290 AGRICULTURAL BACTERIOLOGY 



increase in value as people become more alive to the eco- 

 nomic importance of the disease, and seek to purchase only 

 sound cattle. At present it is difficult, if not impossible, to 

 avoid the introduction of tuberculosis into a herd where 

 considerable numbers of cattle are purchased. The wide- 

 spread distribution of the disease, even though sparingly 

 present in any particular herd, always raises the question 

 as to whether any animal purchased from such a herd may 

 not be in the incipient but non-reacting stage. The limita- 

 tions of the tuberculin test are such that it will not enable 

 one to recognize every case, no matter in what stage the 

 disease may be. Individual animals may show a plainly 

 negative reaction to the test at the time of purchase be- 

 cause of the dormant form of the disease, because the period 

 of incubation has not been completed, or because of dis- 

 honest practices by the seller. These animals, if main- 

 tained in the herd, are quite certain in time to develop open 

 tuberculosis, and be a source of loss if their condition is not 

 detected. 



The only effective way to prevent the introduction of the 

 disease is to purchase, only from herds that are known to be 

 absolutely free from tuberculosis. It is certain that loss 

 can be largely avoided by the testing of all animals at the 

 time of purchase, by keeping the animals separate from the 

 herd for at least three months with a retest at the end of 

 this period, and by the annual testing of the entire herd. 

 In this way, the' use of tuberculin serves as a cheap kind of 

 stock insurance, and should be maintained regularly as an 

 annual duty in the herd. Where such vigilance is prac- 

 tised, little danger from the disease need be feared. 



Tuberculosis of hogs. As has been stated, the infection 

 of the hog takes place with the greatest ease by way of the 

 alimentary tract, hence the lesions are most likely to be 

 found in the glands of the throat, or in the mesenteric 



