The hygiene hypothesis is a theory that attempts to explain the increased prevalence of allergy and asthma in affluent, industrialized nations. It also strives to elucidate factors that are responsible for the development of asthma in individuals. The British epidemiologist David Strachan advanced the beginnings of the hypothesis in 1989 after studying the health records of 17,414 British children born during a week in March 1958 and followed up to the age of 23 years. “
Hay Fever, Hygiene, and Household Size,” published in the November 18, 1989 issue of The British Medical Journal, sought to correlate the presence or absence of childhood hay fever and eczema with data on 16 perinatal, social, and environmental factors. The resultant hypothesis proposes that the rising prevalence of asthma and allergic diseases parallels the decreasing prevalence of infections in childhood. Over the last 100 years, urbanization, advances in public health, improved sanitation, and the adoption of cleaner living environments, along with the introduction of antibiotics, have all led to reduction in infectious illnesses in children. During the same period, the occurrence of asthma and allergic diseases has increased. The hygiene hypothesis links the two observations.
The hypothesis suggests that the reduced exposure to “dirty” environments and to infectious agents at a specific point in childhood leads to less stimulation of certain parts of a growing child’s immune system. Changes consequently fail to take place in the maturing immune response, and the absence of those changes, in turn, predisposes that child to an increased risk of developing allergies or asthma. Epidemiologic studies lend credence to the hygiene hypothesis. Exposure to a farming environment, for example, and to farm animals in particular, appears to protect against the development of asthma. Children raised on farms encounter a different range of organ-isms (animals, viruses, bacteria) than do children raised in industrialized urban centers. Several studies have shown that the children from farming communities have a lessened occurrence of asthma, hay fever, and allergy. Another example that strongly supports aspects of the hygiene hypothesis is provided by the recent work of Dr.
Martin Blaser, an infectious dis-ease specialist and prominent researcher who studies Helicobacter pylori. H. pylori is a bacterium found world-wide; it is usually acquired early in life. H. pylori is a cause of recurrent stomach and duodenal ulcers and is associated with stomach cancer in adulthood. With the advent of improved living conditions and of antibiotic therapies in contemporary westernized societies during the 20th century, the rates of childhood infection with H. pylori have decreased dramatically. That decrease has occurred against the backdrop of increasing childhood asthma (and allergy) leading to the hypothesis that childhood acquisition of H. pylori is associated with reduced risks for asthma and allergy. Further, Dr. Blaser and his colleagues reviewed data collected between 1988 and 1994 from 7663 persons as part of the U.S.
Centers for Disease Control and Prevention’s third National Health and Nutrition Survey (NHANES) as well as that from 7412 individuals from a NHANES follow-up in 1999–2000. The findings demonstrate a significant correlation between the absence of H. pylori infection and early onset asthma in children and teens. The association suggests that acquisition of H. pylori in childhood does indeed confer protection from asthma and allergic conditions. The exact cellular mechanism of how exposures protect a person from developing asthma and allergy is unclear. One possibility is that increased numbers of infections or exposure to farm animals (or pets) might stimulate the child’s immature immune system to develop along immunologic pathways that lead away from asthma. Research continues in the area of the hygiene hypothesis.
An ambitious and far-reaching, ongoing study in that regard is the GABRIEL study launched in 2006, which seeks to identify the genetic and environmental causes of asthma in the European community. GABRIEL consists of a collaboration among 35 partners at major scientific research institutions across the European community, and has recently added partners from Ecuador, Russia, and Hong Kong. The study investigates the genetics, epidemiology, and immunology of asthma in children and adults across several countries.
It also will specifically address the hygiene hypothesis. You can follow the researchers’ progress at http://www.gabriel-fp6.org. Is there an age at which a child’s immature immune sys-tem needs to be stimulated in a specific way, by certain environmental agents, in order for asthma not to develop? If such were the case, specific interventions or medications could perhaps be developed to modify a child’s risk of asthma. The hygiene hypothesis remains controversial and represents an intriguing theory that is far from definitive at the present. Asthma practitioners require scientific evidence to validate the theory, and so no clinical recommendations can be advanced right now, based on what remains a very interesting conjecture.