THE pk()i;li:.m o/ iioksk sickness. 8i 



according to the body-weight, injected some days subse(|uently. 

 The most suitable period proved to be the sixth and tenth day. 

 The serum checks the development of the virus and modifies 

 the disease. In our experiments with a selected virus and 

 serum, we were able to obtain in several instances lOO per cent, 

 recoveries, and this on a fairly large number of animals. The 

 problem of successful inoculation could thus be solved, but. alas! 

 not that of successful immunisation. Indeed, the immunity is 

 not a complete one, not even in horses, which passed through 

 the Dikko]:) form of horse-sickness due to the inoculation. This 

 fact has nothing surprising in the light of previous statements. 

 For some time means were devised to overcome the difficulty. 

 In a horse successfully inoculated with a series of diiterent 

 viruses the chances of breakdown are reduced in proportion to 

 the number and quality of viruses injected. Based on the 

 observation shown before that the ordinary virus gave a better 

 immunity than any other, experiments were undertaken to 

 incorporate this virus into the process of inoculation. It was 

 found that, simultaneously with the second dose of serum, the 

 injection of ordinary virus could be undertaken with a certain 

 amount of risk, varying in the various experiments. The 

 immunity so obtained was better than that obtained with one 

 virus alone, but still not so good as in mules immunised with 

 the same virus. It is possible that under the influence of the 

 active immunity produced by the first Tzaneen virus injection, 

 together with the passive immunity of the two subsequent serum 

 injections, the develoinnent of the ordinary virus is completely 

 arrested in many horses injected, and hence the immunity not 

 improved. That this must be so can be seen from subsequent 

 tests with ordinary virus, when breakdowns are produced which 

 no longer should occur. The superplanting of a second immunity 

 on the first one ofifers. nevertheless, a good prospect for the 

 practical solution. The experience of this year supports this : 

 Of 49 horses possessing an immunity to the highest generation 

 of ordinarv virus exposed to the verv severe natural conditions 

 prevalent this year, 4 per cent. died. This is probably the 

 optimum result we can expect, but it could only be obtained at 

 the expense of a high mortality from immunisation. This 

 latter difficulty has not yet been overcome. It should be clear 

 that in horse-sickness we cannot expect an absolute immunity, 

 vis., an immunity which protects all horses under all conditions 

 against the disease. The protective inoculation has therefore 

 its limits, but it has also, as it stands to-day, its limits from a 

 technical point of view. Three inoculations are required to 

 convey and modify the two necessary attacks of horse-sickness, 

 and a period of four weeks to recover. Furthermore, only a 

 limited quantity of serum can be made under present conditions. 

 It is evident that the protective inoculation cannot yet be looked 

 upon to be the final solution of the problem, even if we are able 

 to pass all horses through an attack of the disease, and we can 



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