Factors affecting the development of the carrier state in leptospirosis

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Abstract
The growth curves of young mice experimentally infected with L. australis A, L. australis B, L. grippotyphosa or L. icterohaemorrhagiae fell into two types, a ‘lethal’ pattern of continued weight loss until death, or a ‘carrier’ pattern of retardation of weight gain. The ‘carrier’ pattern could be changed to the ‘lethal’ by increasing the size of the infecting dose of a given serotype, or vice versa. Infection with L. icterohaemorrhagiae in newborn mice was fatal, while adults became carriers; susceptibility to infection with L. australis B decreased with increasing age.The numbers of infecting leptospirae and the age of the mouse determined the outcome of infection with a given serotype in a standard test animal. The basis of species susceptibility is unknown, but it is suggested that variation in the ability to produce lesions is the basis of the differences in virulence between serotypes. The ‘host of election’ in leptospirosis is regarded as the result of a quantitative rather than qualitative adaptation of host and parasite to one another.