372 PRINCIPLES OF VETERINARY SCIENCE 



Cause. — The disease is caused by the tubercle bacillus dis- 

 covered by Robert Koch in 1882. This organism is too small to 

 be seen with the unaided eye and belongs to the acid-fast group. 

 It is difficult to isolate directly from the tissues and grows slowly 

 on artificial culture media. 



Infection is usually acquired by eating food or drinking fluids 

 contaminated by discharges from tuberculous animals. Calves 

 and pigs contract the disease by drinking milk from tuberculous 

 cows, particularly raw by-products of creameries, cheese factories, 



Fig. 96. — This cow reacted to the tuberculin test five times, but never showed 

 symptoms of tuberculosis. When slaughtered, extensive lesions of the disease 

 were found. 



and skimming stations. Hogs having access to the excreta of 

 cattle which have tuberculosis are certain to become infected. 

 When hogs were fed on milk containing the tubercule bacilli for 

 only three days the post-mortem examination held 107 days later 

 showed that 83.3 per cent, of the animals had developed 

 tuberculosis. 



Symptoms. — A tuberculous animal often shows no physical 

 signs of the disease (Fig. 96) . This is especially true when only 

 one or two organs are involved. Even in cases of generalized 

 tuberculosis there may be no symptoms of the disease. When 

 the lungs are badly affected a cough and some difficulty in breath- 



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