492 INFECTION AND RESISTANCE 



into the exudate obtained by scratching the skin of the individual to 

 be vaccinated. This method has practically gone out of use, and 

 to-day the ripened glycerin pulp prepared as above is taken up in 

 small capillary glass tubes and from these blown upon the vaccina- 

 tion scratch. The efficiency of vaccine virus can be tested for po- 

 tency by the inoculation of the ears of rabbits before use. 



ACTIVE PROPHYLACTIC IMMUNIZATION IN RABIES (HYDROPHOBIA) 



Although many modifications have been suggested and actually 

 used in different parts of the world, the most common method of 

 immunizing against rabies still remains that originally devised by 

 Pasteur. The Pasteur treatment takes advantage of the prolonged 

 incubation period of rabies and is planned to confer immunity be- 

 tween the time of inoculation and the time at which the disease 

 would naturally appear. Since this period in ordinary street infec- 

 tion by dog bite is usually 40 days or more, a considerable interval 

 for active immunization is available. Formerly much of this time 

 was lost in that the diagnosis of hydrophobia in the dog or other 

 animal that had caused the injury could not be made with certainty 

 until the results of rabbit inoculations had been obtained. Nowadays 

 the ease with which a diagnosis of hydrophobia can be made within 

 a few minutes by finding negri bodies in the hippocampal and cere- 

 bellar cells has added considerably to safety in that it has made pos- 

 sible a gain of almost two weeks in determining whether treatment 

 should be instituted or not. 



Here again, although the infectious agent of rabies is not known 

 with certainty even at the present day, the method of Pasteur de- 

 pends upon active immunization by means of an attenuated virus. 



In standardizing the virus for the purpose of treatment Pasteur 

 first produced what he calls the "virus fixe." This consists of the 

 ordinary street virus as obtained from rabid animals passed through 

 a considerable series of rabbits (20-30) until its virulence for these 

 animals has reached a maximum. After a sufficient number of such 

 rabbit passages the incubation time after intracerebral inoculation is 

 reduced to 7 or 8 days, but can no longer be shortened by further 

 passage. The brain and cord material of rabbits dead of rabies after 

 such repeated passages constitutes virus fixe. This can be preserved 

 for considerable periods in 60 per cent, glycerin, and this is the 

 initial material from which the attenuated preparations for treat- 

 ment are produced. 



In preparing the material for treatment a small amount of virus 

 fixe is injected subdurally into rabbits, about 0.2 c. c. of a salt solu- 

 tion emulsion being given. The inoculation is very easily made 

 through a small trephine opening in the skull, and contamination 

 is very easily avoided. Just before the rabbit dies when completely 



