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It's Time to Abandon the Hygiene Hypothesis

Sally Bloomfield

8/18/2016


By Professor Sally Bloomfield


Chairman of IFH and Honorary Professor, London School of Hygiene and Tropical Medicine, London, UK

The July issue of Perspectives in Public Health reviews the latest evidence showing that the hygiene hypothesis is a misleading and dangerous misnomer. Not only does it undermine attitudes to hygiene at a time when antibiotic resistance threatens our ability to treat infections, it also hinders the search for ways to reverse the recent dramatic rise in allergies and other chronic inflammatory diseases. These arguments are set out in a new review summarising the consensus findings of experts who presented at a conference organized by the Royal Society for Public Health and the International Scientific Forum on Home Hygiene in February 2016.

Particularly worrying is that this loss of confidence in hygiene is happening at a time when hygiene is becoming more rather than less important. Co-author Dr. Rosalind Stanwell-Smith says, “The need for better hygiene is being driven by increasing numbers of vulnerable groups at increased risk of infection living in the community, and increasing amounts of healthcare being delivered in out-of-hospital settings. Agencies worldwide, developing strategies for dealing with emerging infections such as influenza, SARS and Ebola, recognize hygiene as the first line of defence during the early critical period before mass measures such as vaccination become available.  Hygiene is also seen as a means to reduce antibiotic prescribing, as part of tackling the global antibiotic resistance issue.”

The conclusion of the peer review study suggests a combination of strategies, including natural childbirth, breastfeeding, increased social exposure through sport, other outdoor activities, less time spent indoors, diet and appropriate antibiotic use, may help restore the microbiome and perhaps reduce risks of allergic disease.  The term ‘hygiene hypothesis’ must be abandoned.  This premise of the term hygiene hypothesis – that the exposures we need are to infectious disease, and that we no longer get these exposures because of our ‘modern obsession with cleanliness’ – is now discounted.

In 2003, co-author Professor Graham Rook proposed the Old Friends Mechanisms, which now offers a more rational explanation. The Old Friends Mechanism proposes that the exposures which the immune system requires to build immune tolerance are the largely non-harmful human, animal and environmental microbes and intestinal worms that co-existed with humans during primate evolution when the immune system was evolving.  

Epidemiological studies suggest that altered or reduced exposure to Old Friends microbes has decreased in humans due to various changes that have occurred over the last 200 years. These include sanitation and clean water (which have cut us off from Old Friends as well as harmful microbes), rapid urbanization, altered diet, caesarean rather than natural childbirth, bottle feeding and excessive antibiotic use, all of which have had profound effects on the human microbiome (the millions of microbes which live on or in our bodies), leading to failure of immunotolerance and increased risk of inflammatory diseases.

There is no good evidence that hygiene, as the public understands it, is responsible for the decrease of Old Friends microbes. If day-to-day hygiene and cleanliness habits contribute, their role is likely to be small relative to other factors. 

Knowing the likely causes does not necessarily mean the reverse is true in that encouraging natural childbirth, breastfeeding, healthier diets, reduced antibiotic prescribing and increased outdoor exposure will reverse trends in inflammatory diseases.  Professor Fergus Shanahan is optimistic but cautions: “much further work is needed to evaluate this.” 

If the Old Friends Mechanism is correct, this is good news for hygiene, but it does mean we need to develop a new approach that maximizes protection against infection whilst at the same time sustaining exposure to our Old Friends.

To achieve this we need two approaches:

  • Targeted Hygiene: Co-author Elizabeth Scott says, “we need smarter approaches to hygiene than the ‘scrupulous cleanliness’ approach advocated by Florence Nightingale. Both Professor Elizabeth Scott and I believe good hygiene is based on knowing how harmful microbes move around the home and other environments. This knowledge will help us to target hygiene practices in the places and at the times that matter to eliminate pathogens that make us sick and prevent them from spreading. It is most important to prevent the spread of pathogens most particularly at times associated with activities such as food, respiratory, hand and toilet hygiene. This has come to be known as targeted hygiene.  Effective hygiene practices involve either removal of microbes from surfaces (e.g with detergent and running water) or killing them in situ (using a disinfectant or hand sanitizer) or a combination of the two. Whereas use of sanitizers and disinfectant in home and everyday life is opposed by some experts, we need to take a balanced view.  There is little evidence to support the widespread, non-targeted use of antibacterial products for routine daily cleaning to give “a bit of extra hygiene,” but the data shows that failing to use sanitizer and disinfectant products in “targeted” situations where pathogens cannot be eliminated by dry wiping or detergent-based cleaning alone can increase spread of infectious agents.
  • Public Perception of Clean: Equally important, we must change public, public health and professional perceptions. The ill-defined term ‘hygiene hypothesis,’ is still strongly associated in the minds of the media, the public and many so-called experts with reduced exposure to infectious diseases caused by excessive home cleanliness.  This term and perspective must be replaced by a more appropriate term such as the Old Friends Mechanism, which refers to a specific scientifically supported concept, which is significantly different from the hygiene hypothesis and does not prejudge that the cause of the problem is “hygiene.” We will not be able to tackle the issue of promoting better hygiene, whilst we remain wed to the original hygiene hypothesis concept; that children protected by “too much hygiene” are more likely to get allergies – and this will not happen until we start using a different term to describe the link between microbe exposure and development of immune tolerance.

To view the July issue of Perspectives in Public Health, visit http://rsh.sagepub.com/

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