238 PATHOGENIC BACTERIA IN MILK 



The milk in udder disease. — In the early stages of udder 

 disease the milk remains to all appearances quite normal. One 

 of the first signs of abnormality is the diminution in the yield. 

 Previously to this it is said there is an actual increase in the 

 quantity of milk. As soon as the disease begins to have effect, 

 there is a definite decline in the yield. For example, a cow 

 which in health gave, say, fifteen litres of milk, falls to one 

 half or one quarter of that amount. The milk also changes 

 in consistence, becoming thin, watery, and serous. At the same 

 time the colour may turn to yellow, and the flocculi and flakes 

 which occur in milk from a healthy udder are present in larger size. 

 As the yield diminishes, the consistence of the fluid becomes more 

 and more irregular, the flocculi predominating. If such milk be 

 allowed to stand in a vessel, a deposit of solid matter, composed of 

 these fragments, settles down, leaving a superficial layer of thin 

 fluid at the top. Finally, the consistence becomes sero-purulent 

 and then purulent. Hence, previously to suppression we get a 

 thick yellow purulent fluid, having an alkaline reaction, coagulated 

 casein, and diminution of lactose. As a rule, tubercle bacilli are 

 readily found, and whether that is so or not the milk is highly 

 infective.^ 



The Entrance of Tubercle Bacilli into Milk 



There are two main sources of the tubercle bacilli found in 

 milk, namely, a bovine source and a human source. The two 

 common channels respectively are a tuberculous udder and a 

 phthisical lung. From the former, milk may derive a direct and 

 abundant supply of tubercle bacilli, from the latter, milk may 

 become indirectly contaminated by the particulate matter of dried 

 sputum. 



Tuberculosis may be introduced into healthy cows in a variety 

 of ways. The most common method is by means of a tuberculous 

 animal, from the excretions and discharges of which infection may 

 be conveyed to soil, water, air, fodder, and general surroundings. 

 In this way not only other animals cohabiting with a tuberculous 

 animal become infected, but premises, stables, and utensils may 

 also become infected. The milk of a tuberculous animal may also 

 be consumed by other animals on the farm, and so a vicious circle 

 of infection is completed. Ravenel has shown that by the cough 

 of a tuberculous cow tubercle bacilli may be distributed. Of thirty- 

 four examinations carried out on five tuberculous cows tubercle 

 1 See also Report of Royal Commission on Tuberculosis, 1896, part iii., p. 142. 



