l. The simultaneous injection of serum 0 and adequate virus resulted in typical horse-sickness reactions, and the immunity obtained therefrom prevented a reaction when the adequate virus is subsequently injected.
2. The immunity obtained by a reaction after injection of serum O and virus 0 (adequate serum and virus) does not prevent a reaction due to a subsequent inoculation of virus of a different strain (Tzaneen and Bulawayo).
3. The simultaneous injection of serum 0 and virus Tzaneen (inadequate serum) is succeeded hy reactions which proved not to
be more fatal to animals than that due to virus 0 (adequate serum).
4. The immunity obtained by this reaction was in no way complete. It did not prevent a reaction either due to a subsequent inoculation of a different strain (0 or Bulawayo), nor did it prevent reactions when the same strain of Tzaneen virus of a later generation was utilised.
5. The fact that a reaction was noted due to the subsequent inoculation of virus in an animal immunised with the same strain of virus can he explained either by accepting that the test virus, being of a higher generation and derived from horses, is of a greater virulency, or that the virus Tzaneen is already of a complex nature containing certain constituents of the 0 virus which are deviated by the 0 serum and accordingly during the immunisation leave no impression on the system of the animal. A subsequent inoculation of the same strain would then not meet the corresponding antibodies and a reaction would result.
6. In some of the animals a single injection of virus, together with adequate or nadequate serum, produced complete immunity
against subsequent inoculations, hence a factor jn the animal has also something to do with the creation of immunity.
7. In the foregoing experiments it has been noted that a virus can attenuate and completely lose its virulency.
8. Virus T of the 12th generation has increased enormously in virulency, which shows itself in the immunisation and in the tests.
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