No clinical evidence base to support the hygiene hypothesis

Point-counterpoint. The triumph of the null hypothesis: epidemiology in an age of change

The recent confusion concerning the relation between hormone replacement therapy and cardiovascular disease has stirred a new wave of debate about the value and future of epidemiology. Opponents of epidemiology suggest an ever-diminishing role in an age of small risks and complex diseases, yet proponents are not in consensus about how to adapt their discipline to the challenges associated with ageing societies and changing disease patterns. While epidemiology is likely to be increasingly called upon to make sense of the risks involved with these changes, wading into this era with a mindset and tools that were derived from epidemiology's 'golden era' of tackling major risk factors, has created more confusion than understanding. Increasingly, we need to downsize epidemiology to what is testable, measurable, and relevant, based on robust methodology and public health rationale. Applying an evolutionary perspective, that views health problems of modernity as a manifestation of the mismatch between our ancient genes and hi-tech lifestyles, can provide guidance for a 21st century research agenda.

[Allergic diseases. Results from the German Health Interview and Examination Survey for Children and Adolescents (KiGGS)]

In the German Health Interview and Examination Survey for Children and Adolescents (KiGGS) from 2003-2006 information was collected on allergic diseases (asthma, atopic dermatitis, hay fever, allergic contact eczema) from a population-based sample of 17,641 0-to 17-year-olds, and blood samples were studied for specific IgE antibodies to 20 common allergens. The lifetime prevalence (LTP) of at least one atopic disease was 22.9 % (95 % CI: 22.0-23.7 %), the 12-month prevalence (12MP) was 16.1 % (15.4-16.8 %); boys (17.3; 16.3-18.2 %) were more frequently affected than girls (14.9; 14.0-15.8 %). Children with a background of migration were less often currently affected by an atopic disease, as were children from families of low social status. According to parents' accounts, the LTP of allergic contact eczema was 9.9 % (9.4-10.5 %); girls (13.8;12.9-14.8) were more frequently affected than boys (6.2; 5.6-6.9 %). Of the 3- to 17-year-olds, 40.8 % (39.6-42.0 %) were sensitised to at least one of the allergens tested; boys (45.0; 43.5-46.5 %) more frequently so than girls (36.4; 35.0-37.9 %). In the states of the former FRG, the LTP of allergic contact eczema was higher (10.2; 9.6-10.9 %) than those in the former GDR (8.4 % 7.4-9.6 %); otherwise there were no East-West differences. The KiGGS data provide the first nationally representative data on allergic diseases and sensitisation. The differences in prevalence observed correspond to a great extent with previous studies and may support the hygiene hypothesis. The prevalences in East and West Germany now seem to have equalised.

Mechanisms of the hygiene hypothesis--molecular and otherwise

The hygiene hypothesis was initially formulated to explain the steep increase in the prevalence of allergies in Western societies observed over the past few decades. It proposes that the microbial environment interfaces with the innate immune system and modulates its ability to impart instructions to adaptive immune responses, particularly when such interactions occur in utero and/or in early life. Recent research on the molecular mechanisms of the hygiene hypothesis highlights the role of Toll-like receptor ligands in modulating allergic inflammation and the importance of bacterial products in directing the development of the host's immune system. Genetic analysis, by contrast, clearly shows that the response threshold of the immune system to environmental stimuli is controlled by natural genetic variation and gene-environment interactions, suggesting that the complex interplay between the organism and the environment might not be regulated by a single mechanism.