A new scientific report out October 3 from the International Scientific Forum on Home Hygiene (IFH)[i] dismantles the myth that the epidemic rise in allergies in recent years has happened because we're living in sterile homes and overdoing hygiene.
But far from saying microbial exposure is not important, the report concludes that losing touch with microbial 'old friends' may be a fundamental factor underlying rises in an even wider array of serious diseases. As well as allergies, there are numerous other 'chronic inflammatory diseases' (CIDs) such as Type 1 diabetes and multiple sclerosis which seem to stem from impaired regulation of our immune systems. Deficiencies in microbial exposure could be key to rises in both allergies and CIDs.
This detailed review of evidence, accumulated over more than 20 years of research since the 'hygiene hypothesis' was first proposed, now confirms that the original notion is not correct.
Presenting the report findings in Liverpool at Infection Prevention 2012, the national conference of the UK and Ireland's Infection Prevention Society, co-author of the report and Honorary Professor at London School of Hygiene and Tropical Medicine, Professor Sally Bloomfield[ii] said: "The underlying idea that microbial exposure is crucial to regulating the immune system is right. But the idea that children who have fewer infections, because of more hygienic homes, are then more likely to develop asthma and other allergies does not hold up."
Another author of the report, Dr Rosalind Stanwell-Smith, also from London School of Hygiene and Tropical Medicine[iii] said: "Allergies and chronic inflammatory diseases are serious health issues and it's time we recognised that simplistically talking about home and personal cleanliness as the cause of the problem is ill-advised, because it's diverting attention from finding workable solutions and the true, probably much more complex, causes." If worrying about 'being too clean' results in people needlessly exposing themselves and their children to pathogens that can make them ill, this would clearly be dangerous.
Professor Graham Rook, also co-author of the report,[iv] who developed the 'Old Friends' version of the hypothesis, said: "The rise in allergies and inflammatory diseases seems at least partly due to gradually losing contact with the range of microbes our immune systems evolved with, way back in the Stone Age.[v] Only now are we seeing the consequences of this, doubtless also driven by genetic predisposition and a range of factors in our modern lifestyle -- from different diets and pollution to stress and inactivity. It seems that some people now have inadequately regulated immune systems that are less able to cope with these other factors."
Dr Stanwell Smith explains the probable reasons why this has happened: "Since the 1800s, when allergies began to be more noticed, the mix of microbes we've lived with, and eaten, drunk and breathed in has been steadily changing. Some of this has come through measures to combat infectious diseases that used to take such a heavy toll in those days -- in London, 1 in 3 deaths was a child under 5. These changes include clean drinking water, safe food, sanitation and sewers, and maybe overuse of antibiotics.[vi] Whilst vital for protecting us from infectious diseases, these will also have inadvertently altered exposure to the 'microbial friends' which inhabit the same environments."
But we've also lost touch with our "old friends" in other ways: our modern homes have a different and less diverse mix of microbes than rural homes of the past. This is nothing to do with cleaning habits: even the cleanest-looking homes still abound with bacteria, viruses, fungi, moulds and dust mites. [vii] It's mainly because microbes come in from outside and the microbes in towns and cities are very different from those on farms and in the countryside.
"The good news," says Professor Bloomfield "is that we aren't faced with a stark choice between running the risk of infectious disease, or suffering allergies and inflammatory diseases. The threat of infectious disease is now rising because of antibiotic resistance, global mobility and an ageing population, so good hygiene is even more vital to all of us." [viii]
"How we can begin to reverse the trend in allergies and CID isn't yet clear," says Professor Rook.[ix] "There are lots of ideas being explored but relaxing hygiene won't reunite us with our Old Friends -- just expose us to new enemies like E. coli O104."
"One important thing we can do," says Professor Bloomfield, "is to stop talking about 'being too clean' and get people thinking about how we can safely reconnect with the right kind of dirt."
