Our results show that the respiratory morbidity of children with a previous history of severe bronchiolitis associated with viral coinfection is, in the first 6–9 years of life, significantly higher than that of children with a simple viral infection. This is suggested by the higher frequency of recurrent wheezing, symptoms in intercrisis periods, chronic asthma treatment requirement, number of hospitalizations due to a respiratory cause, and prevalence of current asthma in children with viral coinfection compared to simple infections.
Although the pathogenic mechanism is not well known and the possible causal relationship between infant bronchiolitis and the subsequent development of asthma is not yet well defined, there is unquestionable scientific evidence of the existence of an association between both entities. Many studies have reported this relationship, initially with RSV and later with other viruses such as hMPV, HBoV and especially HRV (8–11, 25), which, when identified in bronchiolitis, is associated with up to ten times the risk of developing asthma at 6 years of age (10, 26).
However, most of the large prospective studies have focused their analysis on single viral infections, and very few have analyzed the role of viral coinfections in the later development of recurrent wheezing and asthma.
Amat et al., (27), in a 3-year follow-up study of 154 children with a previous history of bronchiolitis (inpatient and outpatient), observed that 46.8% of them had been diagnosed with recurrent wheezing at age 3, identifying as risk factors only a family background of atopy and living in an apartment. In contrast, neither the type of virus identified, nor the single or multiple viral infections showed association with recurrent wheezing development. In contrast, RSV-HRV viral coinfection was independently associated with allergic sensitization at 3 years. In previous studies of atopy phenotypes, allergic sensitization was associated with an increased risk of later development of asthma in children (28–32). Lee et al. found that sensitization to outdoor allergens is associated with an increased risk of new-onset asthma and bronchial hyperresponsiveness (33). In addition, allergic sensitization is one of the major criteria of the modified Asthma Predictive Index (mAPI), used to predict asthma at 6, 8 and 11 years in children < 3 years with recurrent wheezing (34). Our own results show a 4-fold increased risk of asthma in children with allergic sensitization. Therefore, although Amat et al., (27) did not find association between viral coinfection and asthma at 3 years, the association of coinfections with allergic sensitization could be a predictor of asthma development in these coinfected children later in life.
The other study that evaluated the medium-term respiratory morbidity of viral coinfections is that of Petrarca et al., who, in a retrospective, follow-up telephone study at 36 months after admission for bronchiolitis, found no association between coinfection and recurrent wheezing, despite the higher family history of asthma in patients infected with a mixed infection (35). As in Amat´s study, the length of the follow-up period was shorter than ours and no data regarding asthmatic treatment or admissions for asthma were provided.
According to our results, the probability of developing asthma at age 6–9 years was almost twice as high in children with coinfection, who had an asthma prevalence of 31% compared to 18% in the group with single viral infections. Moreover, viral coinfection was an independent risk factor for the diagnosis of current asthma at school-age. In addition, patients with viral coinfection showed greater respiratory morbidity than patients with single infection, since they not only developed asthma more frequently, but the course was more severe, as demonstrated by the higher rate of admission for asthma, double that of children with single infection. Patients in the coinfection group also reported more symptoms in the intercrisis periods, the difference being almost significant (p = 0.06) and needed more frequent maintenance treatment corresponding to a higher level of asthma severity. All these data strongly suggest that coinfections are associated with increased respiratory morbidity, at least until 6–9 years of age.
Regarding maintenance treatment for asthma, Bergroth et al., (36) found that 45% of children previously admitted for bronchiolitis reported prescription of asthma control medication, mainly inhaled corticosteroids, within 48 months after hospitalization. The proportion of treated children was higher in patients with HRV (47%), than in those with RSV (15%) or with non-RSV/HRV bronchiolitis (26%). In our series, 45% of children required treatment coinfection being an independent risk factor that doubled the probability of receiving asthma maintenance treatment and tripled the probability of receiving the combination inhaled glucocorticoid/long-acting beta2 agonist (ICS/LABA). It should be noted that the combination ICS/LABA is usually prescribed as preferred initial treatment in step 4 of asthma treatment (37). Again, both, increased prescription of ICS and ICS/LABA suggest greater severity of asthma in children with a history of bronchiolitis associated with viral coinfection.
