442 CHRONIC INFECTIOUS DISEASES 



are too vague and indefinite to be depended upon with any 

 degree of certainty. A positive diagnosis of tuberculosis, 

 therefore, from the clinical symptoms alone is not tenable. 

 Other aids to diagnosis must be employed, the principal ones 

 of which are the following: 



A. The Tuberculin Reaction. — ^Tuberculin may be applied 

 in several different ways. The most important methods of 

 application given in the order of practical importance are 

 the following: (a) The subcutaneous; ,(6) conjunctival, and 

 (c) dermal. 



(a) The subcutaneous application of tuberculin, which 

 consists in injecting the tuberculin in proper dosage into the 

 loose connective tissue under the skin, is the best known 

 and most reliable method. The reaction following is general 

 in that it induces in tubercular animals (best in cattle) a 

 febrile temperature which usually begins in six to eight hours, 

 reaches its acme in twelve to twenty hours, and lasts for 

 twenty-four to forty hoiu-s after the injection. (For detail 

 of technic, interpretation, etc., see Malkmus's Clinical 

 Diagnostics.) Tuberculin properly used is a very reliable 

 diagnostic agent. In 98 per cent, of the cases a positive 

 reaction indicates the presence of a tubercular lesion which 

 can be determined on necropsy. Tubercular animals may 

 not react to the test under the following conditions: 



1. When the disease is in the period of incubation (Moore). 



2. When the progress of the disease is arrested. 



3. In advanced, generalized cases and the condition of 

 the patient is bad. 



4. When the animal has been just previously (within four 

 weeks) injected with tuberculin. 



In the last two instances cited (3 and 4), by increasing 

 the dose of tuberculin, a reaction is apt to follow. 



(b) The conjunctival application consists in instilling into 

 the conjunctival sac a few drops of tuberculin (undiluted) i 

 In tubercular cattle in six to twenty-four hours symptoms of 

 conjunctivitis develop (congestion, swelling, lacrimosis) with 

 the accumulation of a yellow, flocculent exudate in the inner 

 canthus of the eye. The reaction may last two to four days. 

 It is spoken of as positive when the conjunctivitis is pro- 



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