Please could you give a brief introduction to allergies?
Allergies occur when the body overreacts to substances in our environment, such as pollen, dust mites and so forth. What happens is that when we are exposed to these potential threats, the immune system should react, deal with it, and then shut down. Allergies occur when the immune system does not shut down and instead overreacts to stimuli that should not normally cause an immune response. As a consequence of this over-response, the body experiences unpleasant symptoms like sneezing, irritation and so forth.
There are various forms of allergies, what we looked at were atopic allergies, which appear to have been the main contributor to the steep rise in allergic diseases in the last 40 or so years. The word atopic literally means ‘out of place’. So atopic allergies occur due to an overall body reaction caused by the immune system, rather than just the site where the symptoms occur, such as the skin etc.
How many people do allergies affect and are they on the rise?
It is difficult to give exact numbers as there are various forms of allergies, such as hay fever, allergic asthma, food allergies etc - and also the data is incomplete. Our report summarises the available data from different studies of different types of allergic disease in different countries. Taken together, these studies indicate generally increasing levels of these diseases in the last 100 years or more. The particularly rapid increases in allergies and CIDs since the 1970s remains difficult to explain, particularly as there is evidence that the incidence (new cases) of asthma is now levelling off in some countries.
Just to give some examples, according to NHS choices , the current estimated number of hay fever sufferers in the UK is 10 million. In the UK, 5.4 million people are currently receiving treatment for asthma. That is 1 in every 12 adults and 1 in every 11 children.
Please could you explain the “hygiene hypothesis”?
The “hygiene hypothesis” was first put forward by David Strachan in 1989. He noticed this alarming rise in allergic diseases and he suggested that it could be due to children getting fewer infections. He based this on observational studies which suggested there was a correlation between family size and protection against asthma: the bigger the size of the family, the lower the risk of asthma.
Alongside this, there has been a longer term awareness that children living on farms are less likely to get hay fever. With regards to the larger families, what Strachan was inferring was that children in larger families are more likely to be passing infections around. The core of his hypothesis was that we need exposure to infections in order to protect against allergies and the reason that we have lost these exposures is because of cleaner living conditions, hygiene, higher standards of personal cleanliness and so forth.
What did your recent report reveal about this hypothesis?
In recent years, we seem to have become fixated on this idea that we are too clean for our own good – although no one seems able to define what that really means. What our review does, is to try and move this important issue forwards by highlighting the new data and new thinking which seems to provide more plausible explanations.
The new research data which we looked at suggests that, although the basic idea of the hygiene hypothesis is correct, the microbes that we need exposure to are not infectious diseases, which evolved relatively recently – over the last 10,000 years or so. What Graham Rook, one of the authors of our review, is proposing is that we need to look back much further in history to Palaeolithic times when our immune systems were evolving in response to the microbes around at that time, which aren’t the infectious diseases that we have today.
Things that were around in Palaeolithic times were things like human microbiota gut flora, skin flora, environmental microbes both in the outdoor and in living environments. There were some pathogens, but they were pathogens like hepatitis and worms, which are organisms that we tend to be able to live with and we have to tolerate them rather than try to eliminate them.
These are the types of organisms Rook’s proposing are our “old friends”. Whereas the original form of the hypothesis was saying we need exposure to infectious diseases, Rook’s development of the “hygiene hypothesis” - the “old friends” hypothesis - doesn’t oppose the hygiene hypothesis it just builds on it. The “old friends” hypothesis says it is not infectious diseases, it is our “old friends”, right back from our Palaeolithic past.
The other key point which the report highlights is that although these hypotheses are generally seen as a possible explanation for the rise in allergies, what we are now realising is that microbial exposure may be important for immune regulation in relation to a whole range of diseases not just allergic diseases but other chronic inflammatory diseases like inflammatory bowel disease, type 2 diabetes, multiple sclerosis, and possibly even things like cancer and depression may be linked to microbial exposure. This makes it a much more serious public health problem.
What else did your report reveal?
In reviewing the data, one of the key things we were also looking was to try and understand what may have caused our altered exposure to our “microbial friends” On the basis that this marked rise in asthma and other inflammatory diseases has mainly occurred over the last 100 years or so, we had to think about what happened in this time which might have deprived us of the required microbial exposures. The most obvious answer is that it relates to the Sanitary Revolution of the 19th and early 20th centuries. During the 1800’s we gradually got cleaner water, sanitation, cleaner food, cleaner cities, and all of those led to a decline in infectious diseases. As a result there was the suggestion that allergies may be the price we have to pay for protecting ourselves from these killer diseases.
