IMMUNISATION OF THE HORSE 129 



what volume is required to produce the death of a guinea- 

 pig weighing about 500 grammes in forty-eight hours. This 

 should be effected by ^V °-c-. and if the toxin is at or near 

 this standard, it is ready to begin the process of immunising 

 an animal. 



iTnmunisation of the Animal. — Horses are the best and 

 most convenient animals for this purpose, since they are 

 able to yield large quantities of serum without injury 

 to their health. It is a matter of the greatest importance 

 that the animals should be in a condition of perfect health. 

 They should be particularly examined to ascertain their 

 freedom from glanders and tuberculosis. The horses 

 should not ' react ' when tested by injections of mallein 

 and tuberculin. 



Small but gradually increasing doses of the diphtheria 



toxin are now injected from time to time into the animal, 



the place chosen for the inoculation being the apex of the 



shoulder. A slight swelling makes its appearance, and 



after a few days subsides, when the operation is repeated, 



using a larger quantity of toxin. A swelling again appears ; 



when this has in turn subsided, further injections are again 



made, till it is possible to inject so large a quantity as 



500 c.c. of the toxin without injury to the animal's health. 



Such a degree of resistance as this is developed generally 



after about five or six months' treatment. As pointed out 



by Behring, immunisation takes place most rapidly when 



each injection of toxin produces a reaction in the form of 



locahsed inflammatory swellings ; that is to say, the dose 



should be as large as possible, so long as general injurious 



effects are not produced. When the animal has reached a 



sufficiently high state of immunisation, it is ready to furnish 



supplies of antitoxic serum. For this purpose the blood is 



drawn off from the animal under due aseptic precautions. 



This is done by placing a sterile cannula in the jugular 



9 



