Atopic dermatitis (AD), asthma and allergic rhinoconjunctivitis (ARC) are a major cause of chronic disease in childhood. A revised version of the "hygiene hypothesis" suggests that the pattern of colonisation and the diversity of the intestinal microbiota may be an important factor in the increased prevalence of these diseases observed over the past several decades.[1–3] Subsequently, probiotics have been investigated in the prevention and treatment of allergy related diseases,[3–8] with the strongest evidence emerging for the primary prevention of atopic dermatitis.[3–5] Throughout this paper we refer AD, asthma and ARC as "allergy related diseases", recognising that not all presentations of these conditions are related to a classic IgE-mediated inflammatory process.
Randomised controlled trials (RCTs) testing probiotics in the prevention of childhood allergy related disease are heterogeneous and have used a variety of bacterial strains, administration regimes and varying ages of follow-up. Using information from the first published follow up for each trial, a recent meta-analysis concluded that probiotic administration is protective against the development of AD in infancy. Among the studies with follow-up at or beyond 5 years of age, the greatest protective benefit of probiotics against AD appears to be in early childhood and it is less certain if this effect persists until school age.[9–25] Only one of these studies did not specify a maternal or family history of atopy as an inclusion criteria,[9,10] and there is therefore a particular need for further longer term follow-up studies to determine the ongoing effect of perinatal probiotics in general populations.