The NSP immune response of vaccinated animals after in-field exposure to FMDV.

The aim was to examine the immune response (IR) to non-structural proteins (NSPs), in order to assess the validity of the detection of antibodies to NSPs as a means of diagnosing foot and mouth disease (FMD infection) infection when vaccinated populations are in close contact with clinically sick animals. The study was performed during FMD outbreaks in Israel in January 2004; the IR was examined in vaccinated dairy and feedlot cattle herds under natural field exposure to FMDV, and in vaccinated and unvaccinated sheep flocks. During the 2004 outbreaks, clinical signs were age-related and were noted only among imported calves, although they had been vaccinated; such signs were not found among the local dairy cattle populations. The NSP IR among the feedlot cattle that had been vaccinated more than 4 months prior to the in-field exposure was 86%, compared with only 30% among those feedlot cattle that had received one dose of vaccine less than 4 months before the field exposure. The prevalence of NSP IR indicates that animals vaccinated once, less than 4 months prior to exposure, were clinically resistant to FMDV infection, although possibly still susceptible to subclinical infections, whereas those vaccinated more than 4 months prior to the in-field exposure presented clinical manifestations. This situation is unlikely to occur among repeatedly vaccinated livestock; these remained refractory to FMD exposure, as reflected in the absence of clinical manifestations and a relatively low prevalence of NSP IR compared with that in imported calves.