ORIGINAL ARTICLE, p 616
When Erika Isolauri and colleagues published the first evidence that probiotic bacteria might reduce eczema risk, their data were greeted with a mixture of excitement and scepticism.1,2 The sceptics have been given further fuel by the disappointing results of probiotic trials for eczema treatment.3 However, there is a persistent signal from randomized controlled trials suggesting that probiotics may be effective for eczema prevention, at least in infants at high risk of developing allergic disease.4–8 In this issue of the Journal, Dotterud and colleagues9 report reduced odds of eczema (odds ratio 0·51) in the children of probiotic-supplemented mothers. Their trial is unusual in two ways. First, its use of a general population sample means that their findings might be generalized to inform a public health intervention in communities where eczema is prevalent. Second, they treated pregnant and breastfeeding women with probiotics for just 4 months, without any treatment of infants. Again this takes us one step closer to a public health intervention, as maternal supplementation is easier than infant supplementation. So why aren’t we recommending probiotic treatment for all pregnant and breastfeeding women in countries with high eczema prevalence? The difficulty is that clinical trial results are inconsistent – while in broad terms this intervention fits with the ‘microbiota version’ of the hygiene hypothesis and appears to be effective, the details are confusing. Selection of the right probiotic appears to be critical,5 yet use of the same probiotic in two different populations gave very different results.1,10 Likewise, while some studies suggest that direct infant supplementation is effective, others suggest that maternal treatment or even prenatal treatment alone may be effective.4,9 Indeed, no single probiotic eczema prevention study has replicated the results of another one, using the same probiotic(s) in the same way. As Dotterud and colleagues have shown, probiotics are our most promising intervention for reducing the burden of eczema via primary prevention, yet we don’t really understand how they work, or what we are trying to achieve at an immunological or microbiological level. We may not need to understand the precise mechanism of action in order to turn this into a public health intervention, but we do need some of the positive trials to be replicated so that we can confirm which probiotic(s) we should use and when to use them.