The serological response against foot and mouth disease virus elicited by repeated vaccination of dairy cattle.

In Israel, cattle are annually vaccinated against foot and mouth disease (FMD). If infections with FMD virus occur in dairy farms it mainly involves heifers and calves, while older dairy cows seldom become infected. We hypothesized that this difference in susceptibility between adult cows and the young heifers and calves is due to stronger and more stable immune response elicited by multiple vaccinations. In order to test this hypothesis, 99 dairy cattle, divided into six groups according to number of prior vaccinations, were annually vaccinated with a trivalent vaccine (A, O and Asia-1) and followed during two consecutive years. In total 988 sera were sampled at 11 time points. Virus neutralization tests (VNT) were performed in order to determine the neutralizing antibody titers (NAT) against the vaccine homologous serotypes: O-4625, O-Manisa, Asia-1-Shamir and the heterologous serotype A-Turkey-20/2006. A similar NAT pattern was observed to all serotypes and therefore statistical analysis was restricted to O-4625 serotype. In the 'high vaccination' groups (cows that were vaccinated at least four times before the study), high NAT were found on the beginning of the trial and no or only a mild increase of NAT was observed following further vaccinations. Additionally, in the 'high vaccination' groups, the percentage of cows that had a NAT higher than 2.0 (log10) by the end of the 1st year was significantly higher than in the 'low vaccination' groups (cows vaccinated only three times or less before the study). We conclude that starting from the 5th vaccination, the NAT increase following vaccination is mild and NAT are persistent, suggesting reduction of the frequency of routine vaccination after multiple vaccinations is possible.