154 PRINCIPLES OF VETERINARY SURGERY 



Natural immunity is not absolute and complete; for ex- 

 ample, the horse is immune to anthrax as a rule, but under 

 conditions whereby his vitality is lowered, either by over- 

 work, by some other disease, or by malnutrition, he can 

 be infected with anthrax. Until within the last year or two, 

 it was thought that animals were immune to venereal dis- 

 eases ; but recently Metchnikoff and Roux have succeeded 

 in inoculating the chimpanzee with syphilis, and before the 

 medical society of Berlin a horse was recently shown to 

 have been inoculated with syphilis from the human. More- 

 over, an aninial immune to a certain disease by nature, may 

 acquire it if inoculated with bacteria of unusual virulence. 

 Pathogenic bacteria, after passing through a series of sus- 

 ceptible animals, finally becomes so virulent that, together 

 with a lowered resistance on the part of the animal, they 

 can overcome one which naturally has a considerable de- 

 gree of immunity. Absolute immunity, however, can be 

 acquired either by vaccination or as a result of an attack 

 of the disease. As a rule an animal having been thus pro- 

 tected cannot acquire the disease, no matter how lowered 

 his vitality, or how virulent the bacteria. For example, it 

 is possible for an animal protected against tetanus to with- 

 stand a dose of toxin that is sufficient to kill a thousand 

 unprotected animals. • Physical exhaustion, hunger, ex- 

 posure to cold, pestilence, and famine are factors which may 

 reduce the natural resistance of an animal. 



Acquired Immunity. — This may be artifical (active and 

 passive) or natural (active only). If the immunizing sub- 

 stance is developed in the body of the immune animal, we 

 call it active immunity; if it has been elaborated in the 

 body of another animal and then inoculated into the animal 

 to be immunized, we call it passive immunity. Active im- 

 munity may also be produced naturally as the result of 



