Allergens and Bacteria in Basophil Activation
As stated before, respiratory tract infections caused by viruses have been shown to be epidemiologically associated with asthma in the following ways: first, viruses associated with infantile wheezing have been theorized to lead to the inception of the asthmatic phenotype; second, infants who experienced severe viral respiratory infections are more likely to develop asthma later in childhood.
Several studies, generally based on the recently described multiplex molecular biology-based techniques, have observed a relationship between various viral infections and asthma exacerbations.
Among respiratory viruses, human rhinovirus (HRV) infections preceded as many as 50% of asthma exacerbations in children. However, other authors were unable to detect any difference in microbial pattern between patients with asthma exacerbations and 'nonasthmatic' patients. In this context, three updated reviews describing the present features of the infection of asthmatic patients are now available. Of note, HRV infections seem to aggravate allergic asthma. In addition, the intercellular adhesion molecule-1, which is variably expressed on respiratory epithelia during allergic inflammation, is the major receptor for human rhinoviruses. Very recently, it has been shown that allergic sensitization precedes HRV wheezing but the opposite is not true.
INV also accounted for substantial morbidity (up to 200 annual outpatient visits per 1000 children) in children with asthma and other chronic conditions. Nevertheless, a meta-analysis failed to support the protective effect of influenza vaccination in asthma exacerbations.
PIV have been identified as pathogens potentially involved in asthma exacerbation since the original observations in 1995. From that moment, a number of recent works have confirmed their role.
It has been demonstrated that 94% of children with steroid-resistant asthma had detectable adenovirus antigens compared with 0% of controls. In adults, both with and without asthma, evidence of adenoviral infection has been observed to be as high as 50% of the individuals tested. Nevertheless, the virtual absence of any new report on adenoviruses infections in asthma exacerbation in the recent bibliography may indicate either a well known role of this pathogen in at-risk patients or the absence of any significant association detectable using molecular biology tests.
Newly discovered respiratory viruses such as human metapneumoviruses may also play a role. Further studies are necessary to define the actual role of these viruses that, in clinical experience, are slowly but continuously spreading in pediatric population.
A more recently described parvovirus called human bocaviruses was identified in children with lower RTIs, but its relation to asthma exacerbation is uncertain.
Respiratory viruses, such as human coronaviruses (CoV), have been described in asthma exacerbation. A similar finding was described in the first work on this topic. Even for CoV, the use of large cohorts of patients and highly sensitive and specific assays is needed to better define the actual role.
RSV is another major pathogen causing asthma exacerbation. RSV is virtually the most frequent virus isolated in small patients with bronchiolitis, wherein bronchospasm is frequent. Of note, RSV was used in a recent set of experiments that showed that, in mice, an early exposure to an infectious agent might be responsible for asthma in the following months.
Pathogens Involved in Asthma Exacerbation
As stated before, respiratory tract infections caused by viruses have been shown to be epidemiologically associated with asthma in the following ways: first, viruses associated with infantile wheezing have been theorized to lead to the inception of the asthmatic phenotype; second, infants who experienced severe viral respiratory infections are more likely to develop asthma later in childhood.
Virus Infections
Several studies, generally based on the recently described multiplex molecular biology-based techniques, have observed a relationship between various viral infections and asthma exacerbations.
Human Rhinoviruses
Among respiratory viruses, human rhinovirus (HRV) infections preceded as many as 50% of asthma exacerbations in children. However, other authors were unable to detect any difference in microbial pattern between patients with asthma exacerbations and 'nonasthmatic' patients. In this context, three updated reviews describing the present features of the infection of asthmatic patients are now available. Of note, HRV infections seem to aggravate allergic asthma. In addition, the intercellular adhesion molecule-1, which is variably expressed on respiratory epithelia during allergic inflammation, is the major receptor for human rhinoviruses. Very recently, it has been shown that allergic sensitization precedes HRV wheezing but the opposite is not true.
Influenza Viruses
INV also accounted for substantial morbidity (up to 200 annual outpatient visits per 1000 children) in children with asthma and other chronic conditions. Nevertheless, a meta-analysis failed to support the protective effect of influenza vaccination in asthma exacerbations.
Parainfluenza Viruses
PIV have been identified as pathogens potentially involved in asthma exacerbation since the original observations in 1995. From that moment, a number of recent works have confirmed their role.
Adenoviruses
It has been demonstrated that 94% of children with steroid-resistant asthma had detectable adenovirus antigens compared with 0% of controls. In adults, both with and without asthma, evidence of adenoviral infection has been observed to be as high as 50% of the individuals tested. Nevertheless, the virtual absence of any new report on adenoviruses infections in asthma exacerbation in the recent bibliography may indicate either a well known role of this pathogen in at-risk patients or the absence of any significant association detectable using molecular biology tests.
Metapneumoviruses
Newly discovered respiratory viruses such as human metapneumoviruses may also play a role. Further studies are necessary to define the actual role of these viruses that, in clinical experience, are slowly but continuously spreading in pediatric population.
Bocaviruses
A more recently described parvovirus called human bocaviruses was identified in children with lower RTIs, but its relation to asthma exacerbation is uncertain.
Coronaviruses
Respiratory viruses, such as human coronaviruses (CoV), have been described in asthma exacerbation. A similar finding was described in the first work on this topic. Even for CoV, the use of large cohorts of patients and highly sensitive and specific assays is needed to better define the actual role.
Respiratory Syncytial Virus
RSV is another major pathogen causing asthma exacerbation. RSV is virtually the most frequent virus isolated in small patients with bronchiolitis, wherein bronchospasm is frequent. Of note, RSV was used in a recent set of experiments that showed that, in mice, an early exposure to an infectious agent might be responsible for asthma in the following months.
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