400 DISEASES OF CATTLE. 



the milk of tuberculous cows ; (3) during coition when the sexual organs 

 are tuberculous ; (4) from the tuberculous mother to the fetus in the 

 uterus. Inhalation is by far the most common mode of infection, since 

 statistics show a large percentage of primary lung disease in cattle. 

 Thus in the German report quoted the lungs were found 14.J times more 

 frequently diseased than the digestive organs. 



The bacilli can only get into the lungs when inhaled. They must, 

 therefore, be thoroughly dried and pulverized before currents of air can 

 carry them. It is well known that the bacilli withstand drying for 

 months before they lose their power of producing disease. They leave 

 the body of diseased animals in several ways. There may be a little 

 discharge occasionally coughed up from the diseased lungs, or milk may 

 be spilt, or there may be a discharge from the vagina when the genital 

 organs are tuberculous. The bacilli from these sources may become 

 dried and pulverized, and carried in the air of the stable and into the 

 lungs of still healthy cattle where the disease then develops. 



The disease of the stomach, intestines, and meseuteric glands is very 

 probably the result of food infection. Tubercle bacilli may have been 

 scattered upon the feed by diseased animals. But the most common 

 source of such infection is the milk of tuberculous cows. Calves may 

 become infected in this way. The disease may remain latent until the 

 animal becomes older. The not infrequent occurrence of tuberculosis 

 of the uterus and ovaries makes it probable that the disease may be 

 transmitted by a diseased bull, or carried by a healthy bull from a dis- 

 eased cow to a number of healthy cows. 



The source of infection is always some previous case of the disease, for 

 the latter can never arise spontaneously. Hence, in those stables in 

 which there is frequent change of cattle, the introduction of tuberculo- 

 sis by cattle coming from other infected stables is the most frequent 

 source of infection. Since the bacilli, when dried, can be carried by the 

 air it is not necessary that healthy animals should come in direct con- 

 tact with cases of disease to become infected. 



We Avill now briefly consider the various conditions which favor the 

 bacilli in their attack. Unsanitary conditions, such as overcrowding 

 in poorly- ventilated and poorly-lighted stables, and feeding of food 

 which is not nutritious, are not insignificant in this respect. Condi- 

 tions which injure the lungs are favorable to the development of tuber- 

 culosis. Among these are the inhalation of dust and smoke, and all 

 conditions which may induce chronic inflammation of the bronchial 

 tubes, with abundant secretion and subsequent pneumonia (broncho- 

 pneumonia). Among the other causes which are said to favor tuber- 

 culosis is the overproduction of milk, too many births, the improvement 

 of stock by continual inbreeding, and the consequent inheritance of 

 certain constitutional characters of a debilitating nature. Animals 

 living in the lowlands are more subject to this disease than the more 

 robust races living in elevated mountain regions. Similarly, animals 



