Human bocavirus: As an emerging respiratory pathogen

Abstract

Acute respiratory infections (ARIs) are the main cause of morbidity and mortality worldwide, especially among children. The human bocavirus (HBoV) is a non-enveloped DNA virus that was recently identified as a respiratory pathogen associated with respiratory tract infections (RTIs), predominantly in infants and young children. It is also detected from the gastrointestinal tract in children. The prevalence of HBoV1 acute respiratory tract infection varies across age groups, ranging from 10.3% to 12.51% in individuals under 3 years of age. The spectrum of clinical presentation includes mild upper RTIs, acute exacerbation of asthma, bronchitis, bronchiolitis, pneumonia, and multi-organ failure. Although HBoV is often detected in patients with ARIs who have other respiratory viruses (17%-85%), recent studies have identified it as the sole aetiology for mild to severe ARIs. Children with pre-existing medical conditions infected with HBoV often have a risk of severe illness. HBoV infection is diagnosed primarily by detecting viral DNA in respiratory samples using molecular methods. Currently, there is no specific antiviral treatment for HBoV infections and the cases are managed symptomatically. General preventive measures used for the prevention of viral RTIs are applicable, as there is no effective vaccine against this virus. The HBoV has been implicated in RTIs, particularly in children, and has also been detected in cases of gastroenteritis. Despite its global prevalence, the exact pathogenic role of HBoV remains unclear due to frequent co-infections with other viruses. This mini-review discusses the virology, epidemiology, clinical manifestations, diagnosis, and potential treatment approaches related to HBoV infections.

Key Words: Human bocavirus; Acute respiratory tract infections; Co-infection; Plastic bronchitis; Respiratory viruses

Core Tip: Human bocavirus is a recently identified respiratory pathogen associated with respiratory tract infections. It can cause a wide range of clinical manifestations in the respiratory tract, either as a sole pathogen or in co-infections. High infection rates are detected among children less than three years of age. This mini-review describes virology, epidemiology, clinical manifestations, diagnosis, and potential treatment approaches related to human bocavirus respiratory tract infections.

INTRODUCTION

Acute respiratory infections (ARIs) are the main infectious cause of morbidity and mortality in the world[1]. Around 2.5 million of the global population lose their lives from ARIs each year, and most of these deaths are due to lower respiratory tract infections (LRTIs), which have become the fifth most common disease with a high mortality in the world[2,3].

ARIs are caused mainly by viruses: Human rhinoviruses, influenza A and influenza B viruses, respiratory syncytial virus (RSV), parainfluenza virus, human coronavirus, human metapneumovirus, human enterovirus and adenoviruses are the most common viruses detected from respiratory specimens in patients with ARIs[4,5]. Primary infections with viral pathogens can lead to secondary bacterial infections, which contribute to high mortality. The commonly reported bacteria include Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae[6,7].

Infectious respiratory viruses can cause outbreaks, epidemics and even pandemics. One such pandemic was coronavirus disease 2019, caused by the severe acute respiratory syndrome coronavirus 2, which has led to a global public health crisis. Initially, the disease progression was analyzed based on early clinical and diagnostic observations. For instance, Sabatino et al[8] reported findings from high-resolution computed tomography of the first 64 coronavirus disease 2019 patients, which were crucial for identifying diagnostic features and shaping early response strategies. Similarly, this pandemic underscored the importance of recognizing lesser-known viral agents capable of causing disease, particularly in children. One such pathogen is human bocavirus (HBoV), which is closely related to bovine parvovirus and canine minute virus[9]. The HBoV is recognized as a causative agent of respiratory infections in individuals of all ages, especially in children[10,11].

