420 DISEASES OF CATTLE. 



A worse aspect of the case is that there are some diseased animals in which 

 tuberculin fails to discover the existence of tuberculosis. In most of these, no 

 doubt, the deposits are old, insignificant, and generally calcified, or they are 

 cases where the disease is arrested and perhaps in process of recovery, and 

 which are possibly incapable of disseminating the contagion. But it is known 

 that there are cases, not altogether rare, where tuberculin fails to cause a 

 reaction in a highly tuberculous animal, and consequently one in which the 

 disease exists in an extremely contagious form. For this reason a clinical 

 examination should always be made of an animal Avhich does not give a reac- 

 tion but which shows symptoms indicating that, notwithstanding the test, it 

 may suffer from tuberculosis. 



Nocard, of Paris, wrote also in 1898 as follows : 



The degree of certainty of the indications furnished may be stared in precise 

 terms. The observation of a clear reaction to tuberculin is unequivocal ; the 

 animal is tuberculous. The pretended errors imputed to the method are ex- 

 plained by the extreme sensitiveness of the reagent, which is capable of detect- 

 ing the smallest lesion. It often requires prolonged and minute researches in 

 the depths of all the tissues to discover the few miliary centers, the presence 

 of which has been revealed. The reaction is absolutely specific. In those cases 

 where it is observed with animals which show lesions of another disease (acti- 

 nomycosis, hydatid disease, verminous bronchitis, distomatosis), it may be 

 affirmed that there exists, in addition to these conspicuous changes, a tubercu- 

 lous center which alone has provoked the reaction. 



The failure to react does not necessarily imply absence of tubcrc%ilosis. Such 

 failures of tuberculin are very exceptional. They are seen most frequently with 

 animals affected with tuberculosis in a very advanced stage and made evident 

 by plain external signs. Sometimes, also, there are found at the post-mortem 

 examination of animals which have not reacted small fibrous or calcified lesions 

 in such a condition that one is tempted to believe them cured. "Whether sterile 

 or not, these legions have no tendency to increase, and they are not very danger- 

 ous from the point of view of contagion. 



These opinions of two eminent authorities, living in different 

 countries, after long experience of their own and after studying the 

 results of the many tests made in different parts of the world, should 

 have great weight. They are essentially the same throughout. 



In 1897 Voges compiled statistics of tuberculin tests, the accuracy 

 of which had been determined by post-mortem examination. Of 

 7,327 animals tested, it appeared that errors had been made with 204, 

 or 2.78 per cent. In the work of the Pennsylvania Live Stock Sani- 

 tary Board post-mortem examinations were made on about 4.400 

 reacting cattle and the disease was found in all but 8 of those which 

 had given characteristic reactions. 



The results of a much larger number of tests might be compiled 

 at this time, but they would not materially change the average of 

 those already mentioned. It is plain that tuberculin is a remarkably 

 accurate test of tuberculosis, that the animals which react may be 

 safely considered as tuberculous, and that when a careful clinical 

 examination is practiced in addition to the test there are few animals 

 in a dangerous condition which escape detection. 



