138 MILK AND THE PUBLIC HEALTH chap. 



with lesions not communicating with the exterior, so that 

 no tubercle bacilli are excreted ; (b) cows apparently healthy 

 but suffering from " open " tuberculosis and as such excreting 

 tubercle bacilli ; (c) cows with distinct clinical tuberculosis. 



The last group is the source of most massive infection, 

 but the second, being usually unrecognised, may have the 

 greatest range of infectivity. The actual channels of infection 

 are apparently sometimes through the respiratory and some- 

 times through the digestive tract. The opportunities for 

 infection will vary with the condition of the byre and the 

 degree of association of the cows in it. In well-lit, well- 

 ventilated, and properly constructed byres with well looked 

 after animals the chances of infection are of course reduced. 

 Del^pine, for example, records that a larger number of 

 samples of milk containing tubercle bacilli were obtained 

 from farms in an insanitary condition. 



In addition to direct infection from cow to cow there is 

 another, much less common, but very dangerous method of 

 spread : this is from the use of skim -milk infected with 

 tubercle bacilli to feed calves. In cases in which the skim- 

 milk is returned from a milk factory or creamery, very wide- 

 spread infection may in this way result. 



Eussell ^ records a very interesting illustration of this 

 method of spread in Wisconsin State, U.S.A. It was the 

 practice of cowkeepers supplying creameries to take back the 

 skim -milk to feed the calves. This skim -milk is mixed 

 indiscriminately at the creamery, and thus affords a ready 

 means for disseminating any pathogenic bacilli in the milk 

 from one herd to another. By means of the tuberculin reaction 

 Eussell studied the distribution of tuberculous cows in the areas 

 of certain creameries. These creameries received their milk 

 from a number of different cowkeepers, and by separating the 

 figures for the different creameries he obtained definite 

 evidence of infection through the skim-milk. This is seen in 

 the following table, which shows that 34-5 and 24-0 per cent 

 of the cows in connection with the Medina and Oak Park 

 creameries were tuberculous, as compared with 8'5 and lO^O 

 per cent from other creameries. In the case of the two 

 infecting creameries nearly all the animals were raised on the 



1 Wisconsin University, Agriculhiral Experiment Station Bull. 143, 1907. 



