MECHANISM OF INFECTION IN TUBERCULOSIS 353 



Bacilli from equine sources, whilst agreeing in their cultural 

 characters with the bovine type, give rise when inoculated into 

 animals to lesions of moderate severity such as are caused by 

 the so-called human bacillus. 



There appears to be the strongest evidence, therefore, that 

 the bacilli of human and bovine tuberculosis are varieties of one 

 and the same organism. 



Study of variation has been far more easy in the case of the 

 tubercle bacillus than in the case of other common pathogenic 

 micro-organisms. The slowness of its growth, the mild chronic 

 type of lesions which it causes, the delay which is apparent in 

 its response to altered surroundings are all reasons which have 

 led to an undue share of attention being given to characters 

 probably in themselves not essential which in more rapidly 

 growing bacteria escape notoriety. 



So far as the discovery in a human lesion of bacilli either of 

 " bovine " or of " human " type can afford evidence of infection 

 from one or the other source, the position remains unchanged 

 from that which existed before the work of the Commission was 

 undertaken. 



In view of the proven stability of the tubercle bacillus, it is 

 probable that a child suffering from tuberculosis due to bacilli 

 of " bovine " type may infect a number of other children and 

 give rise in them to tuberculosis due also to " bovine " bacilli. 

 In such secondary cases, it would be of little use to attack the 

 milk supply and neglect the obvious source of infection pro- 

 vided by the first sufferer. 



Certain of the anomalous cases, in which the bacilli ex- 

 hibited both bovine and human characteristics, were attributed 

 by the Commission to a mixed infection with the two types of 

 organism. 



In this connexion, the evidence of Von Pirquet's skin reaction 

 mentioned before is of interest. 



In carrying out this test, it is customary to inoculate tuber- 

 culins from a bovine and a human source on separate sites. As 

 a result, in the vast majority of subjects, if a reaction occur, it is 

 positive to both human and bovine tuberculin. It is incredible 

 that in all cases a mixed infection should be operative. 



In those cases in which one reaction or the other is alone 

 positive, there appears to be no clear correlation between the 

 site of the lesion and the type of reaction. Abdominal lesions 



