26 ACTIVE IMMUNIZATION. 



still living cultures, "vaccins" so-called, to immunize against hen cholera, 

 swine plague, and anthrax. The same principle underlies Pasteur's antirabic 

 vaccination. 



Antirabic Vaccination. 



In all civilized countries their exist at present, special institutions, 

 either directly under the city control or appointed by the city, where the 

 Pasteur treatment for rabies is conducted. It is the duty of the general 

 practitioner, on getting a suspicious case of rabies to advise his patient to 

 undergo this special therapy and to send the rabid animal, its head or brain 

 preserved in glycerin, to the institute as soon as possible for the purpose of 

 ascertaining the presence of rabies. Up to the present the actual cause 

 of hydrophobia is unknown. Most recently Negri has described parasites, 

 known as Negri bodies, in the large nerve cells of the cerebral 



cortex > cerebellum, etc. Pasteur found, that rabies can be 



Treatment transmitted to dogs by injecting them subdurally with the 

 brain substance of rabid animals. This ordinary virus con- 

 taining material is known as Street Virus. 



The incubation period of rabies is very long. It varies from about three 

 weeks, to [possibly], some years. By passing the virus through monkeys, 

 the incubation period is considerably increased. After the successive 

 passage through five or six animals, the virus becomes so weakened that 

 infection is almost impossible. Reversely, increase of the virulence may 

 be affected by passing the virus through a successive number of rabbits 

 which are very sensitive to the disease. After passage through a large 

 number of such animals, the incubation period is gradually shortened from 

 about three weeks or a little less to a constant period of six or seven days. 

 Further diminution in the period of incubation was impossible and there- 

 fore Pasteur called this " Virus fixe" His first experiments in immunization 

 were made by passing the weakened monkey virus through rabbits and then 

 treating dogs with the spinal cords of the latter. 



Later on, Pasteur discovered that instead of passing the virus through 

 monkeys, he could diminish its virulence by drying the spinal cords derived 

 from rabid animals, for varying periods of time. In this way he could 

 prepare an entire series of graduated strengths. The material used for 

 this drying was not the street virus, but that obtained by successive passage 

 through rabbits or "virus fixe" which possessed very constant immunizing 

 and infectious properties. By drying the "virus fixe" over caustic potash 

 at a temperature of 23 to 25 C. for five days, its regular incubation period 

 of 7 days was very much prolonged. Increase in the length of drying 

 caused the entire loss of virulence in the spinal cord. 



Pasteur immunized dogs as follows: He began with the injection of a virulent 

 spinal cord which had been dried for thirteen days and every following day injected 



