Prevalence of Childhood Asthma across Canada

Transcription

1 International Journal of Epldemtotogy International EpMemioioglcal Association 99 Vol. 23, No. Printed in Great Britain Prevalence of Childhood Asthma across Canada ROBERT E DALES, MARK RAIZENNE, SAWSAN EL-SAADANY, JEFF BROOK AND RICHARD BURNETT Dale R E (The Health Protection Branch, Hearth and Welfare Canada), Raizenne M, El-Saadany S, Brook J and Burnett R. Prevalence of childhood asthma across Canada. International Journal of Epidemiology 99; 23: Background. A large cross-sectional study provided an opportunity to estimate the prevalence of childhood asthma in several regions across Canada. Methods. In 9, approximately 000 questionnaires were distributed to the families of - year old children in 30 communities from the following six regions across Canada: the interior of British Columbia, southeastern Saskatchewan, southwestern, the central region of, southern Quebec, and the Maritimes (Nova Scotia and Prince Edward Island). These communities were free of point-source air pollutants and selected to represent a range of ambient sulphate concentrations. In all 9 questionnaires were returned representing an 3% response rate. Results. Currently present, physician-diagnosed asthma was reported for.7% of children by their parents. Persistent wheezing was reported for 3% and persistent cough for.9%. Asthma was most common in the two Maritime provinces (7.%), and least common in British Columbia (3.3%) and Quebec (3.%). Similar regional differences were seen for persistent cough, persistent wheeze, and also hospital separation rates for asthma which were approximately 00 per for the Maritimes and 39 per for British Columbia. Differences persisted despite adjustments for several host and environmental (indoor and outdoor) characteristics. Conclusions. The east coast of Canada may be an endemic area of asthma in Canada. If confirmed by objective measures of asthma, a detailed aetiologic investigation could enhance understanding of this phenomenon and the major environmental determinants of asthma morbidity in general. Little information exists on the prevalence of asthma in Canada. Although many epidemiological studies have been carried out in Canada, most by design have addressed unique populations such as adult workforces " 2 while others have been limited to geographical areas. 3 " Questionnaire-reported asthma prevalences were % among adults in 93 in Chilliwack, British Columbia, 3 % among Montreal children in 93, and % among adults and 9% among children in southwestern Alberta in 9. To study respiratory health and its relation to indoor air quality, questionnaires were administered in 9 to children, aged - years, residing in 30 communities in sixregionsspanning British Columbia to Prince Edward Island. " Regions were chosen to be free of point source pollutants which could influence respiratory health. The presentreportdescribes the prevalence of asthma in these regions. The Health Protection Branch, Health and Welfare Canada. Reprint request] to: Dr Robert Dales, Toxic Air Pollution Health Effects Section, Environmental and Occupational Toxicology Division, Health and Welfare Canada, Room 33, Tunney's Pasture, Ottawa,, Canada K. A 0L2. METHODS Characteristics of the Regions Six regions spanning the east coast to the western interior of Canada were chosen for study. Within each region, five communities each of inhabitants were selected. Theseregions,illustrated in Figure, were: the interior of British Columbia, southeastern Saskatchewan, southwestern, the central region of, southern Quebec, and the Maritimes (Nova Scotia, and Prince Edward Island). The regions were selected torepresenta gradient of exposure to sulphates, a long-range transported air pollutant. Communities within these regions were chosen to be free of point source air pollutants. Environmental and air quality characteristics, expressed as annual averages, were obtained from Environment Canada (Table I). 9 Characteristics of the Study Population - The study population has been described previously. Children aged - years who attended kindergarten to Grade 3 between March and April of 9 were given questionnaires at school to be completed by the parent or guardian most familiar with the child's health. TTie Questionnaire The questionnaire used for the present study was devel- 77

