THE MORE COMMON AILMENTS OP CATTLE 42I 



ously affected, the animals may fall away in flesh and 

 carry a staring coat. But so varying are the symptoms 

 of tuberculosis, and so unreliable, that in a great major- 

 ity of instances the only absolutely assured diagnosis is 

 that which is obtained by means of the tuberculin test. 



Tuberculosis is a communicable rather than a trans- 

 missible disease. In rare instances only is it congenital 

 as when the generative organs are diseased. With the 

 exception named, it cannot reach cattle or other animals 

 in the absence of germ infection communicated from an 

 outside source. There are, however, predisposing 

 causes through which animals become less germ 

 resistant, so to speak, than they would otherwise be. 

 Any influence that lowers the stamina of the animal 

 lowers also its germ-resistant power. Prominent among 

 this class of influences are stables dark, damp, or ill- 

 ventilated, injudicious inbreeding, lack of proper exer- 

 cise, and poor food. 



The germs may reach the animal: (i) Through 

 the medium of the lungs by inhalation; (2) through 

 the stomach and intestines with food and drink; (3) 

 through inoculation, as in contact with broken skin or 

 mucous membrane; and (4) through infection in utero. 

 Infection from the lungs probably never comes as the 

 result of quiet inhalation. Germs may be in the saliva, 

 in nasal discharge, in matter from the bronchial tubes, 

 as when coughing, and in the faeces. When these be- 

 come dry, they may float in the air with dust and thus 

 reach the lungs. Similarly they may come in contact 

 with the food and through saliva or nasal dis- 

 charge may get into the water. Especially are they 

 liable to reach calves which take the unsterilized milk 

 of tuberculous cows, either in the form of whole 

 milk or of skim milk. During recent years tuberculosis 

 in swine has greatly increased through feeding to them 

 unsterilized skim milk from creameries. The instances 

 of infection through inoculation when in utero are rare. 



