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Emerging Diseases and the Hygiene Hypothesis

The media has been full of stories warning the public of a new “super bug” that is threatening the health of our communities. Despite the advances in cleaning products, antimicrobial agent soaps, and an emphasis public health, these infectious diseases continue to thrive and seek out new targets for disease. In the late 1990’s West Nile Virus gained concern as the never before pathogen, spread by mosquito bite and carried by wild birds, began showing up in wild birds across the United States. Our awareness to biological agents was thrust to the forefront of our culture as a result of the anthrax attacks following 9/11. Avian flu, or H5N1, has had our attention on and off for the last few years as the hotly debated question of mutation and the possibility of pandemic is discussed. Several incidents of illness and death have been cited due to pathologic Escherichia coli (E. coli) O157:H7, and most recently, the so-called super bug Methicillin Resistant Staphylococcus aureus or MRSA.

Methicillin Resistant Staphylococcus aureus or MRSA, along with its counterpart ORSA (Oxacillin Resistant Staphylococcus aureus) have been nosocomial, or hospital acquired infections of concern for quite some time infecting the patient who has undergone an invasive procedure or the long term care patient with a compromised immune system. Hospital and other health care workers can become infected with ORSA/MRSA as a result of the performance of their duties. Health care providers may become infected with ORSA/MRSA by 1) contact with colonized or infected patients, 2) colonized or infected sites of the personnel (clothing or unprotected areas) and, 3) exposure to contaminated surfaces or items such as medical devices, or environmental surfaces. The good news is that practicing standard body substance isolation and good personal hygiene (hand washing) can prevent many of these exposures.

The question remains, however, as to why these diseases continue to flourish in a society rich with countermeasures such as anti-microbial soaps and cleaners. Why do they continue to evolve to resist traditional treatment and migrate to infect otherwise healthy people in the community?

Certain theories propose that cleaner is not always better. One such theory is the Hygiene Hypothesis which began circulation as a theory on the relationship between exposure to viruses and bacteria and the development of a healthy immune system around 1989. The Hygiene Hypothesis proposes that exposure to environments that are less than germ-free as an infant or toddler builds a stronger immune system and reduces the risk of developing disease later in life.
How can this be possible? It seems counterintuitive to claim that a more microbe-free environment is not automatically equated to better health. We can, in fact, look to a simple analogy of the immune system for a better perspective. Let’s compare the immune system to voluntary muscle for a moment. When voluntary muscle is stressed, as in the case of exercise, the muscle becomes stronger; the hygiene hypothesis is merely suggesting the same principle applies to the immune system. The exposure a young, developing immune system to repeated small doses of various virus and bacteria endemic to a given environment can be considered as exercise for the immune system. “That which does not kill us makes us stronger” may be a better name for the hypothesis.

Although debated, several findings suggest that early infection with certain bacteria have reduced the severity of later immune response to allergens in mice.
The Hygiene Hypothesis has been implicated as a factor in the rise of asthma and some community acquired immunologic diseases. Research suggests that our society’s overuse of anti-microbial/anti-biotic materials, such as cleaners and soaps, has caused people to be exposed to fewer diseases during immune system development. Scientists point out that asthma and other diseases linked to the immune system have been on the rise for the last 30-40 years despite exposure to fewer infectious organisms. The thought is that these organisms are a key component in training and creating a healthy immune system. Without early exposure, the developing immune system fails to respond at an appropriate level and may actually overreact to irritants that would otherwise be inconsequential. The result is an increased overreaction of the immune system equating to conditions such as asthma. Other research has indicated that children who are exposed to high levels of the bacterial compound, endotoxin, found in house dust may actually be less likely to develop eczema during their first year of life3. Finding such as this support the hygiene hypothesis by indicating that early exposure to infectious or inflammatory agents cause changes in the immune systems and reducing the risk of developing allergy-related conditions later in life.

The hygiene hypothesis remains unproven but continues to be tested by a variety of researchers. I think my analogy comparing the immune system to the voluntary muscular system make a lot of sense. For an immune system to work property it must be trained and experienced, but if training and experience is lacking, the immune system defaults to “going to war” when all that may be needed is to fire a single shot. Our approach to develop items such as anti-bacterial soaps is not entirely bad…I think the marketing and media hold much of the responsibility for this situation.

1.Bacterial Infections Alter Allergic Response; Findings Support Hygiene Hypothesis. Science Daily/national Jewish Medical and Research Center accessed 10 October 2007 accessed 10 October 07
2.Bacterial toxin may protect infants from asthma. National Institutes of Health; Medical News Today accessed 10 October 07
3.Infants who have fevers become children with fewer allergies. National Institute of Allergy and Infectious Diseases; Medical News Today accessed 12 October 07
4.S.B. Levy: Antibacterial Household Products: Cause for Concern. Emerging Infectious Diseases vol. 7, No. 3 Supplement, June 2001 accessed via PDF 12 October 07
5.Erwin W. Gelfand, MD. The Hygiene Hypothesis Revisited: Pros and Cons from Selected Coverage of the 60th Anniversary Meeting of the American Academy of Allergy, Asthma and Immunology accessed 13 October 07 via Medscape
6.Study dishes the dirt on hygiene’s role in disease. CBC News 2006/16/rats-hygiene.html accessed 12 October 07
7.The hygiene hypothesis: Are cleaner lifestyles causing more allergies for kids? Science Daily accessed 12 October 07
8.Edward Willett: The hygiene hypothesis. Online weblog accessed 12 October 07
9.MRSA in Healthcare Settings, Centers for Disease Control and Prevention accessed 15 October 07
10.Information About MRSA for Healthcare Personnel, Centers for Disease Control and Prevention accessed 15 October 07