120 Bulletin 250. 



of the animals, and whether they multiply continuously or become 

 checked in their activities depends upon the degree of the resisting 

 force of the animal and possibly the invading ability of the organisms. 

 It is a struggle between two opposing living forces, and sometimes one 

 and sometimes the other is in the lead. When this parasitic nature of 

 tuberculosis is understood, there will be less difficulty in comprehending 

 many of the seemingly contradictory manifestations- It is the neglect 

 of these natural but subtle tendencies and powers of tubercle bacteria 

 that has enabled them to make headway and gain entrance to the tissues 

 of cattle when the owners have thought they were very careful to guard 

 against them. For example, after a herd has been tested and the reacting 

 animals destroyed, it is possible that no further attention has been 

 given to the remaining cattle, some one or more of which may later 

 develop the disease and spread it to the other animals in the herd. 

 This unfortunate condition is not the fault of the first test, but of the 

 failure to make subsequent ones. The purpose must be to avoid the 

 possibility of infection. 



6. The control of tuberculosis. 



The real problems relating to tuberculosis before the cattle owners 

 are the prevention and the eradication of this disease. They resolve 

 themselves into the best methods to follow under two distinctly different 

 conditions, namely: (i) when the herd is free from tuberculosis, and 

 (2) when a greater or less number of animals are already affected. 



The protection of Jiealtliy herds. 



In preventing the entrance of a specific disease, it is simply necessary 

 to keep out the microbe that produces it. The important question to 

 consider in this connection is, how to keep it out. I have already 

 mentioned the two most common channels that are know^n through 

 which tubercle bacteria gain entrance to a herd of uninfected cattle, 

 namely, (i) through the feeding of calves with the unsterilized milk 

 from creameries and possibly the whey from cheese factories where the 

 milk from tuberculous cows is received, and (2) the introduction into 

 the herd of a tuberculous animal or animals. The prevention by cutting 

 off these channels of infection is not difficult nor expensive compared with 

 the elimination of the disease if introduced or the loss it will occasion if 

 allowed to remain. 



To guard against the first it is necessary to sterilize the milk fed to 

 calves, unless it is known that the animals from which it came are free 

 from the disease. 



To avoid the second danger, it is necessary to have all the animals 

 carefully examined and tested with tuberculin before bringing them into 

 the herd. The tuberculin should be applied by a competent person who 



