Geographical variation in the incidence of Crohn's disease and ulcerative colitis according to the latitude of residence has been reported in Europe. 

However, there are no comparable data in the USA. 

Dr Andrew Chan and colleagues from Massachussetts, USA assessed the incidence of Crohn's disease and ulcerative colitis in relation to latitude in a geographically diverse population of women enrolled in two large prospective studies in the USA.

The team undertook a prospective study of women enrolled in the Nurses' Health Study I (NHS) in 1976 and in the NHS II in 1989.

The hazard ratios for women residing in southern latitudes was 0.5 for Crohn's disease

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Information on state of residence at the time of birth, at age 15 years and age 30 years was collected in 1992 in NHS I and in 1993 in NHS II.

 

Reported diagnoses of incident Crohn's disease or ulcerative colitis to the end of 2003 were confirmed by medical record review.

In both cohorts, among 175,912 women reporting their residence in 1992, 257 cases of Crohn's disease and 313 cases of ulcerative colitis were documented over 3,428,376 person-years of follow-up.

The team found that incidence of Crohn's disease and ulcerative colitis increased significantly with increasing latitude, with residence at age 30 years more strongly associated with risk.

Compared with women residing in northern latitudes at age 30, the multivariate-adjusted hazard ratios for women residing in southern latitudes was 0.5 for Crohn's disease, and 0.6 for ulcerative colitis. 

 

The researchers observed that the effect of latitude of residence on risk of Crohn's disease and ulcerative colitis did not vary according to smoking history.

Dr Chan's team concludes, "In a population of US women, increasing latitude of residence was associated with a higher incidence of Crohn's disease and ulcerative colitis."