432 CHRONIC INFECTIOUS DISEASES 



infected when out of doors following tubercular cattle. As in 

 other contagio-infectious diseases, darkness, dirt, and foul air 

 are conducive to the propagation of tuberculosis, while the 

 opposite conditions tend to inhibit its development. For 

 these reasons life in the open is a useful preventive and cura- 

 tive measure especially in the earlier stages of the disease. 

 In advanced cases it helps relatively little. (See Treatment.) 



Susceptibility. — As noted, tuberculosis is the most widely 

 prevalent disease of cattle and very common in swine and 

 fowls. On the other hand, horses, dogs, and cats are seldom 

 infected, and in sheep the disease is extremely rare. A high 

 resistance offered to tubercular infection may be racial or 

 individual. It is believed that certain breeds of cattle, for 

 instance, are less predisposed than others. The semi-wild 

 strains from the Russian steppes and the native cattle of 

 Japan seem more resistant than those of more refined origin. 

 The long-horn of Texas and the West was apparently an 

 immune. However, the opportunity for original infection and 

 subsequent spread of the disease has been much more difficult 

 among these cattle than it is in the more thoroughly domesti- 

 cated European breeds from which our better American cattle 

 sprang. Among the tamer breeds (Jerseys, Shorthorns, etc.) 

 no racial differences in resistance have been noted. The 

 manner of caring for and the use to which the animal is put 

 probably has as much to do with the susceptibility to tuber- 

 culosis as any racial peculiarity. Selection in breeding opera- 

 tions with only precocity in development, or an unnaturally 

 great milk production in view, to the exclusion of other factors 

 (good constitution, etc.), will produce a race of low resistance 

 to any infection. If tuberculosis happens to be the disease 

 to yphich this race is exposed, infection is the more apt to 

 take place. 



Individual immunity against tuberculosis is commonly 

 observed. In notoriously infected herds a few animals will 

 sometimes remain healthy, although surrounded by every 

 opportunity to take the disease. Whether this immunity is 

 acquired or congenital is difficult to state. Accurate experi- 

 ments to determine whether or not it may be handed down to 

 future generations are wanting. 



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