156 PRINCIPLES OF VETERINARY SURGERY 



and air ; the virus of hydrophobia, by desiccating the spinal 

 cords of affected rabbits. 



The second method of producing artificial active im- 

 munity is by the injection into an animal of gradually in- 

 creasing doses of the products of bacteria — their toxins — 

 either with or without' the dead bodies of the bacteria 

 themselves ; the toxin of tetanus, for example. The viru- 

 lence of the toxin may be diminished by heating it ; by mix- 

 ing it with organic extracts or with certain chemical sub- 

 stances ; by greatly diluting it ; by adding small doses of 

 already prepared antitoxin, or by adding blood serum. 



The third method is by recovering from an attack of 

 an infectious disease. If an ox recovers from anthrax in- 

 fection, thereafter he is immune. This active immunity 

 may be permanent or temporary. In case of anthrax, the 

 immunity will last for a lifetime, whereas a horse that has 

 recovered from tetanus is immune for only a few weeks, 

 In the majority of cases, the active immunity conferred 

 upon an animal by an attack of disease is permanent ; it may 

 be temporary, however, and on the other hand, it may 

 make the animal more susceptible, as in pneumonia. A 

 horse having had pneumonia once is more apt to have it 

 again. Many times one attack of disease renders future 

 attacks very mild; but they may prove fatal. 



Acquired passive immunity differs only from the active 

 in that the immunizing substance is developed first in the 

 body of another animal and then injected into the animal 

 we wish to immunize. This immunizing substance is anti- 

 toxin, and it is in the blood-serum where it is developed. 

 Hence the blood-serum of an animal immune to tetanus 

 injected into another animal will make it imimune also. 

 This is passive immunity. We immunize an animal with 

 a substance which has been developed in the serum of an- 

 other animal. 



