Epidemiological and molecular investigation of non-seasonal influenza-related viral respiratory infections in Northern Italy during four consecutive influenza seasons (2014-2018)

Background Besides seasonal influenza A and B viruses (IAV and IBV), several pathogens are involved in clinically undistinguished respiratory infections, including respiratory syncytial virus (RSV), enterovirus (EV), parechovirus (PeV) and influenza C virus (ICV). This study aimed at investigating the contribution of these viruses in influenza-like illness (ILI) in Lombardy (Northern Italy) during four consecutive influenza seasons (2014-2018). Materials and Methods 1047 IAV/IBV-negative respiratory swabs collected from ILI outpatients in Lombardy within the Italian Influenza Surveillance Network during four seasons (from 2014/2015 to 2017/2018) were tested to detect RSV, EV, PeV and ICV by specific real-time RT-PCR. Positive samples were sequenced and molecularly characterised. Results 19.5% of specimens tested positive to at least one virus: RSV, EV, PeV and ICV contributed to 12.9%, 6.5%, 1.8% and 0.3% of IAV/IBV-negative ILIs, respectively. Multiple virus detection was observed in 2% of cases, most (66.7%) was RSV+EV. Except for ICV that was identified sporadically and only in two seasons, the other viruses were detected in every season: the broadest RSV and EV circulation was in 2016/2017 (15.4% and 8.9%, respectively) and 2017/2018 (15.2% and 8.8%, respectively), while PeV in 2015/2016 (2.5%). These viruses circulated mainly in November/December (RSV, EV, PeV) and February/March (RSV). The highest risk of infection from RSV/EV/PeV was observed in children