The Hygiene Hypothesis is a theory that suggests a young child’s environment can be “too clean” to effectively stimulate the developing immune system to respond to various “threats” such as allergens.

The history of this theory began in the lab with a series of animal studies.  Rats and mice that were genetically prone to develop diabetes did not get the disease when exposed to infections or normal lab facilities, but they did develop diabetes if raised in sterile conditions.  Similar studies with infections showed mice raised in sterile conditions would sicken and die when exposed to bacteria that would not affect normally raised mice. But if these mice raised in sterile conditions were gradually exposed to the same bacteria, they were able to tolerate this.

Human epidemiology studies were done by several investigators.  Strachan in England looked at 17,000 children born in 1958 and followed, and found that the more siblings they had in their family, the less likely these children were to develop hay fever.

Von Mutius in Germany compared East and West German children for development of asthma.  Her theory was that the more “dirty” and rural East would have higher rates of childhood asthma, but she found exactly the opposite.

The science behind this hypothesis is rooted in the concept that there is a developed imbalance in the developing immune system, perhaps via the T helper cells (Th1, Th2) so that unless both are stimulated in the developing child, there is overreaction leading to autoimmune and atopic conditions.

Epidemiological studies have shown that having more than one sibling seems to protect children against hay fever, asthma, multiple sclerosis and type 1 diabetes.  Daycare attendance in the first 6 months of life lessens the risk of asthma and atopic dermatitis.  Farm exposure protects against atopic diseases, especially if the mother was exposed during pregnancy.  Parasitic infections seem to protect against asthma.

Some other examples of this possible effect are the “treatment” of autoimmune diseases, such as patients with active Crohn disease and ulcerative colitis given parasite eggs, and their symptoms improved compared to those given placebo.  In a Finnish study, probiotics given to pregnant women 2-4 weeks prior to delivery and then for 6 months after birth of their child, reduced atopic dermatitis in their children for 7 years.  Hepatitis A infection seemed to protect children from developing atopic diseases.  Sardinian children, who were hospitalized with non-typhoidal salmonellosis, were less likely to develop hay fever and asthma.

Some other interesting facts that support this hypothesis are that 37% of infants less than one month old have C. difficile in their stool, yet they do not become ill.  A recent study in England had peanut protein given to high risk infants and children for the first five years of life, those receiving the peanut protein were many times less likely to develop peanut allergy than the infants who avoided peanuts entirely.

There have been concerns that the name should be changed, as we do not want to stop handwashing to prevent spreading bacteria and viruses..  “Microbial exposure or microbial deprivation” have been suggested, but I think the ring of “hygiene hypothesis” will probably stick.

References:

            Davis, C.P. (2014).  The Hygiene Hypothesis, MedicineNet.com.

Bloomfield, S.F., Stanwell-Smith, R., Crevel, R.W.R., & Pickup, J. (2006).  Too Clean, or Not Too Clean:  The Hygiene Hypothesis and Home Hygiene, Clinical and Experimental Allergy, 36, 402-425.

Okada, H., Kuhn, C., Feillet, H. & Bach, J.-F. (2010). The “Hygiene Hypothesis” for Autoimmune and Allergic Diseases: an Update, Clinical and Experimental Immunology, 160, 1-9.

Mirsky, S. (2011). Can it be Bad to be Too Clean?:  The Hygiene Hypothesis, Scientific American, podcast http://www.scientificamerican.com/podcast/episode/can-it-be-bad-to-be-too-clean-the-h-11-04-06/

Backgrounder brief from PBS series “Evolution,” Show 4 “The Evolutionary Arms Race”, Chapter 8; “Good Germs” discussing Dr. Von Mutius’ work

Clostridium difficile Infection in Infants and Children, (2013). Pediatrics, 131, 196-200.

Consensus Communication on Early Peanut Introduction and the Prevention of Peanut Allergy in High-risk Infants (2015). Pediatrics, 136, 800.