2 77 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY SCALE OF KILOMETERS FIGURE Map of Canada showing the location of the six selected regions: Maritime provinces (), Quebec (2), Central (3), Southwestern (), Saskatchewan (), and British Columbia () oped using respiratory symptoms/disorder questions, occasionally modified, from previously established questionnaires. 0 " 2 This questionnaire has shown a moderate to high degree of reliability as measured by test-retest repeatability. When administered twice over a 2-month period, percentage agreements were greater than 90% for the reports of persistent wheeze, persistent cough, and current asthma. 3 Evidence of validity has been provided by the ability of the questionnaire to demonstrate increased respiratory symptoms associated with active smoking, exposure to environmental tobacco smoke, reported allergies, and reported asthma. "* Asthma was considered present if there was a positive response to both; 'Has a doctor ever said this child had asthma?' and 'Does he or she still have asthma?' Symptoms of asthma which did not require medical interpretation or a physician's diagnosis were also used. Persistent wheeze was considered to be present when a child was reported to have wheezing most days or nights, or wheezing apart from colds, or attacks of shortness of breath with wheezing, but without doctor-diagnosed asthma. Persistent cough was considered present when a child was reported to usually cough for as much as 3 months of the year. Variables which may influence regional asthma prevalences were also obtained by this questionnaire. These included host and environmental factors. Host indicators were: age, sex and race of the child (Caucasian, other); parental education (some post-secondary education obtained by either parent, other); physiciandiagnosed childhood history of allergies (any, other); physician-diagnosed childhood history of an allergic reaction to dust or mould; and a physician-diagnosed allergy or asthma in the biological parent. Environmental indicators were the presence in the home of: smoking; dampness, water damage, or mould; furry or

3 CHILDHOOD ASTHMA IN CANADA 777 TABLE Regional estimates ofambient ozone, sulphate, temperature, and relative humidity Separation database. It was compared, by province, to the survey prevalence data. Region British Columbia Saskatchewan Central Southwestern Quebec Maritimes Ozone ppb Environmental factors* Sulphates Temperature Ug/m "C Humidity % "Dry bulb temperature ( Q and relative humidity (%) were 30-year averages measured at 300 h by the Canadian Climate Centre. Ozone (ppb) estimates represent daytime levels May-August, Sulphate (ug/m 3 ) estimates represent daily levels May-August, Ozone and sulphate data were obtained from estimations by Environment Canada using all available data sources. feathered pets; and crowding (number of occupants per room). Comparative Data Data on hospitalization rates for asthma and pulmonary diseases among children -9 years old, for 9-99, were obtained from the Statistics Canada Hospital Statistical Analysis Analysis was done using the Statistical Analysis System. Symptom prevalence can vary among communities within a region more than would be predicted by binomial variation alone. In such cases, the symptom indicators (asthma, persistent cough and wheeze) should not be treated as independent random variables. Thus standard statistical methods of analysing such data are not appropriate. One approach appropriate for this type of experimental design was suggested by Ware and Stram. 3 Using their methods, differences in symptom prevalence between the six regions were examined by a mixed effects logistic regression model for binary data after adjusting for the covariates listed in Table. These models were characterized by a random intercept on the logit scale for each community, with the covariates listed in Table and indicator variables for the six regions assumed to be fixed effects. This method yielded estimates of the log-odds of each of thefixedeffects and the variance-co variance matrix of these estimates, which incorporated the design effect due to clustering of symptoms within communities. A x 2 test with five degrees of freedom was used to examine the hypothesis that symptom prevalence differed by region. TABLE 2 Host and environmental characteristics of children - years old living in selected regions across Canada in 9 Host characteristic mean age, years (SD) %maie % Caucasian % any allergy % dust allergy % mould allergy % having a parent with some post-secondary education % having a parent with asthma % having a parent with at least one allergy Home environmental characteristic % household smoking % home dampness/mould %pets mean crowding index (SD) British Columbia (n = 2099) () (0.) Saskatchewan (n= 9) () (0.) Central (n = 0) () (0.) Region Southwestern (n = 229) () (0.) Quebec (n = 0) 7() (0.) Mantimes (n = ) 7() (0.) All Regions (n= ) () (0.)

