Danielle Murri / ANTH 3370 / Final Paper -Hygiene Hypothesis
Modern Disease and the Balance of Cleanliness
Can being too clean be dangerous? The hygiene hypothesis states that there may be a such thing as being too clean. The hygiene hypothesis is the notion that as human society has evolved from small, early civilizations into the modern, developed environment, there has been considerably less individual exposure to microbes and pathogens, which may be causing a rise in autoimmune conditions, chronic inflammation, and several diseases. It proposes that childhood exposure to germs and certain infections or pathogens helps the immune system develop properly. Exposure to these stimuli teaches the body to differentiate harmless substances from the harmful ones that trigger certain diseases. In theory, exposure to certain germs will teach the immune system not to overreact and directly will prevent inflammation and autoimmune processes. The hygiene hypothesis is an area of science recently increasing in research, as understanding the hygiene hypothesis and immune priming may help scientists come up with better ways to treat or prevent many illnesses.
In order to understand the basis to the many forms of the hygiene hypothesis and understand the importance of the research being done with the theory itself, one must understand the value of the immune system. According to the National Institute of Allergy and Infectious Diseases, a subsection of the U.S. Department of health and human services,” The immune system is a network of cells, tissues, and organs that work together to protect the body from infection. The human body provides an ideal environment for many microbes, such as viruses, bacteria, fungi, and parasites, and the immune system prevents and limits their entry and growth to maintain optimal health.“ This network of cells, tissues, and organs is precious to the human body. “The immune system provides protection against a variety of external (infectious disease) and internal threats (cancers), but has sizeable costs associated with maintenance and activation. Furthermore there are consequences when immune processes are misdirected and contribute to allergy, asthma, and autoimmune disease” (Stinson, Bogin, & O’Rourke, 2012, pg. 334). The way that the immune system develops is directly correlated to the way an individual’s health will be throughout their lives.
The immune system is meant to detect intruders called antigens, which are substances (usually proteins) that live on the cells of viruses, bacteria, fungi, or parasites. Antigens may also be present on toxins, drugs, and chemicals. Once these antigens have been detected, white blood cells arrive on scene. A type of white blood cell that is critical for the immune response is the lymphocyte. Type B lymphocytes turn into cells producing antibodies, which will attach to specific antigens, making an easier target for other immune response cells to attack. Type T lymphocytes attack the antigens directly and release cytokines which help control the entire immune response. Both B and T cells will multiply once they have been triggered, and will help the immune system create a “memory” so that the body can be immunized in a way from whatever antigen was introduced. For example, those who were exposed to and developed chickenpox will be immunized from it in the future due to the white blood cell immune response memory. The immune response can overreact or lack in response causing immune system disorders or allergies. Inefficient immune responses allow for the development of diseases, whereas overactive immune responses can lead to antibodies attacking the body’s own tissues or autoimmune diseases.
The hygiene hypothesis began in the late 1980’s when scientists were observing that older siblings in families tended to have lower incidences of hay fever. These findings were believed to be consistent with a protective influence of childhood infection that may become lost in the world of modern hygiene. One of the scientists leading these investigations was epidemiologist David P. Strachan (also known as D.P. Strachan). David P. Strachan’s original formulation of the hygiene hypothesis was based on the idea that smaller families provided insufficient microbial exposure; partly because of less person-to-person spread of microbes, but also due to higher standards of home and personal cleanliness. He examined urban living situations and farming families. Strachan's and other studies led to the view that microorganisms and macro-organisms from the environment whether it be soil, animals, or feces play a critical role in immunoregulation and in preventing inappropriate immune responses to allergens and the body itself. It was due to this portion of his research that he named it the “hygiene hypothesis”. D.P Strachan proposed that there is such a thing as too much cleanliness, and that a lack of early exposure to germs and biological stimuli that used to be experienced frequently causes a weakened immune system and response, and therefore causes allergies and similar conditions like hay fever and asthma. In developing nations and previous eras, families typically were much larger than they are today with more than one or two children. When there are more children in a family, the older children tend to expose the younger ones to more germs and the children end up developing stronger immune systems as they have been exposed to stimuli early on in life. This could be due to the fact that children are exposed to germs at school, their friends homes, and when they are physically active outdoors. They then bring these microbes home with them, creating an environment of exposure for their entire families, but especially their siblings with whom they will be around most of the time. This concept of exposure has also been used to research children who attend daycare at early ages and their development of allergies, asthma, and autoimmune conditions. According to many studies, daycare children tend to develop fewer allergies than those who grow up in a more sterile, controlled environment.
Not only did Strachan research health and hygiene among families, he also researched the ‘hygiene revolution’ that began in the early nineteenth century when the health system began to focus more on daily hygiene, which then led to the decrease in medications and more drastic medical interventions and treatments. The concept of personal hygiene also led to many public health changes like the sanitation of water, garbage collections, and the separation of waste areas from living areas. Although these public health changes reduced the prevalence of life threatening diseases like cholera and typhoid fever, it also began depriving people from their “old friends”, the microbes that used to inhabit the same environments. The original formulation of this hypothesis is attributed to D.P. Strachan, but has evolved into the “old friends hypothesis” (also called the biome depletion theory) proposed by Graham Rook in 2003. This new version of the hypothesis provides a rational link between multiple inflammatory disorders and microbial exposure. The original hypothesis only examined reduced microbial exposure in correlation with allergic diseases like asthma, hay fever, and seasonal allergies. The newer proposals apply to a much broader range of chronic inflammatory illnesses such as diabetes, depression, anxiety, and autoimmune disorders like multiple sclerosis. Evidence may also hint that contribute levels of microbe exposure trigger autism and similar disorders. Graham Rook also looks at broader implications of inflammation caused by improper immune responses. Chronic inflammation can trigger the growth of cancer cells, and therefore some forms of cancer could also be linked to his version of the hygiene hypothesis.
