Objectives: It has been speculated that exacerbations of inflammatory bowel disease are influenced by seasonal variations.
Methods: This hypothesis was tested in a large data base of the Health Care Financing Administration (HCFA), consisting of all hospitalized US Medicare beneficiaries from four consecutive years.
Results: The total of hospital admissions (for all diagnoses) was characterized by a marked seasonal variation occurring similarly in each consecutive year. It was high during winter and low during summer. Peak and trough of admission rate varied by 10%. In striking contrast to this general pattern, both Crohn's disease and ulcerative colitis showed an annual pattern of variability without any clear-cut seasonality. However, the time trends of both diseases showed a striking parallelism concerning their short-term changes (affecting 1-3 consecutive months) and long-term changes (affecting 4-8 consecutive months). The parallelism of the temporal changes affected different age and sex groups alike.
Conclusions: The parallel monthly variations suggest that exacerbations in the activity of inflammatory bowel disease may be modulated by exogenous factors, and that Crohn's disease and ulcerative colitis may be influenced by identical modulators of disease activity.