president's address. 1 1 



immunity in the animal affected, but the relapse was due to the 

 different' nature of the virus. This means from a biological 

 point of view, the ultravisible micro-organism will also follow 

 the laws of other organisms, vxz^ that of variability or muta- 

 bility, but which can show itself to our eyes only by a different 

 viruiencv in the animal it attacks. Accordingly more than one 

 variety of horse-sickness organisms exist, and although from a 

 pathological point of view we only recognise one disease, yet 

 there are as many diseases as there are varieties of ultravisible 

 organisms. At one time we thought that the variation was 

 simply due to the influence of environment, but, based on a 

 number of experiments, we came to the conclusion that the 

 cause of the variability of a particular strain lies in the horse 

 from which it is collected. The host represents, so to say, its 

 environments. The passage through a horse determines whether 

 there will be a decrease or an increase in virulency. This fact 

 established, the further conclusion was made that there must be 

 certain strains or varieties of which the viruiencv would not be 

 so pronounced, and accordingly that a greater number of animals 

 would recover when infected. This, indeed, proved to be the 

 case. The variability of the organism has now been made use 

 of for the inoculation of horses in connection with the method 

 as applied to mules. The method was introduced into practice 

 last Near, and only in the experimental manner; it has not yet 

 stood the brunt of the severe tests of the practice. 



The experience just now alluded to teaches us that under 

 the conditions of the practice breakdowns in immunity will 

 occur. It remains to be seen to what extent they do occur, or, 

 in other words, what percentage of inoculated horses will be 

 protected against the naturally acquired disease. The same 

 principle was made use of in the preparation of the blue-tongue 

 vaccine, and again recently in the method of inoculation against 

 anaplasmosis of cattle, a disease generally known as gall-sick - 

 ness. This latter was found to be caused by parasites attacking 

 the red corpuscles of the blood. The remarkable observation 

 was made that two different varieties of organisms could be 

 distinguished under the microscope, and the tests proved that 

 whereas one species was very virulent, the other one was very 

 much less so. and this latter protected an animal to a great 

 extent against the former. The vaccines used against the 

 various diseases therefore represent by no means anything 

 artificial ; they are specially selected germs producing the disease 

 in a milder form, which give a great amount of immunity, but 

 by no means a complete one. owing to the existence alongside 

 of still stronger varieties of the same species or genus. 



A cure or an inoculation against a disease always appeal - 

 to the mind of the layman, and more credit is attached to such 

 an inoculation than to other methods of prevention or controlling, 

 the disease which perhaps are more rational but mo r e tedi^Qjj 

 and cumbersome. A good illustration of this is afford e«^N^v/^oa~J> 



1±/|LIERARY 



