METHODS OF DISTRIBUTION. 267 



to his boots will be sufe to be knocked off when dry, and will thus 

 be carried everywhere that the farmer goes. They will be certain 

 to be dislodged near a healthy cow and may become mixed with 

 her food which is commonly thrown on the floor in front of her; or the 

 particles may become dry and be distributed through the barn as 

 dust. In short it is inevitable that the bacilli voided with the 

 excrement will in time come in contact with every healthy animal 

 kept in the same barn. 



Once distributed from infected animals, the bacilli may find 

 entrance into the healthy animals, by a variety of channels. Some 

 find entrance to the lungs, either by the dust particles or by the 

 bacilli-laden moisture drops from coughing animals, which are 

 breathed by healthy animals. The bacilli which find their way 

 into the watering trough will be swallowed, and the same will be 

 true of those which the animal takes into its mouth by licking its 

 infected neighbor. These two means of entrance are doubtless 

 responsible for most cases of bovine tuberculosis, and it is very 

 easy to understand how a single diseased animal in a barn may, 

 in time, infect most of the herd. 



Abundance of Bovine Tuberculosis. Tuberculosis is widely 

 distributed among cattle, although it is by no means universally 

 found in countries where cattle are kept. It is said not to occur 

 in Africa, and until recently it has been absent from China and 

 Japan, having lately been introduced with imported cattle. In 

 the western part of the United States, among the cattle living out 

 of doors most of the time, it is rare or absent. 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. 



It is practically impossible to state the percentage of cattle 

 suffering from tuberculosis. Among the animals examined in the 

 slaughter houses of Denmark it has sometimes appeared that more 

 than half of the cows are tuberculous. From these high figures 

 the percentage has ranged down to 10 per cent, or even lower in 

 some places, and, in fact, is so variable that no general averages 