In a previous study by Bergroth et al., (38) HRV etiology was associated with more courses of systemic corticosteroids during the follow-up, implying a greater number of asthmatic exacerbations. Although the use systemic steroids was not among the variables in our study, we found a significantly higher rate of recurrent wheezing admissions in the coinfection group, whose treatment usually involves the administration of systemic steroids. Therefore, indirectly, we can also state that patients with viral coinfection (mainly RSV-HRV) require more admissions and receive more treatments with systemic steroids. These data strongly support once again that, in our study, viral coinfections were associated with greater clinical severity in the medium term.
Previous studies have analyzed the association between RSV bronchiolitis and pulmonary function sequelae, mainly obstructive airways disease with varying degrees of bronchodilator reversibility (6–10, 36, 39–41). However, there has been very little standardization between studies (42). A recent systematic review by Verwey et al. (42), including 31 studies, whose primary outcome was the evaluation of long-term pulmonary sequelae measured by pulmonary function test, in children with previously RSV respiratory infection during the first 3 years of life, found no association between RSV infection and abnormal pulmonary function in 13 studies, while 16 reported this association (42). Although abnormal measurements varied across studies, the most commonly described was an obstruction to airflow with or without bronchodilator reversibility. Most of our patients had normal lung function values at 6–9 years and no significant differences were found between the coinfection and the simple viral infection groups. Since in our study lung function was compared among children with severe bronchiolitis with coinfection and with simple infection rather than with a healthy control group, as is usual in other studies, it is not surprising that no difference in lung function was observed between the two groups.
The fraction of exhaled nitric oxide has been suggested as a non-invasive biomarker of eosinophilic inflammation (43). Although some authors found that FeNO measurements in infants with recurrent wheezing episodes were associated with persistence of wheezing through age 3 years (44), others like Mikalsen et al., (45) found no differences between 11-year-old children hospitalized for bronchiolitis and the control group. FeNO, in the study of Mikalsen et al., was associated with atopy, but not with asthma in both groups. Our results also showed no differences regarding FeNO levels between coinfection and single infection groups. By contrast, children with allergic sensitization had significantly higher FeNO values than not sensitized ones. No child with asthma but without allergic sensitization had elevated FeNO. These results, in line with those obtained by Mikalsen et al., (45) suggest that only children who develop atopic asthma have eosinophilic airway inflammation, translated by elevated levels of FeNO.
All our data suggest that bronchiolitis with viral coinfection is associated with increased respiratory morbidity in the medium term. The pathogenic mechanism is unknown but probably the different immune response triggered by double or multiple infections may play a role. Our group, in a previous study in 213 hospitalized infants with bronchiolitis, showed that RSV-HRV coinfected infants exhibited the highest levels of thymic stromal lymphopoietin (TSLP), that has been identified as a master switch for allergic inflammation and is an important cytokine in the development of allergic asthma. We also found that infants with dual RSV + HRV infection were 9 times more likely to have detectable nasal TSLP and this association was independent of other factors such as age or illness severity. These findings suggest that the immunological response in acute bronchiolitis is partly dependent on virus-specific factors and could partly explain the worse mid-term evolution of coinfections (46).
The main limitation of our study is the small number of bronchiolitis with single infections other than RSV or HRV, given the high prevalence of coinfections among some viruses like HBoV. The main strength is to be able to include a cohort of patients, included on admission for bronchiolitis, with clinical and virological data prospectively collected on admission.
In summary, having severe bronchiolitis with double or multiple positive viral detections is an independent risk factor for higher frequency and greater severity of asthma at 6–9 years. The early identification of viral etiology of severe bronchiolitis might facilitate the early prediction and treatment of asthma in school age.