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Let Them Eat Muck - The Hygiene Hypothesis

Last modified 2014-04-26 13:53


My late father-in-law Sydney was convinced that toddlers would benefit by contact with muck. While he did not go out of his way to introduce his grandchildren to the good old Yorkshire soil he did not deter them. This caused great consternation to their mothers. I held him in high regard and I take great pleasure in reporting to you that his ideas that “muck gives you antibodies” is an up to date concept.

There is rarely an opportunity to study whole populations of similar people who have been divided by history into two groups where each group has had a recently different culture and upbringing. The two Koreas and the two Germanys are the only ones that spring to mind.

Dr Erika Von Mutius, a German researcher thought that the children of the cities in the former East Germany who had grown up in an environment that was seemingly poorer, less healthy and dirtier would have a higher incidence of asthma and allergies than those brought up in the western part. She was amazed to find that the opposite was true and had to rethink her ideas. She was aware that there were many cultural differences between the two Germanys. An important one in the context of her research was closer contact with other children in kindergartens and larger families in the eastern part. She reversed her views and suggested that in some way early contact with other children and infectious diseases protected against allergies and asthma in later life.

In this way the “Hygiene Hypothesis” was born. It says that the cleanliness of the western way of life interferes with the way that the immune system matures over the first few years of life. The first few months of life seem especially important.

Hygiene Hypothesis

Rather briefly, the general theory is that cells of the baby’s immune system reach a point in their development when they have to “decide” what sort of cell to develop into. The decision is determined by their exposure to infection. The more infections the baby is exposed to the more cells will turn into the correct anti-infection cells. The remainder will turn into cells that will react to foreign protein by producing the allergic response.

In common with many theories the early research studies were contradictory. Infection from bacteria that were associated with food seemed to protect the babies from allergy, but airborne infections did not.

This seemed to upset the theory until the various groups of researchers found that the lowest incidence of allergies showed up in children who were born and brought up on farms. They studied that further and it was apparent that the important thing was not just the rural upbringing but contact with poultry and animals. Another study by Dr Von Mutius and her team showed that there was a much greater amount of a substance called endotoxin in the dust from the homes of these children.


Endotoxins are fragments of the outer walls of a particular group of bugs called gram-negative bacteria. Bacteriologists use gram stain to colour bacteria. The classes of bacteria are classified by their ability to take up that stain. Gram-positive bacteria take up the stain and gram-negative bugs do not. In vertebrates there is a very sensitive mechanism that is triggered by cellular fragments from all sorts of environmental viruses, bacteria, parasites, moulds and even fungi. This triggering advances the development of the immune system. It seems that exposure to some kind of environmental cellular fragment is a necessary thing for proper immune system maturation.

The theory needed to be broadened as it became clear that it was exposure to these fragments and not necessarily an actual infection that was the important stage in the process. These fragments of infectious agents appear to be present in all dusts. It easy to see that there will be a greater and more varied load in environments of crowding with humans and animals. This would be found in larger families where people are contributing to the viral and bacterial fragments with each cold or cough. Furthermore it would be at its highest in cultures where children mix closely with each other at an early age as in the former East Germany’s kindergartens.

Supporting Evidence

This supporting evidence is piling up, but each paper on its own cannot prove anything. However like circumstantial evidence in a criminal case each part of the jigsaw moves us a little closer to understanding the whole picture.

There are many published papers that support the hygiene hypothesis. I give a selection below of those that I found interesting. I include the references so that you can study the full paper if you so wish. This research is ongoing.

