HBoV was the second most commonly detected virus in our prospective, longitudinal study of children in daycare and was detected in 33% of respiratory illnesses. The virus was frequently redetected in individual children over extended periods of time. Our study calls into question the role of HBoV as a respiratory pathogen, in light of the high frequency of codetection of other viruses, the detection of asymptomatic carriage, the lack of correlation with illness onset, and the lack of correlation of viral load with severity of illness.
Coinfections with respiratory viruses were present in 72% of HBoV+ illnesses. This estimate is in range with previous findings of up to 83%.[16
] The high percentage of HBoV detections that are concurrent with detections of known respiratory viruses calls into question the specificity of observed associations between HBoV and disease. We also found that HBoV detections did not have a distinct seasonal pattern when compared to the overall distribution of all respiratory illnesses over two years, in contrast to other reports.[8
] This pattern indicates that HBoV may be detected with a relatively consistent prevalence among respiratory illnesses caused by other viruses.
Our HBoV prevalence in asymptomatic samples was surprisingly high, and was very similar to a Canadian study that reported HBoV in 43 (43%) of 100 asymptomatic children.[20
] Von Linstow et al has also reported a HBoV rate in samples from asymptomatic children above that found in symptomatic children[8
]. We did not find a difference in HBoV prevalence between asymptomatic samples and the next illness sample within each child. This analysis design has the advantage of allowing for the control of confounding by individual factors such as age. Several studies have evaluated the association between HBoV and illness with comparisons to asymptomatic control groups and found a low prevalence of HBoV in children without respiratory illness symptoms.[16
] Our results contradict these findings.
HBoV detection did not consistently correspond with the onset of respiratory illness. One-fifth of HBoV+ illnesses had an HBoV- swab at illness onset. We identified HBoV infections that spanned the incidence and resolution of multiple respiratory illnesses. Several HBoV infections persisted for an extended period of time. We documented shedding for up to 75 days with regular sampling. This reflects reports of extended shedding for at least 2 months in 32% of children with HBoV+ respiratory illness who underwent monthly testing[8
]. We also described extended shedding events that spanned the onset and resolution of multiple consecutive respiratory illnesses. The persistence of HBoV shedding beyond the resolution of single respiratory illnesses illustrates the difficulty in correlating an isolated HBoV+ result with an incident illness in a cross-sectional study. Given that we have demonstrated that multiple HBoV events separated by HBoV- swabs are possible in individual children, it is possible that persistent HBoV detections reported here and elsewhere[8
] may be due to repeated infections rather than a single extended period of shedding. More frequent sample collection is needed to investigate this fully.
Viral load was not significantly lower during asymptomatic periods compared to periods of respiratory illness, and we did not see increased illness severity with increased HBoV viral load. We found increased viral load among single HBoV infections among individual specimens, similar to other reports [17
] but this comparison was not statistically significant at the illness level. Other groups have been unable to establish a link between HBoV viral load and disease severity as well.[26
While we did not identify any differences in reported symptoms between HBoV+ and HBoV- illnesses, HBoV did appear to contribute to the severity of respiratory illness. Illnesses with HBoV detection, alone or in combination with other viruses, were more likely to last >7 days and specifically were more likely to have cough present for >7 days. HBoV infections with a viral load of at least 10,000 copies/mL were also associated with increased visits to a health care provider. These findings indicate that HBoV may exacerbate respiratory illnesses caused by other pathogens, or that more severe illness may initiate HBoV shedding events.
The identification of HBoV shedding in the stool of children with vomiting and diarrhea [29
] has led to speculation that HBoV may be a causative agent of enteric disease. [6
] We did not find any association between HBoV detection and vomiting or diarrhea as reported by parental diaries and interviews. However, our study was designed for surveillance of respiratory diseases, and illnesses with vomiting or diarrhea without respiratory symptoms may have been missed.
Our rate of HBoV detection in respiratory illness is higher than other published estimates of 2.7% to 21.7%.[30
] In a more direct comparison to published cross-sectional prevalence estimates, HBoV prevalence in our study among first incident respiratory illnesses was 35%. Notably, over one-fifth of the HBoV+ illnesses did not have detectable HBoV until one week after illness onset. Studies that only collect a sample at illness onset may have missed these positive detections. To our knowledge, this study is the first longitudinal study to test for HBoV in a daycare setting. We evaluated all respiratory illnesses, including mild illnesses that are often missed by studies of hospital or primary care patients. It is possible that our high rate may be due to the very young age of our study participants (6 weeks to 24 months at enrollment). We do not know if this high prevalence is universally reflected in young children in other regions or not attending daycare.
Our reported duration of shedding may be an underestimate. HBoV testing was performed retrospectively, and weekly swabbing for respiratory viruses may have been discontinued before the resolution of the HBoV shedding. Secondly, testing began at the start of respiratory symptoms. We would have missed the onset of the HBoV infection if it preceded the start of illness. Our HBoV incidence may also be an underestimate if respiratory illnesses were missed during the surveillance by study staff and parents. However, we believe that very few illnesses were missed, due to the regular contact with the daycare providers and the on site study nurse who promptly identified illnesses. By focusing our study on the daycare population, we may have missed primary HBoV infections in very young children. The symptoms of primary HBoV infection, if any, are unknown and merit further study in young infants.
Few studies have longitudinally studied HBoV in children with mild illness symptoms. [8
] To our knowledge, no longitudinal studies to date have utilized weekly sampling during respiratory illnesses and paired analysis to compare asymptomatic to symptomatic periods. HBoV was not independently associated with particular respiratory symptoms; however our findings indicate that HBoV shedding may increase the duration of respiratory symptoms caused by other pathogens. We also documented long periods of persistent HBoV shedding, independent of the onset and resolution of respiratory illness. Overall, detection of HBoV was not associated with the presence of respiratory illness or with specific respiratory symptoms in this prospective study of infants and toddlers attending center-based daycare.