35 8 AGRICULTURAL BACTERIOLOGY. 



But even the cleanly kept barn would merely reduce, not re- 

 move, the chance of infection. When the disease is in the 

 lungs the animal is commonly attacked by a cough. In 

 ordinary breathing the bacilli are not exhaled with the breath, 

 but in coughing, quantities of the organisms are dislodged 

 from the mouth and nose, and thrown out into the air adhering 

 to particles of moisture. These float around for some time 

 and form a most ready means of infection on the part of other 

 animals in the vicinity. Indeed the only method of prevent- 

 ing the distribution of the bacilli from an animal whose lungs 

 or intestines are infected, is to isolate her completely from the 

 rest of the herd, and to see that no possibility is left for her 

 feces to be carried into the stalls of the healthy animals. 



Once distributed from the infected animals, the bacilli may 

 find entrance into the healthy animals by a variety of channels. 

 Doubtless some of them find entrance to the lungs, either by 

 dust particles, or by bacilli-laden moisture drops from coughing 

 animals, which are breathed by healthy animals. If they find 

 entrance in this way they are able to start an infection in the 

 lungs or glands of the neck. The fact that these regions are 

 so commonly affected in cattle shows that this is one of the 

 common means of infection. The bacilli which get into the 

 food or 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. Finding their way thus to 

 the intestine they may start an intestinal disease. Such a 

 means seems to be very common, judging from the frequency 

 of intestinal tuberculosis. These two means of entrance are 

 doubtless responsible for most cases of bovine tuberculosis, but 

 there is still a difference of opinion as to which is the more 

 common. It by no means follows that all animals receiving 

 the bacilli in their bodies will develop the disease, for many 

 of them may have a sufficient resisting power to drive 

 them off. But it is very easy to understand from these facts 



