Hygiene Hypothesis

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The "hygiene hypothesis" is the theory that the comparatively high rates of asthma and autoimmune diseases in developed nations are due to the fact that our immune systems weren't challenged enough when we were young. If a child lives in a home that is scoured with anti-bacterial cleaners and never comes into contact with microbes, animals or other "dirt," the immune system will atrophy like a muscle that isn't used, or so the thinking goes. It is also thought that certain parasites and childhood infections may trigger mechanisms that help prevent asthma. Thus overly clean environments may be detrimental rather than beneficial to the health of young children. While this does explain overall higher rates of asthma in the United States, it does not fully explain all aspects of increased asthma rates, such as why poor inner city children have higher asthma rates than their well-off suburban counterparts. Not everybody is convinced that the hygiene hypothesis tells the truth, at least not the whole truth, about the development of our immune systems. One study counters some of the tenets of the hygiene hypothesis by showing that in some cases exposure to animals and infections can actually increase the risk of asthma in children.

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  PRO: Respiratory Reviews "Hygiene Hypothesis Gains Support in the United States and Europe "

  Two new studies further bolster the hygiene hypothesis, which says that reduced microbial exposure because of increased sanitation and cleaner lifestyles has facilitated the rise in asthma and allergic disease in the Western world. One of the studies, conducted by Paolo M. Matricardi, MD, and colleagues, is the first to lend credence to the hygiene hypothesis in a US general population sample. [...]

  "Eating dirt or moving to a farm are at best theoretical rather than practical clinical recommendations for the prevention of asthma," remarked Scott T. Weiss, MD, in an editorial that accompanied Dr. Braun-Fahrländer's study. "However, a number of environmental factors are known to be associated with a lower incidence of allergic disease early in life," added Dr. Weiss, Professor of Medicine at Harvard Medical School in Boston. ...read the full article

  Two new studies further bolster the hygiene hypothesis, which says that reduced microbial exposure because of increased sanitation and cleaner lifestyles has facilitated the rise in asthma and allergic disease in the Western world. One of the studies, conducted by Paolo M. Matricardi, MD, and colleagues, is the first to lend credence to the hygiene hypothesis in a US general population sample.

  [...]

  "Eating dirt or moving to a farm are at best theoretical rather than practical clinical recommendations for the prevention of asthma," remarked Scott T. Weiss, MD, in an editorial that accompanied Dr. Braun-Fahrländer's study. "However, a number of environmental factors are known to be associated with a lower incidence of allergic disease early in life," added Dr. Weiss, Professor of Medicine at Harvard Medical School in Boston.

  Those factors include oral supplementation with Lactobacillus ruminus, the presence of a dog or other pet in the home since before birth, and attending day care during the first year of life. "The challenge will be ... to determine the extent of exposure that will ensure safety and have the desired outcome — the development of a healthy child with a very low risk of autoimmune disease," Dr. Weiss said.

  Clean food and water and the treatment of infection are the most common reasons for the reduced microbial exposure associated with allergic disease in the modern world, Jean-François Bach, MD, suggested in an interview. "However, such advances are essential because they keep us from losing children the way less developed countries do," stressed Dr. Bach, Professor of Immunology at the Necker Research Institute in Paris and author of a new paper on the role of infection in autoimmune and allergic disease.

  Rather than eliminate progress, it will be necessary to develop a treatment that substitutes for the infections that formerly protected against autoimmune and allergic disease. People should also not worry too much about cleanliness and maintaining bacteria-free environments, Dr. Bach said. "That is not to say that dirt is good, but too much cleanliness is not really necessary," he concluded. click to close

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  THERE MAY BE SOMETHING TO IT: HealthLink: Medical College of Wisconsin "Hygiene Hypothesis: Are We Too 'Clean' for Our Own Good?"

  Increased hygiene and a lack of exposure to various microorganisms may be affecting the immune systems of many populations — particularly in highly developed countries like the US — to the degree that individuals are losing their bodily ability to fight off certain diseases. That's the essence of the "hygiene hypothesis," a fairly new school of thought that argues that rising incidence of asthma, inflammatory bowel disease, multiple sclerosis and perhaps several other diseases may be, at least in part, the result of lifestyle and environmental changes that have made us too "clean" for our own good. ...read the full article

  Increased hygiene and a lack of exposure to various microorganisms may be affecting the immune systems of many populations — particularly in highly developed countries like the US — to the degree that individuals are losing their bodily ability to fight off certain diseases. That's the essence of the "hygiene hypothesis," a fairly new school of thought that argues that rising incidence of asthma, inflammatory bowel disease, multiple sclerosis and perhaps several other diseases may be, at least in part, the result of lifestyle and environmental changes that have made us too "clean" for our own good.