DISCOVERY AND OVERVIEW OF HBOV

HBoV, a member of the Parvoviridae family and Bocaparvovirus genus, was first discovered in 2005 in Sweden from nasopharyngeal aspirates of seventeen children with LRTIs[9]. HBoV1, the first identified and the most studied genotype, is commonly associated with respiratory tract infections (RTIs) and affects both the upper and lower respiratory tracts[12]. Following this discovery, three further genotypes (HBoV2, HBoV3, and HBoV4) were subsequently detected in fecal specimens[13]. HBoV2, first reported in 2009, from fecal samples of children in Pakistan and the United Kingdom[14], has since been frequently associated with acute gastroenteritis (AGE) in children[15]. Additionally, co-infections with known human gastroenteritis viruses have frequently been observed, along with fecal discharge of asymptomatic children[16,17]. Therefore, the exact role of HBoV2 as an enteric pathogen remains uncertain. However, some studies show HBoV2 has been detected in fecal samples of children suffering from RTIs and is rarely present in respiratory tract samples[18,19]. HBoV3 was initially detected in a fecal sample in 2009[15]. Several other studies have confirmed the presence of HBoV3 in fecal samples of patients with gastroenteritis. However, detection rates have been lower than those of HBoV2[17,20,21]. HBoV4 was discovered recently in fecal samples of both children and adults, but the clinical significance of this virus remains unclear[21].

The HBoV1 has been proposed as a viral pathogen linked to respiratory infections, with global occurrences varying from 0.8% to 56.8% in children[10,11,22]. HBoV1 infection is commonly identified in patients experiencing pneumonia, acute wheezing, asthma, or bronchiolitis, and occasionally in severe lower respiratory infections that are life-threatening[23]. Additionally, HBoV1 has been detected in stool samples, indicating the potential for gastrointestinal involvement[24]. In recent years, this virus has become increasingly recognized as a causative agent of respiratory infections, affecting individuals of all ages[13,25,26]. Various studies have supported the presence of HBoV1 as a potential pathogen in LRTIs, mainly in children under the age of three. HBoV1 has also been detected in asymptomatic children, a finding initially disputed. However, research indicates that HBoV1 can persist in the nasopharynx for extended periods, leading to inaccurate polymerase chain reaction (PCR) diagnoses[27,28]. It is a co-infectious agent, often found with other respiratory viruses, and can also act as an isolated viral pathogen during RTIs[7]. Detection and treatment of co-infections depend on factors like illness severity and the presence of common pathogens[29].

Recent studies have shown that the most common Bocavirus in respiratory samples is HBoV1, compared to other genotypes, HBoV2, HBoV3, and HBoV4. While HBoV1 infections are often mild and self-limiting, severe cases can occur, as demonstrated in the study by Ziemele et al[26], especially in individuals with underlying health conditions. Some studies indicate the need for continued monitoring of HBoV in children with ARIs due to the seriousness of HBoV co-infections. Clinical observations suggest that HBoV infection might impact organ function in the liver, kidneys, heart, and blood acid-base balance. Accurate identification of the virus aids in providing specialized care and raises awareness of its risks in respiratory infections[26]. As demonstrated in the study conducted by Simon et al[30], the identification of HBoV in specimens obtained from the respiratory tracts of children did not necessitate a targeted antiviral treatment. However, it did prevent the need for additional intravenous antibiotics, thereby mitigating potential side effects and minimizing unnecessary costs associated with antibacterial medications[30]. Furthermore, raising awareness about the importance of hospital disinfection, sterilization and standardizing procedures is crucial. Further investigation is warranted to better understand the interactions between HBoV and other pathogens in the future[31]. Research on this virus is crucial for comprehending respiratory illness and developing potential treatments or vaccines. Access to updated and precise information is essential to alleviate strain on healthcare systems and enhance patient care for ARIs.

VIROLOGY AND GENOMIC CHARACTERISTICS

HBoV has been classified as a member of the Bocaparvovirus genus in the Parvoviridae family[13]. It exhibits genetic similarity to bovine parvovirus and canine minute virus[13]. The HBoV is icosahedral in shape and is a non-enveloped virus. The viral genome comprises a single-stranded DNA of approximately 5.3-5.5 kb[9,32].

