SUMMARY: Freshly isolated strains of vary considerably in their virulence for mice when tested by the intranasal route; this variation is continous and must be regarded as a quantitative not a qualitative character. In order to titrate virulence it is necessary to achieve and maintain a standard technique with cultures and to use ‘standard’ mice kept under standardized conditions. It was not possible to show a correlation between virulence (as measured by the LD50) and either the viable count or the presence of toxin of haemagglutinin in the suspensions used in the virulence test. Nor was it possible to show any correlation between virulence and the ‘age’ of the culture, either in number of subcultures, time elapsing between swabbing and testing, type of swab or duration of illness (up to three weeks) when the swab was taken. Some strains kill mice more quickly than others; this ability is attributed to a ‘quick-killing factor’.


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