Tuberculosis in Cattle. 7 



than in one of the same genus. The most extreme example of 

 this is found in the bacillus of the bird which can only with dif- 

 ficulty be made to grow in the system of the mammal. 



But even in the mammal the virulence of the bacillus for 

 other mammals of a different genus or species may be very varied. 



Theobald Smith obtained, from a pet bear that had been owned 

 by a tuberculous master, bacilli which seemed to have no ill effect 

 when inoculated on cattle, and had a somewhat reduced virulence 

 for Guinea pigs. Kruse found bacilli from human sputum, and 

 others from the lungs of cattle which produced only local tuber- 

 cle in Guinea pigs. > 



Clinical observations show that the same is true as between 

 different individuals of the same genus and species, and hence 

 we find instances of tuberculosis in given herds, which continue 

 for a number of years with few cases showing generalized and 

 fatal results ; and other instances of herds in which the disease 

 makes rapid progress, soon affecting all or nearly all of the ani- 

 mals, and proving fatal to a number in rapid succcession. 



This modification of the germ by its surroundings is again 

 well shown in the common experience that it is usually difficult 

 to start (on artificial media in flasks) the growth of tubercle 

 bacilli taken direct from the animal, but when once started and 

 accustomed to grow on such new materials, it may be started 

 again in fresh culture with great certainty. 



In stating, therefore, that the one and only cause of tuber- 

 culosis is the tubercle bacillus, it is not to be understood that it is 

 affirmed that that bacillus is at all times, under all circumstances 

 and to all animals, equally virulent and destructive. If the con- 

 ditions are favorable it will prove very deadl^^ while, if unfavor- 

 able, it ma}^ linger for a time without producing much obvious 

 effect on the general health. Its presence, however, in any 

 herd is a constant menace to all members of the herd, to the 

 attendants, to the consumers of the meat and dairy products of 

 the herd, and to other herds into which members of this herd 

 may be sent. It is also worthy of note, that the power of 

 adaptation of the germ to its surroundings, introduces this 

 further element of danger that, as it becomes adapted to its life in 

 a given animal or in the different members of a closely bred 



