78 IMMUNITY 



medulla, which are not found in any other disease. This dispels all 

 doubt as to the fact that hydrophobia is a real clinical entity. 



It is possible to immunize animals and man against this disease, 

 by the use of attenuated virus. In common with many other 

 viruses, that of hydrophobia can be weakened through the action of 

 either heat, drying, light, or chemicals. Pasteur found that by dry- 

 ing the spinal cords of rabid animals for two weeks, they become 

 totally avirulent. If the cord is dried but three or four days, the 

 virulence is but slightly modified. Immunity to rabies can be pro- 

 duced by injecting minute quantities of the poison, and then 

 gradually increasing the dose until virulent virus can be employed. 



Modification of the amount of poison used may be affected by 

 employing equal quantities of spinal cords from rabid animals that 

 have dried varying lengths of time. The vaccine consists of pieces 

 of cord, i cm. in length, from rabbits that have been killed by inocu- 

 lation with fixed virus. This is emulsified with sterile salt solution. 

 Cord that has dried for fourteen days is first injected, after which 

 cords that have dried fewer and fewer days, until, finally, one that 

 has dried only three days is injected. 



In cases of bites by rabid dogs on the face or head, the vaccination 

 must be rapid, so two injections per diem are given. In Berlin the 

 weakest injection used (the first) is made from a cord that has dried 

 but eight days, and the course is much quicker. The effect of this 

 mode of inoculation is to produce in the bitten individual a very 

 rapid active immunity, quicker in its action than the infection. The 

 treatment is solely prophylactic and in no way curative. If symp- 

 toms of rabies have set in, the treatment is of no avail. In rabies 

 the incubation period is about six weeks, so that there is plenty 

 of time to immunize the patient by injection with attenuated virus. 



Since the immunizing process is always begun after the bite of a 

 rabid, or supposedly rabid dog, it differs from other vaccinations, 

 which are resorted to before infection. 



Results of Treatment. In rabies the total mortality before the 

 introduction of vaccination was not less than 10 percent. Among 



