TUBERCULOSIS IN CATTLE. 45 
In some instances of advanced tuberculosis the tuberculin fails 
to give any reaction. These cases are rare, however, and are 
of comparatively little importance, because when they do occur 
the clinical symptoms are so well developed that the animal 
will be condemned independent of tuberculin. If, therefore, 
we are simply thinking of the matter of the detection of the 
disease, the failure to detect advanced cases is of little signifi- 
cance. Occasionally it may perhaps fail to detect a case not so 
far advanced. At the other extreme it has been claimed that 
there are some animals which react to tuberculin but are not 
suffering from the disease at all. This is apparently a mistake, 
although, of course, it is an extremely difficult thing to disprove. 
There are quite a number of cases of reacting animals in which 
the disease has not been detected after the slaughter of an ani- 
mal, but anyone who knows the difficulty of making a thor- 
ough examination of a dead carcass will see at once that such 
evidence is at least unsatisfactory. If the case is an incipient 
one, the question of discovering it by post mortem examina- 
tion is almost directly dependent upon how careful a search is 
made by the veterinarian. Sometimes he will examine an 
animal for two or three hours without success, but finally, 
after a search of several hours, will discover somewhere a 
small lymphatic gland which has evidently become tubercu- 
lous. Now, stich an animal is certainly infected with the dis- 
ease, and, bearing in mind that we are only here considering 
the matter of the accuracy of the test and not its value in other 
respects, it is very clear that the fact that we do have such cases 
of seeming mistakes is a simple testimony of its accuracy. It 
is the general belief among those who have used tuberculin 
most that if the test is made in a proper way animals do not 
give the reaction unless they have the disease at least in an in- 
cipient form. If the test does give a reaction in animals that 
have seemed to have no signs of the disease, this is because the 
disease is so incipient that it escapes the attention of the in- 
spector. If there are cases where a healthy animal. reacts, 
such cases are at all events extremely rare, and probably due 
to improper use of the tuberculin. But to be of value the test 
should be used only by persons skilled in its use. For various 
reasons there is a growing belief that it should be used only 
under the direction of veterinarians or officials. In detecting 
the presence of tuberculosis in our animals, then, tuberculin 
is very accurate; too accurate, indeed, to be a guide for the in- 
discriminate slaughter of reacting animals. 