[i] The International Scientific Forum on Home Hygiene (IFH) is a not-for profit, non-governmental organisation which is working to develop and promote home hygiene practice based on sound scientific principles. For more details about IFH visit
[ii] Professor Sally Bloomfield is also Chairman of IFH
[iii] Dr Rosalind Stanwell-Smith is Honorary Senior Lecturer at the London School of Hygiene and Tropical Medicine
[iv] Professor Graham Rook is from the Centre for Clinical Microbiology, University College London, UK
[v] The "Old Friends hypothesis" as developed by Rook in 2003, proposes that the microbial exposures vital for immune regulation are not the infectious diseases (respiratory infections such as colds, influenza, measles etc. and gastrointestinal infections such as cholera, polio, Campylobacter, norovirus etc.), which have evolved and spread over the last 10,000 years as we came to live in ever denser urban communities, but rather the microbes with which we co-evolved, and that were already present in Paleolithic, hunter gatherer, times -- the era when the human immune system was developing, in close association with the abundant microbes present in that era. Put simply, in the same way that we have evolved to become dependent on Vitamin C in our diet (something which most mammals could, and most non-primates can still, synthesise for themselves) because it became plentiful in our food, we have also evolved dependency on microbial exposure, such that our immune systems cannot now function properly without it. It is hypothesised that the "old friends" include commensal organisms (the normal microbiota of the skin, gut and respiratory tract of humans and animals) and some potentially pathogenic organisms such as helminths (worms), which establish chronic infections or carrier states. These latter have to be tolerated, because attempts by the immune system to eliminate infections that it cannot remove only lead to pointless tissue-damaging inflammation. The OFs also include environmental saprophytes i.e. species which inhabit our indoor and outdoor environments. Other scientists are now suggesting that we need constant exposure to a diverse range of old friends, not just specific microbes. The most likely explanation of this need for daily exposure to microbes, particularly in early life, is because they interact with the regulatory systems which keep our immune system in balance. Without this our immune systems may overreact, or react inappropriately, or fail to switch off completely when no longer needed, which is an underlying cause of these diseases.
[vi]. Antibiotics are relevant here because there is evidence that they affect immunoregulation by reducing the diversity of gut microbiota.
[vii] Microbiological studies show that routine daily or weekly cleanliness habits have no sustained effect in reducing levels or altering the types of microbes in our home environment. Frequent showering and bathing might be a contributing factor, but there is no good evidence of a link, other than in worsening an existing risk of eczema. The idea that we could create a 'sterile' environment in our homes through excessive cleanliness is just not credible: as fast as they are removed by cleaning, the microbes in our homes are being replaced, via dust and air from the outdoor environment, by microbes that are constantly shed from the human body and from our pets and from contaminated foods and other items brought into our homes.
[viii] Infectious gastrointestinal, respiratory and other diseases circulating in the community continue to exert a heavy toll on health and prosperity. We live in an increasingly crowded and mobile world where any new infectious agent and antibiotic resistant strains can spread easily and quickly. More details can be obtained from section 7 of the summary report.
[ix] Various therapeutic approaches are being investigated, as yet at an early stage, including trials of probiotic drinks/ tablets and low dosages of helminth eggs. Lifestyle changes which could re-establish our exposure to Old Friends range from encouraging natural childbirth and sustained breast feeding to designing/ running our homes to ensure more interaction with our environment to encouraging more sport and other outdoor activities. Increased emphasis on outdoor sport in young people, which is at the heart of the post Olympic movement, could also have a direct beneficial physiological action on our immune system. Despite growing scientific support for the role of old friends in immune regulation, we are still some way from conclusively demonstrating how we can begin to reverse the rise in allergies and CIDs though exposure to the right kinds of microbes. This means that we need a clearer understanding before particular approaches could be strongly advocated.
Materials provided by London School of Hygiene & Tropical Medicine (LSHTM). Note: Content may be edited for style and length.
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