title: Environmental exposures and the risk of multiple sclerosis investigated in a Norwegian case-control study.
abstract: BACKGROUND
  Several environmental exposures, including infection with Epstein-Barr virus, low levels of vitamin D and smoking are established risk factors for multiple sclerosis (MS). Also, high hygienic standard and infection with parasites have been proposed to influence MS risk. The aim of this study was to investigate the influence of various environmental exposures on MS risk in a Norwegian cohort, focusing on factors during childhood related to the hygiene hypothesis.
  
  
  METHODS
  A questionnaire concerning environmental exposures, lifestyle, demographics and comorbidity was administrated to 756 Norwegian MS patients and 1090 healthy controls. Logistic regression was used to calculate odds ratio (OR) with 95% confidence interval (CI) for the risk of MS associated with the variables infectious mononucleosis, severe infection during childhood, vaccination and animals in the household during childhood. Age, gender, HLA-DRB1*15:01, smoking and infectious mononucleosis were included as covariates. General environmental exposures, including tobacco use, were also evaluated.
  
  
  RESULTS
  Infectious mononucleosis was confirmed to be significantly associated with increased MS risk, also after adjusting for the covariates (OR = 1.79, 95% CI: 1.12-2.87, p = 0.016). The controls more often reported growing up with a cat and/or a dog in the household, and this was significant for ownership of cat also after adjusting for the covariates (OR = 0.56, 95% CI: 0.40-0.78, p = 0.001). More patients than controls reported smoking and fewer patients reported snuff use.
  
  
  CONCLUSIONS
  In this Norwegian MS case-control study of environmental exposures, we replicate that infectious mononucleosis and smoking are associated with increased MS risk. Our data also indicate a protective effect on MS of exposure to cats during childhood, in accordance with the hypothesis that risk of autoimmune diseases like MS may increase with high hygienic standard.
authors.forename: Marte Wendel
authors.initials: MW
authors.lastname: Gustavsen
authors.forename: Christian Magnus
authors.initials: CM
authors.lastname: Page
authors.forename: Stine Marit
authors.initials: SM
authors.lastname: Moen
authors.forename: Anja
authors.initials: A
authors.lastname: Bjølgerud
authors.forename: Pål
authors.initials: P
authors.lastname: Berg-Hansen
authors.forename: Gro Owren
authors.initials: GO
authors.lastname: Nygaard
authors.forename: Leiv
authors.initials: L
authors.lastname: Sandvik
authors.forename: Benedicte Alexandra
authors.initials: BA
authors.lastname: Lie
authors.forename: Elisabeth Gulowsen
authors.initials: EG
authors.lastname: Celius
authors.forename: Hanne F
authors.initials: HF
authors.lastname: Harbo
mesh_terms: D000328:Adult; D000818:Animals; D016022:Case-Control Studies; D002415:Cats; D004285:Dogs; D004781:Environmental Exposure; D005260:Female; D006801:Humans; D007244:Infectious Mononucleosis / Q000150:complications; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000453:epidemiology* / Q000209:etiology*; D009664:Norway / Q000453:epidemiology; D016017:Odds Ratio; D057805:Pets; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects; D011795:Surveys and Questionnaires; D055815:Young Adult