186 ANNUAL RIorORT OP THE Off. Doc. 



to demonstrate the j)resence of the virus, rabbits arc generally used. 

 Subdural injections are easily made withoul any j>ain to the experi- 

 mental animal. A 2 per cent, cocaine solution is usually injected 

 subcutaneously over the forehead as a local anesthetic. An incision 

 is made parallel to the long axis of the head, less than 2 c. n. long, 

 a little to one side of the medium line, thiough the skin and perio- 

 steum over the frontal bone. The soft tissues are spread apart and 

 a small hole is drilled through the bone or i)referably a small tre- 

 I'hine is used and a disc of bime is removed about 0.5 cm. in diam 

 eter. The point of a syringe needle through which ihe emulsion con- 

 taining the virus is forced from a syringe is inserted through the 

 meninges and several drops of the emulsion is injected over the 

 brain. A curved hypodermic needle (Pravaz) attached to a Koux 

 syringe is preferred. The emulsion is usually made Avitli a piece 

 of brain tissue, diluted with twice its volume of sterile normal salien 

 solution. Of 117 rabits so inoculated with brain tissue containing the 

 ^drus of heads sent to the laboratory during 1905 and 1909 inclusive, 

 107 (91.3 per cent.) died of rabies on an average of 25.3 days, one 

 in nine days and one as 134 days after the inoculation. These figures 

 do not represent the length of the period of incubation, in that none 

 of the rabbits were kept so as to observe the onset of the first symp- 

 toms. The period of incubation is, therefore, shorter by at least 4 

 daj's, which would make the period twenty-one days on an average. 

 Several unusually long periods from the time of injection to death 

 were observed among rabits included in the 107, of these 66, 85, 110 

 and 134 days were the longest. Ten (9.7 per cent.) of the 117 rabbits 

 resisted infection. It is the general experience that every now and 

 then a rabbit will resist infection, and for this reason two rabbits are 

 usually inoculated. The mate of each of those that resisted infec- 

 tion died of rabies. For a rabbit to develop rabies 100 days or more 

 after inoculation is execeptional, and it is entirely safe to assume 

 that the virus was not in the suspected material inoculated, or that 

 rabies did not exist if both rabbits are alive at the end of 100 days. 

 In the routine diagnosis of specimens received at the laboratory of the 

 Pennsylvania State Livestock Sanitary Board during 1905 and 1909 

 inclusive, 313 rabbits were inoculated subdurally; 107 of which de- 

 veloped rabies ; 200 failed to give any positive proof of the existence 

 of rabies; of these 129 outlived the 100 days allotted; 53 died of 

 unknown causes; IS died of septicemia: 3 of cerebral hemorrhage; 

 2 of coccidiosis and 2 of enteritis. 



Wilson of the New York Board of Health (1898) was the first to 

 use guinea pigs in the place of rabbits, as the period of incubation 

 in these animals is shorter thaii in rabbits. Of 36 guinea i)igs dead of 

 rabies in our laboi-atory, the average period from the time of injec- 

 tion to death was 15.6 days; shortest period 8 days; longest period 

 24 days. The actual period of incubation would necessarily be a 

 trifle less. Arms ^. from a large number of guinea pigs ke])t under 

 observation, estimates that 1.69 days longer is required for death 

 to take place following the appearance of the earliest symptoms, 

 which would place the average period of incubation at a trifle less 

 than 14 days for guinea pigs. 



