Immunity to leptospirosis: renal changes in vaccinated cattle given challenge inoculum.

Resistance to renal leptospirosis was demonstrated in cattle vaccinated with Leptospira interrogans serotype pomona bacterin. Fewer vaccinated cattle given challenge inoculum of virulent serotype pomona leptospires 12 months after vaccination had kidneys with gross focal lesions in the cortex than did nonvaccinated controls. Histopathologic changes characteristically associated with renal leptospirosis occurred less frequently and renal tissue damage was less severe in vaccinated cattle than in nonvaccinated controls. The isolation of serotype pomona from only 1 of 29 vaccinated cattle, compared to 7 isolations from 11 nonvaccinated cattle, at 26 days after challenge inoculum was given, indicated that mild renal infection occurred only infrequently in vaccinated cattle. It appeared that challenge inoculation of vaccinated cattle with virulent serotype pomona leptospires stimulated an accelerated secondary immune response in which immunity limited the multiplication of leptospires in the kidney.