Retrospective Analysis of the Detection of Pathogens Associated with the Porcine Respiratory Disease Complex in Routine Diagnostic Samples from Austrian Swine Stocks

SIMPLE SUMMARY: Multiple viruses and bacteria can cause respiratory disease in pigs. We aimed to report how frequently certain viruses and bacteria were detected in samples from pigs with respiratory disease in the course of routine diagnostic procedures at the University of Veterinary Medicine in Vienna between 2016 and 2021. While Mycoplasma (M.) hyorhinis (55.1%) had the highest detection rate, influenza A virus had the lowest detection rate (6.1%) in the investigated samples. Lung samples tested positive for PRRSV RNA were also more likely to be positive for M. hyopneumoniae and Pasteurella (P.) multocida. Samples tested positive for M. hyopneumoniae were more likely to be positive for P. multocida and Streptococcus suis, but less likely to be positive for M. hyorhinis. In conclusion, lung samples that were positive for a primary pathogenic agent were more likely to be positive for a secondary pathogenic agent. ABSTRACT: The diagnostic workup of respiratory disease in pigs is complex due to coinfections and non-infectious causes. The detection of pathogens associated with respiratory disease is a pivotal part of the diagnostic workup for respiratory disease. We aimed to report how frequently certain viruses and bacteria were detected in samples from pigs with respiratory symptoms in the course of routine diagnostic procedures. Altogether, 1975 routine diagnostic samples from pigs in Austrian swine stocks between 2016 and 2021 were analysed. PCR was performed to detect various pathogens, including porcine reproductive and respiratory syndrome virus (PRRSV) (n = 921), influenza A virus (n = 479), porcine circovirus type 2 (PCV2) (n = 518), Mycoplasma (M.) hyopneumoniae (n = 713), Actinobacillus pleuropneumoniae (n = 198), Glaesserella (G.) parasuis (n = 165) and M. hyorhinis (n = 180). M. hyorhinis (55.1%) had the highest detection rate, followed by PCV2 (38.0%) and Streptococcus (S.) suis (30.6%). PRRSV was detected most frequently in a pool of lung, tonsil and tracheobronchial lymph node (36.2%). G. parasuis was isolated more frequently from samples taken after euthanasia compared to field samples. PRRSV-positive samples were more likely to be positive for PCV2 (p = 0.001), M. hyopneumoniae (p = 0.032) and Pasteurella multocida (p < 0.001). M. hyopneumoniae-positive samples were more likely to be positive for P. multocida (p < 0.001) and S. suis (p = 0.046), but less likely for M. hyorhinis (p = 0.004). In conclusion, our data provide evidence that lung samples that were positive for a primary pathogenic agent were more likely to be positive for a secondary pathogenic agent.