  [...]

  "The hygiene hypothesis suggests that the more hygienic one becomes, the more susceptible one is to various autoimmune diseases. The autoimmune diseases, the diseases that result from all the activation of your immune system, are increasing. The hygiene hypothesis — and we don't yet have a proof of it — acknowledges that the maturation of the immune system needs some kind of hardening, some kind of resistance. Put another way, you cannot really build up good muscles without doing exercise."

  [...]

  "What has happened now, with globalization and human migration, people move to areas that are very, very clean. Within one generation we have moved into a different environment. What we have been finding out is that in the second generation of Asian, Latin American and African children, where the first generation had been exposed to those kinds of parasites and early childhood infections, the second generation that has moved to 'cleaner' countries has not been exposed. The incidence of Crohn's disease, multiple sclerosis, and chronic asthma is as common in the second generation from the third world as in those with European or North American backgrounds, and in some cases even higher."

  Dr. Kugathasan and others interested in hygiene hypothesis have not proposed that "playing in the dirt," or making society less hygienic in general, are useful goals in medicine. But they do propose that taking the impact of reduced immunological strength into account for certain diseases could be beneficial.

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  It's important that a child go through normal childhood illness, for example, notes Dr. Kugathasan. "When we visit the doctor to suppress a lot of things like colds, rather than, in effect, letting nature run its course, we're making immediate treatment the priority rather than long-term prevention, using the analogy of immunological 'muscle-building.' We know that antibiotics wipe out normal cells, too, but you don't want to destroy what medical science has accomplished. Maybe there's no going back, but it's important that we take what the hygiene hypothesis is telling us into account when treating our children." click to close

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  NONCOMMITTAL BUT INTERESTED: American Baby "What Is the Hygiene Hypothesis?"

  One of the initial studies leading to the Hygiene Hypothesis was the observation that, before the Berlin Wall came down, East German children had much lower rates of asthma than did West German children, despite higher rates of air pollution, more tobacco smoking, lower rates of childhood immunizations, poorer socioeconomic status, and poorer public health conditions. The more "hygienic" society seemed more allergic! ...read the full article

  One of the initial studies leading to the Hygiene Hypothesis was the observation that, before the Berlin Wall came down, East German children had much lower rates of asthma than did West German children, despite higher rates of air pollution, more tobacco smoking, lower rates of childhood immunizations, poorer socioeconomic status, and poorer public health conditions. The more "hygienic" society seemed more allergic!

  Another interesting study from the University of Arizona compared asthma rates in children who grew up in daycare where respiratory illnesses, coughing, and wheezing were often the norm, versus children who grew up in more sheltered environments and did not experience frequent respiratory illnesses. The asthma rates were surprisingly higher in children who did not grow up in daycare conditions, again showing that a more hygienic infancy period predisposed children to a more allergic outcome later in childhood.

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  Some studies in Europe are now under way, examining the possible use of "probiotics," so-called good bacteria, in pregnancy and infancy as a way of turning off the "Hygiene Hypothesis switch." I am sure that this very active area of research will yield very interesting and useful findings in the near future. click to close

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  SKEPTICAL: The Scientist "Hygiene Hypothesis Questioned"

  A new study by a team at Stanford questions the controversial hygiene hypothesis, which states that raising children in an overly clean environment leads to the development of asthma. But others in the field say the paper, in the February 16 Nature Immunology, does little to challenge the theory.

  Martin Dahl and colleagues at Stanford University School of Medicine report that infection with influenza A increases the likelihood of developing allergic disease, potentially undermining a key tenet of the hypothesis: namely, that being infected with viruses protects against other diseases. ...read the full article

  A new study by a team at Stanford questions the controversial hygiene hypothesis, which states that raising children in an overly clean environment leads to the development of asthma. But others in the field say the paper, in the February 16 Nature Immunology, does little to challenge the theory.

  Martin Dahl and colleagues at Stanford University School of Medicine report that infection with influenza A increases the likelihood of developing allergic disease, potentially undermining a key tenet of the hypothesis: namely, that being infected with viruses protects against other diseases.

  [...]

