INFECTIOUS DISEASES OF CATTLE. 403 



prevents the animals from developing and maintaining the highest 

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

 ment are sufficient to canse 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 contaminated 

 food; (3) during coition when the sexual organs are tuberculous; 

 (4) from the tuberculous mother to the fetus in the uterus. Inhala- 

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

 bacilli can only reach the lungs by inhalation when they are thor- 

 oughly dried and pulverized and in a condition to be carried by cur- 

 rents 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 discharge 

 occasionally coughed up from the diseased lungs, or there may be 

 ulcers of the intestines from which many bacilli escape and are car- 

 ried off with the excrement, 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 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 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 tuber- 

 culosis 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 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 arise 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- 

 quent source of infection. Since the bacilli when dried can be car- 

 ried by the air, it is not necessary that healthy animals should come 

 in direct contact with cases of disease to become infected. In general 

 the greatest number of cases occurs in the immediate environment of 

 cities, where there are not only abundant opportunities for infection, 

 owing to the frequent introduction of new animals into herds, but 

 where the sanitary conditions may be regarded as the poorest. 

 The bacillus of tuberculosis was discovered by Robert Koch in 1882. 



