118 



Agricultural Bacteriology. 



Tuberculin test. The only way by which it can be 

 determined with certainty whether an animal has tuber- 

 culosis or not is by the use of the tuberculin test. Tu- 

 berculin is prepared in bacteriological laboratories by 

 growing the tubercle organisms in beef broth containing 

 glycerin. The organism produces a substance in the 

 broth, which, when injected beneath the skin, has a 

 peculiar effect on a diseased animal, causing a fever for 

 a few hours; while in the healthy animal it has no 

 appreciable effect. The beef broth is heated, and filtered 



FIG. 13. A TUBERCULOUS ANIMAL. 

 A cow that has had the disease for five years. 



through porcelain, so that the tuberculin as used con- 

 tains neither living or dead bacilli, but is simply an 

 extract of their cells. It cannot produce the disease in 

 healthy animals nor does it cause the disease to spread 

 in affected animals. 



The test is made by taking a series of temperature- 

 readings on the animal, injecting the tuberculin beneath 



