The Hygiene Hypothesis

By Shawn Pichette

Introduction

• Purpose:  To determine whether childhood exposure to infections has a direct effect on the increase of atopy (also known as the “hygiene hypothesis), specifically, asthma.

• History: Proposed in 1989 article in British Medical Journal by David P. Strachan.– Hay Fever and Eczema less prevalent in children from large

families than families with one child.– Conclusion: Children from large families are exposed to more

infectious agents from siblings, therefore, an increased exposure to infectious agents leads to a decreased susceptibility to atopy.

– Since 1989, the hygiene hypothesis has been studied extensively by immunologists, and epidemiologists.

Hazard Identification

• Atopy: A type I hypersensitive or allergic reaction for which there is a genetic predisposition… Hay fever and asthma are two of the most commonly inherited allergies… As with all type I hypersensitivity reactions, IgE is the primary antibody involved .

• Asthma: A disease caused by increased responsiveness of the tracheobronchial tree to various stimuli, which results in episodic narrowing and inflammation of the airways.

• Infection: A disease caused by microorganisms, especially those that release toxins of invade body tissues.– (All definitions were taken from Taber’s Cyclopedic Medical

Dictionary)

Exposure Assessment

• Complex interaction of genetic and environmental factors.

• Causes unknown, however, one hypothesis is the hygiene hypothesis.

• Usually contracted orally, when children touch unclean objects and touch their mouths.

• Also contracted via inhalation of pollen, or pet dander from exposure.

Hazard Assessment • Atopy: 7% of American

population are affected by asthma (about 22 million people).

• Cause for over 2 million emergency room visits every year (WebMD).

• Prevalence of asthma rose 75% from 1980 to 1994.

• From 2001 to 2003, there was an average of 4,210 deaths associated with asthma annually (CDC).

• In 1980, there were 2,891 deaths, with more deaths occurring every 5 year period until 1995.

• Infection: Infant mortality dropped from 15% in 1900 to less than 1% today.

• Children used to die from typhoid, diphtheria, polio, others.

• Controlled today by indoor plumbing, vaccinations, and antibiotics.

Risk Characterization Analysis

• Journal of Allergy Clinical Immunology, 2006:– Study by the National Institute of Environmental Health

Sciences of 9,835 subjects 12 years of age and older.– Epidemiological study based on levels of IgG:

Immunoglobulin that is responsible for producing immunity in infant before birth, to Porphyromonas gingivalis, found in the oral cavity, and responsible for periodontal disease.

– Findings: Higher IgG antibodies to p. gingivalis were associated to children with lower prevalence of asthma.

Risk Characterization Analysis Continued

• Journal of Epidemiological Community Health 2004:– Study by the Centre for Public Health Research in

New Zealand of 1,584 children notified to public health services between ages 0 and 4, and 2,539 children from the general public.

– No major distinction between two groups for wheezing (23.5% to 24.3%, respectively), or major site of infection.

– There was a correlation between children who used antibiotics and asthma (odds ratio: 1.78).

Conclusion

• Hill’s Criteria– Strength – Medium– Consistency – Medium– Specificity – Medium/High– Temporality – Medium/High– Biological Gradient – Unknown– Plausibility – Medium– Coherence – Medium/High– Experiment – No Available Human Experiment– Analogy – Genetic Causes Should Also Be Considered

• Conclusion: Since there is an overall medium association, and because both childhood infection and atopy are serious, moderation between germs and sterility should be practiced.