Presidential Address. 103 



animal's life, even where large quantities of immune serum are given 

 in the hope of holding the infective process in check. 



Where then is the best point for attack in this problem of the 

 protection of the horse ? 



My impression — formed from following closely the work of 

 Edington and Koch, as well as from my own laboratory experiments 

 — is, that serum therapy will not assist us greatly. Serum is uncer- 

 tain in its effect, often over-restraining by a passive immunity when 

 we seek to produce a reaction, and as often failing to exert a suffi- 

 ciently restraining influence and so permitting a dangerous reaction 

 to ensue. We cannot standardise it, and so adjust the exact dosage, 

 because the virus of horse-sickness varies greatly in its virulence 

 from one locality to another, and there is further the difficulty that 

 the horse (which is practically the only animal suitable) differs 

 greatly in his degrees of susceptibility, so that a given dose of our 

 uniform virus would produce in one case a mild attack but cause 

 death in another. This prevents our estimating or prearranging — 

 except in the most general way — the restraining or protective influence 

 of such serum in any given case of infection. I dare not attempt 

 to dogmatise or to put forward my views on this point as final, but 

 I am not sanguine that the application of serum-therapy to the 

 immunisation of horses against horse-sickness will be attended with 

 any very practical or satisfactory results. 



The point of attack, therefore, has for some time past seemed 

 to me to lie rather in the direction of attempting the modi-fication of 

 the actual virus itself, and by reducing it to its lowest degree of 

 virulency, without impairing its activity, to render it safe for the 

 inoculation of even the most susceptible horse. As the more suscep- 

 tible among horses will naturally be the first, to fall victim to the 

 conditions of natural infection, I am sanguine that it will be proved 

 that the establishment of an immunity sufficient to tide over this 

 degree of susceptibility, will be sufficient to meet the practical needs 

 of the question. In other words, if we can protect our most suscep- 

 tible horses, the less susceptible can be left to look after themselves. 



Such an attenuated virus would be looked upon as a vaccine, 

 and it is by the use of such a vaccine, that I hope this very knotty 

 problem in preventive medicine will be solved. Speaking again with 

 reservation, I feel sanguine that the production of such an attenuated 

 virus — a vaccine in fact — is possible. 



Temperature charts were exhibited, showing the pronounced 

 reactions produced by different strains of virus which had been 

 attenuated so as to produce a definite though controllable form of 

 horse-sickness. A small percentage of the horses inoculated in this 

 manner proved refractory, but I am satisfied that if a horse 

 fails to react to large doses of this vaccine, he will prove refractory 

 to the conditions of natural infection. 



I am satisfied that if modified attacks of horse-sickness confer 

 any substantial degree of immunity (a point on which all workers 

 with the disease seem agreed), the main difficulty of producing with 

 safety a modified attack of horse-sickness has been overcome. 



