PRINCIPLES OF VETERINARY SURGERY 155 



recovery from an attack of disease. Artificial active im- 

 munity may be produced in three ways : 



1st. By vaccination. 



2nd. By injection of toxins. 



3rd. By recovering from an infectious disease. 



Vaccination is a form of preven-tive inoculation. It was 

 the knowledge that mild and severe attacks of an infectious 

 disease are equally efficacious in producing immunity, which 

 has given rise to our present methods of preventive inocu- 

 lation. Jenner discovered vaccination : Pasteur was the 

 first to place it on a practical basis. He demonstrated that 

 the virulence of bacteria can be varied by experimental 

 procedures. By these means, the bacteria can be very much 

 weakened or attenuated and still produce immunity against 

 subsequent attacks, if inoculated into an animal. They 

 may be attenuated ii:i several ways : By cultivation at tem- 

 peratures above their optimum ; by successive inoculation 

 into non-susceptible animals; by prolonged artificial culti- 

 vation in the presence of oxygen; by exposure to certain 

 inorganic chemical substances, as diphtheria to trichlorid of 

 iodine, anthrax to bichromate of potash, and tetanus to a 

 weak solution of carbolic acid; by exposure of cultures to 

 organic extracts, such as that of thymus gland; by drying, 

 as is done in making the virus of hydrophobia; by exposure 

 to sunlight, electricity, X-ray, radium, etc. By these meth- 

 ods the bacteria are not killed; their capacity for producing 

 disease is simply lowered. 



After they have been attenuated by any of the fore- 

 going methods, they may be inoculated into the animal 

 with but a slight reaction on its part. Later, gradually in- 

 creasing doses may be injected until the animal is immune. 



No single method of attenuation is suitable for all bac- 

 teria. Anthrax bacilli may be attenuated by cultivating at 

 42° C; the bacillus of chicken cholera, by exposing to light 



