TUBERCULOSIS OF THE COW 137 



This is not really a source of error, it only appears to be 

 one. These instances usually occur among animals for which 

 extensive dissection is impracticable owing to their having to 

 be dressed for sale. 



Nocard,^ a strong supporter of the tuberculin test, makes 

 this very clear. He remarked : " Speaking for myself I have, 

 up to now, personally and publicly performed 500 autopsies 

 of animals which had reacted to tuberculin. In three cases 

 only have I failed to find the lesion sought for ; but some- 

 times it has taken a long time, half-an-hour, three-quarters 

 of an hour, and more, before I could put my hand on some 

 miliary granulations buried in the depth of the lung or 

 scattered in the thickness of a lymphatic gland." 



Stated another way, it is sometimes objected to the test 

 that it is too sensitive and detects even the slightest and 

 most latent cases of tuberculosis. From the point of view of 

 clearing a herd of tuberculosis it is very necessary to re- 

 cognise that these early and latent cases will, almost all 

 eventually and some rapidly, develop into marked cases which 

 will extend the disease and become a source of infection to 

 the milk. 



The above sources of error can, for the most part, be 

 guarded against, and rather illustrate the need for care in 

 using the test than any unreliability in the test itself 



During the last few years additional tests have been 

 advocated for the diagnosis of tuberculosis in both man and 

 animals. Those include the ophthalmo-tuberculin test and 

 Von Pirquet's skin reaction test. These tests appear to be 

 of but limited value in the diagnosis of bovine tuberculosis, 

 but extended experience is desirable. 



The Methods of Spread of Bovine Tuberculosis 



Infection is usually introduced into a herd by the 

 addition of an animal suffering from tuberculosis. The in- 

 fectivity of the introduced animal will depend upon the 

 extent to which it is excreting tubercle bacilli. From this 

 point of view reacting tubercular cows may be divided into 

 three groups : (a) slight cases reacting to tuberculin, but 

 1 Trans. Br. Congress on Txiberculosis, 1901, vol. iv. p. 10. 



