568 DISEASES OF UNKNOWN ETIOLOGY 



virulence was fully established when passage of the virus from rabbit 

 to rabbit caused each successive animal to sicken in about six or seven 

 days, and to die regularly on the ninth day. No further increase in 

 pathogenicity for the rabbit could be induced, and the virus at this 

 level of virulence was called "virus fixe" by Pasteur. The spinal cord 

 of such a rabbit, dried for two weeks over caustic soda at room tem- 

 perature, lost its virulence for rabbits, although cords dried for a week 

 or ten days killed the animal when injected subdurally; the period 

 of incubation was, of course, increased when the partly dried cords 

 were used. 



The original Pasteur treatment consisted in grinding a piece of 

 dried cord half a centimeter in length in 5 c.c. of sterile salt solution, 

 and injecting the emulsion subcutaneously, preferably on the abdo- 

 men of the patient. Daily injections, using fresher and fresher cords 

 were used, until finally a cord from a rabbit dead but twenty-four 

 hours furnished the material for inoculation. The entire treatment 

 required about three weeks, at the end of which time a very decided 

 degree of immunity was induced. The incubation period of the 

 naturally acquired disease is usually not less than six weeks; the 

 advantage of instituting treatment at the earliest possible moment is 

 obvious. 



The mortality from rabies among those treated by the Pasteur 

 method of immunization is less than 0.5 per cent.; the average mor- 

 tality of untreated cases is about 16 per cent. 



Modifications in the original Pasteur treatment, principally along 

 the lines of injecting more virulent material, have been made from 

 time to time, and the tendency at present is to administer a shorter 

 treatment to mild cases (judged according to the location of the bite 

 and the extent of local injury) on the one hand, and to administer 

 a much more intense treatment in the severe cases. The present 

 routine followed in the Pasteur Institute of Paris is shown in the 

 accompanying table 1 (see page 569). 



Statistics indicate that a considerable degree of immunity is devel- 

 oped by the end of the second week of the treatment. The duration 

 of the immunity has not been definitely established, but it appears 

 to last for several years. Exposure to extreme cold and excesses of 

 various kinds, especially alcoholism, are said to be dangerous imme- 

 diately after the treatment is completed; they may reduce the 



1 Kraus and Levaditi, Handbuch der Technik und Methodik der Immunitatsforschung, 

 1908, i, 713. 



