240 PATHOGENIC BACTERIA IN MILK 



depends upon infection in the herd, that is upon contagion. But it 

 is probable that, as a channel for the conveyance of infection, 

 the udder is more common than any other. When the udder 

 is affected, the milk invariably contains large numbers of bacilli, 

 and it will be understood that when one cow in a herd is so 

 diseased, the entire volume of mixed milk from the herd will be 

 contaminated. The presence of the bacilli in the milk is not 

 always proportionate to the extent of the disease in the animal, 

 especially when diagnosed clinically. The reason of this is the 

 difficulty of clinical diagnosis between chronic interstitial mastitis 

 and tuberculous udder. There can, however, be little doubt that 

 the chief source of tubercle bacilli in milk is the tuberculous 

 udder. 



Finally, milkers affected with phthisis may readily infect the 

 milk, either by the repulsive habit of spitting on their hands prior 

 to milking, or by dried expectoration in cowshed, dairy, or milk- 

 shop. After distribution, milk is exposed in a variety of ways to 

 dust, and it cannot be doubted that such dust does at times contain 

 particulate matter derived from dried tubercular expectoration, and 

 that therefore in this way also it is possible for milk to become 

 infected. 



The requirements for infection by tuberculosis may be sum- 

 marised very simply as follows : — First, the causa causans, without 

 which there can be no tuberculosis, is the tubercle bacillus of Koch. 

 Secondly, the conveyance of tubercular infection from man to man 

 and from animal to animal in an infected fluid or dried infective 

 discharge ground into fine dust. Thirdly, an important condition 

 of transference and of site of disease is that the part of the body 

 of man or animal in which the infective germs begin to grow must, 

 as a general rule, have been in some way weakened or injured. 

 And lastly, to gain foothold and produce disease, the infection itself 

 must be of some degree of intensity, or have been in operation for 

 some period of time. It is these conditions which are the control- 

 ling factors in the limitation, such as it is, of the ravages of the 

 disease. 



The possibility of infected milk without clinical udder 

 disease. — It was believed by many, and is still held by some, that the 

 milk of tuberculous cows is infective only when the udder is affected. 

 Ernst, Theobald Smith, Bang, and Hirschberger have, however, 

 shown that cows having generalised tuberculosis but apparently 

 unaffected udders may, in fact, yield a small percentage of cases of 

 tuberculous milk. The last named found this percentage to rise as 



