ii8 Bulletin 250. 



(3) It is known that cows that have reacted, may, because of the 

 arrest of the disease, fail to react subsequently but still later the disease 

 may start up again, when the animal will react. We have records of many 

 cases of this kind. Great care must be exercised, therefore, in the inter- 

 pretation of negative results, especially of tests made in herds where 

 tuberculosis exists, and where it is possible that the animals failing to 

 react have already been infected. 



In applying the present knowledge of tuberculosis to the purchase or 

 exchange of cattle on the tuberculin test, it is safe to consider animals 

 that react to be suffering with active tuberculosis ; but if they do not 

 react they must not be considered to be absolutely free from infection, 

 especially if they are taken from a herd in which tuberculosis exists. 

 The longer the disease has been in a herd and the larger the number of 

 animals that are infected, the greater are the chances that those which 

 fail to respond will react if tested at a later date, from the fact that 

 tuberculosis which has become latent or healed may " break out " or 

 become active again, or because the animals have been infected but have 

 not yet developed the disease. For example, a few years ago a large 

 herd was tested and many of the animals reacted. Those that did not 

 react were placed in a new barn and after three months they were retested 

 and several of them responded. Three months later a few more reacted. 

 These later tests discovered the individual animals that were infected but 

 in the period of incubation, or those in which the disease was tem- 

 porarily arrested at the time of the first test. When there are very 

 few reactions at the first examination, the subsequent ones are usually 

 negative. Tubercle bacilli in a herd of cattle are much like weeds in a 

 garden, — • the more there are at the time of the first weeding the more 

 there are liable to be at the second cleaning. When most of the animals 

 in a herd respond to the test, all the others must be regarded with 

 suspicion. When only a few in any herd respond, it is less likely that 

 any of the remainder are infected. 



In a herd where the Bang method was being carried out, the retest of 

 the animals that reacted gave a negative result in about 20 per cent of 

 the cases. Some of these continued not to react for one, some for two 

 and others for three years ; but the greater number reacted again after 

 one or more negative tests. These facts are convincing that animals 

 which react once to tuberculin should not be considered safe to return to 

 the sound herd, although some of them may recover. 



The subsequent reactions and non-reactions are often difficult to 

 understand by those not versed in the nature of tuberculosis. They 

 are, however, not more mysterious than many other phenomena. 

 They are in perfect harmony with the natural course of the disease. 

 Tuberculosis is the result of little organisms living on the tissues 



