MANUFACTURE OF VACCINES 731 



six vaccinated pigs should live. Blackleg Aggressin may also be sub- 

 jected to potency test by injection of calves with 5 c.c. each, or with 

 varying doses. If prepared according to standard methods from care- 

 fully selected strains of blackleg bacillus, the calves receiving dosage 

 of 3 to 5 c.c. should live, following the inoculation of the animals with 

 virulent blackleg virus, ten days after receiving the aggressin. 



Blackleg Filtrate. Blackleg nitrate is a cultural product con- 

 taining the metabolic substances resulting from the growth of the 

 blackleg bacillus in liquid culture media. After incubation, usually 

 about seven days, or until optimum growth has taken place and the 

 cultures are checked for purity, a preservative is added and the organ- 

 isms are removed by passage through Berkefeld niters. The method 

 of testing for potency is similar to that for blackleg aggressin. 



RABIES VACCINE. The successful preventive treatment for rabies, 

 or hydrophobia, resulted from the brilliant researches of Pasteur. 

 The method devised by Pasteur in 1885, with some modifications, con- 

 tinues to be the only practical, specific preventive treatment for rabies. 

 This treatment consists of a series of vaccinations, each vaccination 

 involving the application of rabies virus having a known degree of 

 attenuation. In each succeeding application of modified rabies virus 

 the patient receives increasingly more virulent material until finally 

 active immunity is acquired and subsequent attack from the disease 

 is 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 anaesthe- 

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

 the suspension just under the exposed dura mater. The inoculation of 

 ordinary rabies virus usually produces symptoms of "dumb rabies " 

 and the death of the animal in fourteen to eighteen days. In order to 

 increase the virulent properties of the same strain of rabid material, it 

 is transmitted from rabbit to rabbit by subdural inoculations until 

 the incubation period is shortened to about six days. Experience has 

 shown that when the virus has reached its maximum degree of virulence 



