By: Professor of Generic Epidemiology, King’s College London
A battle is waging about how often and how carefully we should wash our hands. Many allergy experts say it is vital for our future health and that of our children to reduce hand washing and allow friendly microbes back into our mouths. Some authors go as far as urging us to eat dirt. But other experts, such as those working on infectious diseases, say this is highly irresponsible and dangerous advice because of the recent rise in cases of food poisoning and transmission of viruses.
So, who is correct? We desperately need some sensible guidance.
Allergies have been rising since the 1970s in all developed countries and problems caused by previously harmless pollen, house dust mites and various foods are reaching epidemic proportions. Remarkably, while the first case of a documented food allergy was reported only in 1969, nut-free schools are now commonplace. Originally, we were told to deal with allergies by eliminating or avoiding them – vacuuming, disinfecting, removing pets, restricting diets and staying indoors. But this has done nothing to stop rates rising.
The hygiene hypothesis
Some 25 years ago, a paper suggested a different mechanism to explain allergy – which has since been called the “hygiene hypothesis”. It discovered that children in large, poor families living in rural farms, surrounded by animals and dust, had less allergies. These findings have been replicated multiple times around the world in different environments. Children from small families in rich, urban neighbourhoods are consistently found to have the highest allergy rates.
The initial idea was that children exposed early on to pathogens must have better tuned immune systems, which don’t overreact when later exposed to harmless proteins like pollen or peanuts. However a mechanism for this was never proven and recent scientific breakthroughs suggests the infections themselves may not be the main story.
Instead, the realisation that the functioning of our immune system is totally dependent on the normal inhabitants of our guts – in particular the 100 trillion microbes in our colon (known as the microbiome) – has changed our view of hygiene. These microbes are key to digesting food and producing vitamins and chemicals that keep our immune system in check. When our normal microbiome gets disturbed, we lose species diversity and this makes us prone to an inappropriate response to harmless proteins – and allergies and autoimmune diseases ensue.
This has happened over the last 30 to 40 years in developed countries due to a deadly combination of events that disrupt microbes – antibiotic overuse, processed sterile diets, reduced fibre intakes, reduced breast feeding and high caesarean section rates. We are estimated to have 40% less gut and mouth microbial species diversity than hunter gatherers and pre-medieval people. This is compounded by trends of increasing urban living moving us ever further away from our natural outdoor environment of dust, animals and soil.
How dangerous is dirt?
So is not washing your hands the answer? It will certainly increase transmission of microbes between people. And, although unproven, it could increase gut diversity and health. But clearly this is very unwise for high-risk individuals such as the elderly or immune deficient – or during a norovirus outbreak or in environments such as hospitals.
There is also no reason to alter advice and habits on good toilet hygiene. Some people believe that transmission of cold and flu viruses could be reduced by regular hand washing – but the data is inconclusive and could be offset by the reduced immunity to viral infections caused by a potentially reduced microbiome. Still, if you are a professional food worker, hand hygiene is vital and we still have fatal outbreaks from fast-food and to remind us.
The nature of food poisoning outbreaks is changing and new threats emerge due to the modern way we make and eat food. The microbe Campylobacter used to be rare. Now it is commonplace in refrigerators and causes an estimated 100 deaths and around 300,000 serious infections a year, costing the UK £900m and the US several billion. Pork is another regular problem as shown by a recent MRSA outbreak in the UK.
A 2015 food safety survey found that Campylobacter was present in 73% of the supermarket chickens tested and, of those, the majority were resistant to some antibiotics and many with high enough levels of microbes to cause infections. The meat industry says they can’t eradicate the problem and keep prices low. While we have a culture of cheap processed food at any price, the consumer and taxpayer take the risk – and 11m days a year off work just in the UK is the result.
Educating parents and children about refrigerator hygiene is vital if we insist on eating cheap meat. Until the meat is cooked thoroughly it should be treated like a radioactive substance from the moment you touch the wrapper in the supermarket and ensuring surfaces, utensils and hands washed with soap, detergent and hot water.
Vegetables are much less of a problem. For the trendy modern semi-vegetarian family who knows where their vegetables come from, the only significant risk they run is a bit of grit while gaining millions of temporary soil microbes which may be beneficial. The only greens I would always avoid are sprouted foods (bean sprouts, watercress) used in salads which you shouldn’t trust. They are usually grown in a warm, moist environment where bacteria thrive. Sprouts contaminated with E.coli caused the worst outbreak in modern history killing 51 Germans in 2011.
If you’re healthy you don’t need to wash your hands after going on public transportation, chopping veg, gardening or a walk in the woods. But do be careful around meat, refrigerators, toilets and disease outbreaks. And if your kid drops something on your floor and you are sure it isn’t in a puddle of chicken blood – then the five second rule should still be fine. If it’s a fallen dummy sucking it yourself has been shown to reduce allergies. Playing with the dog and other animals and getting dirty outside in the garden or park should also be encouraged.
As we increasingly discover the benefits of increased exposure to microbes we urgently need more information and education so we can make informed decisions on the foods we buy, the antibiotics we ingest and the lifestyle choices we make to reverse our decline in diversity. All this has to be balanced against taking too many risks with modern intensively farmed foods which should now come with health warnings.