748 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



the incubator and sterilized in streaming steam. The killed cultures 

 are then evaporated over a water bath to one-tenth the original volume, 

 the bacteria are removed by passing the cultures through paper and 

 Berkefeld filter and a preservative is added. For cattle the dose of 

 tuberculin concentrated by evaporation to one-tenth the original vol- 

 ume, is 0.25 c.c. to 0.7 c.c. Because of the fact that the material is 

 thick and syrupy in consistency and the dose is inconveniently small, 

 it is usually diluted with seven parts of weak carbolic acid solution. 

 During the preparation it may be evaporated to four-fifths the original 

 volume and preserved by the addition of i per cent, carbolic acid of 

 sufficient volume to dilute properly. The United States Department 

 of Agriculture, Bureau of Animal Industry, requires that a dose of 

 tuberculin for the subcutaneous test of cattle shall not be less than the 

 equivalent of 0.5 g. of Koch's Old Tuberculin. According to this 

 requirement the dosage for cattle must depend upon the amount of 

 Koch's Old Tuberculin contained in any given brand of the product. 

 The dose of tuberculin prepared by most laboratories should not be 

 less than 5 c.c. as each c.c. usually contains the equivalent of o.i g. 

 Koch's Old Tuberculin. The product should be tested for activity 

 by injecting known tuberculous animals with the tuberculin under test. 

 The presence of typical reactions in tuberculous animals indicates the 

 reliability of the product. Tuberculin may be subjected to experi- 

 mental test for activity by the method suggested by the Bureau of 

 Animal Industry, the essentials of which are as follows: 



A series of guinea-pigs are inoculated with a suspension of fresh 

 tubercular material taken from lesions in tubercular cattle. After 

 the development of tuberculosis in the guinea-pigs a second series of 

 guinea-pigs may be injected with a suspension of tubercles from the 

 first series of infected guinea-pigs. These animals should be weighed 

 at regular intervals and carefully examined for evidences of tubercu- 

 losis. After a period usually of about three or four weeks, a series of 

 guinea-pigs should be selected which show unmistakable evidence of 

 tuberculosis. This series together with controls may be injected with 

 varying doses of tuberculin under test, in order to determine the degree 

 of sensitiveness to the lot of tuberculin in question. One group of 

 tubercular guinea-pigs should be injected with Bureau of Animal In- 

 dustry Tuberculin of known potency which serves as a standard. The 

 (lose of tuberculin to be injected in the test guinea-pigs should be the 



