Self-reported prevalence of childhood allergic diseases in three cities of China: a multicenter study

Over 25% of the world's population suffer from allergic diseases including asthma, allergic rhinitis, eczema, and drug reactions [1]. Allergic diseases have become one of the top three conditions demanding a major effort toward prevention and control in the 21^st century, according to the World Health Organization. In recent years, the prevalence of allergic diseases has increased annually throughout the world [2‐6], seriously affecting the quality of life of children, and creating a serious burden on both families and society. If not controlled, allergic diseases can be fatal [7]. Such conditions constitute a challenge for both public health organizations and healthcare providers [8].

In 1997, an ISAAC steering committee studied the prevalence of allergic diseases among children aged 13-14 years in 56 countries, and found that the prevalence of asthma in China was nearly 2.1%, but that in some Western countries including the UK and New Zealand was over 20% [9, 10]. Economic development and urbanization in China over the past have dramatically changed both the environmental and lifestyle features of children, but few data are available on the current status of allergic diseases in young Chinese. The aim of the present study was to determine the prevalence rates of these diseases in children from three major cities in China, to provide baseline information for clinicians and health policymakers.

Allergic diseases are very common in children worldwide [9]. A survey by the World Allergy Organization, covering 1.2 billion children, found that 250 million suffered from allergic diseases [11]. Such conditions may limit the ability of a child to play, to learn, and to sleep. Allergic conditions require potentially complex and expensive therapeutic intervention, and are thus associated with both direct and indirect costs (e.g., missed school and work days) [12‐14]. During the 1990s, the prevalence of childhood allergic diseases increased considerably in several countries [15‐18]. Such diseases are a major burden on affected children and their families, and are becoming a serious challenge to public health organizations and healthcare providers [8].

ISAAC conducted a multiphase cross-sectional study, beginning in 1991, to facilitate research into asthma, allergic rhinitis, and eczema, and to allow comparisons of the prevalence of allergic diseases between populations in different regions. ISAAC developed a standardized epidemiologic tool to measure the prevalence of various childhood allergic conditions [19, 20]. In 1997, the ISAAC steering committee investigated the prevalence rates of allergic diseases in children aged 13-14 years in 58 countries, and found that asthma prevalence differed 20- to 60-fold across countries [9]. Among the countries in the ISAAC study, the United Kingdom, Australia, New Zealand, and some other Western countries showed prevalence rates of asthma of more than 20%, and the rates of individuals who complained of symptoms of asthma in the 12 months prior to analysis attained 30% [10]; these rates were higher than the corresponding rates in other countries. In an earlier survey, the prevalence rates of asthma and rhinitis in children in Mainland China were 2.1% and 9.1%, respectively [10]. However, we found that the prevalence rates of asthma in Beijing, Chongqing, and Guangzhou were 3.15%, 7.45%, and 2.55% respectively; and the rates of rhinitis were 14.46%, 20.42%, and 7.22% respectively. In 1997, the prevalence rates of wheeze/exercise-induced wheeze and dry cough among children, at any time in the past year, were 1.4%, 6.6%, and 7.1%, respectively, in Mainland China. In our present survey of children in Beijing, Chongqing, and Guangzhou, the prevalence rates of wheeze were 5.54%, 3.48%, and 3.93%, respectively; the rates of exercise-induced wheeze were 6.80%, 7.14%, and 8.45%, respectively; and the rates of dry cough were 8.71%, 8.65%, and 8.79%, respectively.

Many factors, including genetic features, environmental influences, and social status, affect the prevalence of allergic diseases, making it very difficult to offer a comprehensive explanation of observed trends. Previous studies suggested that the observed worldwide increase in the prevalence of allergic diseases might be linked to lifestyle changes associated with urbanization [21]. For example, a population-based study in Mongolia found that the prevalence of allergic sensitization increased significantly as population density increased, being 13.6% in villages, 25.3% in rural towns, and 31.0% in a city [22] In the past few decades, the economy of China has gone through a period of unprecedented rapid development, leading to great changes in lifestyle, development of urbanization, and increasing "Westernization" of environmental factors [23]. Changes in maternal diet, smaller family size, the occurrence of fewer infections during infancy, lowered exposure to rural environments, and improvements in sanitation may partly explain the observed trend; however, further research is needed to achieve a full understanding.

In our survey, children in Chongqing showed significantly higher prevalence rates of asthma and allergic rhinitis compared to children living in Beijing or Guangzhou, consistent with the findings of a previous national epidemiological survey, conducted in 2000, of asthma prevalence in children aged 0-14 years [22]. This geographic variation may be part-explained by variations in environmental and climate factors [24]. Chongqing is in southwest China, where the humidity is relatively high, thus favoring the growth of mites that cause allergic disease [25]. Other social and environmental factors should be examined in future studies. We also found that the prevalence of asthma in males was significantly higher than in females in each city, indicating a gender-based difference in the prevalence of allergic diseases in children, in line with data from previous studies. It is probable that both hormonal changes with age, and genetic susceptibility, contribute to such differences [26]. Besides, boys, who tend to engage in more outdoor activities, are better protected against development of allergic diseases [27]. As the age of children increased, the prevalence rates of asthma, allergic rhinitis, and eczema showed different trends. The prevalence of asthma was low in both very young and older children. Allergic rhinitis was relatively uncommon in children aged less than 5 years. The prevalence of eczema gradually decreased as children grew. Such variations suggest that healthcare and governmental authorities may need to construct age-dependent preventative strategies to control allergic diseases in children.

In recent years, some scholars have proposed that asthma and allergic rhinitis share a pathological mechanism, and that the conditions are different (but closely allied) clinical manifestations of chronic inflammation of the upper and lower respiratory tracts [28-59]. In some Western counties, 30-90% of patients with asthma also have allergic rhinitis [31, 32], and the latter condition has been proposed to be an independent risk factor for asthma [30, 32]. In our survey, 49.54%, 50.14%, and 34.83% of asthmatic children suffered also from allergic rhinitis in Beijing, Chongqing, and Guangzhou, respectively. This observation is in line with the above suggestion. In addition, the proportions of asthmatic children who also had eczema, and those of asthmatic children who suffered from both allergic rhinitis and eczema, were high in all three cities, indicating that allergic diseases often occur in combination.

A common shortcoming of questionnaire-based studies is selection bias. In the present study, our subjects included children in kindergartens, primary and secondary schools, and those under kindergarten age. Sampling method is also a potential source of bias. In the present study, we measured child numbers in each age group, and randomly selected subjects with reference to these numbers, to ensure that our data was representative. One potential limitation of the survey is that rural-dwelling children were not included. Also, although we conducted our work in three widely separated cities--Beijing, Chongqing, and Guangzhou--differences in the risk factors for asthma/allergic rhinitis/eczema, and the reasons for variations in disease prevalence among the three cities, were not further investigated.

We used an ISAAC questionnaire to study the prevalence rates of allergic diseases in children aged 0-14 years living in three major cities in China. We found that the prevalence rates of such diseases were significantly higher than in previous surveys, and that the gap that previously existed in the prevalence of allergic diseases between China and Western countries is gradually narrowing. Environmental and lifestyle changes may have contributed to this trend. Further studies are necessary to discern the precise reasons explaining the observed trend. We also found that the prevalence rates of allergic diseases were relatively high in very young children. In addition, all of asthma, allergic rhinitis, and eczema, the principal allergic diseases in children, often coexist, and may thus mutually interact to increase the severity of each individual condition.