The Virulence of Haemophilus pertussis for Mice by the Intranasal Route

Abstract

H. pertussis strains were tested for virulence by the intranasal inoculation of mice. Four dilutions were used and virulence was measured by computing the LD50 on the 28th day. Standardization of the technic, including the use of mice of the same strain and of the same wt. is of upmost importance. The author emphasizes that virulence is a quantitative and not a qualitative character of strains of H. pertussis. There is no correlation between virulence and the viable count or the toxin or hemagglutinin content of the suspension used to infect mice, nor is there any correlation between virulence and the history of the strains tested either in the number of subcultures, time elapsing between swabbing and testing, duration of illness when the swab was taken, or type of swab. Some pertussis strains kill mice more rapidly than others of equal LD50. This ability is attributed to a "quick killing factor.".