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Most physicians who have cared for atopic patients over the last two decades recognize the sharp increase in the prevalence of allergic conditions, such as rhinitis, asthma, eczema, and food allergies. Several international epidemiologic studies demonstrate the magnitude of this increase:
- Atopy defined by increased sensitization to aeroallergens increased at least 50% over the past 30 years, correlating with increased allergic rhinitis.
- Asthma cases in the US are up 60% since 1980 and more than doubled in other countries around the world.
- Allergic skin disorders, especially eczema, have tripled in many global areas since 1990.
- Food allergy (particularly peanut) mediated by IgE has increased in prevalence of sensitization and positive food challenges by two- to three- fold in the past decade.
- Life-threatening anaphylaxis, rarely reported as isolated case histories up to 1960, now occurs in as many as one per 1000 patients due to drugs, stings, and foods.
Numerous hypotheses and theoretical mechanisms have been offered for this apparent global epidemic of allergic diseases of the last two decades. Since the time frame is generally too short to attribute primary causality to genetic factors, most experts consider environmental changes to be the main influence driving this heightened atopic tendency. Several influences have been noted to increase the development of atopy:
- Cigarette smoking in the home (especially by mother) increases an exposed child’s risk for allergic sensitization and asthma.
- Formula feeding and early introduction of solids reduce exclusive nursing, which is known to protect breast-fed babies from allergic sensitizations and atopic diseases.
- Factors associated with climate change have effects on growth of vegetation, amount and antigenicity of pollen produced, and dispersal patterns that may worsen allergies.
- Air pollutants such as ozone, sulfur dioxide, volatile organic chemicals, and diesel exhaust particles directly aggravate underlying respiratory inflammation and may act as adjuvants to enhance IgE production.
- Various chemicals from pesticide and fertilizer applications, as well as food additives, preservatives, flavor and color enhancers, etc., have numerous potential effects on increasing allergies.
The so-called “Hygiene Hypothesis” remains the most popular theory to account for how environmental changes lead to the recent allergy epidemic. During this 30 year timespan, significant advances in public health, sanitation, immunization, antimicrobial usage, and personal hygiene around the world have led to major reductions in exposure to infectious micro-organisms, especially early in childhood. The basic theoretical tenet – that we have made our environment “too clean” – involves a potential shift in the production of T helper cells from Th1 (infection fighting) to Th2 (allergy enabling) subtypes. Thus, without as much needed stimulus to fight infection, the immune system realigns itself to enable allergic responses to predominate. Several observations support this theory:
- The risk for allergic disease is greater in children from small families of high socio-economic status and lower in large families when older sibs attend school or daycare.
- The prevalence of sensitization and allergic diseases increased dramatically in children living in former East Germany when they became westernized within five years after re-unification.
- The incidence of allergy is very low in children growing up on farms with livestock, especially if exposed in utero or as young infants to barn animals.
- Atopic diathesis, particularly eczema and asthma, increases in children receiving antibiotics very early in life, or prenatally when their mothers took antibiotics.
- Although evidence is conflicting, most studies support the use of probiotics during pregnancy or infancy to decrease risk and severity of allergic diseases later.
In conclusion, the epidemic of atopic disease has led to greater prevalence rates of both IgE sensitization and clinical allergic disorders, especially in countries with improving health and economic conditions over the past several decades. The “Hygiene Hypothesis” offers the most plausible reason for immunologic changes favoring allergic mechanisms in populations no longer at such high risk for infectious diseases. However, there are many unanswered questions regarding how and why allergies develop in the first place. The allergenic nature of the sensitizing substance, the route of exposure, and the age at which a patient comes into contact all seem to be important factors in determining allergic disease. The complexity of mechanisms involved in this epidemic of allergy shows the difficulty in developing effective means to prevent allergies in the first place!
Mark L Vandewalker, MD, FACAAI
Allergy and Asthma Consultants
Columbia, Missouri, USA
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Bio:
Dr Mark Vandewalker is a board-certified pediatric allergist working in the largest private allergy group in Missouri for the past 25 years. He actively conducts clinical trials of new allergy and asthma therapies, teaches medical students and residents, and has served in leadership positions at both state and national levels of his specialty, Allergy/Immunology.
Resources:
- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2841828/
- http://www.allergyclinic.co.nz/guides/71.html
- http://www.asehaqld.org.au/index.php/allergy-information/allergy-epidemic/125-lifting-the-lid-on-the-allergy-epidemic
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