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

## TL;DR

This study investigates how vaccinating pigs against two respiratory pathogens affects their immune responses and disease outcomes.

## Contribution

The study reveals that PRRSV viremia does not impair the immune response to APP vaccination in pigs.

## Key findings

- APP vaccination during PRRSV viremia did not affect serological responses to APP antigens.
- Pigs vaccinated during PRRSV viremia had lower lung lesion scores at slaughter.
- All APP vaccines induced presumably protective opsonophagocytic antibodies.

## Abstract

Vaccination of fattening pigs against the respiratory pathogens Actinobacillus pleuropneumoniae (APP) and Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is common practice. For APP vaccination, inactivated or subunit vaccines are used, whereas modified live vaccines (MLV) are applied for PRRSV vaccination. Despite vaccination against APP, severe disease outbreaks may still occur, and the underlying causes of vaccine failure are often unclear. The hypothesis of this study was that APP vaccination during MLV-PRRSV-induced viremia affects the serological response against APP. However, no differences were observed in serological responses to various APP antigens or in presumably protective opsonophagocytic antibodies between pigs vaccinated against APP with three different vaccines during MLV-PRRSV viremia and pigs vaccinated against PRRSV six weeks after APP vaccination, at a time distant from MLV-PRRSV viremia. Lung lesion scores at slaughter were lower in pigs vaccinated against APP during MLV-PRRSV viremia at the beginning of the fattening period. All APP vaccines elicited antibody responses. In conclusion, this study found no effects of MLV-PRRSV viremia on the serological responses induced by APP vaccination.

The reasons for disease outbreaks caused by Actinobacillus pleuropneumoniae (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 (n = 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 (p = 0.127), APP-LPS (p = 0.120), or opsonophagocytic antibody responses on day 40 of fattening. Lung lesion scores at slaughter were significantly higher (p = 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.