MILK-BORNE INFECTIONS 447 



contain tubercle bacilli than to determine the actual percentage of 

 milk infected. 



Since tubercle bacilli are frequently present in milk, it is self- 

 evident that milk products may also contain them. Bacilli rise 

 with the fat globules; consequently, cream contains more tubercle 

 bacilli than the milk from which the cream was separated. Schroe- 

 der and Cotton have found that tubercle bacilli disappear from the 

 skimmed milk and are found in the sediment and cream, whether 

 the cream be gravity or separator cream. Therefore tubercle 

 bacilli may be in higher concentration in butter than in milk. 

 Furthermore, the bacilli retain their virulence in butter for a 

 considerable period. According to the same authors, they are 

 not materially attenuated in ordinary salted butter for forty-nine 

 days and are highly virulent even after three months. Since 

 butterine and oleomargarine contain some butter made from 

 cream or milk, these products are not exempt from the possi- 

 bility of harboring tubercle bacilli. 



Since cream is liable to contain tubercle bacilli, any product 

 prepared from it becomes a menace. Ice-cream, therefore, should 

 be made only of heated cream. In buttermilk the danger is prob- 

 ably not great, because, as stated before, the bacilli rise largely 

 with the fat which is removed as butter. But there remains the 

 possibility that buttermilk may be contaminated with tubercle 

 bacilli, and it is not known whether the acid in buttermilk has 

 sufficient germicidal power to destroy tubercle bacilli. 



Milk, cream, and buttermilk are always consumed within a 

 few days, at the most, and living tubercle bacilli may be present. 

 Cheese, however, is usually consumed after a period of ripen- 

 ing, and it has been found that tubercle bacilli die before the end 

 of the ripening period. Harrison, after a series of experiments, 

 decided that there is no danger from tuberculosis in Emmenthal 

 and Cheddar cheese, as they are rarely consumed before they are 

 four months old or more. However, the author found that cream 

 cheese is not entirely free from danger, and it is reasonable to 

 assume that cheeses which are eaten when fresh, as cottage cheese, 

 for example, may contain virulent tubercle bacilli. 



In the main, tubercle bacilli enter milk from either excreta or 

 diseased udders. They may be derived from excreta, whether 

 these are dislodged from the coat of the animal during milking or 

 from dust in the air. 



In a previous chapter it was shown that there is conclusive 

 evidence that bacteria cannot pass through the healthy mammary 

 gland. Investigators were puzzled for a long time by the pres- 

 ence of tubercle bacilli in milk from cows whose udders showed 

 no tuberculous lesions. There are two factors which account for 



