76 THE PK015LEM OF HORSE SICKNESS. 



the injection and tests. With Tzaneen virus the percentage of 

 II corresponds to 3 per cent, of the original number; accord- 

 ingly it can safely be concluded that out of every 100 horses 

 taken at random, about three or four will resist all injections of 

 virus, and can be considered as completely immune. 



2. The fact that a horse does not react to one injection 

 iloes not necessarily mean that it possesses any immunity ; in 

 fact, it can be seen from the figures given that the disease was 

 contracted by 80 per cent of animals in the case of ordinary 

 virus, and by 89 per cent, of animals in the case of Tzaneen 

 virus, of which 56 per cent, died in both instances. In other 

 words, out of 100 horses that do not react to one injection, 56 

 Vv'ill (lie when tested later. 



:;. ( )\ the horses that reacted to the original injection, con- 

 sidering both Dunkop and Dikkop forms together, a far better 

 immunity was given by ordinary virus, the survivors amount- 

 ing to 79 per cent, as compared with 61 ]Der cent, from Tzaneen 

 A'irus. It is, however, evident that even in the case of the 

 strong virus (ordinary), a reaction does not constitute immunity 

 to subsequent injections. That is to say, out of 100 horses 

 that recover from horse-sickness, and which are considered 

 " salted," anything from 26 to 81 can be expected to contract 

 the so-called "relapse " or aanmaanings, the number varying 

 according to the strength of the virus, conferring what I may 

 call the " ground " immunity. 



4. On comparing the results of the tests on the horses that 

 obtained their ground immiuiity as a result of a Dunkop re- 

 action with those that obtained it from a Dikkop re- 

 action, the advantage of a Dikkoo immunity is at once noticed. 

 In the case of ordinary virus only 17 per cent, of the Dikkop 

 reactors died on test, as compared with 25 per cent, of the 

 Dunkop reactors, whilst with Tzaneen virus the difference is 

 even more marked, the respective figures being 25 per cent, as 

 compared with ^2 per cent. From a practical point of view, 

 therefore, a horse that has " salted " from the Dikkop form of 

 horse-sickness has a better chance of resisting later infections 

 than a horse that salts from the Dunkop form. Recovery from 

 one attack of Dikkop is. however, not a guarantee against later 

 attacks of the same form. Amongst the horses referred to, 

 15 per cent, of Dikkop reactors contracted the same form for 

 the second time. 7 per cent, died at the seconcl attack, whilst 

 1.5 per cent, contracted it three times, of Avhich 0.5 per cent, 

 died at the third attack. 



5. I have not given anv special statistics relating to the 

 " breaking power " of any particular virus, and have onlv men- 

 tioned the one case of the breaking power of Tzaneen virus on 

 the mule that salted to ordinary virus. This point was also 

 experimentallv investigated, and whilst it held good that Tzaneen 

 virus could break the immunity given by ordinarv virus, yet 

 the reverse also applied when ordinary virus broke the immunity 



