TUBERCULOSIS. 359 



how a single diseased animal in a barn may, in time, infect 

 most of the herd. 



ABUNDANCE OF BOVINE TUBERCULOSIS. 



Tuberculosis is widely distributed amoyg cattle, although it 

 is by no means universally found in countries where cattle are 

 kept. Some countries appear to be free from it. It is said 

 not to occur in Africa, and until recently it has been absent 

 from China and Japan, having been lately introduced with im- 

 ported cattle. In the western part of the United States, where 

 the cattle live out of doors most of the time, it is rare or ab- 

 sent. In general it is most abundant in localities where the 

 cattle are housed for a considerable part of the year. It is 

 consequently most abundant in northern countries, and appears 

 to be most widely distributed in northern Europe, Denmark 

 and Sweden. In warmer climates the disease is less common, 

 due probably to the fact that the animals live in the open for 

 a large part or the whole of the year. But even in the warmer 

 countries of Europe it is more prevalent than could be wished. 

 The disease is found, in short, in nearly all countries where 

 cattle are kept, and is almost proportional to the length of time 

 in which the cows are kept housed during the winter season. 



It is practically impossible to determine the percentage of 

 tuberculosis among the cattle of any country. There is no 

 method of detecting its presence among living animals except 

 by the use of the tuberculin test (see page 368), and this has 

 not been used sufficiently in any country to make it possible 

 to draw any conclusions as to the amount of the disease. The 

 only reliable data for determining its presence come from ex- 

 aminations of slaughtered cattle, and this method is open to 

 serious objections, the chief of which is the variation in the 

 accuracy of the inspection in different places. The evidence 

 from this source shows such variations that it would be rash 

 to attempt to give any distinct figures. It may be stated that 



