DISEASES OF CATTLE 247 



The milk of reacting cows may be used if it is first boiled or 

 heated to a point sufficient to kill the germs. This heating to a point 

 less than boiling is called pasteurizing, and is safe provided all the 

 milk reaches the required degree of heat and is kept there sufficiently 

 long. For this it is necessary to keep the milk for 20 minutes at 149 

 F. or for 5 minutes at 176 F. 



This system of dealing with tuberculosis in a herd was planned 

 by Prof. Bang, of Denmark, and has been very successfully followed 

 in that country for some years. It has the advantage of allowing the 

 reactors to be made use of while a sound herd is being built up. Under 

 this system the sound herd increases in numbers as healthy calves are 

 added to it, while the diseased herd becomes smaller as the reactors 

 die off or are killed as open cases of tuberculosis. Finally a point is 

 reached where only a very few reactors remain, and the owner will 

 then find it to his interest to kill them rather than have the trouble of 

 keeping them isolated. 



Some time is required for the successful carrying out of the Bang 

 system, and the owner must be prepared to follow it steadily and 

 faithfully for the whole time that is needed, which may be several 

 years. During this time the healthy herd must be tested every six 

 months and any reactors removed to the diseased herd. At the same 

 time a sharp lookout must be kept for animals showing definite symp- 

 toms of the disease. These should be destroyed promptly, as they are 

 the most dangerous source of infection. 



Dealing With a Badly InfectedHerd. Where the testshows more 

 than half the number diseased, a somewhat different plan is required 

 than the Bang system. This herd is so badly affected that the non- 

 reactors can not safely be considered healthy. Many of them are sure 

 to have been infected with the disease quite recently, so that the test 

 fails to detect it. These will react at the next test, and in the meantime 

 may develop the disease so rapidly as to infect others. This will re- 

 peat the difficulty occurring at the first test, and it would be a long and 

 tedious process of weeding before even a small but perfectly healthy 

 herd could be established. 



For these reasons it is better to treat such a herd as if it were 

 entirely diseased and to begin with the newborn calves to build up a 

 healthy herd. The method from this point is exactly the same as the 

 Bang system, except that as there are no healthy cows to act as foster 

 mothers the calves must be raised on pasteurized milk. At 6 months 

 old the calves are tested and reactors are transferred to the other herd. 

 This plan was devised by a German veterinary surgeon named Oster- 

 tag, and is known as the Ostertag system. It is very successful when 

 carefully carried out. 



While getting rid of the disease by whatever system may be 

 adopted, an animal should never be bought for the healthy herd 

 unless known to be healthy. The tuberculin test should be applied, 

 and if possible the animal should be selected from a herd that is 

 known to be free from tuberculosis. New purchases should be iso- 

 lated or kept apart from the healthy herd, and if possible from each 



