Early‐life hygiene‐related factors affect risk of central nervous system demyelination and asthma differentially

Summary The increasing prevalence of immune‐related diseases, including multiple sclerosis, may be partly explained by reduced microbial burden during childhood. Within a multi‐centre case–control study population, we examined: (i) the co‐morbid immune diseases profile of adults with a first clinical diagnosis of central nervous system demyelination ( FCD ) and (ii) sibship structure in relation to an autoimmune ( FCD ) and an allergic (asthma) disease. FCD cases ( n  = 282) were aged 18–59 years; controls ( n  = 558) were matched on age, sex and region. Measures include: history of doctor‐diagnosed asthma; sibling profile (number; dates of birth); and regular childcare attendance. FCD cases did not differ from controls with regard to personal or family history of allergy, but had a greater likelihood of chronic fatigue syndrome [odds ratio ( OR)  = 3·11; 95% confidence interval ( CI) 1·11, 8·71]. Having any younger siblings showed reduced odds of FCD ( OR  = 0·68; 95% CI: 0·49, 0·95) but not asthma ( OR  = 1·47; 95% CI: 0·91, 2·38). In contrast, an increasing number of older siblings was associated with reduced risk of asthma ( P trend = 0·04) but not FCD ( P trend = 0·66). Allergies were not over‐represented among people presenting with FCD . Sibship characteristics influence both FCD and asthma risk but the underlying mechanisms differ, possibly due to the timing of the putative ‘sibling effect’.