Agricultural Bacteriology. 



said to have "open" tuberculosis, and are a special 

 menace to the remainder of the herd. "When the dis- 

 ease is confined to parts of the body that have no exter- 

 nal opening, as lymph glands, the cow is said to have 

 41 closed" tuberculosis. As long as the disease remains 

 "closed," the animal is not a source of danger. It is 

 impossible to foretell when the "closed" type will 

 change to the "open," as it is certain to do sooner or 

 later with the continued development of the disease. 

 Hence every affected animal must be considered a menace, 

 present or potential, to the herd and to the public health. 



Infection of the animal. The bacteria that come 

 from diseased animals are carried into healthy animals 

 in the dust from polluted mangers and dried manure. 

 The dust may enter the lungs or it may lodge in the 

 throat. The food may be soiled by the sputum of an 

 animal, or by dust. The milk fed to calves and hogs 

 may contain the bacteria, which then pass through the 

 walls of the intestine into the lymph and blood streams 

 and are carried to various parts of the body, especially 

 the lungs and lymphatic glands. For this reason these 

 are the organs most often affected. Hogs acquire the 

 disease very readily when fed on contaminated milk, 

 a single feeding of milk containing many tubercle 

 bacilli suffices to infect hogs. Because of the content of 

 manure in tubercle organisms, hogs running after cattle 

 in the feed-lot are as likely to be diseased as those fed 

 on skim milk. Animals may acquire the disease by 

 contact with a diseased animal, as for instance by licking 

 each other. Very rarely are the reproductive organs 

 affected, and the calves from tuberculous dams are 

 usually healthy. Because of this fact, it is possible to 



