INFECTIOUS DISEASES OF CATTLE. 419 



There are undoubtedly predisposing conditions which contribute 

 toward the development of the disease ; some of these are found in the 

 animal body and others in the environment. An enfeebled condition 

 due to insufficient food, exposure to great extremes of atmospheric 

 temperature and insanitary surroundings, or the drain occasioned by 

 heavy production of milk appear to aid the development 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 penetrated 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 tuberculosis and this bacillus penetrates 

 the tissues of their bodies. 



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

 in the order of their importance, may be considered under four 

 heads: (1) By inhalation into the lungs; (2) by taking into> the 

 digestive tract in the milk of tuberculous cows or with other con- 

 taminated food; (3) during coition when the sexual organs are tuber- 

 culous; (4) from the tuberculous mother to the fetus in the uterus. 

 Inhalation appears to be by far the most common mode of infection. 

 The bacilli can reach the lungs only by inhalation when they are 

 thoroughly dried and pulverized and in a condition to be carried by 

 currents of air. 



It is well known that the bacilli withstand dr^nng 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 pul- 

 verized 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 food infection. Tubercle bacilli may have 

 been scattered upon the feed by diseased animals. But the most com- 

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

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



