390 CHRONIC INFECTIOUS DISEASES 



free from the disease. Modern transportation facilities are 

 important agents in spreading the disease among cattle. The 

 illegitimate use of tuberculin by unscrupulous persons is a 

 further factor in the spread of bovine tuberculosis. Cattle 

 reacting to the test are frequently sold as healthy to unsus- 

 pecting buyers, who thus introduce the disease into their 

 herds. Tuberculosis of animals is not yet as prevalent in 

 the United States as in other countries. It is constantly 

 increasing, however, especially in States where no adequate 

 measures have been inaugurated to combat it. Swine are 

 infected from tuberculous , cattle in two ways, vi'z^: (a) By 

 being fed milk containing tubercle bacilli and (b) by feeding 

 on the excrements or offal of tuberculous cattle. 



The prevalency of animal tuberculosis can be estimated 

 with approximate accuracy from abattoir statistics and the 

 results of tuberculin testing. In Germany nearly 21 per cent, 

 of the cattle and 3 per cent, of the swine killed for food have 

 been found affected. The tuberculin test showed over 50 

 per cent, reacting. Fully 25 per cent, of the cattle of Ger- 

 many are infected, and in France over 10 per cent. In the 

 United States 1 per cent, of the cattle are found tuberculous 

 on slaughter and 2.5 per cent, of the hogs. Results from tu- 

 berculin tests on 400,000 head of cattle gave 10 per cent, 

 reacting. It is very probable that 1 per cent, of the beef 

 cattle and 10 per cent, of the dairy and stud herds of this 

 country are tuberculous. 



Etiology. — The cause of tuberculosis is the Bacillus tuber- 

 culosis of Koch. Three types of this bacillus are fairly well 

 defined, viz. : , (&) Typus humanus, (6) typus bovinus, and 

 (c) typus gallinaceus. (See Bacteriology.) 



Natural Infection. — A tuberculous animal can spread the dis- 

 ease only by throwing off tubercle bacilli. Such are spoken of 

 as cases of "open tuberculosis." On the other hand, where 

 the animal is tuberculous but no tubercle bacilli are passing 

 from it, the case is one of "closed tuberculosis." Bacilli may 

 not pass continuously from "open" cases. "Closed" cases 

 may at any time change to "open" ones. 



Modes of Infection. — Tubercle bacilli are taken into the 

 body: (a) via digestive tract with contaminated food and 



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