THE ACID FAST GROUP 377 



of nonvirulent strains of the human type. The immunity, 

 however, does not last for more than a year. 



Diagnosis of the Disease. — Tuberculosis is most com- 

 monly recognized by taking advantage of the fact that 

 animals affected with the disease show marked hypersus- 

 ceptibility or anaphylaxis, that is, when an extract of the 

 tubercle bacillus is injected into an animal having the 

 disease a marked reaction generally results. 



Such an extract of tubercle bacilli used for diagnosing 

 the disease is known as tuberculin. The standard tuber- 

 culin most commonly used in the testing of cattle, the 

 so-called Koch's old tuberculin, is prepared by inoculating 

 a flat-bottomed flask containing 5 per cent glycerin broth to 

 a depth of 2 to 3 centimeters with a culture of Mycobac- 

 terium tuberculosis. Careful inoculation of the surface 

 from a pure culture and incubation at blood heat results in 

 the growth of a film or pellicle. This is well developed in 

 a few weeks. The broth is then evaporated in a porcelain 

 dish on a water bath until the glycerin concentration 

 becomes about 60 per cent, when it is filtered through 

 paper or a porcelain filter. It is evident that the extract 

 contains the growth products of the tubercle bacillus to- 

 gether with glycerin, salt and various substances present 

 originally in the broth. Purified tuberculins and tubercu- 

 lins made from the portions only of the tubercle bacillus 

 have been prepared in a great variety of ways. 



Diagnosis of tuberculosis in cattle is most frequently 

 effected by the subcutaneous injection of tuberculin. The 

 dose given is about one-fourth cubic centimeter of the 

 Koch's old tuberculin. Before the injection of the animal 

 the normal temperature should be determined. After six 

 or eight hours the temperature is again taken and there- 

 after every two hours during the test. Animals in heat or 

 advanced in pregnancy or suffering from other diseases 

 should not be tested. In general a positive test will be 



