Can antibody responses in cattle vaccinated with a multivalent leptospiral bacterin interfere with serologic diagnosis of disease?

A commercially available leptospiral bacterin containing 5 serovars (canicola, grippotyphosa, hardjo, icterohemorrhagiae, pomona) was used per label directions to vaccinate 45 steers and bulls. In animals in which the prevaccination sera were negative for microscopic agglutinating antibody, the increase and decrease of titers were similar among all serovars. A serologic response was stimulated by a single dose and enhanced by the 2nd dose. Most titers peaked at 2 weeks and then fell rapidly by 15 weeks after the 2nd dose. Thirty-four animals with prevaccinal titers to serovar hardjo responded with high microscopic agglutinating titers for a sustained period. Of these cattle, 31 still had titers between 200 and 400 by 29 weeks after the 2nd dose. These results were interpreted as representing an anamnestic response initiated by the bacterin to a sensitization that had followed natural exposure.