Vaccination of MDA positive pigs with a live modified PRRSV vaccine

Since it emerged, PRRSV has had an important economic impact on the swine industry. Proper vaccination against PRRSV can significantly improve the production results in feeder finishing pigs. Ideally, fattening pigs are vaccinated at 6 weeks of age, when no maternally derived antibodies (MDA) are present anymore. However, when there is a high PRRSV wild-type pressure, it might be beneficial to vaccinate earlier. In this study, we investigated how antibodies influence vaccination with a modified PRRSV vaccine. To examine the effect of antibodies on the vaccine take, four groups of approximately 50 pigs each were vaccinated at 3 to 4 weeks of age with Porcilis PRRS (Intervet Int. b.v., Boxmeer, The Netherlands). At this period, various levels of PRRSV specific antibody titres were found. One group was vaccinated intradermally with 1 dose of Porcilis PRRS and 3 groups were vaccinated intramuscularly with 0.1, 1 and 10 doses of Porcilis PRRS, respectively. One group of 10 pigs was left unvaccinated. Blood was taken at various weeks post vaccination. PRRSV specific immunofluorescence test (IFT) antibody titres were determined from all samples and PRRSV neutralising antibody titres only from the samples taken at vaccination. Based on these data, it could be determined whether or not a pig seroconverted by vaccination and whether the vaccine had taken or not. To determine the effect of the antibody titres on vaccine take, all pigs were categorised according to their overall MDA titre and their PRRSV neutralising antibody titre at vaccination. Then the percentage of seroconverted animals per category was calculated. Regardless of the dose and route, 65% or more of the pigs seroconverted by vaccination, a percentage high enough to give sufficient herd protection. No consistent relation could be found between the virus neutralising (VN)-titres and the percentages of takes. When the effect of the total antibody titre on vaccine take was evaluated, high MDA titres were correlated with a high percentage of takes. At medium titres, the least number of takes were found. When the titres further decreased the percentage of takes again increased. We interpreted that these parabolic curves were the resultant of the positive effects of the opsonising PRRSV specific antibodies and the negative effects of PRRSV neutralising antibodies on vaccine take. Basically the virus can use two independent routes to enter its target cell, the macrophage. First, it can use the PRRSV specific receptor. Alternatively, in the presence of PRRSV specific antibodies, it can enter by the Fc-receptors on the macrophage. However, the influence of antibodies is opposite in both routes: high antibody levels will facilitate a better uptake of the virus by the Fc-receptors but will inhibit the uptake of the virus in the macrophage by the PRRSV specific receptors. At low antibody titres, the reverse situation occurs: virus uptake by the receptors is enhanced and uptake by the Fc-receptors is minimised. With this mechanism, the lowest percentage of takes will be obtained at medium antibody titres. High percentages of takes will be found at either high antibody titres, facilitating a good uptake by the Fc-receptor, or at low antibody titres, facilitating a good uptake by the PRRSV specific receptor. Because of the opsonisation of the virus by PRRSV specific antibodies, it will be possible to vaccinate young, MDA positive piglets