BOVINE AND HUMAN BACILLI 337 



tuberculosis, as has been recently stated. A number of observers 

 have shown that whilst it is true that ill-ventilated, dark, damp cow- 

 sheds predispose to infection, milch cows living entirely in the open 

 do not, on that account, escape the disease.* It depends upon infec- 

 tion in the herd, that is, upon contagion. But it is probable that, 

 through more than any other channel, the udder is the most common 

 one for the conveyance of infection. When the udder is affected, the 

 milk invariably contains large numbers of bacilli, and it will be 

 understood when one cow in a herd is so diseased, the entire volume 

 of mixed milk from the herd may be contaminated. The presence of 

 the bacilli in the milk is not always proportionate to the extent of the 

 disease in the animal, especially when diagnosed clinically. The 

 reason of this is the difficulty of clinical diagnosis between chronic 

 interstitial mastitis and tuberculous udder. There can, however, be 

 little doubt that the chief source of tubercle bacilli in milk is the 

 tuberculous udder. 



Finally, milkers affected with phthisis may readily infect the 

 milk, either by the repulsive habit of spitting on their hands prior to 

 milking, or by dried expectoration in cowshed, dairy, or milk-shop. 

 After distribution, milk is exposed in a variety of ways to dust, and 

 it cannot be doubted that such dust does at times contain particulate 

 matter derived from dried tubercular expectoration, and that there- 

 fore in this way also it is possible for milk to become infected. 



The Bovine and Human Tubercle Bacillus Compared. The 

 morphology of the bacilli in cultures of bovine origin is more 

 uniform and constant than in cultures from man. The bovine bacilli 

 are thick, straight, and short, seldom more than 2 ^ in length, and 

 averaging less (Theobald Smith). In the early generations many 

 individuals are seen which are oval, their length not more than 

 double their breadth. They are less granular than those from a 

 human source. They stain evenly and deeply with carbol-fuchsin, 

 beading being almost always absent from young cultures, and often 

 from old ones. In culture they have fairly constant and persistent 

 peculiarities of growth and morphology (Eavenel). 



The human bacilli are, on the other hand, much longer, thinner, 

 and tend to increase in length in sub-cultures. They are generally 

 more or less curved, sometimes showing S-shaped forms. They 

 stain less intensely with carbol-fuchsin, but beading is generally 

 seen, even in early growths, and is often very well marked. 



The above characteristics are most evident and persistent in 

 cultures grown on blood serum. On glycerine agar, glycerine 

 bouillon, and glycerine potato, bovine and human tubercle bacilli 

 approach each other in cultural features and morphology much more 

 closely, and by continued cultivation the differences tend to become 



* Report on Bovine Tuberculosis, Government of New Zealand, 1900 (Gilruth). 



Y 



