256 MICROBIOLOGY 



of temperature. In cows that do not react there is no effect 

 upon the flow of milk.* 



The interpretation of the reaction. When a tuberculin 

 reaction occurs it indicates that the animal is suffering from 

 a tuberculous infection and that most likely the process is 

 active. This reaction may occur when the tuberculous focus 

 is very small. It is possible that in those individuals where 

 a reaction takes place and where no evidence of the disease is 

 found in a gross examination of the viscera, a more careful 

 examination of the organs and intermuscular and subcu- 

 taneous tissues as well as the bones and nervous system might 

 reveal the presence of lesions. 



In cases where there is no reaction the interpretation is 

 more difficult. Tuberculin, however, does not give a reaction 

 (1) during the period of incubation; (2) in many, if not all r 

 cases where the disease is arrested; and (3) possibly not when 

 it is very advanced. This means that the records of tested 

 herds do not necessarily point out all the animals that are in- 

 fected, or those which may subsequently develop 'an active 

 form of the disease. The latent cases can not be detected until 

 the disease becomes active. To detect them as soon as they 

 become active requires repeated tests. It is not known how 

 long the tubercle bacteria will remain alive in latent lesions. 

 We have cases where the lesions became active three and one- 

 half years after the animal ceased to react and also a case 

 where an apparently healed calcified tubercle contained living, 

 virulent tubercle bacteria three years after the animal ceased 

 to react. It is such cases as these that have caused tubercu- 

 losis to reappear in many herds after the reacting animals have 

 been destroyed. The owners of such herds are inclined to 

 blame the tuberculin or the man who used it. The fault is 

 not with the first application of this reagent, or with the 



* Gilliland (Proceedings Am. Vet. Med. Asso., 1907, p. 371) made 

 a careful study of the effect of the tuberculin test on lactation. He 

 found it did not reduce the flow of milk of non-reacting cows and the 

 slight reduction in the reacting ones he attributed to the rise of 

 temperature. 



