322 VETERINARY BACTERIOLOGY 



the test, but in practice frequently but one or two preliminary 

 determinations are made. After six or eight hours the tempera- 

 ture is again to be taken every two hours for the remainder of the 

 twenty-four hours after injection. Care must be exercised that 

 the animals are kept under normal conditions during the test, and 

 that they remain quiet. Animals in heat or advanced in preg- 

 nancy or suffering from other diseases should not be tested. A 

 positive test should show an increase of at least 1.5 above the 

 previous maximum recorded temperature. The temperature 

 usually begins to rise in about eight hours, and reaches its maximum 

 in from ten to eighteen hours after injection, then gradually 

 subsides. There are many theories of the mechanism of the tuber- 

 culin reaction, but it is now believed to be explained best on 

 the basis of anaphylaxis. The fact that the amount of tuberculin 

 used in the test is not appreciably poisonous to a healthy animal 

 indicates that the infected animal has become sensitized against 

 the bacterial constituents, probably proteins The presence of a 

 specific allergin has not been satisfactorily demonstrated, but some- 

 thing of that nature is probably present and renders the tissues 

 sensitive, principally about the lesions. That this sensitiveness 

 extends to other tissues also is shown by the ophthalmic and other 

 reactions to be described presently. It is a well-known fact that 

 one injection of tuberculin will prevent a reaction to a second 

 injection made shortly after. This fact is made use of by dishonest 

 cattlemen in vitiating the tuberculin test. The probable explana- 

 tion of this fact is that the body is in a condition of anti-anaphy- 

 laxis, that the allergin has been exhausted by the preceding dose, 

 and there has been insufficient time for the accumulation of a new 

 supply. There is no evidence that the use of tuberculin as or- 

 dinarily practised ever results in the sensitization of the animal. 

 This is an important point, as the test can be used repeatedly in a 

 herd of cattle, and the results may be relied upon. Animals in 

 advanced stages of the disease frequently fail to react. In such 

 cases it is probable that the body is in a state of immunity to the 

 proteins of the tubercle bacillus. As was stated in the discussion 

 of anaphylaxis, the mechanism of this immunity is not well under- 

 stood. This immunity to injections of tuberculin must not be 

 confused with immunity to the disease, for it seems that these 



