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Thursday, 11 October 2012

Hygiene Hypothesis out and "Old Friends" are in

For some years now, increased rates of childhood asthma and eczema have been observed and the finger of suspicion has pointed to a lack of microbes in overly-clean homes. This idea is known as the hygiene hypothesis.
A recent report from the International Scientific Forum on Home Hygiene has concluded that this hypothesis is not correct.
It seems that microbes - or rather a lack of them - may well be implicated in the increase of allergies but it is not a reduced quantity of bacteria, viruses and intestinal worms that seems to be the problem.
We all have a vast population of "friendly" microbes that live on our skin and in our body cavities - nose, mouth, gut and so on. The bacteria alone are said to outnumber our body cells by ten to one. Without exposure to this vast array of microscopic life, we cannot be healthy. We know this, in part, because mice brought up in a sterile environment reliably fail to develop normal, healthy immune systems.
These "friendly" microbes stimulate the immune system in many interesting ways. They train it in infancy and help to keep it finely tuned throughout life.
There is a further range of microscopic life that lives in our homes – on surfaces, on dust flakes and so on. Living alongside these familiar life forms is normal and healthy. Of course there are also occasional pathogens that can cause illness. But every day babies and toddlers get away with licking floors and other household surfaces without picking up diseases.
The report concludes that the hygiene hypothesis should be replaced by the "Old Friends" hypothesis. This states that allergies are on the increase because the mix of microbes on and around us bodies has changed. It is no longer the same rich brew that co-habited with our ancestors and is no longer quite what the developing immune system needs.  Our immune systems evolved alongside these old friends, developing  complex and subtle symbiotic partnership.
My grandmother grew up in the country, playing on the dung heaps in the yard, in a home that teemed with rural bacteria. When she had ear infections, the treatment was to pour her own urine into her ear. As her young immune system dealt with this environment, it learned to deal with a mix of bacteria that had probably changed little through the millennia.
My grandchildren, growing up in an urban environment, have encountered a different range of microbes. If you could analyse their microbe population it would not be smaller, but would certainly be different to that of their great, great grandmother. There will be an absence of some of the "old friends" that evolved alongside their ancestors for millennia.
It is not just an absence of dung heaps that has caused this change. It has become normal for children to have several courses of antibiotics for chest or ear infections. This will have affected their blend of gut bacteria. Hygienic caesarian birth probably has an effect and so might bottle-feeding from a sterilised teat instead of a mother's microbe-rich breast. Factors like this are probably far more significant than how often their parents clean the bathroom.
You may say this is not a revolutionary change of hypothesis, so does it matter? Should we worry about hygiene, and if so where and when?
Well yes, food and kitchen hygiene are important if we want to avoid food poisoning. Hand washing can reduce the number of “tummy bugs” and colds we contract. If someone in the family has a compromised immune system, then extra vigilance is needed. And if someone in the family is already asthmatic, then reducing house dust exposure might be helpful.
But we still have a lot to learn about friendly bacteria and intestinal parasites. We don’t know the identity of these old friends and whether it would be helpful to hold a reunion or not. It is a branch of science that is relatively new and is only just starting to yield interesting information. It will be a while before it comes up with any practical suggestions.
So if you were worried about your house being too clean, then you can indulge in a bit of anxiety-free cleaning. And if you were worried about a degree of household squalor, then worry no more. The friendly bacteria will carry on doing what they do, and cleaning, or not cleaning will probably have no effect whatsoever. 
To read more, here is a link to Science Daily

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