Introduction

Respiratory tract diseases (RTDs) are a leading reason for morbidity in young children and are caused by a broad spectrum of microbial agents. Viruses account for the largest number of respiratory tract infections (RTIs). The so-called respiratory viruses include influenza A and B viruses (IAV, IBV), human parainfluenza viruses (HPIVs), human respiratory syncytial virus (HRSV), human adenoviruses (HAdVs), human rhinoviruses (HRVs), human coronaviruses (HCoVs), enteroviruses (EVs) and human parechoviruses (HPeVs). In the 21st century, using large-scale molecular virus screening, several novel viruses have been discovered in patients with respiratory infections. These viruses include human metapneumovirus (hMPV), polyomaviruses KI and WU, several coronaviruses (SARS-CoV, HCoV-NL63, HCoV-HKU1, MERS-CoV) and human bocavirus (HBoV) as described by Allander et al. in 2005 [1,2,3]. The DNA virus HBoV is a member of the family Parvoviridae, genus Bocaparvovirus. HBoV is classified into genotypes 1 through 4. HBoV1 is predominantly found in respiratory tract secretions from children with RTD, and HBoV2-4 are found mainly in stool samples from patients with gastroenteritis [3, 4]. HBoV is predominantly present in winter and spring [5]. The average prevalence of HBoV in respiratory tract samples ranges from 1.0% to 56.8%, depending on the country. The worldwide estimate for the total prevalence of HBoV in respiratory infections is 6.3% [6]. HBoV has been reported worldwide in all age groups; however, it has mainly been detected in children who presented at the hospital with RTD. A high HBoV viral load could be an etiologic agent for severe LRTI, and these patients may develop bronchitis and pneumonia with fever, cough and peribronchial infiltrates detected on a chest X-ray [17].

In order to assess whether HBoV showed characteristics of a respiratory pathogen in the coinfected group, we chose to focus in detail on cases in which HBoV was expressed at a high viral load or a higher viral load then it copathogens. In these patients, a viral URT prodrome followed by breathing difficulties and wheezing were the most frequently reported clinical signs, and bronchi(oli)tis and recurrent wheezing were the leading diagnoses. In this group, a relationship was observed between HBoV and LRTD. Some previous studies have found high viral loads to be associated with more-severe symptoms [9], while others have failed to find a clear relationship [5, 7, 23]. In our opinion, it is not possible to establish a clear relationship between viral load and severity of LRTD. In the past, many scoring systems have been developed in an attempt to quantify respiratory distress objectively, but few have demonstrated any predictive validity [36]. Since clinical parameters such as respiratory rates or use of accessory muscles were sometimes lacking in the medical files, it was not possible to make a 100% correct statement about severity. We may assume that all children who needed hospitalization, and certainly those who developed hypoxia with a need for extra oxygen or ventilator support, had rather severe clinical signs.

In conclusion, HBoV is frequently found in NPSs of children with RTD. In our study, we found that HBoV infection at high viral load in infants is associated with bronchi(oli)tis and recurrent or episodic wheezing (P = 0.013, Cramer’s V = 0.613).