568 Veterinary Medici?ie. 



resistance, and further, this resistance is not permanent but must 

 be renewed at short intervals. The immunity may be in part ac- 

 quired in the womb of the infested dam, in the last months of 

 gestation, but it is chiefly post-natal through the attacks of the 

 ticks. 



a. Infection of sucking calf. The indigenous cattle acquire 

 immunity mainly through the attacks of the boophilus, in the 

 first month or two when they are still on an exclusively milk 

 diet, that renders the piroplasma practically harmless. Following 

 a parallel method, calves, living on milk alone, can be taken into 

 the infected regions and exposed to the attacks of the ticks with 

 safety, and with the result of protection for the future. Escaping 

 the first invasion, they continue to harbor as many ticks summer 

 after summer, as will reinforce yearly their acquired power of re- 

 sistance, so that they continue measurably safe though spending 

 the life in the area of the infection. 



Francis and Connoway applied this to Jersey calves of two to 

 six weeks old applying to each 25 to 50 ticks. It led to slight 

 hyperthermia, some dulness and inappetence, but on recovery 

 they all gained flesh and condition. Two died from exposure but 

 necropsy showed no sign of Texas fever. The following summer 

 all were infested with 200 to 500 ticks apiece but not one sickened 

 in consequence. These were Jersey calves (the least susceptible 

 breed) and the experiments were made in cool weather in autumn. 

 The limitation of the practice to the cool fall or winter months 

 renders the operation much more safe. 



b. Infecting Older Animals by a limited number of Ticks. 

 Yearling Jerseys, Holsteins and Shorthorn were subjected to 25 

 to 50 ticks in July, they showed only slight rise of temperature, 

 and later resisted the free exposure to tick infestation. It must 

 be recognized that these were still young animals, with pre- 

 sumably greater resisting power than the mature, but on the other 

 hand they were of the susceptible northern herds, they were first 

 infested in the hottest season, and the acquired resistance appears 

 to have been perfect throughout the succeeding summer. The 

 added precaution of subjecting them, in late autumn or winter 

 only, to the ticks raised in a warm room or thermostat, would add 

 greatly to the safety of the operation. After recovery from the 

 effects of the first crop of ticks, a second crop of 50 to 100 



