Asthma and the hygiene hypothesis
Asthma is a widespread chronic health condition, and the major cause of the hospitalisation of children in Australia. Asthma deaths may be declining, but the disease is becoming more common throughout Australia and the world: its prevalence has doubled throughout the world in only 15 years.
For many people with asthma, it is an allergy that brings on (‘triggers’) their asthma symptoms. In fact, allergy is a likely cause of asthma in 90 per cent of children with asthma and 50 per cent of affected adults. Exposure to the substance to which you are allergic (the ‘allergen’) is considered to be an asthma trigger.
The increasing levels of asthma and allergic diseases have been attributed by some researchers to environmental factors such as a lack of exposure to infections in the first years of life due to our more hygienic environment.
The so-called ‘hygiene hypothesis’ blames the environmental conditions of modern society, such as higher standards of hygiene, altered patterns of infant feeding and use of antibiotics, for the worldwide increase.
How does the hygiene hypothesis explain rising levels of asthma?
The theory is that our lifestyle has changed, and we live in a more hygienic environment, but this affects the maturing of our immune system, allowing the more primitive allergic responses to persist beyond infancy.
To understand this theory, it’s helpful to understand a little about the immune system.
Some research indicates that when we are born, the majority of our immune responses are of the type known as Th2. Scientists believe Th2-type responses are contributors to allergy and atopy (the inherited tendency to be ‘allergic’).
As we are exposed to infections in childhood the body’s immune system develops the ability to fight off bacteria and other potentially harmful foreign substances by producing antibodies. Once created, these antibodies become a part of our body’s immune system so that next time we are exposed to the same substances, our body is already prepared to fight them off. This type of response is called a Th1-type response and it commonly develops in the first few years of life. The maturation of our immune responses from Th2-type to Th1-type responses depends on both environmental and genetic factors.
The hygiene hypothesis suggests that our germ-free environment means children are less exposed to infections and are less likely to develop these Th1-type responses. This theoretically allows the persistence of the Th2-type responses which result in allergy and atopy.
Evidence for this hygiene hypothesis comes from a number of studies which have shown that children who have had frequent colds and respiratory infections are less likely to develop asthma and allergy as they get older.
Is your home too clean?
Some researchers have suggested that exposure to a component of dust — bacterial endotoxin — early in life may protect against the development of allergen sensitisations, such as asthma.
For example, one US study of 61 babies with a history of wheezing found that the homes of the children who were most sensitive to allergens (16 per cent) had significantly lower concentrations of bacterial endotoxin than those of children who were not sensitised to any allergens.
This supports the idea that early exposure to bacteria could help prevent us from becoming overly sensitive to potential allergens.
Can antibiotic use lead to asthma in children?
Other studies have suggested that the use of antibiotics in childhood may lead to some children developing asthma and other allergic diseases. This is because early childhood infections are thought to have a protective effect against asthma by means of the development of antibodies. Using antibiotics may prevent the body from developing this response.
For example, one study analysed the prevalence of asthma and allergies in 1800 children aged 7 or 8, taking into account use of antibiotics in early childhood and other potentially important elements such as the history of respiratory infections. The study found that early childhood use of antibiotics may increase the risk of developing asthma or allergies if the child is already predisposed to these types of ‘allergic’ responses, say from having a family history.
How does day care affect your child?
Evidence suggests that there’s a reduced chance of a child developing asthma or frequent wheezing later in childhood if they have been exposed to older siblings or other children at day care facilities.
In one US study, more than 1000 children were studied from birth. Researchers found that having at least one older brother or sister at home or having attended day care (defined as a child-care setting where at least 6 unrelated children were present) in the first 6 months of life protected against the development of asthma later in childhood.
This suggests that exposure of young children to older children at home or day care exposes them to more infections during early childhood, prompting their immune system to kick into action, and therefore helping to protect them from developing asthma and wheezing later on.
How can the hygiene hypothesis help people in the future?
Although the hygiene hypothesis is still controversial, some researchers believe that by more fully understanding the effects of our environment on our body’s immune responses, we can also learn more about the role our genes play in the development of conditions such as asthma.
By more fully understanding the roles of both genetics and environment, it’s possible that new methods of treating and preventing asthma could be developed in the future. For example, doctors might be able to help people who are genetically susceptible to allergy and asthma by exposing them to harmless substitutes for bacterial infections. If the hygiene hypothesis is correct, this exposure could help their body’s immune system to protect them from developing asthma and allergies.
Last Reviewed: 25 March 2003
Sponsored links
