used for immunisation and hyperimmunisation in mules was forty-three 

 generations ; in horses, seventy generations. 



It is usually observed in other diseases that the passage of virus 

 from animal to animal of the same species increases the virulency, and 

 it may be expected that such virulency can break the immunity obtained 

 by a lower generation. However, the virus of the Ordinary strain of a 

 high generation, and even the hyperimmunisation with such had no 

 effect on the animal immunised with a lower generation. 



Conclusion. — The immunity of mules and horses obtained by the 

 recovery from the injection of Ordinary Virus protected completely 

 against the infusion of large quantities of virus of a higher generation. 



(The reservation should be made here, that in the passage of virus 

 through the different generations, mules and horses were used, and it is 

 possible that this may account for the stability of the virus.) 



Experiences in Practice. — A. — Tzaneen Virus. — Once it had been 

 settled that the immunity in mules could not be broken by huge quantities 

 of virus, it was expected that such an immunity would stand natural 

 exposure, and accordingly the method was introduced into practice. 



During the horse-sickness season, 1905-06,* a total number of 3,235 

 mules had been inoculated, and were exposed to natural infection. At 

 the end of the season, of this number 21 were reported to have died of 

 horse-sickness, and 45 had shown symptoms of relapses (aanmanings). 

 There was but little doubt that death was due to horse-sickness. Post- 

 mortem reports in several instances were obtained from the District 

 Veterinary Surgeon. At the same time some horses which had recovered 

 from experiments, and had also been hyperimmunised, were exposed to 

 be tested. One of these died in Tzaneen, the Government estate in the 

 Letaba Low Veld, a notorious country for horse-sickness. Blood from 

 the dead animal was obtained, and named " Tzaneen Virus." It was then 

 used in the following experiments. A number of mules, totalling 139, all 

 immune against Ordinary Virus, were tested with Tzaneen Virus, with the 

 result that 12 showed typical horse-sickness fever reactions, 4 with lesions 

 of dikkop, and 1 died of horse-sickness. A number of horses, totalling 17, 

 all immune against Ordinary Virus were tested with Tzaneen Virus, with 

 the result that 5 showed reactions and recovered, 3 had reactions with 

 symptoms of dikkop, 1 died from the pulmonary and 1 from the dikkop 

 form of horse-sickness. There were only 7 animals out of the 17 which 

 did not react. Some of the tests were made within a few months after 

 recovery from Ordinary Virus, and on animals (horses) which had been 

 previously hyperimmunised once, twice, ar.d three times with Ordinary 

 Virus. 



Annual Kepovt, G.V.B., fransvaal, 1906-07, 



