Respiratory tract infection etiology is complex and diverse, and new pathogens are continuously being reported. Over the past few years, several novel respiratory viruses including human metapneumovirus (hMPV) [1], severe acute respiratory syndrome (SARS) coronavirus [2], human coronavirus NL63 (HCoV-NL63) [3, 4], and coronavirus HKU1 (HCoV-HKU1) [5–7] have been identified.

In 2005, Allander et al. [8] reported a previously undescribed human parvovirus, human bocavirus (HBoV) that belongs to the genus Bocavirus, in respiratory secretions of children with respiratory tract disease in Sweden. HBoV is a single-stranded deoxyribonucleic acid (DNA) virus with a small genome size of approximately 5.3 kilo-bases (kb), which has three open reading frames (ORF) encoding two non-structural proteins NS1 and NP1, and the two structural proteins VP1 and VP2. VP1 and VP2 are located within the same ORF but have different initiator codon positions [8].

Subsequently, HBoV was reported in respiratory samples from different countries and regions worldwide [9–14], where HBoV was detected in 1.5%-8.3% of respiratory samples from individuals with acute respiratory tract illness (ARTI), especially young children and infants. The virus was also found in stool samples from patients with gastrointestinal illness [15–22]. These reports suggest that HBoV might be associated with upper and lower respiratory disease and gastrointestinal illness throughout the world. In 2009, two viruses closely related to HBoV, named HBoV2 [23] and HBoV3 [24], were found in stool samples, and suggested HBoV diversity.

HBoV infection has recently attracted increasing attention all over the world. However, the incidence and clinical presentation of this infection varies widely, and often involves co-infection with other potential pathogens [9–22]. Such characteristics have led to debate over the role of HBoV as a true pathogen. Therefore, additional evidence and studies are needed throughout the world to gain a better understanding of this virus. In this study, 2811 respiratory samples were collected from patients (with an age range of 9 days to 84 years) with ARTI in Guangzhou, southern China, from November 2009 to November 2010 to analyze the characteristics of HBoV-positive patients.

Samples in this study were taken as part of standard care. The First Affiliated Hospital of Guangzhou Medical University Ethics Committee approved the experimental design and patient involvement in this study.

HBoV is a novel parvovirus first described in 2005 by Allander and colleagues [8]. Since that time, it has been associated with upper and lower-respiratory tract disease and gastrointestinal illness throughout the world. However, most studies were focused on young children and infants [9–14], and only a few papers have described the characteristics of HBoV infection in adult patients [16, 25, 26]. Our study successfully analyzed the characteristics of HBoV-positive samples from ARTI-infected patients with a wide age distribution from Guangzhou, southern China for the first reported time. Similar to previous study [16], the detection rate in pediatric patients (<18 years old) was significantly higher than that in adults (≥18 years old) (p < 0.001), and most HBoV-positive patients were ≤3 years old. HBoV was detected in four adult patients, including 60 and 70 year-old patients. This suggested that older people were also susceptive to HBoV infection, although at much lower positive rates. Four adult patients also presented with systemic influenza-like symptoms, which might suggest that HBoV infection in adults is a more complex and serious problem than in children.

While seasonal peaks of HBoV infection vary among different counties and regions because of climate and other factors, many previous studies suggest a higher detection rate in winter [27, 28]. In our study, a higher frequency of HBoV was observed between May and June during the 13-month testing period (Figure 2). This result was similar to the report of Choi and colleagues, in which a seasonal peak was observed between May and June [29].

Characteristics of HBoV-positive patients in our study were also similar to previous reports [9–14]. The male:female ratio in the HBoV-positive patients did not differ significantly from the HBoV-negative patients. In all clinical characteristics, seven major symptoms (cough, fever (≥38°C), abnormal pulmonary breath sound, dyspnea, expectoration, snivel and pneumonia) had the highest ratio (ratio > 10%) (Table 3) and were common as URTI and LRTI.

As previous studies [13, 29–34], co-detection with other potential pathogens was common in HBoV-positive patients. In this work, 43.1% (28/65) patients were co-detected with other 16 potential pathogens, and there would be higher ratio if human rhinovirus were concerned [16]. Furthermore, not only co-pathogens but also single HBoV groups had a high ratio of major symptoms (Table 3), which might suggest HBoV is an important pathogen in URTI and LRTI. A statistic difference was found for the symptom of "pneumonia" between the two groups "Single HBoV" and "Co-pathogens" (Table 3), which might further suggest HBoV is an important pathogen. Further studies were required to determine whether HBoV played a causative role in these co-infections or acted as an exacerbation factor.

Before 2009, little variation was found in the surface protein of HBoV (VP1-VP2), and some researchers predicted that HBoV infection might only occur after the subsequently development of life-long immunity via the neutralization of the target antibody [16]. However, novel types of HBoV2 and HBoV3 were described successively in 2009 [23, 24], suggested diversity within this group of viruses. Furthermore, we successfully sequenced the complete HBoV genome (GU338055) for an isolate from an adult patient and phylogenetic analysis revealed the existence of three lineages: group I, HBoV; group II, HBoV2; group III, HBoV3, and GU338055 was located in group I.

The frequency and clinical presentations of HBoV infection vary widely [9–14, 16, 25, 26] and are typically associated with other pathogens [13, 29–34]. Such characteristics have subsequently led many scientists to question the role of HBoV as a potential pathogen. This lack of information is largely because of the inability of HBoV to grow in standard cell lines [35]. To confirm the effects of HBoV, more studies are required throughout the world, focusing on various aspects of this infection, including epidemiology, serology, molecular biology, in vitro culture and animal models.

HBoV was detected from children and adults with ARTI from Guangzhou, southern China, and the features were described in this study. HBoV was confirmed in elderly patients (60 and 70 years old), suggesting that older people were also susceptive to HBoV. All four adult patients with HBoV positive in this study presented systemic influenza-like symptoms, which potentially suggest that HBoV infection in adults may develop more serious symptoms than those in children. Phylogenetic analysis suggested that HBoV-GU338055 from an elderly patient is in a single lineage with other HBoVs.