Seroepidemiology of porcine reproductive and respiratory syndrome virus infection on commercial farms

Abstract - The serum neutralisation (SN) antibody levels to porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) in commercial swine farms are not well understood. In diagnostic laboratories, the SN test has not been routinely used, because the test requires specific skills and longer times (3-5 days) to obtain the results. Unlike other serologic tests, the SN method uses intact live virus as the antigen. The SN antibody titers are correlated with the antibody response to the major envelope glycoproteins of PRRSV. Following an infection with PRRSV, pigs initially develop non-neutralizing antibodies concurrent with the viremic period. Once pigs develop neutralizing antibodies, viremia is usually absent. The presence of SN antibody and absence of viremia suggest that the SN method could likely measure protective immunity for PRRSV infection. In this paper, the SN responses in pigs following experimental or natural infection with PRRSV are reported. Also, sow SN antibody profiles from different commercial farms are presented. Sera from pigs inoculated with different wild-type PRRSV, or with a modified live virus (MLV) vaccine and a killed vaccine were tested for SN antibody to homologous and heterologous PRRSV. Differences in SN titers were observed when monospecific sera were tested against PRRSV isolates with different RFLP patterns, while there was no major difference in SN titers when sera from pigs inoculated twice with PRRSV of heterologous origin or sera from farms with endemic PRRSV infection were tested. The SN titers were negative or markedly low when the same sera were tested against the Lelystad virus (LV). These results indicate that similar SN titers would be expected, irrespective of the reference virus used, if pigs are exposed to multiple PRRSV strains under field conditions. Monthly SN titers from 20 sows of an endemically infected farm without vaccination were tested. Mean log ₂ SN titers decreased to less than 1.0 for the first 3 months and increased to more than 3.0 for the next 3 months. No major difference was noted for SN titers when compared using a farm-specific virus and a MLV. It is interesting to note that a minor clinical PRRS outbreak was reported during the third month on this farm. The SN antibody levels on 20 different farms were investigated. Thirty to 60 sera from randomly selected sows along with information concerning previous clinical PRRS outbreaks and the current vaccination program were collected from each farm. Of 944 sera from the 20 farms, 496 (52.5%) were positive by the SN test. Five of the 20 farms did not have SN positive pigs and were found to be free of PRRS virus infection. The remaining 15 farms were SN positive, with the prevalence ranging from 17.5 - 100% with mean log ₂ SN titers of 0.2-5.3. Farms with recent clinical PRRS had high mean log ₂ SN titers (1.9-5.3). One farm had a mean SN titer of 1.0, and clinical PRRS was experienced 3 months later. Five farms vaccinating MLV followed by subsequent autogenous killed vaccine for their introduced gilts showed mean SN titers of 0.2-3.0. On commercial swine farms, herd SN profiles for breeding pigs could be classified into 3 major patterns; high for farms with recent clinical PRRS, a mixture of high, low and negative for farms with endemic infection, and negative or low for farms with no clinical PRRS. On endemically infected farms, SN titers will fluctuate, with infection being maintained among susceptible subpopulation and newly introduced pigs. The SN titers will be low or negative following an outbreak with a single strain of PRRSV, and infection will be stabilized in such herds. Similarly, a naïve farm using only a MLV vaccine will have no or low SN titers. On the other hand, herds with high SN titer profiles ( 1:4) could indicate infection with multiple strains of PRRSV. It has been reported that SN antibody becomes detectable after cessation of viremia in pigs following a PRRSV infection. In our previous study, the challenge virus was not isolated from the sows with detectable SN antibodies. For this reason, we propose that SN antibody may be used as an indicator for protection from viremia, and that herd SN antibody profiles be used to measure the risk levels in swine herds.