The newer proposals of the hygiene hypothesis like the “old friends” theory, argue that vital exposures are to not common childhood infections or recently evolving pathogens as the original hypothesis suggested, but rather microbes that were present in hunter-gatherer times when the immune system was just beginning to evolve. These microbes evolved with the human race, and eventually people became dependent on these microbes in order for their immune systems to develop and function properly. This view of the hygiene hypothesis is also known as the Darwinian approach.
Not much research has been made public on the newer versions of the hygiene hypothesis, however quite a bit of research has been conducted on the original version focusing on the necessity of microbe exposure at a young age.
Although it is a hard leap from mice to humans in a study, researchers at Brigham and Women’s Hospital in Boston have been comparing mice living in ‘normal’ environments with others who have been kept in germ free environments. The germ free mice were found to have high levels of special white blood cells (iNKT) in their lungs and intestines indicating levels of inflammation. These types of cells and inflammation are common in autoimmune diseases as well as asthma, (in the lungs), and ulcerative colitis (intestines). The germ free mice proved to be much more susceptible to disease than the mice living in an environment with natural microbes present. Even when the germ free mice were introduced to a normal environment with bacteria later in their lives, they still had higher levels of inflammatory cells and diseased lungs and intestines (“Early Bacteria Exposure Important for Building Immunity, Study Says”). This evidence backs up the concept of “immune priming”, and that there must be a window at a very early age when microbes must be present in order to create a healthy, strong immune system and response.
Numerous studies are being conducted on infants to try to determine when this “window of opportunity” is as far as the best timing to expose individuals to pathogens and microbes. These studies also provide different avenues of microbe exposure, specifically examining the impact of household pets and siblings on infant microbiota in their gut (Azad, et. al, 2013).
Although to date, the hygiene hypothesis is still unproven and therefore a hypothesis still, there is substantial data being gathered to look at the implications of such a concept. The hypothesis has been used to look at whether or not vaccinations are beneficial, as well as whether hyper-cleanliness at a young age may trigger life long chronic conditions or diseases. Recently, the field of medicine has been using the concept of the hygiene hypothesis to try alternative treatment measures such as probiotics, helminthic therapy, and oral capsilated microbiota transplantation (frozen fecal pills). The idea behind all of these treatment or preventative measures is to reintroduce microbiota to the body in order to heal, reverse, or prevent illnesses. They are used to trigger immune responses, and the hope is that if the immune response can be triggered appropriately, the immune systems may begin to positively adapt to the reintroduction of microbes and become strong. Although there are no health treatments for disease directly responding to changes in hygiene, many lifestyle changes can increase individual and familial exposure to microbes. There is a healthy balance as with anything, and risks associated with allowing more microbe or pathogen exposure. Some simple lifestyle changes could be to have more physical interaction with others (especially as a child), to breastfeed babies, have more natural childbirths, and encourage children to play and interact with non sterile natural outdoor environments. There is still much to be researched and tested about the hygiene hypothesis before it can become scientific fact, however, the concept has already begun to revolutionize the way that people think and react with bacteria in their environments and has also changed the focus of medicine in regards to some autoimmune and allergic conditions.
References
Azad, M., Konya, T., Maughan, H., Guttman, D., Field, C., Sears, M., ... Kozyrskyj, A. (2013, January 1). Infant gut microbiota and the hygiene hypothesis of allergic disease: Impact of household pets and siblings on microbiota composition and diversity. Retrieved December 6, 2014, from http://www.aacijournal.com/content/9/1/15
Bloomfield, S., Stanwell-Smith, R., Crevel, R., & Pickup, J. (5, September 25). Summary. Retrieved December 11, 2014, from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1448690/
Human Energetics. (2012). In S. Stinson, B. Bogin, & D. O'Rourke (Eds.), Human biology: An evolutionary and biocultural perspective. New York: Wiley.
Immune response: MedlinePlus Medical Encyclopedia. (2014, May 1). Retrieved December 1, 2014, from http://www.nlm.nih.gov/medlineplus/ency/article/000821.htm
Mirsky, S. (2011, April 1). Can It Be Bad to Be Too Clean?: The Hygiene Hypothesis. Retrieved December 1, 2014, from http://www.scientificamerican.com/podcast/episode/can-it-be-bad-to-be-too-clean-the-h-11-04-06/
Rook, G. (2012, April 1). A Darwinian View of the Hygiene or “Old Friends” Hypothesis. Retrieved December 2, 2014, from http://www.microbemagazine.org/index.php?option=com_content&view=article&id=4700:a-darwinian-view-of-the-hygiene-or-old-friends-hypothesis&catid=950&Itemid=1301
Welsh, J. (2012, March 22). Early Bacteria Exposure Important for Building Immunity, Study Says. Retrieved December 7, 2014, from http://www.livescience.com/36217-early-bacterial-exposure-immunity.html