The evidence on trigger factors for exacerbation of asthma comes from small case-controlled studies. Viruses appear to be a common trigger, whilst bacterial infections are a much less common cause [GINA, 2006; SIGN and BTS, 2011]:
- Small case-controlled studies in children have shown respiratory syncytial virus to be associated with wheezing in infancy. Rhinovirus (the cause of common cold) is the principal cause of wheezing and worsening asthma in older children and adults. Other viruses, such as parainfluenza and enterovirus, are also associated with worsening asthma symptoms.
- Small case-controlled studies have identified atypical bacteria (Chlamydia pneumoniae and Mycoplasma pneumoniae) as a much less frequent cause of worsening asthma, but their exact role remains uncertain.