Epidemiology and molecular characterization of influenza viruses, human parechoviruses and enteroviruses in children up to 5 years with influenza-like illness in Northern Italy during seven consecutive winter seasons (2010-2017).

Besides the influenza virus (IV), several other viruses are responsible for influenza-like illness (ILI). Although human parechoviruses (HPeVs) and enteroviruses (EVs) may impact on ILI, limited data on their epidemiological characteristics are available. During seven consecutive winter seasons (from 2010-2011 to 2016-2017), within the framework of an influenza surveillance system (InfluNet), 593 respiratory swabs were collected from children ≤5 years of age with ILIs. Molecular detection showed that 58.3 % of swabs were positive for at least one of the viruses under study: 46 % for IV, 13 % for EV and 5.4 % for HPeV. A single virus was identified in 51.3 % of samples while more than one virus was detected in 7 % of the samples. The risk of contracting IV was higher than the risk associated with EV, which in turn was higher than the risk of contracting HPeV. The risk of developing an IV infection was twofold greater in children >3 years than in those ≤3 years, who had higher risk of EV/HPeV infection. The frequency of EV/HPeV-positive swabs increased significantly during the 2016-2017 winter season compared to the previous six seasons. Sixteen EV genotypes were identified belonging to species A and B. HPeV-1 was the most frequently detected genotype, followed by -6 and -3. In this study, IV was mainly responsible for ILI, however EV and HPeV were also involved and particularly affected children ≤3 years of age. Influenza surveillance samples could provide us with valuable insight into the epidemiological features of viruses involved in ILI.