Commentary: A step towards understanding asthma in low- and middle-income countries

Abstract

Children growing up on a farm in Europe or North America have rather consistently been observed to less often suffer from asthma and allergic diseases. The hypothesis of possible protective effects of early life contact with a farming environment has opened a way for new understanding of the asthma and allergy epidemic in the Western world in the past few decades. A recent publication showed that children living on farms were exposed to a wider range of microbes, and this explained a substantial fraction of the inverse relation between asthma and growing up on a farm. This suggests that within a complex farming environment, microbial diversity may be of central importance for the observed protective effects. Microbial stimulation is essential for the development of immunological competence early in life. A rapidly expanding field of research relates the human microbiota to immunological maturation and systemic inflammation, with potential impact on chronic pulmonary disease as well as a number of other chronic inflammatory diseases. Farm environment and animals are, however, very complex exposures, including a variety of biological exposures and subject to selection. There is some evidence that consumption of raw farm milk is of importance for observed protective influence of farm exposure on asthma. The role of allergens in allergic diseases has been the focus of research for decades, and a farming environment certainly provides exposure to a number of potent allergens. Airways irritants like ammonia and dusts are prevalent in a farming environment, as are a number of chemical substances like pesticides. In adult farmers, the farming environment may cause asthma, respiratory symptoms and mucosal symptoms like eye irritation. The ‘farming environment’ exposure is further complicated by the fact that a farming environment and contact with animals are not random exposures, but subject to selection based on a variety of mechanisms. Complex selection might have taken place over several generations. In a Western world population of today almost everybody descended from farmers, whereas a comparatively small number are farmers themselves. Our knowledge on asthma and allergic diseases almost entirely relates to the Western world. With the exception of a few published papers from lowand middle-income countries (LMICs) the major scientific effort to elucidate these diseases is based on studies from Europe and Northern America. However, there is increasing concern about the rise of non-communicable diseases including chronic lung diseases in the non-affluent parts of the world. In fact, the largest burden of chronic diseases today is present in LMICs and not in the rich parts of world (www.who.int/nmh/en). The ‘10/90 dilemma in health science’ points to the problem that only 10% of medical research concern the health problems of 90% of the world’s people. The article by Brunekreef et al. in this issue of the IJE, investigating the farming allergy association in affluent and non-affluent societies, is a step towards a global research agenda for asthma and allergic diseases. The analyses show increased risk for childhood symptoms of wheeze, rhinoconjunctivitis and eczema in relation to contact with farm animals among children living in non-affluent countries, whereas no such association was found in affluent societies. The findings are based on the International Study of Asthma and Allergies in Childhood (ISAAC) study including study centres across the world, mostly in urban and sub-urban societies. The authors conclude that there is a great need for further understanding of asthma and allergic diseases in non-affluent parts of the world. The comparison of a well-known risk association between affluent and non-affluent societies is most important, addressing whether scientific results from the Western world may be applicable in the much less investigated non-affluent parts of the world. The current article shows significant differences, which render such generalization impossible with regard to this issue. Published by Oxford University Press on behalf of the International Epidemiological Association