Serological Findings in Pigs Vaccinated Against Actinobacillus pleuropneumoniae and the Porcine Reproductive and Respiratory Syndrome Virus

The reasons for disease outbreaks caused by <i>Actinobacillus pleuropneumoniae</i> (APP) in vaccinated pigs are often unknown and remain a challenge for farmers and veterinarians. One hypothesis for APP vaccine failure is the timing of APP vaccination during field or vaccine-induced viremia with Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), which may negatively affect the immune response to APP vaccination. In this study, fattening pigs were vaccinated with a modified live vaccine (MLV) against PRRSV either at the beginning of the fattening period (group G1) or six weeks later (group G2). All pigs were vaccinated against APP five days after the start of fattening, which coincided with MLV-PRRSV viremia in G1. Within both G1 and G2, four subgroups of pigs (<i>n</i> = 10) were vaccinated with three different APP vaccines or remained unvaccinated to assess serological responses to various APP antigens. MLV-PRRSV viremia had no significant effect on APP-ApxII (<i>p</i> = 0.127), APP-LPS (<i>p</i> = 0.120), or opsonophagocytic antibody responses on day 40 of fattening. Lung lesion scores at slaughter were significantly higher (<i>p</i> = 0.004) in pigs from G2 (1.82 ± 2.38) compared with those from G1 (0.65 ± 0.88). All APP vaccines elicited presumably protective opsonophagocytic antibodies. In conclusion, no effects of MLV-PRRSV viremia on serological responses following APP vaccination were observed.