PROPHYLACTIC VACCINATION 283 



be employed for human rabies, but results attendant upon this method 

 of treatment have been unsuccessful. Therefore, at the present time, 

 efforts are directed toward producing an active immunity in those who 

 have been exposed to the disease. It is of interest to note that laboratory 

 inoculations in man rarely, if ever, lead to the development of the 

 disease. It is probable that in order for infection to occur the virus 

 must be implanted with animal sputum or some other form of con- 

 tamination. Bites from rabid dogs are relatively infrequent, and it is 

 therefore unnecessary to immunize an entire population. Further- 

 more, man is somewhat resistant to infection with rabies. Statistical 

 evidence in regard to the frequency with which rabies follows the bites 

 of rabid dogs are unreliable because of uncertainty as to whether or 

 not the animal was rabid. Doebert found that in Prussia, where data 

 had been very carefully collected, there was a mortality of 14.8 per 

 cent, in 122 untreated persons bitten by rabid animals between the years 

 1902 and 1907. Other estimates conform closely to this. More recently, 

 however, Marx has expressed the opinion that the rate of mortality 

 probably does not exceed 6 per cent, to 10 per cent, of untreated 

 bitten persons. The mortality and morbidity rate are practically identical. 

 Fortunately the period of incubation of rabies is of sufficiently long 

 duration so that active immunization may be effected during the period 

 of incubation. 



The period of incubation in man is variable and depends to a con- 

 siderable extent upon the site of the bite or scratch. According to 

 Bauer, the average period of incubation in 510 cases was seventy-two 

 days. In very rare cases the period of incubation may be less than 

 nineteen days and in more rare instances it may be one year or more. 

 Of seventy-three cases of bites about the head and neck the average 

 incubation was fifty-five days ; of 144 cases of bites on the upper 

 extremities the average period was eighty-one and one-half days ; 

 and of seventeen cases of bites on the lower extremities the average 

 period of incubation was seventy-four days. 



Active Immunization. Preparation of Material. As has been 

 indicated above it is necessary, because of the failure of passive 

 immunization, to produce an active immunization. In spite of the fact 

 that laboratory accidents practically never lead to the development of 

 rabies it is considered dangerous to inoculate man with the living virus. 

 Ferran and subsequently Proescher have, however, employed a method 

 whereby the active fixed virus is employed. Both these investigators 

 stated that no accidents had followed the use of unmodified fixed virus. 

 Hogyes has successfully employed dilutions of fresh fixed virus. The 

 majority of investigators, however, have employed virus which has 

 been attenuated by a variety of methods including heat, partial diges- 

 tion, the action of bile, the action of glycerol, of anti-rabic serum, of 

 phenol and of mechanical disintegration. Nevertheless, the original 

 method of Pasteur is employed almost uniformly throughout the world. 

 For this purpose the virus is passed through rabbits until it acquires 



