incidence rates of asthma, rhinitis and eczema symptoms and influential factors in young children in...

Acta Pædiatrica ISSN 0803–5253

REGULAR ARTICLE

Incidence rates of asthma, rhinitis and eczema symptoms and influentialfactors in young children in SwedenM Larsson ([email protected])1, L Hagerhed-Engman2, T Sigsgaard3, S Janson1, J Sundell4,5, C-G Bornehag1,2,4

1.Karlstad University, Health and Environmental Sciences, Karlstad, Sweden2.SP Technical Research Institute of Sweden, Boras, Sweden3.Institute of Public Health, Aarhus University, Department of Environmental and Occupational Medicine, Denmark4.Technical University of Denmark, International Centre for Indoor Environment and Energy, Denmark5.University of Texas at Tyler, Texas, USA

KeywordsAsthma/allergy, Baseline group, Children,Incidence, Influential factors

CorrespondenceM Larsson, Karlstad University, Health andEnvironmental Sciences, S-651 88 Karlstad,Sweden.Tel: +46-54-700-25-34 |Fax: +46-54-700-22-20 |Email: [email protected]

Received15 November 2007; revised 22 April 2008;accepted 21 May 2008.

DOI:10.1111/j.1651-2227.2008.00910.x

AbstractAim: To estimate the incidence rates for asthma, rhinitis and eczema symptoms and to investigate

the importance of different influential factors for the incidence of these symptoms.

Methods: The Dampness in Building and Health study commenced in the year 2000 in Varmland,

Sweden with a parental questionnaire based on an ISAAC protocol to all children in the age of 1–

6 years. Five years later a follow-up questionnaire was sent to the children that were 1–3 years at

baseline. In total, 4779 children (response rate = 73%) participated in both surveys and constitute

the study population in this cohort study.

Results: The 5-year incidence of doctor-diagnosed asthma was 4.9% (95% CI 4.3–5.3), rhinitis was

5.7% (5.0–6.4) and eczema was 13.4% (12.3–14.5). However, incidence rates strongly depend on

the health status of the baseline population. Risk factors for incident asthma were male gender and

short period of breast-feeding. Allergic symptoms in parents were also a strong risk factor for incident

asthma, as well as for rhinitis and eczema.

Conclusion: When comparing incident rates of asthma between different studies it is important to realize that

different definitions of the healthy baseline population will give rise to different incident rates.

INTRODUCTIONAsthma and allergy affect up to 30–40% of the children inSweden (1). Recent data report incidence rates for child-hood asthma in the range of 0.9–1.6 children/100 children/year (percent/year) in Sweden (2–5). Regarding rhinitis andeczema there is practically no valid information on inci-dence rates in childhood. It is difficult to compare the inci-dence of asthma between studies when baseline definitionsas well as the outcome definitions are different (6). The firstperiod of life is suggested to play a critical role for the de-velopment of asthma and allergies (7,8). Specifically, familyhistory of allergy, a low birth weight and parental (mother’s)smoking are important risk factors for the development orworsening of childhood asthma (2,8–10). Breast-feeding isreported to be a protective factor in some studies (7,8,11),but a suggested risk factor in other studies (12,13). Risk fac-tors have generally been estimated by association in cross-sectional studies (2,4).

The objective of this study has been to estimate the inci-dence rates for asthma, rhinitis and eczema symptoms dur-ing a 5-year period among 4779 children, aged 1–3 years atbaseline, in a rural area of Sweden. Another objective was toinvestigate whether the baseline health status and the def-inition of the health outcome affect the incidence rate andthe importance of different influential factors for incidentasthma, rhinitis and eczema.

MATERIAL AND METHODSWith the aim of identifying health-relevant exposures inhomes an epidemiological study ‘Dampness in Buildingsand Health’ (DBH) started in the year 2000 in Sweden. Thehealth focus of the study was on asthma and allergies amongsmall children and their parents. The DBH study is dividedinto three phases, which are described elsewhere (14). Thisstudy is based on a 5-year follow-up questionnaire study ofthe first phase of DBH-study (2000), carried out in March2005.

