MILK-BORNE INFECTIONS 449 



sunlight and injured by diffuse daylight. Consequently, when 

 they are present in feces they are protected from light and retain 

 their virulence. Briscoe found tubercle bacilli to live in cow 

 manure for seventy-three days when exposed to weather condi- 

 tions in a pasture in the shade, and for forty-nine days when ex- 

 posed to sunshine. It is, therefore, desirable to construct stables 

 so as to admit plenty of light and roomy enough to avoid crowd- 

 ing. Railway cars in which cows are shipped may be the means 

 of communicating the infection on account of crowded conditions 

 and the close contact of animals. 



2. The plague may find a permanent foothold if diseased 

 animals are ushered into a sound herd or if a tuberculous bull, 

 for pairing, be introduced into the herd. Especially is this true 

 in large herds. New animals are being purchased constantly, 

 and these may be in the initial stages of tuberculosis. This can 

 be avoided only by purchasing from herds known to be free from 

 the scourge, but whether it is possible to know that a herd is 

 entirely free from the scourge is questionable. Therefore all 

 new animals should be tested with tuberculin and retested after 

 about six weeks, since the injection of tuberculin renders them 

 immune for a period up to six weeks, a fact which has led cattle 

 dealers to deceive purchasers. In the intervals between test- 

 ings the cows should be kept separate from the healthy herd, 

 and it should be made compulsory by legislation that all animals 

 responding to the tuberculin test be branded in some manner. 



Herds of valuable cattle have been known to contract the 

 disease at public exhibitions as a result of coming in contact with 

 infected animals or infected discharges. Grazing on the same 

 pasture with infected animals and drinking water from a com- 

 mon trough are dangerous. 



3. A most prolific source of infection is through the distribu- 

 tion of whey from cheese factories or of skimmed milk from 

 creameries. In Denmark distribution of these by-products is 

 prohibited by law unless they are pasteurized, as pasteurization 

 destroys tubercle bacilli. It can be readily understood that 

 through the distribution of raw skimmed milk or whey tuber- 

 culosis can be spread from one herd of cows to another. These 

 by-products are frequently used for feeding hogs, and as the latter 

 are highly susceptible to tuberculosis there is a tremendous loss 

 annually from the ravages of this disease. 



There are two principal methods of diagnosing tuberculosis in 

 living cattle: by physical examination and by testing with tuber- 

 culin. 



Physical examination has its limitations. As stated before, 

 cows do not show outward signs of tuberculosis for some time 



29 



