No. 6. DEPARTMENT OF AGRICULTURE. 149 



animal in normal condition. However, if llie injected animal is 

 tubercnlous, a decided rise of tenii;eratuie will follow within a 

 short time after the injeclion of the tuberculin. The subcutaneous 

 tuberculin test is applied by tirst determining- the normal tempera- 

 ture of the animal to be tested. This requires the taking of at least 

 two preinjection temperature measurements, at intervals of two to 

 three hours. 



The proper dose of tuberculin is injected under the skin with a 

 sterile syringe, preferably between the hours of 8 to 10 P. M. The 

 measurements of the post-injection temperatures should begin not 

 later than eight hours after injection and continued at intervals of 

 two hours until the 24th hour if the fullest information is desired. 

 If the injected animal is free from tuberculosis, there will be no 

 decided increase of temperature and no deviation from the usual 

 appearance and actions. In the case of a tuberculous animal there 

 will be a raise of temijerature usually noted at from the 8th to 10th 

 hour, and which reaches its greatest height at from the 14th to 18th 

 hour. In the case of a typical reaction there will be an increase of 

 two or more degrees up to a temperature of 104 degrees and above, 

 in some instances the temperature may read as high as 107 degrees. 

 The amount of increase or the height of temperature is no indication 

 of the extent to which the animal may be atfected. Occasionally an 

 animal will give what has been termed a "suspicious reaction," that 

 is, the increase will be less than two degrees and the temperature 

 under 104. In such cases it is not advisable to positively condemn 

 the animal unless there are some physical indications of tubercu- 

 losis, but it should be held for a retest at from 6 to 8 weeks from the 

 date of the indefinite test. Notwithstanding the proven efficacy of 

 the tuberculin test, it has been subjected to unfavorable criticism, 

 chiefly because applied by incompetent or careless operators and 

 under unfavorable conditions. Its use has become so general that 

 many persons have empolyed it without a careful study of the physi- 

 cal phenomena which cause the reaction to the tuberculin. Conse- 

 quently they are not prepared to avoid extraneous conditions which 

 may interfere with accuracy, nor can they attribute the proper 

 causes for apparent but not real failures. Tuberculin is a delicate 

 diagnostic agent which brings about the reaction by acting upon 

 the heat centers of the nervous system through the medium of the 

 tuberculous material which is present in the body of a tubercnlous 

 animal. These heat centers may be influenced by other factors, and 

 all such causes, so far as known, should be avoided when applying 

 the test. From careful study of many thousands of tests by various 

 investigators, it has been determined that the test when properly 

 applied is accurate to an extent of 98 per cent. The two per cent. 

 of apparent failures may be attributed to various causes which can- 

 not always be definitely determined. Among these we may mention: 



1. An impotent preparation of tuberculin. 



2. When the tuberculin is applied during the incubative period 

 of the disease. 



3. When applied to an animal in which the tuberculosis lesions 

 may be temporarily arrested, healed or encapsulated. 



The causes for apparent failures may be divided into two classes, 

 which we may designate as positive and negative. A failure in the 



