title: Relevance of the hygiene hypothesis to early vs. late onset allergic rhinitis.
abstract: BACKGROUND
  The hygiene hypothesis proposes that reduced exposure to infections in early life increases the risk of developing allergic conditions including allergic rhinitis. We examined the association between markers of the hygiene hypothesis and allergic rhinitis that developed before 7 years of age and allergic rhinitis that developed after 7 years of age.
  
  
  METHODS
  The Tasmanian Longitudinal Health Study (TAHS) is a population-based cohort (n=8583) study of respiratory disease. Participants have been followed from 7 to 44 years of age. Information on potential risk factors, allergies and respiratory symptoms was collected longitudinally. Using multi-nomial logistic regression, exposure to siblings, infections, tonsillectomy and farm residence during childhood were examined as risk factors for allergic rhinitis that developed before or after 7 years of age. All analyses were adjusted for gender, maternal and paternal atopy, mother's age at participant's birth, paternal socio-economic status in 1968 and personal socio-economic status in 2004.
  
  
  RESULTS
  Greater cumulative exposure to siblings before the age of 2 years was strongly inversely associated with early onset allergic rhinitis (<1 year sib exposure: OR=0.6, 95% CI 0.3-1.0; 1-3 years sib exposure: OR=0.6, 95% CI 0.4-0.9; >3 years sib exposure: OR=0.4, 95% CI 0.3-0.8) less so with later onset allergic rhinitis. The risk of early onset allergic rhinitis decreased with increasing viral infections (OR=0.7, 95% CI 0.5-0.9) during childhood. Having a tonsillectomy before 7 years of age increased the risk of early onset allergic rhinitis (OR=1.7, 95% CI 1.2-2.5). None of these factors was associated with later onset allergic rhinitis.
  
  
  CONCLUSIONS
  Exposures relevant to the hygiene hypothesis were important predictors for the development of early onset but less so for later onset allergic rhinitis. The exact mechanisms by which siblings and infections protect against allergic rhinitis are unclear. The stronger findings for earlier onset allergic rhinitis suggest that family structure and infections have most impact on disease risk in early life. Further research should focus on early onset allergic rhinitis when exploring causal explanations for any sibling effect.
authors.affiliation: Centre for Molecular, Environmental, Genetic and Analytic Epidemiology, School of Population Health, The University of Melbourne, 1/723 Swanston Street, Carlton, Vic. 3053, Australia. mcmat@unimelb.edu.au
authors.forename: M C
authors.initials: MC
authors.lastname: Matheson
authors.forename: E H
authors.initials: EH
authors.lastname: Walters
authors.forename: J A
authors.initials: JA
authors.lastname: Simpson
authors.forename: C L
authors.initials: CL
authors.lastname: Wharton
authors.forename: A-L
authors.initials: AL
authors.lastname: Ponsonby
authors.forename: D P
authors.initials: DP
authors.lastname: Johns
authors.forename: M A
authors.initials: MA
authors.lastname: Jenkins
authors.forename: G G
authors.initials: GG
authors.lastname: Giles
authors.forename: J L
authors.initials: JL
authors.lastname: Hopper
authors.forename: M J
authors.initials: MJ
authors.lastname: Abramson
authors.forename: S C
authors.initials: SC
authors.lastname: Dharmage
mesh_terms: D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D001315:Australia; D001424:Bacterial Infections / Q000453:epidemiology; D002648:Child; D005260:Female; D006801:Humans; D006920:Hygiene; D006967:Hypersensitivity / Q000453:epidemiology; D008137:Longitudinal Studies; D008297:Male; D015999:Multivariate Analysis; D016017:Odds Ratio; D010290:Parents; D015995:Prevalence; D006255:Rhinitis, Allergic, Seasonal / Q000453:epidemiology / Q000209:etiology*; D012737:Sex Factors; D035781:Siblings; D012959:Socioeconomic Factors; D014068:Tonsillectomy / Q000706:statistics & numerical data; D014777:Virus Diseases / Q000453:epidemiology; D055815:Young Adult