4 77 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY 0- t Asthma a m a CO CD O c03 cd <D «- o 2- ID N d> a> JZ siste 0- JC O) 3. O e - o e- 0 ' * BC. SASK. C. ONT. SW. OUT. QUE. MAR. SASK. c. ONT. sw. orr. QUE. MAR. * B * BC. SASK. C. ONT. SW. ONT. OUE. MAR. Regions FIGURE 2 The prevalences of asthma, wheeze and cough, by region and community. Region is on the abscissa andprevalence percentages on the ordinate. The small squares represent the communities

5 CHILDHOOD ASTHMA IN CANADA 779 TABLE 3 Prevalences of physician-diagnosed current asthma and asthma symptoms by region for children - years of age in 9 Symptom Region British Columbia Saskatchewan Central Southwestern Quebec Man times Overall Current asthma Persistent cough Persistent wheeze * Adjusted ' * Adjusted Adjusted "Based on all observed responses. ""Prevalences adjusted for the variables listed in Table. c based on samples used for adjusting: all subjects with any missing values were eliminated. RESULTS In all 7 92 questionnaires were distributed and 9 (3.2%) were returned. Current physician-diagnosed asthma was reported for.7% of children while persistent wheezing was reported among 3%. While cough was reported among.9%, a respiratory illness keeping the child at home for at least 3 days was reported in.9% and ever having chronic bronchitis was reported among %. There were many differences in sociodemographic and indoor environmental characteristics between regions (Table 2). Quebec reported the highest parental education (%), and the lowest percentage of children's allergies (3%) and parents with a history of asthma and allergies (3% and % respectively). The greatest prevalences of parental asthma and allergies were found in central (% and 33% respectively) whereas the prevalence of childhood allergies was highest in Saskatchewan (27%). Home dampness or moulds was highest in the Maritimes (0%), household smoking and pets were most common in British Columbia (2% and 0% respectively). Figure 2 presents the prevalences of current asthma, wheeze, and cough for each community within a region. Communities within the same region generally appeared more similar than communities between regions, suggesting a clustering effect. This was confirmed by a two-level analysis of variance. Of the total variability in asthma between communities,.% was due to inter-regional differences and 3.% was due to intra-regional differences. Respectivefiguresfor cough were 7.2% and 2.%, and for wheeze were 7.% and 32.%. Table 3 presents both crude and adjusted prevalences ( 0).0(097).0(097).9( 003) 2(097) 2(097) 3.0(390).(097).(097) by region. After adjustment for all of the covariates listed in Table 2, the prevalences of each response varied by region (P < 0.00). Current asthma, persistent cough and wheeze were all more common in the Maritimes (Prince Edward Island and Nova Scotia) than all other regions combined (P < 0.0). The unadjusted prevalence of physician-diagnosed asthma in the Maritimes (7.%) was twice that of British Columbia (3.3%) and Quebec (3.%), and 0% greater than the average of all the regions studied (.7%). Inter-regional differences in asthma prevalence were not accounted for by differences in sociodemographic characteristics of the children or adult respondents, or indoor air characteristics. The adjusted symptom prevalences were very similar to the unadjusted values (within 2%). Adjustment for the many variables which could influence asthma (described in Questionnaire section), magnified the differences between the Maritimes (7.%) and British Columbia (2.3%). The adjusted odds ratio (9% confidence interval [CFJ) for asthma and the Maritimes (Maritimes, other regions) was 2.2 (.0-2.2). The association between asthma prevalence and individual regional outdoor environmental characteristics was examined after adjusting for the covariates listed in Table 2. Results are presented in Table. The odds ratio between relative humidity and asthma was relatively large at 3.0 (9% a:.-.02). When relative humidity was entered into a model with an indicator variable for the Maritimes (Maritimes, other) the odds ratio for the association between humidity and asthma was reduced to.2 (9% CI: ) but the Maritimes remained statistically significant at. (9% CI:.0-2.). A small positive relationship was observed with

