So THE PROBLEM OF HORSE SICKNESS. 



going through the dip. The losses were only ;},/ horses out of 

 a total number of i,o66. This result is better than we can 

 expect to obtain from preventative inoculation. This observa- 

 tion deserves full attention. Experiments to obtain a Ijetter 

 protective substance to be applied to the skins of horses by means 

 of spraying or dipping have been for some time, and are still, 

 undertaken by the Division. 



The prevention of the disease by rendering the exposed 

 animals immune would undoubtedl}' represent the most economic 

 way, and would meet all requirements, provided a simple method 

 could be found, which could be apjilied with none or but little 

 risk, and which would give the maximum of immunitv. The 

 method which conforms to these requirements does not exist; 

 the inoculation of mules, as introduced about ten years ago, seems 

 nevertheless to answer most practical requirements. This 

 method consists in a simultaneous inoculation of protective serum 

 and virus under the skin, the virus producing the disease, the 

 serum modifying it so that recovery ensues. The virus utilised 

 is the Pretoria strain, which, as previously has been shown, gives 

 a good immunity. Breakdowns and deaths due to natural 

 exposure do occur, and vary in different seasons. A mortality 

 of 5 to 6 per cent, is probably the average loss. 



For horses we have not yet been able to overcome all diffi- 

 culties, and they are more numerous than we anticipated. A 

 simple method, as in the case of mules, seems to be excluded. 



The injection of serum simultaneously with the adequate 

 Pretoria or ordinary virus is followed by heavy mortality. The 

 factor Avhich assists the mule to pass through the disease is 

 absent in the horse. It is probably the factor inherited from 

 the ass. A weak virus, on the other hand, when injected 

 simultaneously with the serum, is acted upon by the serum, and 

 no disease develops and no immunity ensues. Of the Tzaneen 

 strain we possess two varieties, a virulent one and an attenuated 

 one. Both qualities have been obtained by the same process, 

 7'ic.. by passage from one animal to another. It is difficult to 

 say what is the modifying agency which converts the virus into 

 either a weak or a strong virulency. Experience has shown that 

 it is the horse or mule in which the virus develops which 

 supplies it. It is probably a quality of all viruses to undergo 

 these modifications. The virulency of the same generation, 

 obtained from different animals, varies according to the animal 

 from wliich it is obtained. No definite law can be laid down. 

 It is a matter of experimenting to find a virus of suitable viru- 

 lencv. In the case of Tzaneen strain a lower generation will 

 break the immunity conveyed by a higher generation, and in the 

 case of the ordinary strain the higher generation will break that 

 conveyed by a lower. Ordinary virus through a great numl)er 

 of generations has become verv exalted in its virulency. We 

 immunise at present horses by injecting first attenuated Tzaneen 

 virus — we may call it the virus-vaccine — then serum calculated 