The genome contains three open-reading frames (ORF1, ORF2, and ORF3): (1) ORF1, located on the left half, encodes a series of non-structural protein (NS) 1-4; (2) ORF2, a smaller middle reading frame, encodes a unique nuclear protein (NP1), crucial for viral DNA replication and mRNA processing; and (3) ORF3, situated on the right half, encodes structural capsid proteins, viral structural protein (VP) 1, VP2, and VP3, with VP3 serving as the major capsid protein. The NS1 and NP1, which are relatively conserved proteins, play essential roles in viral DNA replication and are targeted for HBoV detection. In contrast, VP1 and VP2 exhibit greater mutational diversity. They are commonly used for phylogenetic analysis, contributing to the classification of HBoV into genotypes HBoV1, HBoV2, HBoV3, and HBoV4 based on nucleotide divergence in the VP1 capsid region. The VP1 is crucial for infectivity, aiding in the release of the virus from endocytic compartments into the nucleus of the host cell[32-35].

The non-enveloped capsid surface of the virus carries host determinants and plays various roles, including host tropism, cell recognition, pathogenicity, intracellular trafficking, genome packaging, assembly, and immune response[36].

EPIDEMIOLOGY

Numerous clinical investigations have recognized HBoV1 as one of the most commonly detected respiratory viruses in infants and young children with RTIs. Most literature indicates that HBoV1 infection is prevalent among children aged 6 months to 2 years, with the highest detection rates occurring during the second year of life[31,37,38].

The prevalence of HBoV1 shows variation across age groups. Studies report a detection rate of 0.58 episodes per child by the age of two[25]. Prevalence rates range from 10.3% to 12.51% with the highest rates often found in children aged 1-3 years[39,40]. Some data show 63.1% of cases in infants under 12 months[40]. Other findings indicate a 28.1% prevalence with a median age of three, while the highest rate, 37.5% was seen in children aged ≥ 5 years[31].

Globally, the HBoV1 infection is reported in both low and high-income nations[25,38-41]. For instance, detection rates in European countries such as Norway and Croatia have remained moderate. Whereas Asian regions, including China and Egypt, have reported a wider range[22,31,42-44]. Additionally, some Asian regions, including Japan and Sri Lanka, have remained mid-range of detection rates[45,46]. A meta-analysis of 35 European studies found prevalence rates ranging from 2.0% to 45.69%, with a pooled rate of 9.57%[47]. In contrast, large-scale surveillance in the Middle East, such as in Saudi Arabia, found HBoV1 to be relatively uncommon among hospitalized children[48].

Regarding the seasonal pattern of HBoV1, several studies have indicated a greater prevalence during the late autumn and winter seasons[13,38]. Most diagnoses occurred during winter months, demonstrating that infections can happen throughout the year but are most common during the winter season[13,25,48]. However, another study in China showed a seasonal pattern, with a higher incidence of HBoV1 infections observed during summer[40]. Specifically, during the study duration, elevated detections of HBoV1 were primarily observed between May and August, as well as November and January[40].

CLINICAL MANIFESTATIONS

HBoV infections present with a range of symptoms, including mild upper respiratory symptoms to respiratory distress. The range of illnesses associated with HBoV infection seems comparable to those reported for most of the other respiratory viruses, with a predominant impact on the upper respiratory tract[28,49].

