22 

 THE TUBERCULIN TEST. 



Tuberculosis develops so slowly that in many cases it is months, and some- 

 times years, before any symptoms are shown. During this period, the infected 

 animals cannot be distinguished from the healthy in any ordinary way. There 

 is a test, however, which does no harm to the healthy, yet detects the diseased 

 ones practically without fail. This is known as the tuberculin test, because 

 the substance used in making it is called tuberculin. 



WHAT IS TUBERCULIN? 



Tuberculin is a fluid containing the products of the tubercle germ without 

 the germs themselves. As it contains no living germ, it cannot convey the 

 disease. Great skill is required in its preparation. A special fluid (or culture 

 medium) is prepared and the tubercle bacilli planted in it, great care being 

 taken to keep all other germs out. The fluid is then placed in a special kind 

 cf incubator and kept at the temperature of the animal body. Under these 

 conditions, the germs grow and multiply. Gradually the fluid becomes filled 

 with the products of the germs. When the right point is reached the fluid 

 is heated sufficiently to kill the germs which are then strained out. The remain- 

 ing fluid is tuberculin. 



Tuberciilin does not harm healthy cattle, even in large doses, but on 

 diseased animals it produces a marked effect. This is shown by a feverish 

 attack which comes on about eight to twelve hours after the tuberculin is 

 administered, lasts a few hours, and then subsides. This temporary fever is 

 called the reaction^ and animals which show it are called reactors. The value 

 of the test lies in the fact that diseased animals react while healthy ones do not. 



RELIABILITY OF THE TEST. 



The tuberculin test in the hands of a competent and experienced man is 

 much more accurate than any other method of detecting tuberculosis. The 

 records of large numbers of tests made by Government ofiicials show that, with 

 certain precautions, it is accurate in 98 per cent of the reactions obtained. 

 This gives a margin of a possible 2 per cent of error and this small number 

 may be still further lessened by care in making the test. For practical purposes 

 any animal that reacts must be considered tuberculous. 



