Simple SummaryProcessing fluids (PFs) are novel diagnostic specimens consisting of serosanguineous exudate obtained from tissues after piglets’ castration. PFs can be used for some diagnostic tests that would otherwise require a blood sample. This has the advantage of not subjecting piglets to an additional stressful procedure and, at the same time, reducing sampling costs. In the present study, the efficacy and reliability of a monitoring plan for porcine reproductive and respiratory syndrome (PRRS), one of the main pig diseases, based on PF sampling has been assessed in the Italian swine production system. Twenty-five breeding herds were monitored for 4–5 months by RT-PCR performed on both PFs and blood serum (the standard specimen used for monitoring). Based on the results obtained from each method, the herds were classified following the standard PRRS status categories. The two methods fully agreed in discriminating between herds with stable and unstable PRRSV circulation. However, we observed a slight discrepancy in classifying high- and low-prevalence herds within unstable herds. We conclude that PFs are a reliable material for the PRRSV surveillance and classification of breeding herds, but in case of unstable circulation, a strategy combining blood and PF sampling is recommended.AbstractThe porcine reproductive and respiratory syndrome (PRRS) control strategy within swine breeding farms is based on herd classification relative to PRRSV infection status. This study aims to assess the efficacy of a monitoring plan based on processing fluids (PFs) by comparing it with the classification of herds based on the analysis of blood serum. Twenty-five breeding herds were enrolled in the study, with at least five consecutive batches sampled from each herd. Each batch was tested for PRRSV by RT-PCR performed on (i) pre-weaning blood serum from 30 piglets and (ii) PFs from all the male piglets in the batch. PRRS categories following the Holtkamp classification were assigned based on the results of each testing protocol. The two protocols assigned the same category to 18 out of 25 herds: while they showed perfect agreement in identifying positive unstable and stable herds, we observed some discrepancy in discriminating between low- and high-prevalence classes within unstable herds. PFs are thus a reliable sample to assign PRRS categories in Italian breeding herds characterized by widespread PRRSV circulation. However, in case of an unstable epidemiological scenario, we recommend the adoption of an integrated monitoring strategy that combines blood sampling with PFs.