The primary symptoms observed in both solitary infection and co-infection with HBoV were fever and cough. Respiratory distress ranked third in terms of frequency among both types of cases[26]. Rhinorrhea was notably more prevalent in cases of single infection compared to co-infection[26]. Conversely, respiratory failure was exclusively detected in cases of solitary HBoV infection[50]. Wheezing occurred more frequently in co-infection cases than in single-infection cases, with a significant contrast. No substantial variation was observed, although pneumonia was more commonly associated with co-infection[38]. Previous studies have highlighted cough, rhinorrhea, and fever as the most prevalent symptoms in patients solely positive for HBoV, followed by severe respiratory distress, which required tracheal intubation[26]. Furthermore, patients infected with HBoV, whether as a solitary infection or co-infection with other viruses, exhibit a clinical presentation similar to those infected with other respiratory viruses such as RSV and human metapneumovirus[38,51]. Symptoms include upper RTIs, bronchitis, bronchiolitis, pneumonia, and acute exacerbation of asthma[26,52,53].

Although HBoV1 is primarily associated with respiratory symptoms, it has been detected in stool samples, indicating the potential for gastrointestinal involvement[24]. In contrast, HBoV genotypes 2-4 are linked with gastrointestinal issues, such as loss of appetite, vomiting, diarrhoea, and nausea[48]. Co-infections with intestinal pathogens, including human rotavirus, noroviruses, and certain strains of Escherichia coli or Salmonella are common, affecting up to 77.6% of HBoV-positive children[54]. However, the exact connection between the HBoV and gastroenteritis is still uncertain, despite these frequent associations[7].

A study conducted with the children who had symptoms of gastroenteritis, acute respiratory tract infection (ARTI) and symptoms of both shows that HBoV1 can be detected in patients with both ARTI and AGE and also patients with AGE alone, in addition to patients with ARTI symptoms. This shows HBoV1 can be found in stool samples during ARTI with or without gastroenteritis[55].

CO-INFECTIONS

HBoV can be frequently detected in the first two years of life, and it is ranked as the fourth most prevalent respiratory virus, following RSV, rhinovirus, and adenovirus[25,56]. The HBoV1 can be detected with one or more respiratory viruses or bacteria (Table 1)[4,5,13,20,25,31,38,39,43,46,48,57-60]. A notable feature of bocavirus infection is its frequent co-detection with other pathogens, such as respiratory viruses and bacteria, likely due to the extended shedding of HBoV1 in the nasopharynx, which can last weeks or even months. Notably, some studies revealed that HBoV1 DNA shedding typically lasted up to four weeks in almost all instances[27]. Moreover, increasing age, winter seasons and attendance at childcare facilities are associated with higher rates of HBoV1 detection[25,27]. The initial release of HBoV1 is linked to mild respiratory symptoms, followed by an extended period of detecting HBoV1 DNA for about a year, which can lead to co-infection. Repeated infections with HBoV1 prolong this shedding process[61]. The prevalence of HBoV co-infections varies across different populations and geographical regions, highlighting the need for further investigation into its epidemiology and clinical significance[62].

Table 1 Summary of reported prevalence ranges of human bocavirus co-infections with viral and bacterial pathogens.

Type of co-infection	Common co-detected pathogens	Reported prevalence range	Clinical significance
Viral co-infections	RSV, human rhinovirus, influenza A/B, human enterovirus, seasonal coronaviruses, human polyomaviruses	17%-86%	Frequently reported in children with respiratory illnesses, RSV co-infection may increase disease severity
Bacterial co-infections	Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Mycoplasma pneumoniae, Escherichia coli, Klebsiella pneumoniae	14%-87%	Associated with more severe respiratory outcomes, including intensive care unit/intermittent mandatory ventilation requirement

RSV: Respiratory syncytial virus.

CO-INFECTIONS WITH VIRUSES AND BACTERIA

The prevalence of mixed viral infections in nasopharyngeal swab samples varies across different studies and regions. Co-infection rates range from around 17% to as high as 86% in twelve studies[4,5,13,25,38,39,43,46,48,57-59].

