No. 6. DEPARTMENT OF AGRICULTURE. 127 



have feeble acliou even on the most susceptible animals, like guinea- 

 pigs,^^ while others are most actively virulent. Besides Culture BB, 

 described in this paper, cultures of exalted virulence have been iso- 

 lated from man by Vagades and Lartigau. Whether or not these 

 cultures were in reality the bovine organism which had infected man 

 we caonot say now. At present our knowledge is so restricted in 

 the matter of differentiation that we must designate the cultures 

 human or bovine according as they are isolated from man or from 

 (tattle. The dictum of Koch that inoculation of cattle will infallibly 

 distinguish one from the other cannot be accepted offhand, as it is 

 far from proven that virulence for cattle is a fixed aad constant char- 

 acteristic. Prof. McFadvean savs: "If a low degree of virulence 

 for cattle is to be taken as the distinguishing feature of human ba- 

 cilli there will be no difficulty in proving that human disease is 

 sometimes transmitted to the lower animals." 



lias the Bovine Tubercle Bacillus more Pathogenic poiver for Man 

 than the Human Bacillus? — In trying to interpret the results ob- 

 tained by inoculatioo of animals we are forced to ask ourselves 

 whether or not they are applicable to man also? Having proven that 

 for practically all experimental animals the bovine tubercle bacillus 

 I^ossesses, as a rule, a much greater pathogenic activity than the 

 human, is it fair to conclude that this increase of virulence will 

 hold good for man also? Until the contrary is proven, or until 

 good reason for believing the contrary is shown, I feel that this must 

 be our conclusion. This matter opens up a most fascinating field for 

 speculation, in which we meet with many apparent contradictions; 

 and until we gain a better understanding of immunity in general we 

 cannot speak dogmatically, but must stick to ascertained facts in 

 each individual case. Virulence is a factor which is relative to the 

 subject, and exaltation of virulence for one species of a«imal does 

 not necessarily prove an increased power for other species. Indeed, 

 the reverse is sometimes true. For example, the strptococcus is said 

 to become increased in virulence for mice by successive passages 

 through these animals, but less virulent for rabbits. So, too, the 

 bacillus of '^rouget du pore," though but slightly pathogenic for rab- 

 bits, rapidly acquires a high degree of virulence for them by succes- 

 sive passages; but when this has taken place we find that all viru- 

 lence for swine, fi-om which it came in the first place, has disappeared. 

 Large doses do not even make them ill. 



Admitting freely all these facts, we must regard them as excep- 

 tional, for it is a firmly established rule in bacteriology that when 

 the virulence of a pathogenic organism is increased for one animal 

 it is increased for all that are naturally susceptible to its action 

 as well as for those in which the disease is known only experi- 

 mental] v. 



