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Aamodt, Geir*; Vatn, Morten H.†
*Norwegian Institute of Public Health, Oslo, Norway; and †University of Oslo, Oslo, Norway.
Ulcerative colitis and Crohn's disease are assumed to be caused by a dysregulation in the immune system due to the commensal bacteria flora in genetically susceptible individuals. Incidence rates of the diseases, however, show large geographic and temporal variability. The temporal increase is often explained by the “hygiene hypothesis”, stating that reduced number of infections in early life will increase the likelihood for developing autoimmune diseases in adulthood, but this hypothesis is less likely to explain differences between countries. We will therefore decompose worldwide incidence rates of ulcerative colitis and Crohn's disease and correlate the incidence values to latitude, temperature, time, and economic conditions.
We used published incidence studies of both Crohn's disease and ulcerative colitis from 1966 to 2007. The incidence rates were modelled as functions of latitude, mean annual temperature for the study areas, year of the study, and gross national product per capita.
Incidence rates of both diseases increase with increased gross national product per capita (P<0.001 and P = 0.046 for UC and CD). Incidence rates of ulcerative colitis were significantly related to latitude (P = 0.028). For Crohn's disease incidence rates were associated to both latitude and mean annual temperature (P = 0.016, P<0.001).
Incidence rates for both diseases are related to economic changes and either latitude and/or mean annual temperature. Mean annual temperature is also assumed to reflect latitudinal change. We postulate that both economic and latitudinal variability are aspects of the same phenomena: microbial diversity. Improved economic conditions are related to a reduced microbial load. Species diversity is also known to be larger closer to equator than far from equator–the latitudinal diversity gradient. Reduced contact with commensal flora or “old friends”, due to reduced microbial load or ecological conditions could therefore explain the worldwide trends in incidence rates.
© 2009 Lippincott Williams & Wilkins, Inc.
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