Presidential Address. ioi 



Edington, of the Cape, who worked long and laboriously at 

 the problem, was the first to shew, in 1900, the possibility of pro- 

 ducing by hyper-immunisation a serum, which would to some extent 

 arrest or modify the course of the disease. Professor Koch, later 

 in 1904, followed on much the same lines, but did not materially 

 advance our prospects of a practical method of inoculation. Dr. 

 Theiler, however, pursuing the same system of serum-therapy, 

 has been able lately to devise a method whereby the mule can 

 be rendered practically immune to the attacks of the disease, 

 and to this worker, who has done so much for science in other 

 branches of preventive medicine, we must accord the honour of lirst 

 practically applying a system of prevention which, though still on 

 approbation as regards the length of immunity conferred, bids fair 

 to solve the difficulty of keeping the mule alive in districts where it 

 previously ran much danger of succumbing to the disease. The 

 system employed, as you are probably aware, is in brief and simple 

 language somewhat as follows : — ■ 



A mule, recovered from the disease, is subjected to repeated 

 and increased injections of blood taken from other mules suffering 

 from the disease. In time, its blood becomes strongly antidotal, and 

 when this blood, or the serum of the blood, is injected into healthy 

 mules, together with a small dose of virulent blood, taken from a 

 mule suffering from the disease, a mild form of the malady follows, 

 which protects against a natural (and probably fatal) attack of the 

 disease. The quantitv of this antidotal serum necessary to immunise 

 one animal, is 300 cc, which is equivalent to about a pint of the 

 original blood of the immune mule, which mule would upon this 

 computation furnish sufficient serum to immunise 8 to 10 other 

 animals at one abstraction of blood. 



The degree of susceptibility of the mule, however, to the disease 

 is considerably less than of the horse, and a system which can with 

 safety be applied in the case of the mule fails when the attempt is 

 made to apply it in the case of the horse. The marked susceptibility 

 of the horse to the disease does not constitute, however, so great a 

 difficulty to the scientific worker as the fact that the degree of 

 susceptibility between different horses is extremely variable, so that 

 preventive measures adjusted safely to one animal, prove dangerous to 

 others. This is the main difficulty which has confronted successive 

 investigators, and checked the devising of a practical system of 

 preventive inoculation. 



In following a case of infection by horse-sickness in the horse, 

 we find that where a fatal issue is about to ensue, certain of the 

 white cells of the circulating blood become greatly reduced in their 

 numbers ; and conversely, when an animal has become infected and 

 has successfully resisted the disease, these particular white cells — 

 the folymorfhonuclear leucocytes — become largely increased in 

 numbers. Successful resistance, therefore, to the disease-attack 

 increases the number of these particular cells, and defeat and 

 approaching death can be foretold by their diminution and 

 disappearance. 



