Early-life risk factors and allergic rhinitis: Comparing European and US data

Allergic rhinitis has been shown to affect 3% to 19% of subjects in various epidemiologic studies of different populations. Eighty percent of those with allergic rhinitis experience symptoms by the age of 20 years, and 40% have symptoms by the age of 6 years. 1 Allergic rhinitis is a substantial burden on both adult and pediatric patients, not only because of the bothersome physical symptoms but also because of the emotional and social effects. 2 , 3 It is also closely linked with the development of asthma. 4 Investigation into early-childhood factors influencing the development of allergic rhinitis raises the possibility of altering its natural history and decreasing this significant negative effect. In this issue of the Journal, Matheson et al 5 present data from the European Community Respiratory Health Study II (ECRHS II) regarding the association between the incidence of rhinitis and early-life exposures. The most striking findings of this study were that a number of early-life exposures seem to be protective against rhinitis. The incidence of rhinitis decreased with increased number of siblings, bedroom sharing with older children before the age of 5 years, exposure to pets before the age of 5 years, and farm upbringing, regardless of atopy status. Risk factors for rhinitis included maternal smoking in pregnancy and childhood and, as one would expect, a parental history of allergic disease. Focusing on atopic subjects only, with respect to the incidence in childhood, adolescence, and adult life, the authors found that having any siblings was associated with a reduced incidence of rhinitis. Pet ownership in the first 5 years of life reduced the incidence of rhinitis in adolescence but not adulthood. Farm upbringing was also associated with a reduced incidence of rhinitis in adolescence. The authors created a combined early-life variable, which included siblings, pets, and farm upbringing, and found that each factor had an additive protective effect. Maternal smoking during pregnancy and childhood was again a risk factor for rhinitis in atopic subjects and remained so in all stages of life. Comparing these ECRHS II data with US data on early exposures and rhinitis reveals striking parallels but also some divergence. The large size of the ECRHS II cohort, which spans many countries, ethnic backgrounds, and socioeconomic levels, is a major strength of this study. Such large studies regarding rhinitis in the US do not exist, which must be taken into account for any data comparison. In the US, as regards number of siblings, a study of African American and Dominican children living in low-income neighborhoods in New York City found no association between birth order and allergic rhinitis prevalence from birth to age 3 years. The authors of this study postulated that there might be other environmental or social factors masking a birth-order effect. They also postulated that follow-up until only the age of 3 years might not have been enough time for a birth-order effect to manifest. 6 A follow-up study examined the association between birth order, atopy, and respiratory symptoms in 4-year-old children from inner-city New York City and did not find a significant decrease in the prevalence of rhinitis among children determined to be atopic by means of serology with or without siblings. 7 Certainly there are differences between these 2 studies and the study by Matheson et al 5 : the cohorts