DISEASES CAUSED BY INFECTED MILK 107 



and, contrary to prevailing opinion, it is impossible to 

 produce tuberculosis with tuberculin. 



Tuberculin is a chemical substance which is quite harm- 

 less to a normal animal, but produces a reaction in an ani- 

 mal having tuberculosis. The principal diagnostic feature 

 of this reaction consists in a rise of temperature. 



The tuberculin test is an exceedingly delicate one. This 

 is both an advantage and a disadvantage. The advantages 

 are evident. The disadvantage consists in the fact that the 

 test is so refined that it does not distinguish between a 

 slight localized lesion — which may be locked up in a 

 gland and never cause the animal further trouble — and 

 a massive, progressive, or generalized type of the disease. 

 This fact has caused considerable confusion and some 

 prejudice in the minds of the farmer with the tuberculin 

 test. One of his cows reacts typically after an injection of 

 tuberculin. The animal is killed and examined and the only 

 tuberculosis found is a small focus the size of a pea in one 

 of the lymph glands of the chest. It is true that such a cow 

 might furnish a safe milk for years; nevertheless it is im- 

 possible to tell when such a focus may break its bounds 

 and spread. The only way to eliminate the disease from a 

 herd is to get rid of every vestige of it: a single spark left 

 may be suflBcient in time to produce an epidemic. 



The tuberculin test is one of the most reliable reactions 

 of its kind in biology. An accurate diagnosis may be estab- 

 lished in over ninety-seven per cent of the cases tested. 

 The three per cent of failures occur both in cattle with 

 tuberculosis which do not react, and in cattle without tu- 

 berculosis which do react. A cow may have tuberculosis 

 and fail to react, especially if in the last stages of the dis- 

 ease, and occasionally in the very first stages. Sometimes 

 tuberculous animals fail to react because they have had 

 a previous injection of tuberculin within six weeks. This 

 fact is sometimes used by unscrupulous persons to trick 

 the inspector, by giving an injection of tuberculin sev- 



