were 

 tested several times without a single specimen developing 

 to repletion. It should be mentioned that these hosts^ 

 showed marked resistance to the attack of other species 

 of ticks. This resistance appears to be due largely to an un- 

 usually strong tendency to form scabs at the point of attack,, 

 thus healing the woTind, and throwing off the tick with the 

 scab. On all hosts sca])s are usually produced as tho 

 result of the insertion of the mouth parts of the ticks. When 

 the larvae first become attached a translucent yellowish 

 exudate is thrown out at the point of attack. The subject 

 of resistance will be discussed at greater length in another 

 publication." This is also mentioned by Tryon (1916, 

 p. 50). We have been unable to find further reference to 

 this other publication, which may throw greater light on 

 the problem. 



Schroeder (1905, p. 57), stated that "" When ticks are 

 numerous it has been observed that a sticky substance 

 frequently accumulates on the skin of cattle : this may be 

 either an exudate from the irritated skin or an accumulation, 

 of the discharges from the ticks. After a while this sub- 

 stance hardens into dark, granular flakes and peels off. 

 It is best seen in the creases of the skin. Whether it exudes 

 from the skin or is discharged from the tick does not alter 

 the fact that its original source is the body of the cow or 

 steer. If it exudes from the skin it is strong evidence that 

 the skin is extremely irritated . . . ." The same 

 author also describes the effect of a tick bite on the skin of 

 an animal — " When a cattle tick is detached from the skin 

 of a cow, a small circular spot, sometimes partly denuded 

 of hair, and slightly reddened, is seen, with a minnte, barely 

 visible puncture in its centre, from which a drop of blood 

 may ooze. When examined under the microscope the skin. 



