234 VETERINARY HYGIENE 



thus rendered easy in crowded, ill-ventilated byres not only by 

 tubercle bacilli contained in moist particles in the expired air, but 

 also, and possibly to a less extent, by the tubercle bacilli contained 

 in contaminated faeces, discharge, &c., which has become dry and 

 has subsequently been raised into the air by draughts. Contagion 

 may also be conveyed from one adult animal to another through 

 the medium of soiled water, food, or expectorate containing tubercle 

 bacilli, and though ingestion at least for adult cows plays a minor 

 part compared with inhalation, its possibility must not be lost sight 

 of. The feeding of calves with raw milk, either whole or separated, 

 obtained from badly tuberculous cows, some of which may be 

 suffering from tuberculosis of the udder, is the most common 

 method by which young stock become affected. 



With regard to the escape of bacilli from the body, use has been 

 made of the expressions " open " and " closed " tuberculosis. By 

 " open " tuberculosis was meant cases in which the lesions were 

 in direct communication with the exterior, and from which bacilli 

 were being more or less continuously discharged. " Closed " 

 lesions were those in which, owing to their nature, such escape was 

 declared to be impossible. The use of such expressions is mislead- 

 ing because often no one can say at any given moment whether 

 bacilli are being discharged or not. The fact is that every animal 

 which reacts to tuberculin must be looked upon as capable of 

 infecting; such animals may appear to be in a perfect state of 

 health, and at the same time be discharging large numbers of 

 bacilli in the faeces, &c. In any case, according to Schrceder* 

 animals which have been infected for three years or more are, with 

 rare exceptions, spreaders of the disease. 



A certain amount of controversy has raged around the question 

 as to the infectivity or otherwise of the milk of reacting cows, 

 which are apparently healthy and whose udders appear normal on 

 clinical examination. Many observers (Ostertag, Hessler, 

 Schrceder, Cotton, and others) as the result of their experiments, 

 concluded that such milk does not contain tubercle bacilli, and it is 

 generally agreed that this is so. In Great Britain, under the present 

 conditions of the disease, it would be impossible on the score of 

 expense to insist that the only milk sold should be derived from non- 

 reactors. There is always of course the risk that the disease process 

 may extend to the udder tissue, but in face of the objection 

 mentioned that risk has to be faced. The possibility still remains 

 that milk which in the udder contains no tubercle bacilli may be 

 contaminated with them during or after the process of milking. 

 *Journ. Comp. Path., 1908, Vol. XXL, p. 1. 



