29 



Conclusion. — The immunity obtained by the Tzaneen Virus did not 

 frotect all the mules and horses against the Ordinary Virus. 



(2) Tests with Bulawayo Virus. — There were tested 13 mules immune 

 to Tzaneen Virus, with Bulawayo Virus, also a comparatively short time 

 after immunisation, with the result that 8 showed horse-sickness reactions 

 and 1 died of horse-sickness. Of 4 horses immune to Tzaneen Virus and 

 tested with Bulawayo Virus, 2 showed horse-sickness reactions, and I a 

 reaction with the symptoms of dikkop. 



Conclusion. — The immunity obtained by the Tzaneen Virus did not 

 frotect all the mules and horses against the Bulawayo Virus. 



D. — Immunity from Bulawayo Virus and Tests with Ordinary Virus 

 AND Tzaneen Virus. 



(a) Three mules immune against Bulawayo Virus and tested with 

 Ordinary Virus all showed reactions ; one immune horse reacted to the 

 virus. 



(6) Of 5 mules immune against Bulawayo Virus and tested with 

 Tzaneen Virus, 3 gave reactions. Of 2 horses, 1 died from the test. 



Conclusion. — The immunity obtained from the Bulawayo Virus did 

 not protect all the mules and horses against a subsequent test of 

 either Ordinary or Tzaneen Virus. 



E. — Tests op Mules immune to Tzaneen Virus with Tzaneen Virus 

 or A Higher Gsneration. 



The experience of a rather large number of animals had shown that 

 the immunity obtained by the Ordinary Virus could not be broken by 

 the same virus, by whatever quantity of virus and whatever generation, 

 and after even a lapse of two years. This proved not to be the case with 

 the Tzaneen Virus ; it is particularly typically pronounced in horses. 

 Of 81 horses which were immunised with the Tzaneen Virus of a lower 

 generation, and within a comparatively short time tested with the same 

 strain of a higher generation, 16 showed reaction typical for horse- 

 sickness, 6 showed reactions with dikkop, 9 showed reactions and died 

 of pulmonary form, and 3 showed reactions and , died of dikkop. A 

 similar observation was made with horses injected twice with the Tzaneen 

 strain of two different generations, first of a lower one and later with a 

 higher one. When tested with the virus still of a higher generation, 

 breakdowns of immunity were noted. Of 40 horses thus treated, 4 showed 

 simple horse-sickness reactions, 3 showed reactions of dikkop, 9 died of 

 pulmonary form, and 3 died of dikkop. The same observation was 

 repeated to a smaller extent even in horses which had received three 

 subsequent injections of Tzaneen Virus of different succeeding generations, 

 as the following statistic shows : Of 15 horses thus treated, 3 showed 

 simple reactions, and 2 died of pulmonary horse-sickness. 



