No. 6. DEPARTMENT OF AGRICULTURE. 123 



certain difference between a culture of tubercle bacilli from a cow 

 and another culture believed to be of human origin. These differ- 

 ences consisted in minute variation in size and shape, in some 

 slight, although well marked, differences in growth upon artificial 

 culture media and, most strikingly and most constantly, in differ- 

 ences in virulence for cattle. It was shown by Smith and also b\ 

 Frothingham, Dinwiddie, Ravenel and de Schweinitz that, while 

 cultures of tubercle bacilli from cattle afflicted with tuberculosis 

 arc, almost invariably, capable of producing progressive tubercu- 

 lous disease when inoculated upon cattle that, on the other hand, 

 cultures of tubercle bacilli from man are usually not virulent for 

 cattle; that is to say, when inoculated upon cattle they produce 

 either no effect or merely local, disease. 



This especial subject received a great deal of attention at tb<- 

 laboratory of the State Livestock Sanitary Board during a period 

 of five years. A large number of experiments were instituted for the 

 purpose of comparing tubercle bacilli from man and from cattle. 

 It was found that tubercle bacilli from cattle are at least as viru- 

 lent and generally very much more virulent, than tubercle bacilli 

 from man for experimental animals including herbivora, carnivora. 

 omnivora and also monkeys of several species. 



This development in the study of tubercle bacilli from different 

 mammals, which had taken place almost entirely in America and 

 which was just becoming generally known, was brought vividly to 

 the attention of the whole world in 1901, by Koch, who, in a paper 

 before the British Congress on Tuberculosis, made the statement 

 that I have already quoted, to the effect that bovine tuberculosis 

 is of such slight importance to the public health that no action 

 need be taken regarding it. In our effort to come to a just decision 

 in this matter, it is necessary to analyze briefly the facts upon which 

 Koch's opinion was based. 



Koch had found by experimentation that American investigators, 

 following the lead of Theobald Smith, were correct in their con- 

 clusions to the effect that tubercle bacilli from cattle are usuallv 

 much more virulent for experimental animals than are tubercle 

 bacilli from man. He was so impressed by this fact that he came to 

 regard tuberculosis of man and cattle as distinctly different types of 

 disease. It had been shown that human tuberculosis cannot, in 

 most cases, be transmitted to cattle, and so he concluded that the 

 converse must be true and that bovine tuberculosis cannot be trans- 

 initted to man. It does not seem that this inference can fairly 

 be drawn from the established facts that the chief and most strik- 

 ing difference between human and bovine tubercle bacilli lies in the 

 fact that the bovine germ is very much more virulent than that from 

 man. so that while most human tubercle bacilli are incapable of 

 causing disease in experimental animals of several kinds, there is 

 no experimental animal (mammal) that is able to resist infection 

 by the bovine germs. So far. then, as this point is concerned, it 

 would appear that the demonstration of the extreme virulence of 

 the bovine tubercle bacillus would point quite as clearly to more 

 danger to mankind than had formerly been attributed to this germ, 

 than to lessened probability of danger. 



Another point that we must consider, that was made by Koch 

 in his London address, is with regard to infection of the human 