Study populationIn DBH-I, the parents of 14 077 preschool children aged1–6 years were invited by a postal questionnaire. Of thestudy population, 100 randomly selected children partici-pated in a pilot study for development of the questionnaire.The response rate for the baseline questionnaire in 2000was 79% (n = 10 851). The nonresponders’ analyses couldnot find any obvious indications for selection bias (14). Inthe follow-up questionnaire study 2005 (DBH-III), the threeyoungest year cohorts in DBH-I were included (n = 7509)(6–8 years). There were 5483 responses from children cor-responding to a response rate of 73%. Of these, 4779 chil-dren participated in both the surveys (DBH-I and DBH-III)and therefore constitute the study population in this cohortstudy. The children are described in Table 1. There is always

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Larsson et al. Incidence rates of asthma and allergy in children

Table 1 Description of the 4779 children

n %

SexGirls 2388 50.0Boys 2391 50.0

Age6–7 years 1557 32.67–8 years 1568 32.88–9 years 1654 34.6

Number of children in the family1 2163 45.82 2262 48.0>2 291 6.2

Allergic symptoms in familyNone 2465 51.6Only father allergy 880 18.4Only mother allergy 944 19.8Both parents 490 10.2

Type of dwellingSingle family house 3473 74.5Attached dwelling 398 8.5Multifamily house 793 17.0

Smoking in familyYes 1044 21.8No 3735 78.2

Note: n varies due to missing data.

a risk for selection bias when participants select themselvesfor a study, as in this study. However, since the responserate was quite high both in the baseline and in the follow-upstudy (i.e. 79% and 73%, respectively), the risk for selectionbias is limited. (Tables S1–S3 ).

QuestionnaireQuestions on symptoms in airways, nose, eyes and skinamong children in DBH-I and DBH-III, were the same asin the ISAAC Study (15). In addition, questions on doctor-diagnosed asthma and doctor-diagnosed rhinitis were in-cluded. Exposure data and influential factors were also col-lected (Tables S1–S3).

Different health status of the baseline population anddifferent health outcomesThe incidence of doctor-diagnosed asthma during the 5-yearperiod (2000–2005) was estimated for three different groupsof children:

1. Children without doctor-diagnosed asthma at baseline(n = 4483).

2. Children without doctor-diagnosed asthma and with-out wheezing ever at baseline (n = 3320).

3. Children with wheezing ever at baseline but no doctor-diagnosed asthma (n = 935).

The incidence of any of the two symptoms; doctor-diagnosedasthma and wheezing ever, was calculated from a baselinepopulation without these two symptoms (n = 3320).

The incidence of rhinitis during the 5-year period (2000–2005) was estimated in two different ways.

1. The incidence of doctor-diagnosed rhinitis 2005 wascalculated among children without such diagnose atbaseline (n = 4692).

2. The incidence of any rhinitis symptoms 2005 was cal-culated in the group of children without any rhinitissymptoms at baseline, (n = 3949). Symptoms includedin the definition of any rhinitis symptoms are doctor-diagnosed rhinitis, rhinitis ever, rhinitis last 12 months,rhinitis on pollen and rhinitis on pets.

Finally, the incidence of eczema ever during the 5-year pe-riod (2000–2005) was calculated for the group of childrenwithout such symptoms at baseline (n = 3705).

Since we collected questionnaire data for a 5-year period,we had no information of the yearly incidence during theintervening years. The yearly incidence was therefore es-timated by dividing the 5-year incidence by five, meaninga mean incidence per year during the period. This way ofmeasuring the yearly incidence is often used in studies ofcumulative incidence (3,4,16).

Statistical analysesAssociations between influential factors and incidence ofsymptoms were estimated by using univariate tests (chi-2)and multiple logistic regression analyses, expressed as crudeodds ratio (OR) and adjusted odds ratio (aOR) with 95%confidence interval. Adjustments were made for sex, age,week of birth, length of the breast-feeding period, age ofthe child when introduced to solid food, family size, day-care attendance, allergic symptoms in parents, any currentsmoker in the family (mother, father or any other person)and type of dwelling (see also Tables S1–S3). In the adjustedanalyses a stepwise method was used (forward: conditional)where only factors remaining statistically significant in thefinal model are reported in the tables. Trends in prevalenceof symptoms at different ages for the children were tested bylinear-by-linear association. All analyses were considered tobe statistically significant when the p-value was less than0.05. Statistical analyses were carried out using SPSS forWindows (version 14.0). The study was approved by themedical ethics committee at Uppsala University, Sweden.

RESULTSThe prevalence of doctor-diagnosed asthma increased from5.2% for children aged 1–3 years to 8.7% 5 years later whenthe children were 6–8 years old (Table 2). The prevalenceof wheezing during the last 12 months declined signifi-cantly from 22.2% at baseline to 10.0% 5 years later andthe prevalence declined with age both at baseline and atthe follow-up group. The prevalence of ‘wheezing ever’ wassurprisingly lower in the follow-up questionnaire comparedto baseline (24.9% in 2000 vs. 24.1% in 2005). We believethat this difference is due to a recall bias. Doctor-diagnosed

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Incidence rates of asthma and allergy in children Larsson et al.

