Etiological Role of Ortho- and Paramyxoviruses in Acute Respiratory Tract Infections Among Children Aged < 4 Years in Bulgaria

Background: Influenza viruses (family Orthomyxoviridae); respiratory-syncytial virus (RSV), human metapneumovirus (hMPV), and parainfluenza viruses (hPIV) type 1, 2 and 3 (family Paramyxoviridae) are among the most common causes of acute respiratory tract infections (ARTI) in infants and young children. The aim of this study was to determine the contribution of these viruses in cases of ARTI requiring medical attention among children aged < 4 years during the 2012/13 and 2013/14 winter seasons in Bulgaria. Methods: A total of 416 nasopharyngeal swabs of children aged <4 years presenting ARTI from different regions of country were tested for influenza MB viruses by real-time RT-PCR. Influenza virus negative samples were examined by individual real-time RT-PCR using specific primers/probes for RSV, hMPV, and hPIV1, 2, and 3. Results: Of the 416 specimens tested, 129 (31%) were influenza virus positive. Influenza A(H1N1)pdm09, A(H3N2), and type B viruses were found in 61(14.7%), 14(3.4%), and 49 (11.8%) of samples, respectively. Of the 287 influenza virus negative specimens, paramyxoviruses - RSV, hMPV, hPIV1, hPIV2, and hPIV3 were detected in 55(19.2%), 28(9.8%), 17 (5,9%), 5 (1,7%), and 14(4.9%) samples, respectively. RSV were the most frequently identified paramyxovirus (p < 0.05). Overall, 15 (6.4%) patients were co-infected with two viruses. The contribution of respiratory viruses in cases of bronchiolitis, pneumonia, and neurological complications was analyzed. Conclusions: Influenza viruses and RSV were the most frequent viral pathogens causing ARTI among children < 4 years of age during the 2012/13 and 2013/14 winter seasons in Bulgaria.