Circulation of enteroviruses and human parechoviruses in children aged up to 5 years with influenza-like illness in Northern Italy during the 5 previous winter seasons

Background: Influenza-like illness (ILI) is a cause of child morbidity during the winter. Predominantly caused by influenzavirus, it can also be caused by different respiratory viruses, such as enterovirus (EV) and human parechovirus (HPeV). Aim: to describe the circulation of influenzaviruses, HPeV, EV and EV-D68 in children aged up to 5 years during the five previous winter seasons (2010/11-2014/15) in Lombardy (Northern Italy). Methods: Throat swabs were collected from 392 children (0-5 years; 45.4% females, median age: 3 years ; IQR: 4 years) with ILI in the framework of the Italian influenza surveillance network InfluNet. RNA was extracted from each sample (Invitek), and amplified for detection of influenzavirus type A/B and EV/HPeV with two one-step multiplex real-time RT-PCRs. EV-positive samples were tested for EV-D68 with a one-step real-time RT-PCR. Results: Of the 392 samples, 46.9% (184/392) were influenzavirus-positive (75% type A), 9.4% were EV-positive and 3.8% were HPeV-positive. EV-D68 was not detected. Among the 208 influenzavirus-negative samples, 11.5% and 3.8% were positive for EV and HPeV, respectively. HPeV-positive children were younger (mean age: 21 months) than both EV-positive (mean: 34 months, p=0.002) and influenzavirus-positive children (mean: 39 months, p<0.001). HPeV was identified in 1.6% (in 2010/11) to 5% (in 2014/15) of samples. EV was similarly detected (9.7% to 11.3%) except in 2012/13 (4.3%). Co-infections were detected in 10.9% (20/184) of cases: 7.1% (13/184) were influenzavirus/EV-positive and 3.9% were influenzavirus/HPeV-positive. None were EV/HPeV. Conclusions: Almost half (47%) of ILI cases in our study were influenzavirus-positive, as expected. No EV-D68 was identified. Of the remaining influenzavirus-negative cases, 15% were positive for EV or HPeV, highlighting the importance of monitoring the circulation of these viruses, generally not included among common investigated pathogens.