Serological response of pigs to a standard and increased dose of foot-and-mouth disease vaccine

Two randomly allocated age-matched groups of 17 conventionally reared pigs derived from vaccinated sows were vaccinated at 10 and 14 weeks of age with a commercially available foot-and-mouth disease vaccine, using either a 1 mL dose or a 3 mL dose. A control group of four pigs was left unvaccinated. Pigs were monitored at regular intervals from birth to 26 weeks of age for antibodies to FMD Type O virus using a liquid phase blocking ELISA. At 12 weeks post vaccination, significantly more pigs vaccinated twice with 3 mL of vaccine had developed antibodies against Type O foot-and-mouth disease virus (at an ELISA titre of 90 or greater) than those vaccinated twice with 1 mL of vaccine (chi-squared test, p = 0.006). Overall, the response to vaccination was poor in both groups of pigs. Four weeks after the first dose of vaccine only four pigs had detectable antibody against the virus. Twelve weeks after the second dose of vaccine only 60% of pigs given the 3 mL dose and 15% of pigs given the 1 mL dose had ELISA titres of 90 or greater. Maternal antibody is considered to have played a role in this poor response, as it was present in 27 of the 34 vaccinated pigs at the time of first vaccination. Two pigs in the unvaccinated control group developed a low level antibody response (antibody titre <90). Infection with field virus was considered a highly unlikely cause of this. These results show, that under field conditions using a widely adopted protocol not all pigs vaccinated develop antibody to foot-and-mouth disease. This, in part, may explain why vaccination programmes against this disease in Hong Kong seem to have a limited impact. The results also suggest, that an increased dose of vaccine has a positive effect on the humoral immune response against FMD virus and may improve protection against this disease. Timing of vaccination needs to be re-evaluated to reduce the impact of maternally derived antibodies. (author)