Effect of urban vs. rural residence on the association between atopy and wheeze in Latin America: findings from a casease control analysis.

Abstract

Background

The association between atopy and asthma is attenuated in non-affluent populations, an effect that may be explained by childhood infections such as geohelminths. Objective

To investigate the association between atopy and wheeze in schoolchildren living in urban and rural areas of Ecuador and examine the effects of geohelminths on this association. Methods

We performed nested case–control studies among comparable populations of schoolchildren living in rural communities and urban neighbourhoods in the Province of Esmeraldas, Ecuador. We detected geohelminths in stool samples, measured recent wheeze and environmental exposures by parental questionnaire, and atopy by specific IgE (sIgE) and skin prick test (SPT) reactivity to aeroallergens. Results

Atopy, particularly sIgE to house dust mite (HDM), was more strongly associated with recent wheeze in urban than rural schoolchildren: (urban, adj. OR 5.19, 95% CI 3.37–8.00, P < 0.0001; rural, adj. OR 1.81, 95%CI 1.09–2.99, P = 0.02; interaction, P < 0.001). The population fractions of wheeze attributable to atopy were approximately two-fold greater in urban schoolchildren: SPT to any allergen (urban 23.5% vs. rural 10.1%), SPT to HDM (urban 18.5% vs. rural 9.6%), and anti-HDM IgE (urban 26.5% vs. rural 10.5%), while anti-Ascaris IgE was related to wheeze in a high proportion of rural (49.7%) and urban (35.4%) children. The association between atopy and recent wheeze was attenuated by markers of geohelminth infections. Conclusions

Our data suggest that urban residence modifies the association between HDM atopy and recent wheeze, and this effect is explained partly by geohelminth infections. Introduction

Atopy is a consistently strong risk factor for asthma in industrialized countries [1, 2], but this association appears to be weaker [3-6] in non-industrialized countries. A predominance of non-atopic over atopic wheeze/asthma has been observed in several Latin American countries including Brazil [3, 4, 7], Peru [8], and Ecuador [9].

The association between atopy and allergic disease appears to be stronger in affluent compared to non-affluent [6] and in urban compared to rural populations [10, 11]. Urban–rural differences in the prevalence of environmental exposures that attenuate atopy could explain these observations. Examples of environmental exposures that may suppress atopy include household pets [12], farming exposures [13, 14], and infections during childhood [15]. Geohelminth infections, that are common chronic parasitic infections of predominantly low-income rural populations in the tropics, may have a role in attenuating the association between atopy and allergic symptoms [16] including wheeze [17, 18].

The aim of this study was to investigate if associations between atopy and recent wheeze are modified by rural vs. urban residence and to explore which environmental exposures including geohelminth infections might contribute to such an effect. To do this, we conducted nested case–control studies comparing schoolchildren living in rural communities, as reported previously [17], with urban neighbourhoods in the Province of Esmeraldas in Ecuador.