Seasonal variations in the onset of ulcerative colitis in Japan.

AIM To investigate seasonal variations in the onset and relapse of ulcerative colitis (UC) in Japanese patients. METHODS Between 1994 and 2006, 198 Japanese patients diagnosed with UC according to conventional criteria in an academic hospital were enrolled for onset evaluation. Among 265 Japanese patients with UC who were observed for more than 12 mo, 165 patients relapsed (239 times) and were enrolled for relapse evaluation. The patients' symptoms were recorded each month for 12 consecutive years. RESULTS There was monthly seasonality in symptom onset during October and March for UC. The onset of symptoms in UC patients frequently occurred during the winter. Variation in UC onset was observed according to both month (P = 0.015) and season (P = 0.048). Relapse commonly occurred in October, and variations in relapse were not significant either in month (P = 0.52) or season (P = 0.12). Upper respiratory inflammation was the main factor responsible for relapse. CONCLUSION Our results suggest that environmental factors associated with winter and spring seasonality may be responsible for triggering the clinical onset of UC in Japan.