CAN you be too clean? That is the question posed by the hygiene hypothesis, which seeks to explain why, as many illnesses have become rarer in rich countries, some have become more common. The hygiene hypothesis posits that the rise of several of these diseases, including asthma, eczema and type-1 diabetes (all of which seem associated with malfunctions of the immune system), has been caused by improvements in hygiene of the sort that have helped get rid of other illnesses. Exactly how that might happen is unclear. But at the AAAS meeting Brett Finlay of the University of British Columbia, in Vancouver, persuasively filled in some of the blanks in the case of asthma.
Asthma is caused by chronic inflammation of the airways, and inflammation is an immune response. The thinking behind the hygiene hypothesis is that a lack of exposure to parasites and pathogens in what has become an unnaturally clean environment means a child’s immune system does not develop appropriately. Evidence that asthma is a consequence of overcleanliness includes the facts that farm-raised children are less prone to it than city-raised ones (farms are full of bacteria and other critters that provoke immune responses), that those born by Caesarean section are more prone than others (they do not receive an initial bacterial inoculation from maternal faeces and vaginal fluids), and that those treated with antibiotics as babies are also more prone. Dr Finlay, therefore, wondered if he could find bacteria which might be involved in asthma protection in the guts of children.
To this end he got in touch with the organisers of the Canadian Healthy Infant Longitudinal Development (CHILD) study, which looks at the development of children from birth to the age of five. He asked if the study’s organisers could include the regular collection of faeces as part of their protocol and he thus obtained stool samples taken at the ages of three months, 12 months and annually thereafter, the bacterial contents of which he analysed.
Asthma does not normally manifest itself before a child is five, but a tendency to wheeze and a reaction to a particular skin-prick test are good indicators that the child in question will eventually become asthmatic. Recording both of these are routine parts of CHILD. Dr Finlay was therefore able to correlate the composition of an infant’s gut flora with the presence or absence of these indicators. When he did so he found that children deficient, at the age of three months, in four relatively rare bacteria, Faecalibacterium, Lachnospira, Rothia and Veillonella, were 20 times more likely than those playing host to these species to manifest the two predictive indicators…