PERIOD OF INCUBATION 590 



who has recently furnished a very interesting contribution to the patho- 

 genesis of rabies in man, states that the shortest period on record is 

 about fourteen days; the average period, eight to twelve weeks. This 

 author has seen a case with a period of incubation of twenty months; 

 Spencer one of two years, four months, and Krikoff one of three 

 years and two and one-half months. While all fully developed 

 cases of rabies in man lead to a fatal issue, man is not very susceptible 

 to the disease. According to older statistics dealing with persons 

 bitten by rabid dogs, 16 to 20 per cent, develop the disease. Hoegyes 

 gives a percentage figure of 13.9, other authors of 5 to 6 per cent. 

 Kirchner's figure for Germany is 2 per cent, to 3 per cent., and 

 Paltauf thinks that the figure is certainly below 10 per cent. These 

 percentages all refer to persons bitten by rabid dogs. The more 

 serious lacerations produced by rabid wolves are much more danger- 

 ous, and it is estimated that 60 per cent, of persons bitten by these 

 animals contract the disease and die from it if not treated. 



Paltauf's recent studies of the pathology and pathogenesis of 

 rabies in man were made on four persons who died shortly after being 

 bitten by rabid dogs from some intercurrent diseases which had 

 nothing at all to do with hydrophobia. 



In all four cases it was found that the medulla, when emulsified 

 and injected subdurally, infected rabbits with rabies, showing the 

 presence of active virus in the patients' nervous tissues; but this 

 virus was in an attenuated condition, since the incubation period 

 in the inoculated rabbits was unusually long, forty days and over, 

 and the type of rabies developed was that of the chronic or "con- 

 sumptive" form. As probably at the highest estimate but one person 

 in ten of those bitten develops rabies, it must be assumed that four 

 consecutive positive findings do not represent latent infection which 

 would have manifested itself had the patients lived longer. It appears 

 rather that these observations indicate that in persons bitten by 

 rabid animals the virus commonly reaches the central nervous 

 system, but that nine times in ten it is there destroyed by the natural 

 defensive agencies without causing symptoms. These agencies may 

 be made more effective by the immunizing process of the Pasteur 

 treatment. In other words, rabies-inoculated men usually develop 

 a latent infection which is overcome without the symptoms of rabies. 

 The medulla of three other persons, who died shortly after the com- 

 pletion of a course of Pasteur treatment, were found to be non-infectious 

 for rabbits, indicating that the virus was destroyed under the influence 

 of the immunization. 



Presumably the virulency of the virus with which the individual 

 is inoculated is one of the main factors in deciding whether it will 

 be overcome or not, for the bites of rabid wolves cause rabies in 

 about 60 per cent, of those bitten as against from 6 to 10 per cent, or 

 less of fatalities from dog bites, and none from subcutaneous inocula- 

 tion of attenuated rabbit virus. Possibly in other cases the failure 



