THE PROBLEM OF HORSE-SICKNESS. 



By Sir Arnold Theiler, K.C.M.G., D.Sc. 



I need hardly apologise for introducing the subject of 

 Horse-Sickness at present, seeing that the country has just 

 passed through a very severe season, perhaps the most severe 

 experienced within the past 25 or 30 years. All farmers long 

 for a remedy, either in the shape of a cure or of a preventive. To 

 lind this remedy seems to solve the problem of horse-sickness. 

 To show that this ])ri)])lem can l)e solved, and how it must be 

 solved, is the object of this ])a])er. Ajjart from its economic 

 importance, the subject is of general biological interest. (Jur 

 present knowledge concerning this disease allows us to draw 

 certain conclusions which suggest the solution. This paper 

 will indicate the lines for furtlier investigation. In order to 

 l)ring my points out clearly, it will be necessary to enter some- 

 what into the nature of the disease, and to show — (i) that it 

 is caused ])\' a micro-organism; (2) how this organism finds 

 its entrance into the susceptible animal; (3) where it will prob- 

 ably be found under natural conditions ( the reserA-oir of the 

 virus). (4) The cjuestion of immunity, the result of our 

 experience, will be dealt with rather extensively. 



At the outset I must admit that many points are as yet of 

 a hypothetical nature, but b}' analogy with other and similar 

 diseases in which our facts are proved, we are justified in accept- 

 ing the theory brought forward about horse-sickness as being 

 well founded. The difficulties of bringing the actual proofs 

 will be ]3ointed out in the course of this ])aper. 



I. The Cause of the Disease. 



The micro-organism of horse-sickness belongs to the group 

 of the so-called ultravisibles, whose size is beyond the resolving 

 powers of our best microscopes. The dark-field illumination 

 has not shown anything specific in those body liquids which we 

 know contains the infection, neither has any attempt to culti\ate 

 it_ been successful, as, for instance, can be done with the filtrable 

 virus of pleuro-pneumonia of cattle, where the micro-organisms 

 show their presence as a cloudiness in a clear, suitable liquid 

 medium. Since neither form nor shape of an organism can 

 be detected, it is the common practice to speak of it as a virus. 

 All horses which suiter from horse-sickness contain this virus 

 in their blood, whether it be the Dunkop or the Dikkop form 

 of the disease. By means of the injection of such blood into 

 fresh, susceptible horses, the disease can be reproduced after 

 an incubation period of three to thirty days in extreme cases. 

 Whether Dunkop or Dikkop develops depends on the virus 

 injected, and on the individuality of the horse itself. Accord- 

 ing to our experience the appearance of Dikkop is somehow 



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