The growth of L. australis B in the kidneys of young mice which become carriers was followed after experimental intraperitoneal infection. There was a primary growth corresponding to generalized acute infection and terminated at the time of appearance of antibody. A secondary growth bf leptospirae followed in the kidneys alone about 7–10 days after infection, coinciding with the onset of leptospiruria and recovery from infection. Subsequently mice carried about 106–107 leptospirae in their kidneys permanently.
The leptospirae in the urine or in the kidneys of carriers were resistant to the action of antibody in the serum or urine of the host animal, or in rabbit antisera prepared against mouse-renal leptospirae or against cultured leptospirae of the infecting strain. No antigenic differences were detected between renal and cultured leptospirae. An analogous situation is the growth in vitro of leptospirae in homologous antiserum. The mechanism permitting growth of leptospirae in homologous antiserum in vivo or in vitro is unknown.
The carrier condition results from the ability of virulent leptospirae to (i) grow in the host and produce lesions in the primary, acute generalized infection, (ii) grow in renal tubules in the presence of antibody.