Interestingly, HBoV1 can also be found in asymptomatic children, which can lead to high rates of co-infections, contributing to the high prevalence of the virus in the pediatric population. The role of HBoV in respiratory infections was questionable, with some studies suggesting it as a potential cause of severe respiratory illnesses in children[63]. In contrast, the other studies show it as a harmless virus[64]. However, recent studies increasingly support the fact that HBoV causes significant illness, even when it's the only infectious agent present. Therefore, timely diagnosis is crucial, especially for pediatricians, since this viral infection can potentially have severe and fatal consequences[26].

Several studies have highlighted the prevalence of bacterial co-infection among individuals who are positive for HBoV[31,38,39]. They have also reported complications such as pneumothorax, pneumomediastinum, and severe respiratory failure necessitating intensive care unit (ICU)/intermittent mandatory ventilation support in cases of HBoV coinfection with other pathogens[50]. Multiple co-infections in the respiratory tract were associated with more severe disease progression and the need for ICU admission[65].

HBOV1 AS A SOLE PATHOGEN

Many studies across China demonstrate that HBoV1 frequently appears as a single infection in people with ARTIs, suggesting it can act as an independent respiratory pathogen. The mono infection rate ranges from 49.34% to 81.2% out of HBoV1 positive cases, reinforcing the virus’s standalone pathogenic potential[5,45,66].

In Sri Lanka, out of 200 respiratory samples from suspected severe acute respiratory syndrome coronavirus 2 patients, only one sample tested positive for HBoV1 with common ARTI symptoms[67].

In the United Kingdom, HBoV1 mono-infected children who received oxygen, nebulizers, and mechanical ventilation; inappropriate antibiotic use was noted, raising concerns about resistance and microbiota disruption[13].

The literature shows that patients with ARTI had either high or low viral load, and HBoV1 single infections are not significantly different from co-infection with respect to clinical features; the virus can be as pathogenic by itself as other respiratory agents[68].

RISK FACTORS AND COMPLICATIONS

Children infected with HBoV often have pre-existing medical conditions that elevate the risk of severe illness, including chronic lung diseases, congenital heart conditions, neuromuscular disorders, cancer, or immunological issues, with prevalence rates ranging from 14% to 77%[69,70].

HBoV1 infection in young children is linked to several factors, including prematurity, passive smoking, winter birth, and family history of asthma. There is a higher risk in children under 5, especially those attending childcare[25,71].

The main complications associated with HBoV infection include progressive respiratory distress leading to acute respiratory failure and dehydration[38]. Additionally, rare but life-threatening conditions such as pneumomediastinum and bilateral pneumothorax can develop in HBoV-infected children[48]. Apart from respiratory failure, the HBoV is also associated with acute heart failure at a significant rate of 10%[53].

Additionally, some children needed ICU admission due to serious events, such as heart failure, myocardial damage, and liver function damage. These studies show the importance of considering HBoV1 as a differential pathogen and diagnosis in pediatric patients presenting with respiratory symptoms[72]. Although most children recover with supportive care, severe and fatal cases have been documented, particularly among immunocompromised or chronically ill patients[73].

DETECTION METHODS

Detection of HBoV infection primarily relies on identifying viral genomes present in human respiratory samples, though serum, blood, stool, and urine samples are also utilized. Various molecular assays, such as PCR, employing specific sets of primers targeting viral genes NP1, NS1, VP1, and VP2, are employed for this purpose[62]. The most prevalent techniques include quantitative PCR and real-time PCR, which quantify HBoV mRNA. Samples from different parts of the respiratory tract are examined in patients with RTIs, ranging from NP aspirates and swabs to bronchoalveolar lavage. Recent advances have introduced multiplex PCR panels and multiplex tandem PCR, allowing simultaneous analysis of samples where HBoV is detected alongside other respiratory viruses[59].

Moreover, serological assays are also employed to measure HBoV1-4 – immunoglobulin (Ig) G and IgM antibodies in plasma samples. These assays utilize in-house enzyme immunoassays, providing a complementary approach to molecular diagnostics for assessing HBoV infection. Such comprehensive diagnostic approaches enhance our understanding and management of HBoV-related illnesses, contributing to more effective patient care and public health strategies[74]. In addition to serological assays, advanced molecular surveillance tools, for instance, a metagenomic approach based on target-independent next-generation sequencing, have become recognized method for both known and novel viruses in clinical samples[75].

