Bronchiolitis is the most frequent disease affecting children of age < 2 years, and it is a leading cause of hospital admissions in this age group. In this study, we conducted a retrospective investigation of 842 children hospitalized with bronchiolitis in order to identify the distribution of pathogens and co-infection. We found that 83.7% of the children had a single pathogen infection, whereas 16.3% had co-infections.
The most common pathogens identify in our study were RSV (53.5%), followed by MP (18.8%) and HRV (17.3%). A longitudinal prospective investigation conducted in the USA revealed that RSV was the causative pathogen for bronchiolitis in 76% of infants; this percentage is much higher than that observed in our study; however HRV was isolated in 18% of the their cases, which is similar to the rate of HRV infection noted in our study.12 In an 11-year study in Spain, 62.7% of the patients (children ≤ 2 years of age with acute bronchiolitis) had RSV infections, which is a percentage slightly higher than that noted in our study.13 In a similar retrospective Slovenian study of children under 2 years with bronchiolitis, RSV (57.5%), HRV (25.6%), and HBoV (18.4%) were identified as the most common pathogenic viruses;14 their results were similar to ours in the case of RSV but higher in the case of HRV. Further, in our study, MP was isolated in 18.8% of our patients, which is similar to the percentage (17.2%) reported in a previous study.15
The current study indicated that a single viral infection (72.9%) was most common type of infection in children under 2 years of age with bronchiolitis. We noted that RSV was the most common virus isolated, especially in infants under 6 months of age. We also noted that the percentage of RSV infection gradually decreased with age, which suggests that younger infants are more vulnerable to RSV disease; this is consistent with the findings of previous studies.16,17 A retrospective cohort study indicated that the reduced exposure of pregnant women to RSV epidemic contributed to more severe RSV-induced bronchiolitis in children under 6 months of age.18 Therefore, RSV-induced bronchiolitis is common in 6 months age; this may be associated with the circulation of antibodies that are not associated with RSV infection during pregnancy. Further investigations are necessary to identify specific susceptibility factors.
One hundred and thirty seven (16.3%) of our patients had infection due to two pathogens. Interestingly, the distribution of multiple viral coinfection and viral-bacterial infection was similar. The probability of co-infection in children ≤ 6 months old was significantly lower than that in children aged between 6 months and ≤ 2 years of age. A prospective study from Turkey identified that the rate of coinfection among children with acute bronchiolitis was 34.2%, which is higher than the percentage observed in our study.19 Similarly, a study from Israel showed that the rate of co-infectin in infants with acute bronchiolitis was nearly 40%, which is also higher than that in our study.20 A UK study reported an even higher percentage of 46%.21 The discrepancies in the rate of co-infection in bronchiolitis may be attributed to differences in the pathogen detection methods and the type of pathogens isolated.
The impact of co-infection on the severity of bronchiolitis still remains questionable. A comprehensive review in London revealed that multiple viral infection was associated with the admission of infants to the pediatric intensive care unit for the management of severe bronchiolitis.22 In contrast, some studies have shown that there is no correlation between the presence of multiple co-infections and severity of bronchiolitis.23 A Taiwanese study also reported that different viral pathogens do not give rise to different clinical characteristics among children with bronchiolitis.24 However, a Brazilian study revealed that both co-infection and RSV load influenced the prognosis of acute bronchiolitis in infants.25 Our findings in this study indicated that the duration of symptoms and duration of hospitalization in cases of simple virus infection were significantly less than those observed in case of combined viral and MP infection. Further, simple virus infection was least likely to induce fever. Thus, we believe that co-infection can aggravate the disease. Further studies are necessary to confirm these associations.
Some of the most common reasons for admission due to bronchiolitis are hypoxia, requirement for supplemental oxygen, poor feeding, and respiratory disease.2,26,27 Small airway obstruction and the resultant mucus plugs and edema are believed to play a crucial role in the pathogenesis of bronchiolitis. In our study, the requirement of oxygen was most frequent among patients having co-infection with both viral pathogens and MP and least frequent among those with simple MP infection. Studies have shown that the high incidence of hypoxia associated with RSV infection may be predictive of a poorer outcome,28 which is consistent with our findings.