Rheumatoid arthritis (RA) incidence varies significantly with place of residence in the USA, a large cohort study reveals.
Women born in New England and the Southeast were 46% and 45% more likely to been diagnosed with RA than those born in the West, an analysis that adjusted for the women's age suggests.
"Potential explanations include regional variation in behavioral factors, climate, environmental exposures, RA diagnosis, and genetic factors," says the team.
Karen Costenbader (Brigham and Women's Hospital, Boston, Massachusetts, USA) and co-workers analyzed data for 83,456 participants in the Nurses' Health Study.
They collected information on the women's region of residence at enrolment in 1976, and also asked them in 1992 about their place of residence at birth and at age 15 and 30 years.
Overall, 706 women developed RA during the follow-up from 1976-2004, the team reports in the Archives of Internal Medicine.
Case patients were diagnosed with RA at an average of 57.6 years of age, but those living in the West were diagnosed an average of 3 years later than those in the East (60.1 vs 57.3 years of age).
Rheumatoid arthritis patients living in the East were more often rheumatoid factor positive at diagnosis than those in the West (60.9% vs 50.5%), the team reports.
For residents who stayed in the same region from birth to age 30 years, those in New England and the Midwest were significantly more likely to develop RA than those who stayed in the West, with an odds ratio (OR) of 1.46 after adjusting for age.
Nurses who lived in the East at all three time points had the highest risk for RA, compared with those who lived in the West at each time point (OR = 1.38).
However, a comparable increase in risk was seen among women from the West if they moved to the East before 30 years of age (OR = 1.34)
"This study represents the most detailed analysis of the differences in RA incidence rates in US women between geographic regions to date," say Costenbader et al.
They conclude: "We hypothesize that the risk of RA among women living in the Midwest and eastern United States from 1921 to 1976 could be attributable to an environmental exposure."
It was one of the explanations..
"Potential explanations include regional variation in behavioral factors, climate, environmental exposures, RA diagnosis, and genetic factors," says the team.