DISEASES COMMUNICABLE IN MILK 43 



test extensively. The latter two authors have made 4,926 tests, including 

 retests on 4,001 head of cattle of which 1,614 reacted, the results being 

 checked by 1,000 subcutaneous tests and 341 autopsies. They consider 

 that if a 5 per cent, or stronger solution of alcoholic tuberculin is used and 

 the tests are performed by experienced operators, the results are of 

 equal accuracy to those obtained by the subcutaneous method and are 

 obtained with economy of time and materials. They do not recommend 

 the test for unskilled practitioners. 



Injection is done with a hypodermic syringe having a needle of 25 

 or 26 gage and a point ^{Q in. long, and is best made into one of the folds 

 of the skin underneath the base of the tail. The application of disinfect- 

 ants to the point of injection should be avoided for they are likely to 

 produce swellings that may be mistaken for the reaction. A positive 

 result is indicated by a thickening of the subcaudal fold or by the appear- 

 ance at the point of injection of a sensitive swelling varying in size from 

 that of a small pea to that of an orange. Small indurations about the 

 size of the head of a parlor match frequently occur in normal non-reacting 

 cattle at the point of inoculation but anything larger, that persists for 72 

 hr. should be considered a positive reaction. A recognizable reaction 

 may be present on the sixth hour after injection but usually it is first 

 clear about the twelfth hour and continues to increase in size till the 

 forty-eighth hour. Delayed reactions that do not appear until the nine- 

 teenth hour have been noted and in some instances early reactions dis- 

 appeared before the forty-eighth hour. To make certain of every case two 

 observations must be made, preferably at the thirty-sixth and seventy- 

 second hours. If only one observation can be made, it should be on the 

 seventy-second hour. Haring and Bell found that the injected cattle fre- 

 quently, but not always, exhibited a well-marked thermal reaction. Thus 

 in 273 cattle from seven herds, 153 reacted locally and of these 130 reacted 

 thermally, but none reacted thermally which did not locally. It has 

 been their experience that the cases which fail to react thermally either to 

 the intradermal or the subcutaneous tests have usually been found on 

 autopsy to have small encapsulated lesions. In the course of their work 

 they have encountered 35 cases that showed a thermal reaction to the 

 subcutaneous test but failed to give a local reaction with the intradermal. 

 One of these cases was autopsied and showed a few active tuberculous 

 lesions. The failure of this and the other 34 cows to react locally is ac- 

 counted for on the theory that a local reaction is weakened when it 

 occurs simultaneously with a thermal reaction though it is pointed out 

 that there were many cases where both reactions were well marked. 



Opinions as to the relative accuracy of the subcutaneous and intra- 

 dermal tests are divided, and more data needed before the question can be 

 decided. Those that have used the latter test extensively incline to the 

 belief that in skilled hands it yields results of equal value to those yielded 



