In a previous communication [this Bull. 3. 538.] it was shown that mules could be protected against horsesickness by a suitable formalinized vaccine. Further work has confirmed these results and indicated that a spleen virus attenuated by 1:2, 000 formalin protected against a 1:3, 500 concentration, while mules which survived the double inoculation were solidly immune. The method was not entirely devoid of danger as a mortality rate from the vaccine of about 5 per cent may be expected. The substitution of Tyrode's solution in place of saline as a diluent for the spleen emulsions failed to improve the vaccine.
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