  The hygiene hypothesis discriminates between viruses that enter via the gastrointestinal tract — for example, hepatitis A virus — and those that enter via the respiratory tract. Dahl and colleagues used the respiratory route, infecting mice with influenza A, allowing the infection to clear, and then inducing allergic disease.

  "We found somewhat surprisingly that the disease was worse in terms of the allergic response, in terms of asthma," said Lewis. "Many of the classic features were definitely worse rather than better, so at least it argues that the hygiene hypothesis has probably been very oversimplified and certainly doesn't seem to apply in a general sense to viral infections."

  "We're not trying to attack the hygiene hypothesis, I think we're trying to set straight what we think are some of the issues," he said.

  [...]

  "It was known already that the respiratory viruses carry the higher risk of allergy development compared with the nonrespiratory viruses. If you would do the same with, say, a gastrointestinal virus, like hepatitis A virus, you would probably see a different response," said Lambrecht, who was not involved in the study.

  [...]

  Gailen Marshall, professor of medicine and pathology at the University of Texas Medical School at Houston, told The Scientist in an E-mail that he too felt that the hygiene hypothesis had not been challenged by the data. click to close

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  HYGIENE HYPOTHESIS IS WRONG: The Collaborative On Health and Environment "Environmental Risk Factors for Asthma: Findings from the Children's Health Study"

  With this paper, Salam et al. report significant increases in asthma risk associated with exposures experienced early in childhood. Of the several risk factors they studied, the strongest associations they found were with exposures to herbicides and pesticides in infancy, and attending daycare before the age of 4 months old. Children with early persistent asthma were 10 times more likely to have been exposed to herbicides before the end of their first year than controls. Several features of their results are inconsistent with the "hygiene" hypothesis... that better health care in childhood prevents the immune system from developing normally. ...read the full article

  With this paper, Salam et al. report significant increases in asthma risk associated with exposures experienced early in childhood. Of the several risk factors they studied, the strongest associations they found were with exposures to herbicides and pesticides in infancy, and attending daycare before the age of 4 months old. Children with early persistent asthma were 10 times more likely to have been exposed to herbicides before the end of their first year than controls. Several features of their results are inconsistent with the "hygiene" hypothesis... that better health care in childhood prevents the immune system from developing normally.

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  Infants exposed to herbicides before the age of 1 were 10 times more likely to develop early persistent asthma. [...] Children exposed to herbicides after their first birthday,in contrast, were no more likely to develop asthma than controls.

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  Results for pesticide exposure were also significant but not as strong. Children exposed before 1 to pesticides were 3.58 times more likely to develop early persistent asthma than controls. As with herbicides, there was no increased risk of asthma if exposure began after the age of 1. Exposures to farm animal, farm crop or dust exposure also elevated the risk of early persistent asthma, although the effect was not as pronounced as for pesticides and herbicides.

  [...]

  They found no association between asthma risk and whether or not the family had pets.

  What does it mean? The most striking aspect of their finding is the strong association with herbicides and pesticides before the age of 1. This is the strongest association yet reported in a body of scientific literature in which there are few studies that have attempted to quantify early exposures to pesticides or herbicides and asthma risk in children.

  The authors observe:

  [...]

  Three aspects of their findings are inconsistent with the "hygiene hypothesis,"

  [...]

  - Children beginning day care before the age of 4 months are more likely to develop asthma. [...]

  - Children exposed to farm animals and dust are more likely to develop asthma. The hygiene hypothesis predicts the opposite. [...]

  - Children with no siblings were at lower risk to asthma than children with one or two siblings. [...]

  [...]

  As the authors point out, at the very least the finding that early day care attendance is associated with heightened risk, and that children with no siblings are at lower risk than those with one or two "suggests the need for a more complex "hygiene hypothesis." [The high prevalence of asthma in inner city poverty centers also undermines this interpretation.] The chief weaknesses of the study are its dependence upon parent recall and the lack of specificity about the details of exposure. Under most circumstances, these factors would make it false negatives more likely. Hence the strength of the associations discovered should be taken seriously.

  While the study falls far short of establishing any causal relationships between exposures and asthma, it adds significantly to the evidence that exposures early in life may increase asthma risk and may thus may be contributing to the burgeoning asthma epidemic. Salam et al.'s work adds more broadly to the weight of evidence that exposures to herbicides and pesticides early in life can have adverse effects and should be avoided in the home and other settings. click to close

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