DISEASES OF CATTLE 245 



readily transmits the disease to young animals. It is very dangerous 

 to children. Hogs and calves are very readily infected by it. 



How a Herd Is Infected. Tuberculosis may be introduced into 

 a healthy herd in & number of ways. 



1. By the purchase of a bull or other animal that is infected with 

 the disease. This animal may be apparently healthy at the time of 

 purchase, but if it contains the germs, the disease may develop and 

 spread to other cattle. New animals should only be bought from a 

 herd that is known to be healthy. 



2. By feeding calves with milk, buttermilk, or whey that has 

 come from tuberculous cows. A farmer may have a healthy herd, 

 but if he brings home skim milk from a creamery and feeds it to his 

 calves he may give them the disease. Such milk should be rendered 

 safe by boiling or pasteurizing it. 



3. By showing cattle at fairs and exhibitions where no proper 

 care is taken to keep out diseased stock or to disinfect the stables. 



4. By shipping animals in cars that have not been disinfected, 

 as these may have recently carried diseased cattle. 



5. By allowing cattle to graze with diseased ones, or to come in 

 contact with them over fences. 



The Tuberculin Test. Tuberculosis develops so slowly that in 

 many cases it is months and sometimes years before any symptoms 

 are shown. During this period the infected animals can not be dis- 

 tinguished from the healthy in any ordinary way. There is a test, 

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

 practically without fail. This is known as the tuberculin test, be- 

 cause the substance used in making it is called tuberculin. 



What Is Tuberculin? Tuberculin is a fluid containing the prod- 

 ucts of the tubercle germ without the germs themselves. As it con- 

 tains no living germs, it can not convey the disease. Great skill is re- 

 quired 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 

 of 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 remaining fluid is tuberculin. 



Tuberculin 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 8 to 12 hours after the tuber- 

 culin is administered, lasts a few hours, and then subsides. This tem- 

 porary 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 num- 

 bers of tests made by Government officials show that with certain pre- 

 cautions it is accurate in 98 per cent of the reactions obtained. This 