Table 2 Prevalence of symptoms among 4779 children responding both in the baseline (2000) and the follow-up (2005) questionnaire

2000 Baseline 2005 Follow-upn = 4779 n = 4779Total 1–2 years 2–3 years 3–4 years p-value Total 6–7 years 7–8 years 8–9 years p-value

Doctor-diagnosed asthma 5.2 3.8 6.3 5.7 0.017 8.7 7.7 9.5 8.8 0.279Wheezing ever 24.9 24.7 26.4 24.4 0.818 24.1 24.4 24.2 24.1 0.836Wheezing last 12 months 22.2 26.3 23.2 18.6 0.000 10.0 11.3 9.7 9.3 0.071Cough at night 7.3 8.0 7.9 6.4 0.093 6.3 6.8 7.3 4.9 0.023Doctor-diagnosed rhinitis 1.4 0.6 1.2 2.3 0.000 6.3 5.8 6.1 7.1 0.127Rhinitis ever 12.0 13.1 11.4 11.9 0.295 20.4 18.2 21.6 21.8 0.013Rhinitis on pets 2.8 1.6 2.5 4.5 0.000 6.3 6.0 5.2 7.7 0.049Rhinitis on pollen 2.5 1.4 2.1 4.0 0.000 11.6 9.9 11.7 13.3 0.003Eczema ever 21.9 16.2 23.7 25.8 0.000 23.2 22.5 21.2 26.1 0.014Eczema last 12 months 18.7 15.0 20.2 20.7 0.000 18.3 17.8 16.6 20.7 0.034

1Trend estimated by linear-by-linear association.

rhinitis and rhinitis on pollen increased significantly withage.

AsthmaThe 5-year incidence of doctor-diagnosed asthma was 4.9%(95% CI 4.3–5.3), and the mean incidence rate per yearwas 1.0 child/100 children/year (1.0%) (Table S1). Whencomparing different age groups (1–2 years, 2–3 years,3–4 years at baseline) the 5-year incidence of asthma wasslightly higher for the younger children (5.5% (4.2–6.7),4.7% (3.7–5.9) and 4.5% (3.4–5.5)). When children withwheezing ever also were excluded from the baseline, theyearly incidence of asthma decreased to 0.6% per year. In thegroup of children with wheezing ever but without doctor-diagnosed asthma at baseline, the mean incidence became2.4% per year. Finally, the yearly incidence rate for any of thetwo symptoms, doctor-diagnosed asthma or wheezing ever,was found to be 2.4% when children with such symptomswere excluded from the baseline group.

In both crude and adjusted analyses, male sex, allergicsymptoms in parents and short period of breast-feeding(<3 months) were significantly associated with incidentasthma. These associations were mainly independent of thehealth status of the baseline group (Table S1). In almostall analyses, allergic symptoms in the mother were morestrongly associated with the incidence of asthma in thechild than such symptoms in the father. The highest inci-dent rate was found in children from families where bothparents were reported to have allergic symptoms. A shortertime of breast-feeding increased the risk for incident asthma.In the baseline group, where doctor-diagnosed asthma wasexcluded, single parenthood was associated with incidentdoctor-diagnosed asthma. Smoking in family was not asso-ciated with incident asthma.

RhinitisThe 5-year incidence of doctor-diagnosed rhinitis was 5.7%(95% CI 5.0–6.4) and the annual mean incidence was 1.1%while the yearly incidence of any rhinitis symptom was 3.7%(Table S2). When looking on different age groups, we did not

find any difference in incident rhinitis. Allergic symptoms inparents and a low number of siblings (or no siblings) at base-line were associated with incidence of rhinitis. The risk of in-cident rhinitis was about doubled if the child had an allergicmother or father, and if both parents had allergic symptomsthe risk of incident rhinitis increased dramatically (aOR4.63 (3.18–6.74)). Male sex was associated with a higherincidence of doctor-diagnosed rhinitis. Living in a multi-family house was associated with an increased incidence ofany rhinitis symptom but not with doctor-diagnosed rhini-tis. Neither the length of breast-feeding period, smoking infamily or day-care attendance was associated with incidentrhinitis.