  1. A significantly lower incidence of asthma and other allergic problems was found in children who had fever episodes in early life. If the fever episodes occurred only in the second year there was a normal rate of asthma and allergy. This supports the ideas that the first year of life is important for the normal development of the immune system (ref1).
  2. The following conclusion was reached in a study from the USA. : 'In the United States serologic (blood test) evidence of acquisition of certain infections, mainly food-borne and orofaecal infection, is associated with a lower probability of having hay fever and asthma. Third National Health and Nutrition Examination Survey data support the hypothesis that hygiene is a major factor contributing to the increase in hay fever, asthma, and atopic sensitisation in westernised countries' (ref2).
  3. In a study from Ireland 25 out of 30 gastroenterologists or surgeons reported seeing no Inflammatory Bowel Disease (Crohn’s disease and Ulcerative Colitis) in their Traveller communities. The authors rightly point out that this could have many causes but that the hygiene hypothesis was a possibility. It is also interesting as there is a feeling that inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) may form a spectrum of disease with the same cause. IBS being at the benign end and IBD at the more severe end (ref3).
  4. A German study came to the conclusion that: 'Our findings support the hygiene hypothesis that exposure to high concentrations of endotoxin very early in life might protect against the development of atopic eczema within the first 6 months of life, along with an increased prevalence of non-specific respiratory diseases'. (ref4)

Lifting the Corner of the Curtain

In a recent copy of the New Scientist there was a news item about some research performed at the Washington School of Medicine in Saint Louis in America. This showed that newborn mice reared in a microbe free environment developed a faulty blood supply to their guts. If they were then injected with appropriate microbes (why not just feed them the bugs?) the blood supply became normal within 10 days. While the work was performed on mice it is another pointer towards possible causes of human diseases. Other authors link the Hygiene Hypothesis to the increase in diabetes and the protective effect of early exposure to intestinal worms or parasites such as malaria to a reduced incidence of allergy.

These small glimpses under the curtain suggest that we need our germs to develop properly in all sorts of areas. Certainly this could mean areas that come into contact with the environment such as the skin. Less obviously the lungs and the bowel have direct contact with the outside world. It is probably then no coincidence that asthma, eczema and IBS are regarded as having allergy as part of their cause. Who knows what organs and bodily functions might be affected and which illnesses and problems might eventually be linked to these causes?

Thinking Outside the Box

Like everyone else doctors need to be able to stereotype the things they contend with, namely diseases, by making a 'diagnosis'. This labelling immediately constrains thinking to 'the box' from which it is difficult to escape. The box then clearly determines the treatment strategies. There is a slowly dawning realisation that many things may be linked to allergy, heredity and stress. The slow emergence of the hygiene hypothesis may make the medical profession scrap a number of boxes like asthma, eczema, classical allergy, food intolerance and so on and replace the many boxes with a single one called stress/allergy/inheritance or whatever. Hopefully medicine will then recognise that symptoms of this new entity may be skin rashes, headaches, bowel upsets and the other things that formerly had their own boxes. We may then be able to apply a unified treatment to the common causes of these things. Sadly that's a long way off but the acceptance of the hygiene hypothesis is another step on that tortuous road.


What is the point of bringing all this to your attention?

Is it of any practical use?

Should we all go out and throw our children into the dirt?

The answers are not really clear and are in most of us sadly only of interest and perhaps regret. If you have really small babies and are very clean then maybe, just maybe, easing off on that cleanliness might be of help in allergy prevention. The greatest prospect is that thinking 'out of the traditional box' could help the scientists to develop new and better treatments. These might give us something useful where we have nothing at present. They might offer something better where all that there is available now is just a symptomatic treatment.

In the meantime Sydney would be smiling a wry smile of satisfaction that he was right after all.


  1. Calvani M Jr, Alessandri C, Bonci E. Fever episodes in early life and the development of atopy in children with Asthma. Eur Respir J 2002 Aug; 20(2): 391-6
  2. Matricardi PM, Rosmini F, Panetta V, Ferrigno L, Bonini S Hay fever and asthma in relation to markers of infection in the United States. J Allergy Clin Immunol 2002 Sep; 110(3): 381-7
  3. McCormick P, Manning D. Chronic inflammatory bowel disease and the 'over-clean' environment: rarity in the Irish 'traveller' community. Irish Med J 2001 Jul-Aug; 94(7): 203-4
  4. Gehring U, Bolte G, Borte M, Bischof W, Fahlbusch B, Wichmann HE, Heinrich J; LISA study group. Lifestyle-Related Factors on the Immune System and the Development of Allergies in Childhood. Exposure to endotoxin decreases the risk of atopic eczema in infancy: a cohort study. J Allergy Clin Immunol 2001 Nov; 108(5): 847-54


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