DISEASES OF CATTLE 275 



There is little danger from meat as tuberculosis does not usu- 

 ally affect the muscles, and besides the heating to which meat is 

 subjected in cooking is sufficient to destroy the tubercle bacillus. 

 The danger from milk is far greater, since it is usually consumed 

 as raw milk, and since it is the food most frequently infected, and 

 forms the chief food of children. 



How Does Milk Become Infected With Tubercle Bacilli? As 

 has been shown, the organisms leave the body of the diseased ani- 

 mal in a number of ways and may be introduced directly or in- 

 directly, into milk. Tubercles in the udder may discharge their 

 contents directly into the milk ducts, and thus the milk is contam- 

 inated before withdrawal from the udder. Proof exists that milk 

 may be infected before withdrawal even though no tuberculosis can 

 be demonstrated in the udder. 



The tubercles in the lungs discharge their contents into the 

 air passages, and the infectious material is coughed up. A small 

 portion is discharged from the mouth during the act of coughing, 

 but the larger part is swallowed ; the tubercle bacilli pass uninjured 

 through the stomachs and intestines of the animals and are elimin- 

 ated in the manure. After being drawn, the milk may become con j 

 laminated by the introduction of manure and dust from the coat of 

 the cow during the milking. The more clean the method of produc- 

 tion of milk, the less important becomes this source of its infection. 

 Milk may also become infected from healthy cows in a tubercu- 

 lous herd, if the coat of the healthy animal becomes soiled with in- 

 fected manure. 



How Frequently Does Milk Contain Tubercle Bacilli? This 

 varies so widely that it is difficult to answer this question. In 

 those parts of the country in which a considerable portion of the 

 milch cows are tubercular, the mixed milk coming from a number 

 of herds is certain to contain tubercle bacilli almost constantly in 

 greater or less numbers. In the case of single herds, the oppor- 

 tunity for infection of milk depends upon the presence or absence 

 of the disease and upon the number of diseased animals in the herd. 

 If a cow has tuberculosis the milk may contain tubercle bacilli. 

 In order to be certain that the milk does not contain them, the 

 cows must be free from the disease. 



Is Milk an Important Means of Spreading Tuberculosis? It 

 is probably second in importance to the transfer of animals. It 

 has been shown that calves and hogs acquire tuberculosis easily 

 when fed on infected milk. Hogs, especially, are most easily in- 

 fected through the food, a single feeding of infected milk being 

 often sufficient to cause the disease. 



How Can the Spread of Tuberculosis by Milk and Other 

 Dairy Products Be Prevented? The patron of a cheese factory or 

 whole milk creamery carries to his calves and hogs a mixture of 

 the milk and whey from all of the other patrons, some of whom 

 may have badly diseased herds. It is important that these by- 

 products be treated so as to destroy any tubercle organisms they 

 may contain. This can be done by heating the skim milk and 



