Foot-and-mouth disease: changes in serum-neutralizing activity of immunized cattle shortly after virus exposure.

After infection of the upper respiratory tract with foot-and-mouth disease (FMD) virus in immunized cattle, a drop in specific serum-neutralizing antibody activity was observed. Vaccinated cattle with high preexposure serum antibody titers had a precipitous drop for the first 4 days after virus exposure, followed by a rise in titer. This drop was also seen in passively immunized cattle with high preexposure titers; however, their antibody titers decreased for 5 to 7 days after exposure to virus. Some vaccinated and passively immunized cattle with low preexposure antibody titers showed fluctuations in their titers shortly after exposure. These data suggest that the course of infection with FMD virus in immunized cattle depends on the rapidity of new antibody formation as well as on the level of neutralizing antibody titer at the time of exposure.