Dictionary of Allergies .. Epidemiology of asthma

Asthma is a highly prevalent chronic respiratory disease affecting 300 million people world-wide. A significant fraction of the cost and morbidity of asthma derives from acute care for asthma exacerbations. In the United States alone, there are approximately 15 million outpatient visits, 2 million emergency room visits, and 500,000 hospitalizations each year for management of acute asthma. Common respiratory viruses, especially rhinoviruses, cause the majority of exacerbations in children and adults. Infection of airway epithelial cells with rhinovirus causes the release of pro-inflammatory cytokines and chemokines, as well as recruitment of inflammatory cells, particularly neutrophils, lymphocytes, and eosinophils.

The host response to viral infection is likely to influence susceptibility to asthma exacerbation. Having had at least one exacerbation is an important risk factor for recurrent exacerbations suggesting an 'exacerbation-prone' subset of asthmatics. Factors underlying the 'exacerbation-prone' phenotype are incompletely understood but include extrinsic factors: cigarette smoking, medication non-compliance, psychosocial factors, and co-morbidities such as gastroesophageal reflux disease, rhinosinusitis, obesity, and intolerance to non-steroidal anti-inflammatory medications; as well as intrinsic factors such as deficient epithelial cell production of the anti-viral type I interferons (IFN-alpha and IFN-beta)[3].The epidemiology of asthma is handicapped by the lack of a universally accepted definition of the disease (See Definitions of asthma,).

Airway hyperresponsiveness to a specific agent is a relatively simple condition to define, though abnormal values will inevitably be set arbitrarily. If this definition is taken, then we must include large numbers of the elderly whose hyperresponsiveness is due to narrowed airways, most often a result of smoking over a prolonged period. Atopy is a strong risk factor for "asthma". In addition to genes which determine atopic status, studies in families showing a greater prevalence of hyperresponsiveness in first degree relatives of asthmatic patients suggest evidence that response to cholinergic agents is udner genetic control. In rat experiments, several strains have been produced exhibiting an abnormal sensitivity to acetylcholine and serotonin as determined by a separate recessive gene. Other factors which enter into the epidemiology of asthma include persistent exposure to allergens leading to increased airways responsiveness and sudden increases in aeroallergens resulting in asthma "epidemics". The effect of dietary sodium on the increase in bronchial response to histamine in atopic men but not in women is a factor not fully explained.

Viral infection including vaccinations increase responsiveness, especially in children. The role of air pollution in asthma is controversial. High concentrations of aeroallergen may produce serious "epidemics" and passive smoking is associated with increased use of services by asthmatic children. Experimental studies suggest that exercising asthmatics are more sensitive to SO2, equally sensitive to O3, and give contradictory results for NO2. The effects of air pollution will however, be modified by the use of medication and changes in behaviour  when pollution levels are high. On the current evidence it is difficult to estimate the effect of air pollution on morbidity from asthma, but it cannot be assumed that it makes a large contribution. The prevalence of asthma is currently increasing, probably due to an unexplained increase in atopic disease. This trend has led to a tendency for death rates to rise in many countries and for greater hospitalization rates, particularly in younger age groups where the increase is most marked.

The prevalence of allergic diseases appears to have been increasing in recent years. The hospitalization rate of asthma in children showed an increasing trend. House dust mites and cockroaches are the two most common indoor aeroallergens in Taiwan. Various kinds of inhaled corticosteroids or combination medications are available, but in clinical practice these have not been used as much as oral beta-2 agonists. Generally 68% of the physicians would follow the asthma treatment guidelines. Because of the comprehensive health care insurance system, the majority of the population in Taiwan can afford the medical expense of diseases. The country's expenditure of asthma care is around USD 83.1 millions per year which is increasing by the year as well. In clinical aspects, asthma education should still be included as part of its treatment.

References

1. Burney, G.J., P.: Epidemiology of asthma. In: Epidemiology of Allergic Diseases. Symposium 5. In the Annual Meeting of the EAACI, Zűrich, Switzerland, May 25-29, 1991.

2.. Yeh KW, Chiang LC, Huang JL. Epidemiology and current status of asthma and associated allergic diseases in Taiwan- ARIA Asia-Pacific Workshop report. Asian Pac J Allergy Immunol. 2008 Dec;26(4):257-64.

3. Dougherty RH, Fahy JV. Acute exacerbations of asthma: epidemiology, biology and the exacerbation-prone phenotype. Clin Exp Allergy. 2009 Feb;39(2):193-202