Details of the results obtained from the injection of horses and mules with neurotropic mouse and guinea pig adapted virus are given. It is shown that the virulence of horsesickness virus progressively decreases for equines as neurotropic fixation takes place by serial passage through mice and guinea-pigs. The attenuation occurs more rapidly through the guinea-pig, but it is not known whether the ultimate level will not be the same. All animals which survive an injection of one infective dose of neurotropic virus, whether or not a demonstrable reaction is produced, are immune to the homologous strain of virus. Immunity to heterologous strains is at most only partial.
No difference in favour of either the subcutaneous or intravenous method of injection could be determined.
It is shown that the subcutaneous injection of as small a dose as 10 c.c. of a 1:10,000 dilution of infective brain emulsion is adequate.
Attention is directed to the phenomenon of a high concentration of infective guinea-pig brain emulsion producing a milder reaction than a low but still infective concentration.
The possibility of developing a polyvalent vaccine is discussed.
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