EczemaThe 5-year incidence for eczema was 13.4% (95% CI 12.3–14.5) and the annual mean incidence was 2.7% (Table S3).When comparing different age groups we did not find anycorrelation between age and incident eczema. In contrastto asthma and rhinitis where boys were at risk, in the caseof eczema, girls were more inclined to develop symptomsduring the 5-year period. Furthermore, allergic symptoms inparents, early introduction of solid food (<3 months) andsingle parenthood were significantly associated with inci-dent eczema. The mother’s allergic symptoms were morestrongly associated with incident eczema in the child com-pared to such symptoms in the father.

DISCUSSIONThe mean incidence rate of parental-reported doctor-diagnosed asthma in our study was 1.0 child/100 children/year (1.0%). Studies from other countries have shown in-cidence rates for childhood asthma in the range of 1.3–2.6 children/100 children/year (percent/year) (10,16,17).Our incidence is similar to other studies in Sweden thathave used the same outcome definition. Larsson (3) showedan incidence of 1.1% in young adults aged 16–19 years, thesame rate as in the study by Smedje et al. (18) who stud-ied children aged 7–13 years. The most comparable studywas by Ronmark et al. (2) who reported an incidence rate



of doctor-diagnosed asthma of 0.9% in children aged 7–9 years. Our study had the same outcome definition asLarsson, Smedje and Ronmark for new cases (doctor-diagnosed asthma), when we estimated the incidence ratefor asthma but we had different baseline definitions. Ourstudy excluded solitary doctor-diagnosed asthma from thebaseline population while the other three studies excludedboth doctor-diagnosed asthma and ever asthma from base-line. In our study, we did not have information on everasthma, so an exact comparison could not be done. If wehad had the same baseline definition as Larsson, Smedjeand Ronmark, our incidence rate would probably have beensomewhat lower, in case ‘ever asthma’ represents real asth-matic disease. Toren et al. (6) suggest that respiratory symp-toms such as wheezing should be excluded from the baselinepopulation to decrease the risk of misclassification bias.When excluding doctor-diagnosed asthma and wheezingfrom the baseline population in our study, the incidenceof doctor-diagnosed asthma became lower (0.6%). Thisindicates that many children with wheezing get asthmaduring the following 5-year period. Furthermore, amongchildren with wheezing but without doctor-diagnosedasthma at baseline, the yearly incidence of asthma was ashigh as 2.4%, also emphasizing that wheezing is stronglyassociated with development of asthma. It can be argued,that there is a continuum of symptoms starting with wheezeand ending up with severe asthma even if not all childrenwith wheezing will develop asthma (19). Within this rangethe chance of being diagnosed of asthma is increasing withseverity. Our study confirms the vast majority of childrenhaving doctor-diagnosed asthma in year 2000 (n = 249) andin year 2005 (n = 411) were also reported to have wheez-ing (95.6% and 94.6%, respectively). However, 20.3% of thechildren with wheezing also had doctor-diagnosed asthmain year 2000 (n = 238) and 34% in year 2005 (n = 389).

The different rates of incident asthma reported from dif-ferent studies could in part be explained by the differencesin health status of the baseline group. Another reason fordiverse incidence rates in studies could be differences indefinition of the studied health outcomes. Ronmark et al.(2) reported different incidence rates when using two differ-ent definitions of incident asthma. The incidence of doctor-diagnosed asthma in Ronmark’s study was found to be 0.9%(percent/year) while the incidence of ever asthma was 1.6%.When we excluded doctor-diagnosed asthma and wheezingfrom baseline we found a large variation whether measuringthe incidence of doctor-diagnosed asthma (0.6%) or the inci-dence of any of the two outcomes; doctor-diagnosed asthmaand wheezing ever (2.4%). This shows that different defini-tion of asthma, both at baseline and as outcome, have aconsiderable impact on the reported incidence rates in dif-ferent studies.

In contrast to many prevalence studies of rhinitis andeczema in children, only a few have investigated the inci-dence rate. A longitudinal study by Johnke et al. (20) showeda cumulative incidence of atopic dermatitis of 11% in in-fant’s sensitized ≥ once during the first 18 month of life.However, most longitudinal studies have been conducted

on adults and are often in the area of occupational health.In our study the incidence of doctor-diagnosed rhinitis was1.1%, which is comparable with another Swedish study byNorrman et al. (4) reporting an incidence of rhinoconjunc-tivitis of 1.0% in teenagers aged 13–16 years in northernSweden. The incidence rates are similar, even though bothage groups, baseline and outcome definitions were different.

