28 



Conclusion.— T^e immunity obtained hy the Ordinary Virus did not 

 protect all tie mules'and horses against the test with Tzaneen Virus, 

 even when this test" was carried out soon after immunisation, and 

 even in horses which had been hyperimmunised. 



B. — BuLAWAYO Virus and Ordinary Immunity. 



This virus was obtained from a mule which had been immunised in 

 Bulawayo during the season 1905-06 with Ordinary Virus. When 

 exposed, it showed a relapse, from which it recovered. 



A number of mules, totalling 36, all immunised with Ordinary Virus, 

 were tested with this Bulawayo Virus, with the result that 7 mules 

 showed fever reactions typical for horse-sickness, 6 animals showed 

 the symptoms of dikkop and recovered, 1 mule had a doubtful reaction, 

 and 6 died of horse-sickness. ' Of 9 horses immune to Ordinary Virus, 

 which were tested with the Bulawayo Virus, 3 showed reactions with 

 dikkop, and 4 died of , horse-sickness. There were 2 animals out of 

 the 9 which did not react. Some of these tests were made within a few 

 weeks after immunisation. 



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

 frotect all the mules and horses against the test with Bulawayo Virus, 

 even when this test was carried out soon after immunisation. 



C. — Immunity from Tzaneen Virus and Tests with Ordinary Virus. 



Shortly after it had been noticed that Tzaneen Virus could break 

 the immunity of the Ordinary Virus, it was concluded that the former 

 would be stronger or, better, more virulent, and therefore probably 

 better suitable for the practice. It was at the same time ascertained 

 that the serum of animals hyperimmunised with the Ordinary Virus 

 could advantageously be used with the Tzaneen Virus, the mortality 

 from inoculation averaging about the same percentage. 



In the first half of the season 1906-07, the Tzaneen Virus was intro- 

 duced, but in the latter half withdrawn, as it was found to have changed 

 spontaneously in virulency. Subsequent to this, for the purpose of 

 ascertaining the immunity obtained from the Tzaneen Virus, tests were 

 made. 



(1) Tests with Ordinary Virus. — A number of mules, immune against 

 Tzaneen Virus, totalhng 59, were tested with Ordinary Virus a com- 

 paratively short time after immunisation. The result was that 47 animals 

 showed fever reactions more or less typical for horse-sickness, 2 animals 

 showed the symptoms of dikkop and recovered, and 5 animals died of 

 horse-sickness. There were tested 19 horses immune to Tzaneen Virus, 

 with Ordinary Virus, of which 7 showed fever reactions typical for horse- 

 sickness, 5 had reactions with symptoms of dikkop, 2 died of horse- 

 sickness, and 5 had no reactions. 



