DIFFKRENTIAI. DIAGNOSIS 213 



to warrant a positive diagnosis. In the living fowl it seems as 

 yet to be impossible to fix upon any diagnostic symptoms. 

 At post-mortem, however, properly stained cover-glass prepa- 

 rations from the tubercles will reveal the presence of tubercle 

 bacteria. This renders the positive diagnosis in the dead fowl 

 a comparatively easy task. 



The positive diagnosis of tuberculosis rests in : 



1. Finding the tubercle bacterium on a microscopic 

 examination of the lesions. 



2. The production of tuberculosis in experimental ani- 

 mals by inoculating them with the suspected tuberculous 

 material. 



3. Obtaining a typical reaction after the injection of 

 tuberculin. 



S 153. Microscopic examinations. The diagnosis by 

 microscopic examinations is possible when one has the dis- 

 charge from a lesion, such as the sputum when the lungs are 

 involved. In case of tubercular abscesses, the examinations 

 should be made from the scrapings of the walls of the abscess 

 rather than from the purulent contents. It is often possible 

 to find tubercle bacteria in sections of the diseased organs. 



A method for staining tubercle bacteria. Stain the cover-glass with 

 fresh carbol fuchsin. Place a few drops of the stain on the film side of 

 the cover-glass preparation and hold it over a flame with forceps until 

 steam is given off. Allow the hot stain to act for from 3 to 5 minutes, 

 or the preparation may be floated on the carbol fuchsin in a watch glass 

 without heat. In this case it is allowed to act for from 10 to 15 minutes. 

 The preparation is then rinsed in water and decolorized by treating it 

 with a 10% solution of nitric or sulphuric acid for from % to i minute. 

 It is again rinsed in water, when it is ready for examination. It can be 

 dried and mounted permanently in balsam. The tubercle bacteria 

 should be stained a deep reddish color. All other bacteria or animal 

 tissue in the preparation should be nearly or quite decolorized. If 

 desired, a counter-stain, such as alkaline methylene blue, may be used 

 after decolorizing ; that is, the preparation should be again stained for 

 about I minute in alkaline methylene blue, rinsed in water, and 

 examined as before. In these preparations the tubercle bacteria are red 

 and the other organiams and cells are blue. A counter-stain is of little 

 value in preparations made for simple diagnostic purposes. When a 



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