410 DISEASES OF CATTLE. 



of the bacillus, and there is also a special individual susceptibility 

 in some cases which may be otherwise described as an inability of the 

 animal tissues to resist and destroy the bacilli when they have pene- 

 trated to the inner recesses of the body. 



Among the conditions of environment which aid the development 

 of tuberculosis may be mentioned stabling with lack of ventilation, 

 damp buildings, the keeping of many animals together, drafts of air 

 which cause colds and catarrhs, and, in general, everything which 

 prevents the animals from developing and maintaining the highest 

 condition of health. None of these conditions of body or environ- 

 ment are sufficient to cause the disease, however, unless the animals 

 are exposed to the Bacillus tuherculosis and it penetrates the tissues 

 of their bodies. 



The ways in which the tubercle bacilli find their way into the body 

 may be considered under four heads: (1) By inhalation into the 

 lungs; (2) by taking into the digestive tract in the milk of tuber- 

 culous cows or with other contaminated feed; (3) during coition 

 when the sexual organs are tuberculous; (4) from the tuberculous 

 mother to the fetus in the uterus. The bacilli can reach the lungs 

 by inhalation only when the bacilli are thoroughly dried and pulver- 

 ized and in condition to be carried by currents of air. 



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 dis- 

 charge occasionally coughed up as a spray from the diseased lungs, 

 or this material may be swallowed and the bacilli carried off with 

 the excrement, or milk may be spilt, or there may be a discharge 

 from the vagina when the genital organs are tuberculous. There 

 may also be ulcers of the intestines, from which many bacilli escape 

 with the feces. 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 mesenteric glands is 

 very probably the result of feed 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 occur- 

 rence 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 one from a diseased cow to a number of healthy cows. 



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

 for the latter can never rise spontaneously; hence, in those stables 

 in which there is frequent change of cattle the introduction of tuber- 

 culosis by cattle coming from other infected stables is the most fre- 



