Assessment of disease-associated Streptococcus suis (DASS) carriage and viral co- infections as risk factors for S. suis disease outbreaks on German and Dutch farms (2021–2022)

Background Disease-associated Streptococcus suis (DASS) is linked to invasive disease in pigs, yet how carriage of DASS predisposes animals to disease remains poorly understood. Viral co-infections with porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), and porcine circovirus type 2 (PCV2) have been suggested as additional risk factors. We investigated whether DASS carriage and/or concurrent viral infections were associated with S. suis disease outbreaks in a longitudinal field study on three Dutch and three German nursery farms. Three consecutive batches were followed per farm and sampling was initiated when two or more animals presented with clinical signs consistent with S. suis disease (outbreak batch). Clinically affected case animals and apparently healthy pen-mates were sampled alongside healthy pigs from an unrelated pen. In subsequent batches without an outbreak, healthy control animals were sampled. Animal classifications were retrospectively verified by post-mortem examination and S. suis culture. Results S. suis disease was confirmed in 12 of 18 examined batches, with serotypes 9, 2 and 7 being the most frequently isolated from lesions. All Dutch farms were positive for serotype 9. Interestingly, systemic isolates of different serotypes were found in consecutive outbreaks on the same farm. Among necropsied pigs clinically classified as cases, S. suis disease was confirmed in 75% in the Netherlands and only 33% in Germany. DASS was detected on all six farms, with batch-level prevalences ranging from 26 to 95%. DASS carriage was not elevated in cases compared to non-diseased animals, although a significantly higher DASS/total S. suis ratio was observed in outbreak batches on two farms. Neither metagenomic sequencing nor qPCR revealed a consistent association between viral presence and S. suis outbreak status. Conclusions No clear risk factors for S. suis disease outbreaks were identified. Discordance between clinical and post-mortem diagnosis, a limited number of cases, and imbalanced outbreak and control batches likely contributed to these null findings. Preceding viral or bacterial infections were not assessed here but should be considered in future study designs. These findings highlight the complexity of S. suis disease and the considerable challenges of field-based risk factor studies.