THE MANUFACTURE OF VACCINES. 473 



pigs which received three-fourths of the usual dose should die, those 

 which received the half dose should show symptoms and recover, while 

 those which were injected with one-third of the usual dose should remain 

 normal. To determine further the potency a heifer may be injected 

 subcutaneously 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 black- 

 leg vaccine a "double vaccine" may be employed. This consists of two 

 vaccines, each possessing different degrees of attenuation, which are con- 

 trolled by the degree of heat and the period of time used in attenuating 

 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 Houghton in 1898, are injected hypoder- 

 matically. 



RABIES VACCINE. The successful preventive treatment for rabies or 

 hydrophobia resulted from the brilliant researches of Pasteur. The 

 original method devised by Pasteur in 1885 has been but slightly modified 

 and still continues to be the only practical, specific treatment for rabies. 

 This treatment consists of a series of vaccinations, each vaccination 

 involving the application of rabies virus having a known degree of attenu- 

 ation. In each succeeding application of modified rabies virus the 

 patient receives increasingly more virulent material until finally active 

 immunity is acquired and the subsequent attack of the disease may be 

 successfully resisted. 



The preparation of rabies vaccine begins with the attenuation of a 

 virus having a known degree of virulence. The material may be secured 

 from an ordinary case of "street rabies." A dog suffering from the disease 

 is killed and a small portion of the brain removed. The brain tissue is 

 emulsified in sterile water or salt solution and a few drops of the material 

 thus suspended in liquid, are injected subdurally into a rabbit. This 

 may easily be accomplished by trephining the skull, after anaesthetizing 

 the animal, and with a small syringe inoculating a few drops of the sus- 