CLINICAL MANAGEMENT

Currently, there is no approved specific antiviral treatment for HBoV infection, and comparative studies on antiviral drugs are lacking in the literature. While research on potential treatments is ongoing, there has been limited exploration of therapeutic options[7].

A study has been conducted to determine the effectiveness of steroids for acute HBoV1 infection. In this study, steroids were given to hospitalized children who suffered from wheezing episodes, who were confirmed to be positive for HBoV1 infection[76]. However, the results showed no effect in reducing hospitalization time, symptom duration, or relapse rates. Therefore, use of corticosteroids in managing acute HBoV1infection has not shown efficacy[76].

A case report showed that the use of cidofovir, used for herpesvirus infections, resulted in the elimination of HBoV1 infection. This report highlights the possibility of using antivirals for gastroenteritis caused by HBoV1, and may eliminate HBoV1, though evidence remains unclear[77].

Supportive measures like bronchoscopic intervention can be effective approaches for HBoV1-associated plastic bronchitis[78,79].

Case studies show that the supportive care is effective for HBoV1 infection, as no targeted antiviral therapy is currently available. Fever was controlled with paracetamol and tepid sponging. Early supportive interventions, particularly oxygen therapy, fever management, and hydration are effective in stabilizing patients with HBoV1 infection and can lead to favourable outcomes[80].

PREVENTION

These findings highlight the urgent need for preventive actions such as vaccines. An experimental study using virus-like particles (VLPs) of HBoV1 and HBoV2 as vaccine candidates in mice demonstrates that HBoV1 VLPs successfully induced immune responses in mice. This has been characterized by the production of high-titre and high-avidity IgG antibodies. HBoV1 VLPs were capable of eliciting balanced T helper type 1/type 2 cellular immune responses. Additionally, cross-reactive antibody responses were observed between HBoV1 and HBoV2, with higher reactivity in HBoV1-immunized groups, suggesting that HBoV1 VP2 VLP-based vaccines represent a strong candidate for the prevention and management of HBoV1 infections. This can be used as a foundation for the development of a vaccine targeting HBoV1[81].

Accurate identification of the virus aids specialized care and raises awareness of its risks in respiratory infections. As demonstrated by Simon et al[30], the identification of HBoV in specimens obtained from the respiratory tracts of children did not necessitate a targeted antiviral treatment. However, it prevented the need for additional intravenous antibiotics, thereby mitigating potential side effects and minimizing unnecessary costs associated with antibacterial medications[30]. Furthermore, raising awareness about the significance of hospital environment disinfection and sterilization and standardizing procedures is crucial. Further investigation is warranted to better understand the interactions between HBoV and other pathogens in the future[31]. Research on this virus is crucial for comprehending respiratory illness and developing potential treatments or vaccines. Access to updated and precise information is essential to alleviate strain on healthcare systems and enhance patient care for ARIs.

CONCLUSION

HBoV1 is increasingly recognized as an emerging respiratory pathogen, commonly detected in young children and capable of causing illness ranging from mild respiratory symptoms to severe complications. Since no specific antiviral therapy exists, management relies mainly on supportive care.

Distinguishing true HBoV1 infection from co-infection or prolonged viral shedding remains essential to avoid misdiagnosis and unnecessary antibiotic use, emphasizing the importance of quantitative PCR, serological markers, and improved clinical interpretation.

Advances in molecular diagnostics, particularly multiplex PCR panels and metagenomic sequencing, have enhanced detection accuracy, while experimental virus-like particle vaccines have demonstrated strong immune responses in animal studies, offering a promising foundation for future preventive strategies against HBoV1.