In 2005, a previously unknown DNA virus was identified in nasopharyngeal specimens from children with respiratory tract infection.1 Researchers used random polymerase chain reaction (PCR) amplification and high-throughput sequencing methods specifically developed for detection of unknown viral sequences. Analysis of the recovered gene sequences showed resemblance to bovine and canine minute parvoviruses, and the virus was named human bocavirus (HBoV). Later, 3 other HBoV were identified in stool and named HBoV 2, 3 and 4.2,3 Disease associations of HBoV are not entirely clear, but recent studies provide evidence that HBoV1 causes pneumonia and other respiratory tract diseases, in particular during primary infections.4
VIRUS STRUCTURE AND BIOLOGICAL PROPERTIES
HBoV is a small DNA virus with a nonenveloped icosahedral capsid similar to other Parvoviridae.5 The 5 kb linear and single-stranded genome is organized in 3 open reading frames that encode 2 forms of the nonstructural protein NS1, a nuclear phosphoprotein NP1, and 2 structural capsid proteins, VP1 and VP2.6 HBoV types 2–4 have similar genomic organizations as HBoV1 and 10%–30% sequence dissimilarities.3
Replication mechanisms of HBoV and the pathogenesis of HBoV infections are poorly characterized. This is largely because no animal model is available and tissue culture of HBoV is difficult, although it has been cultured in primary respiratory epithelial cells.7 The primary replication site of HBoV1 appears to be the respiratory tract, where it has been detected most frequently and in highest copy numbers. HBoV1 can be found also in serum, pointing to a systemic spread.8,9 Viral copy numbers of HBoV1 in stool are low. On the contrary, HBoV types 2–4 have been detected predominantly in stool, but the host cell types are not known.2,3,10
HBoV1 has been detected for up to 6 months in serial nasopharyngeal samples.11,12 Prolonged replication or passive persistence may account for the frequent presence of HBoV1 in asymptomatic children. HBoV1 often is present in samples together with another respiratory virus, which might suggest reactivation of a latent virus by a superinfection. However, there is no documented evidence of establishment of a persistent, latent state by the HBoV.
Globally the prevalence of HBoV1 DNA in young children with respiratory tract infections is around 10%, in some studies up to 33%.4 It occurs year-round, but most commonly in the winter. HBoV1 is more frequently detected in young children (<2 years of age) than in older children or adults. Limited knowledge of transmission, persistence, establishment of latency, reinfections and reactivations cause uncertainties regarding the epidemiology of HBoV1. Of the enteric bocavirus types, HBoV2 is the most prevalent with detection rates of up to 26% in stool samples from children, and 4% from adults.2,3 DNA of HBoV3 and HBoV4 has been detected in <5% of stool samples.
Seroepidemiologic studies have documented that most children have IgG antibodies against HBoV1 by school age.13,14 Differentiation between seroresponses against HBoV types 1 to 4 is, however, difficult because of cross-reactivity.15
Many studies have reported an association between a respiratory tract infection and HBoV1 detected by PCR in the nasopharynx. Clinical manifestations have ranged from mild upper respiratory tract infections to severe pneumonia. However, because of insufficient diagnostic methods, selected patient populations and lack of control groups, the majority of studies are of limited value. The pathogenic role of HBoV1 has been challenged by documentation of other viruses in the same samples (up to 90%) and detection of bocavirus also in asymptomatic individuals (up to 44%).4 In a study of children in day-care centers, 33% of those with respiratory symptoms and 44% of those without symptoms were positive for HBoV1 DNA.11 Furthermore, 70% of the HBoV1 DNA positive children with symptoms were positive also for another respiratory virus, most commonly human rhinovirus. The mere presence of HBoV1 DNA in the nasopharynx is, therefore, not a sufficient evidence of an acute HBoV1 infection and cannot be used for estimating the clinical impact of this virus. Recently, more solid data for the case that HBoV1 can cause disease have been provided by use of PCR in serum and by serology. In a study of wheezing children, 45 of 49 (92%) with HBoV1 DNA in serum had a serologic diagnosis as defined by positive IgM, IgG seroconversion or an >4-fold increase in IgG, whereas 2 of 15 children (13%) with HBoV1 DNA only in their nasopharyngeal samples had serologically confirmed diagnoses.16
Studies using quantitative PCR and serology associate bocavirus with wheezing illnesses and pneumonia. Only a few studies have used serum bocavirus PCR in a study setting with comparison groups without respiratory tract infection. In one such study, detection of HBoV1 DNA in serum was associated with lower respiratory tract illnesses and pneumonia.17A serological follow-up study of 109 children from infancy to early adolescence compared the clinical events during the sampling intervals when seroconversion occurred with the next and prior intervals, and found an association between primary HBoV1 infection and respiratory tract illnesses including acute otitis media.14 Type 2 bocavirus has been detected in stool in 3%–25% of children with gastroenteritis, but often with another enteric virus.2,18,19 HBoV2 has been found also in stool of healthy individuals, and any association with gastroenteritis is weak. Taken together, there is substantial amount of data linking HBoV1 with upper and lower respiratory tract infections, some data linking HBoV2 with gastroenteritis and very few data linking HBoV3 or HBoV4 with any clinical illness. Studies with robust diagnostic methods in controlled populations would be needed to increase knowledge of the clinical impact of bocavirus in children and adults.
HBoV1 infections cannot be clinically differentiated from other viral respiratory infections. Bocavirus isolation in tissue culture is not available for diagnostic use. HBoV can be readily detected by PCR targeting NS, NP or VP genes, and it is included in several commercially available multiplex respiratory virus PCR panels. Type-specific primers or nonspecific primers followed by sequencing of the PCR product can be used for the differentiation between HBoV types. Quantitative PCR may be useful for judging the clinical significance of bocavirus DNA detection, as higher viral loads correlate with acute infections, fewer coinfections and increased disease severity.8,16,20 Serological methods have been developed to detect bocavirus specific IgM and IgG antibodies by utilizing recombinant capsid antigens or viral-like particles.13,16,17,20,21 Past-immunity antibodies toward HBoV2 to 4 cross-react with HBoV1. To reliably detect seroresponses to HBoV1, this should be corrected by depletion of HBoV2–4 reactive antibodies.15
Acute HBoV1 infection is most reliably diagnosed by detection of DNA in serum by PCR and in respiratory tract samples by quantitative PCR, simultaneously with detection of IgM or a diagnostic IgG response in paired serum samples.16 The value of a mere positive PCR result in nasopharyngeal sample is questionable, but very high viral copy numbers (>104 HBoV1 genomes/mL of nasopharyngeal aspirate) may indicate current illness.9 HBoV2–4 viruses can be detected by PCR in stool and by serology, but correlation of virus detection with illness has not been established.
TREATMENT AND PREVENTION
No specific antiviral treatment or prevention by immunization has been reported. Currently, treatment is supportive and directed by the clinical manifestations. Standard precautions should be applied to limit the transmission of HBoV1 by respiratory secretions.
HBoV1 is, according to currently available information, an important causative agent of respiratory tract infections in young children. However, the disease burden caused by HBoV1 is not known yet. Diagnosis of HBoV1 infections needs critical approach and desirably combination of PCR and serological methods.
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