=Economic aspects of bovine tuberculosis.= Not only is this disease invested with much importance because of its inter-relation with the human, but from an economic point of view alone, it is undoubtedly the greatest scourge that affects the dairyman. Its insidiousness makes it exceedingly difficult to recognize. The consequence is that many fine herds become seriously involved before its presence is recognized. In the main, the disease is introduced into a herd by purchase, often by buying in pure-bred stock to improve the quality of the herd. Where the disease has been established in a region for some time, there is also danger that unheated factory by-products, as skim milk and whey, may function in its spread. Where such conditions prevail, the spread of the disease in the creamery district is exceedingly rapid. When once introduced into a herd, the disease sooner or later spreads from the originally affected animal to others in the herd. Close contact, and close confinement in ill ventilated stables facilitate the spread of the disease, and sooner or later, other animals acquire the trouble. This may all occur while all animals appear in a healthy condition. The symptoms of the disease in the earlier stages are quite indefinite. As the disease progresses, the nutritive functions appear to be disturbed, and sooner or later, the body weight begins to decline, and finally marked emaciation ensues. Accompanying this condition, especially when the disease is in the lungs, is a cough, which is generally aggravated with active exercise. While the run-down condition permits frequently of the detection of the disease in the advanced stages, it is wholly impossible with any accuracy to diagnose the trouble in the incipient stages. It is at this stage that the tuberculin test comes to the aid of the stockman. _Tuberculin test._ This test is made by the injecting beneath the skin of the animal a small quantity (about 2 c. c.) of tuberculin, and noting the temperature of the animal, before and after the injection. Tuberculin, a product of the growth of the tubercle bacillus, when injected into the body causes a marked rise in temperature, in the case of an animal affected with the disease, and no such elevation in the case of a healthy animal. The process of preparing tuberculin makes it absolutely free from danger, so far as liability of producing the disease, or in any way injuring the animal, is concerned. Fig. 19 shows the temperature range of both reacting and non-reacting animals. While the test is not absolutely infallible, it is so far superior to any and all other methods of diagnosis that it should take precedence over them.