168 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



distribution of blackleg vaccine by the department. In the past 15 

 years more than 17,000,000 doses of this vaccine have been distributed 

 to stock raisers. In regions where blackleg prevailed the losses for- 

 merly amounted to more than 10 per cent of the annual calf crop, 

 but by the use of the vaccine the losses have been reduced to less than 

 0.5 per cent of vaccinated cattle. 



TUBERCLE BACILLI. 



• The rapid increase of cases of tuberculosis among the animals 

 slaughtered in the various packing houses of the country demanded 

 the careful study of the many questions which were connected with 

 this insidious disease. Consequentl}^ the presence of tubercle bacilli 

 in the milk of cows that reacted to the tuberculin test but without 

 showing any clinical indications of the disease was investigated by 

 means of very extensive experiments. 



The transmissibility and the transformability of the human, bovine, 

 and avian types of tubercle bacilli was made the subject of study; 

 also the different methods of immunization ; the retention of vitality 

 by tubercle bacilli that chance to be lodged in cheese, butter, or eggs; 

 and the occurrence of the different types of tubercle bacilli in cases of 

 natural infection of birds and animals in captivity. 



Other investigations on tuberculosis have thrown much light on the 

 relation between the location of tuberculous lesions in the animal 

 body and the channels through Avhich tubercle bacilli are expelled 

 and disseminated from the bodies of tuberculous animals ; on the per- 

 sistence of the life and virulence of tubercle bacilli under different 

 conditions and in different media; on the relation between tubercu- 

 losis of lower animals and human beings; on the relation between 

 tuberculosis of cattle and tuberculosis among other species of animals; 

 on the persistence of tubercle bacilli in a latent or semilatent state, 

 without l»ss of virulence, in the tissues of living animals; on the 

 causes that are responsible for the increased frequency of tuberculosis 

 among hogs, etc. 



The practical significance of some of this work is shown, for 

 example, by the widespread interest taken in those studies on the 

 elimination and dissemination of tubercle bacilli by tuberculous ani- 

 mals, which led to the discovery that tubercle bacilli are of common 

 occurrence in the feces of even apparently healthy tuberculous cattle. 

 This discovery at once offered an explanation for the occurrence of 

 tubercle bacilli in the milk of tuberculous cows with healthy udders, 

 and made it possible to prove definitely that the feces of tuberculous 

 cattle are a common cause of tuberculosis among hogs. 



HOG CHOLERA. 



For many j'^ears the Bureau of Animal Industry carried on a sys- 

 tematic study concerning the cause of hog cholera. These investi- 