In our study, as in many other studies (2,4,10), allergicsymptoms in parents were a strong risk factor for develop-ing asthma in the child. Our results are also in line withmany studies showing that a short period of breast-feedingincreases the risk for developing asthma (2,7,8,21). In themultidisciplinary review by van Odijk et al. (7), it was con-cluded that breast-feeding reduces the risk for asthma andthe protective effects increase with the length of the breast-feeding period. It could be discussed if the association be-tween a short period of breast-feeding and incidence of al-lergic symptoms in children is underestimated in this study,since there is a broad public opinion in Sweden that breast-feeding protects small children from developing asthma andallergy. Therefore, a mother in a family with allergic symp-toms may tend to breast-feed the child for a longer periodcompared to a mother in a family with no allergic problem.

Our finding, that incident asthma is higher for youngboys than for girls has been shown in many other studies(2,5,16,17). However, in adolescence, the pattern changesand onset of wheeze is more prevalent in females thanmales. Asthma, after childhood, is more severe in femalesthan males, and is underdiagnosed in female adolescents.Possible explanations for this switch around puberty in-clude hormonal changes and sex-specific differences in theenvironmental exposures (22). Finally, we found that chil-dren living with one parent more often developed asthmathan children living with both parents. Single parenthoodcould be a proxy for lower socioeconomic status in Sweden(21). An association between low socioeconomic status andasthma has been shown by Almqvist et al. (23). Environ-mental tobacco smoke exposure has serious consequencesfor children’s respiratory health. Studies shows that incidentasthma and wheezing in children up to 3 years of age arestrongly associated with parental smoking (24,25). However,in our study smoking in family at baseline was not associatedwith incident asthma in the child even when we stratified byage. In Sweden, smoking among parents to school childrenis lower than in most other countries (less than 22% of thechildren in this study live with at least one smoking par-ent) and even if the parents are smoking, most of them donot smoke indoor. This is a possible explanation why therewas no association between asthma and smoking in family,that is Swedish parents are well informed about the risk ofexposing the children to tobacco smoke.

As in other studies (9,26), we found a strong associationbetween allergic symptoms in parents and the incidence ofrhinitis in the child. We also found that male gender wasassociated with rhinitis, which is in line with a study byPeroni et al. (26). In addition to previous reports (27,28) ourstudy indicate that having one or more siblings is associatedwith reduced risk of rhinitis. Such findings have been used


Incidence rates of asthma and allergy in children Larsson et al.

as evidence for the so-called ‘hygiene hypothesis’ (29). Therewas no association between rhinitis symptoms and smokingin family, which was also reported by Peroni et al. (26).However, Biagini et al. (27) found that exposure to smokewas associated with an increased risk of developing allergicrhinitis at age 1.

Allergic symptoms in parents were the strongest risk fac-tor for incident eczema in our study. This is supported byTaylor et al. (30) reporting that parental history of eczemaas well as advantaged family economic status was associatedwith eczema. However, we found that single parenthood wasassociated with incident eczema, where single parenthoodcould be seen as a proxy for low socioeconomic status ina family (21). Factors associated with lower socioeconomicstatus such as living in a multifamily house and smoking wasalso associated with eczema, at least in the crude analyses.

In conclusion, the yearly mean incidence of asthma symp-toms was found to be in the range of 0.6% to 2.4% andthe incidence of rhinitis symptoms were between 1.1% and3.7%. Estimating the incidence rate of allergic symptoms isstrongly dependent of how the baseline population is definedfrom health point of view and the definition of the studiedhealth outcome. The yearly mean incidence of eczema symp-toms was 2.7%. When comparing different age groups the5-year incidence of asthma was slightly higher for theyounger children. Allergic symptoms in parents were astrong risk factor for incident asthma, rhinitis and eczema.Short breast-feeding period and male sex were associatedwith asthma and being a boy was also associated with rhini-tis. Single parenthood and being a girl were associated witheczema. Different definitions also impact the associationsbetween influential factors and allergy. When comparing in-cident rates of asthma between different studies it is im-portant to realise that different definitions of the healthybaseline population will give rise to different incidentrates.

ACKNOWLEDGEMENTSThis study was supported by the Swedish Research Councilfor Environment, Agricultural Sciences and Spatial Planning(Formas), Swedish Asthma and Allergy Association’s Re-search Foundation, and the Swedish Foundation for HealthCare Sciences and Allergy Research. We want to thank Ce-cilia Boman for supporting data entry.

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Supplementary materialThe following supplementary material is available for thisarticle:

Table S1 Incidence of asthma symptoms among young chil-dren and associated influential factorsTable S2 Incidence of rhinitis symptoms among young chil-dren and associated influential factorsTable S3 Incidence of eczema symptoms among young chil-dren and associated influential factors

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