730 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



with the vaccine under test; three each with three-fourths the dose for 

 cattle, three each with one-half dose and the remaining two with one- 

 third dose. 



Temperatures of the test animals should be recorded for three sub- 

 sequent days. Vaccine of proper strength is indicated when thermal 

 reactions occur in practically all the test animals together with local 

 reactions in some instances. None of the animals in the series should 

 die. 



As an additional test for potency a heifer may be injected sub- 

 cutaneously with one dose and a few weeks after the vaccination the 

 animal may be exposed to the disease by receiving an injection of the 

 virulent living organisms. If the animal remains normal the activity 

 of the product is indicated. In order to test the vaccine in regard to 

 safety, heifers may be injected with several doses each. The absence of 

 severe disturbances shows that the material may be used without 

 danger. 



For the purpose of eliminating possible danger from the use of 

 blackleg vaccine a "double vaccine" may be employed. This consists 

 of two vaccines, each possessing different degrees of attenuation, which 

 are controlled by the degree of heat and the period of time used in attenu- 

 ating the organisms in the affected muscle tissue. When the final 

 product, either single or double blackleg vaccine, is ready for use it is 

 usually distributed in the form of a powder, prepared threads or small 

 pills. The latter, first suggested by Hough ton in 1898, are injected 

 hypodermically. 



Blackleg Aggressin. Blackleg Aggressin is a tissue extract con- 

 taining the immunity-producing substances which are naturally present 

 in the tissue of calves, dead from acute blackleg. The tissue juices 

 of calves dead from the disease as a result of inoculation with pure 

 strains of blackleg bacillus are recovered from the affected tissues 

 and rendered free from the blackleg organism and extraneous contami- 

 nation by filtration. The dose for cattle is 5 c.c. 



Blackleg Aggressin may be tested for activity or potency by the 

 injection of a series of guinea-pigs as follows : Two pigs each are injected 

 with i, 2, and 3 c.c. of the aggressin under test. After ten days the 

 series of pigs together with two controls are injected with twice the 

 minimum lethal dose of virulent blackleg culture or virus. The con- 

 trol pigs should die in the usual time and at least four of the series of 



