446 



MILK 



clean animals, good food and water prolong the usefulness of 

 the cow and may retard the progress of the disease. One 

 of the most important means of communicating tuberculosis is 

 through the excretion of large numbers of the bacillus with the 

 feces of tuberculous cattle. It has been estimated that about 

 40 per cent, of tuberculous cattle which give no outward indica- 

 tion of the disease discharge tubercle bacilli with their excreta. 

 After initial infection there is a period without discharge of tubercle 

 bacilli in the dejecta, and this fact complicates the difficulties of 

 ridding a herd of tuberculosis. Some bacilli are also expelled 

 from the mouth and perhaps from the nose. Since tubercle 

 bacilli are discharged in large numbers with feces, it follows that 

 market milk is frequently contaminated with them. 



Examination of raw market milk in large cities in this country 

 and in Europe has shown that tubercle bacilli are frequently 

 present. Earlier observations showed even greater frequency of 

 tubercle bacilli than later ones, but cannot be relied upon, be- 

 cause the specific stain for tubercle bacilli was depended upon 

 for diagnosis. It is now known that there are harmless acid- 

 proof bacilli widely disseminated on food and the bedding of 

 cattle and, therefore, found in milk, which give a microscopic 

 picture similar to that of tubercle bacilli in milk. Later investi- 

 gators have depended for results upon infection of guinea-pigs 

 that have been injected with the milk. To make this test the 

 milk is centrifugalized and the sediment and cream united. Tu- 

 bercle bacilli rise with the cream and sink with the sediment, 

 so these two parts of the milk contain practically all the bacilli 

 present. The mixture is then injected into guinea-pigs. How- 

 ever, other bacteria in the milk also rise with the cream and sink 

 with the sediment, and are liable to cause infection. Therefore 

 in order to obtain as many infections with tuberculosis as pos- 

 sible, at least three guinea-pigs should be injected with one sample. 



Investigators in various cities have given the percentage of 

 market milk containing tubercle bacilli at 5 to 17, and even higher. 

 A conservative estimate might place the average at 6 to 8 per 

 cent. Milk from the same source will not always contain tubercle 

 bacilli, and the relative numbers when they are present will vary. 

 Therefore it is essential that a series of tests be made from each 

 milk-supply. The usual experience is that tubercle bacilli appear 

 in the milk intermittently, and that sometimes they are so scarce 

 that when several guinea-pigs are inoculated but one of them 

 will contract tuberculosis. Milk-supplies which contain tubercle 

 bacilli at intervals are especially dangerous, since a false sense of 

 security may be created by not finding the bacilli in every sample. 

 It is more important to learn which supplies may occasionally 