6 70 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY TABLE Estimated odds ratio and 9% confidence intervals for asthma and outdoor environmental factors, adjustedfor covariates listed in Table 2 Environmental factor Temperature, C Relative humidity, % Ozone, ppb Sulphate, ug/m 3 Range used to determine odds ratio.9 Oddi ratio 9% confidence interval sulphates (odds ratio,.) but it was highly variable (9% CI: ). Figure 3 illustrates that asthma prevalences ranked by community survey data corresponded to prevalences ranked by provincial hospital separation data. Prince Edward Island and Nova Scotia had the highest rates of hospital admissions for asthma; Prince Edward Island also had the highest rate of hospital admissions for pulmonary disease. Similarly, British Columbia and Quebec had the lowest separation rates for asthma and pulmonary disease. DISCUSSION Among children - years old, the prevalence of physician-diagnosed current asthma across six regions and 30 commum'ties spanning Canada was.7%. This value, however, does not reflect the large inter-regional adjusted differences ranging from 2.3% in British Columbia to 7.% in the Maritimes. These observed differences in asthma prevalence are probably real based on the following arguments. Firstly, the selected communities were free of point source pollutants which could variably increase asthma symptoms and reporting. Secondly, differences in the distributions of community physicians' practices can theoretically account for variations in the diagnosis of asthma. Some physicians may recognize and diagnose asthma more frequently than others. In the present study however, chronic respiratory symptoms compatible with asthma, independent of physicians, such as a persistent cough or wheeze showed similar trends. Thirdly, hospital morbidity data also showed differences in asthma prevalences between provinces which were similar to the survey prevalences. What then could cause inter-regional differences in childhood asthma morbidity? Sociodemographic and indoor environmental indicators did not account for the elevated prevalence of asthma in the Maritimes. Genetic risk factors for allergy have been identified in several recent studies. Different patterns of early immigration and settlement could have caused regional differences in genetic predisposition to asthma and allergy. However, our adjustments for parental asthma and allergies, and childhood allergies, should have minimized a genetic effect. It has recently been recognized that acid aerosols can affect respiratory health. 7 In a recent assessment of the levels of strong particulate acid in Canada, the FIGURE 3 Asthma prevalences plotted against hospital separation rates for asthma by province

7 CHILDHOOD ASTHMA IN CANADA 7 Maritimes and southwestern were found to have the highest levels, with western Canada and Quebec experiencing the lowest levels. This may partly explain the higher illness rates in the Maritimes compared to other regions studied. A Canadian acid aerosol network was established in 99. We are currently in the process of estimating historical acid levels based on historical sulphate levels and current acid/sulphate ratios. The strong effect of regional humidity suggests that indoor air biologicals such as dust mites and mould products, not measured in the present study, may also contribute to the observed regional variations. The aetiologic importance of dust mite antigen is repeatedly being confirmed 9 " 20 whereas mould-related health effects are poorly understood at present. Their spatial distributions across Canada are unknown and warrant further study given our observations. In summary, we have presented prevalence data for asthma and chronic respiratory symptoms among children aged - years who live in relatively small communities across Canada. Inter-regional differences in asthma prevalence corresponded to inter-provincial differences in asthma admissions suggesting that these differences were not simply restricted to the particular communities studied. We recommend objective measures of asthma to confirm that asthma is endemic to east coast Canadian communities. If so, investigating regional differences in ambient acid aerosols and indoor biological would enhance the understanding of asthma morbidity in this population and others. ACKNOWLEDGEMENTS We thank Statistics Canada (Cyril Nair, Diane Mulder, Rene Poulin) for preparation of the hospital separation data, S Bartlett for statistical analysis, and L Lemieux for typing the manuscript. 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