But, if the hygiene hypothesis is right then these changes have saved us from infectious diseases, but may have also inadvertently cut us off from the old friends that occupy the same habitats. We just didn’t realise we were doing it as we weren’t monitoring for it, as there was no need to monitor for apparently harmless organisms.
It is possible that other factors such as the increasing use of antibiotics over the past 70 years are also involved, but the simplistic idea that the fundamental cause is “overcleanliness in our homes” does not hold up. If this factor contributes at all, its contribution is likely to be small relative to the other factors.
What can people do to avoid developing allergies?
There are various approaches which are being pursued. Various therapeutic approaches are being investigated, but are as yet at an early stage. Using probiotic strategies (such as probiotic drinks or foods) to reintroduce key microbes to our bodies seems an obvious approach, but further work is required to identify protective organisms, and progress is unlikely until there is much better understanding of which “old friends” are truly friendly – and effective – and safe.
As far as thinking what people could do for themselves, you could suggest that we adjust our lifestyles to improve the chances of being exposed to our “old friends”, by things like encouraging natural childbirth, breast-feeding etc., because it is fairly clear that one of the key times for this exposure is early in life when the immune system is developing. You could encourage children to spend more time playing with each other and playing outdoors so they get plenty of exposure to the microbes from each other and from their environment. But until we can positively identify that this would have a beneficial effect, advocating that everyone should adopt these approaches would be inappropriate. Encouraging lifestyle changes which could increase exposure to Old Friends is a huge challenge and would likely meet with opposition, without more robust evidence of a real health benefit.
Would you like to make any further comments?
I think it is very important to bear in mind that although it appears that microbial exposure is a fundamental factor in increasing the risk of these allergic and other inflammatory diseases, it is not the only factor. Increasingly data shows that things like diet, vitamin C deficiency, pollution, lack of exercise, obesity all can contribute to the immune deregulation.
Genetic predisposition is also a major factor. Together these factors may help to explain why we do not all suffer from these diseases. It is conceivable that for some people the requisite microbial exposure that we experience is sufficient for immune regulation, but for others it is not. Individuals in the latter category may suffer from allergies and other CIDs. These may be triggered, particularly when one or more of the other factors outlined above causes further dysregulation of the immune system
So what are your conclusions about hygiene?
Whilst it is extremely difficult to make firm recommendations about how we might act to reverse or halt the trends in inflammatory diseases, for us, the International Scientific Forum on Home Hygiene, the report is good news because it refutes the suggestion that allergies may be the price we have to pay for protecting ourselves from infectious diseases.
Infectious diseases remain a serious problem which threatens our health and prosperity. In our modern crowded and mobile world infectious diseases spread very rapidly, and our ability to treat them is now threatened by the problem of antibiotic resistance. This means that the need for hygiene is as great as it ever was.
Whist it is difficult to propose measures which might reverse the trends in inflammatory diseases, what we can do is to promote better understanding of the basic principles and practice of hygiene – but this is another story. As Dr Stanwell Smith, one of the authors of our report says, “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” We have to stop using “home cleanliness” as a scapegoat to explain a much more complex set of public health issues which urgently need addressing.
Where can readers find more information?
The report was very long and very detailed and we felt that most health care professionals would not want to read through the whole thing, so we produced a short summary which is freely available from our website: http://www.ifh-homehygiene.org
About Sally Bloomfield
Dr Sally Bloomfield currently acts as a consultant in hygiene and infectious disease. She is Chairman and Member of the Scientific Advisory Board of IFH.
She initially qualified as a pharmacist, but she then went on to develop a career in microbiology, where she found a specific interest in the prevention of infectious diseases and in microbial quality assurance.
Dr Bloomfield was an academic in the Department of Pharmacy, Kings College London for 25 years. Her research interests during this time mainly focussed on the study of antimicrobial agents used in the community and in medicine. Specifically, she was involved in studying the molecular and physiological basis for the development of resistance to antimicrobial agents. She was also involved in a range of projects studying the role of hygiene, including the role of cleaning and disinfection, in preventing the spread of infectious disease in the hospital, food processing, domestic and other environments.
Dr Bloomfield joined Unilever Research in 1997, where she helped to develop a programme involved with raising awareness of the importance of hygiene in the domestic setting. It was also during this time that she was involved in setting up the International Scientific Forum on Home Hygiene (IFH).
Dr Bloomfield was awarded the Rudolf Schulke Hygiene Prize in 1995, for outstanding contribution in the field of Hygiene and Preventive Medicine. From 1990-2003, she was also a member of Comité Européen de Normalisation - CEN TC 216 - Antiseptics and Disinfectants.