386 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



not contract tuberculosis, showing;: that the bacilli in her milk were 

 certainly, through all the six years, mildly virulent germs of the kind 

 originally injected into her udder. The cow illustrates what we may 

 expect to find from time to time in dairy het-ds that are protected 

 against tuberculosis by bovo-vaccination. It seems that, once the 

 udder of a cow is invaded by tubercle bacilli, no matter how much or 

 how little tuberculous disease develops, her milk thereafter must be 

 regarded as permanently infected. 



Studies in the dissemination of tuberculosis through the agency of 

 flies have been made. While we have proved that flies may become 

 contaminated with tubercle bacilli by feeding on the feces of tuber- 

 culous cattle and by feeding on other substances that contain tubercle 

 bacilli, it does not seem in the least degree probable that they ever 

 carry the bacilli in sufficient numbers or in a way that actually con- 

 tributes to the propagation of tuberculosis among cattle or other 

 lower animals or human beings. In this connection we must bear in 

 mind that a few tubercle bacilli may enter the bodies of animals 

 through ordinary channels without causing disease, and that the 

 tubercle bacillus in nature is a strictly parasitic bacterium. Unlike 

 the typhoid-fever germ, it is not capable of multiplying in a sapro- 

 phytic manner in milk and a variety of other substances; hence, in 

 valuing the significance of the fact that flies may become contami- 

 nated with tubercle bacilli, we must bear in mind that the germs we 

 have to deal with on the flies are only the small number 1:hat will 

 adhere to their bodies under conditions which are rapidly fatal to 

 pathogenic bacteria, and not such dangerous colonies of germs as is 

 very likely to be the case with typhoid bacilli, when flies contami- 

 nated with the latter rub them off to grow and multiply on various 

 articles of human food. 



This view of the significance of flies in the dissemination of tuber- 

 cle bacilli is borne out by the experiments relative to the kind and 

 the intimacy of the exposure required to transmit tuberculosis from 

 diseased to healthy cattle. The work of the station almost justifies a 

 hard and fast conclusion that tuberculous infection can not travel the 

 distance of 10 yards through the air unless it has other aids than air 

 currents and flies. Closer proximity than 10 yards betAveen tubercu- 

 lous and healthy cattle offers so many opportunities for the transfer- 

 ence of infectious material from the former to the latter that flies 

 and air currents need not be drawn upon to supply an explanation 

 for the spread of the disease. 



A number of other investigations on tuberculosis have been and 

 still are in progress, one of which is the effect on animal bodies 

 caused by swallowing dead tubercle bacilli in pasteurized milk. It is 

 with some satisfaction that we are able to report that such dead 

 tubercle bacilli seem to be quite harmless, though it also seems that 

 tlie ingestion of living tubercle bacilli, even when they are not the 

 direct cause of tuberculosis, may reduce the normal resistance of 

 the body against the development of tuberculous processes. More 

 work will be done on this subject. 



Some work has been done relative to the different methods in which 

 tuberculin is applied to cattle in its use as a diagnostic agent for 

 tuberculosis and to test the quality of the tuberculin manufactured 

 by different commercial establishments as to its efficiency. The sub- 



