High incidence of inflammatory bowel disease in Northern Australia: a prospective community population-based Australian incidence study in the Mackay-Isaac-Whitsunday region

Background and aims: To determine the incidence of inflammatory bowel disease (IBD) in the Mackay-Isaac-Whitsunday region in Northern Queensland (-21.14° S) and to allow a comparison with Southern Australian and New Zealand data (Geelong, Australia -38.14° S; Tasmania -41.43° S and -42.88° S (Launceston and Hobart) and Canterbury, New Zealand -43.46 °S).

Design: A prospective observational community population-based IBD study was conducted between 1 June 2017 and 31 May 2018.

Outcome measures: Primary includes the crude annual incidence rate of IBD, Crohn's disease (CD), ulcerative colitis (UC) and inflammatory bowel disease-unclassified (IBDU), while secondary includes disease phenotype and behaviour.

Results: Fifty-six new cases of IBD were identified. Twenty-three were CD, 30 were UC and 3 were IBDU. The crude annual incidence rate per 100 000 for IBD, CD, UC and IBDU were 32.2 (95% confidence interval (CI): 24.78-41.84), 13.23 (95% CI: 8.79-19.90), 17.25 (95% CI: 12.06-24.67) and 1.73 (95% CI: 0.56-5.35). When directly age-standardised to the World Health Organisation Standard Population Distribution, the overall CD, UC and IBDU incidence were 13.19, 17.34 and 1.85 per 100 000, with an overall age-standardised IBD incidence of 32.38.

Conclusions: This is the first study to define the incidence of IBD in a Northern Australian cohort and to allow a comparison between North and Southern Australia. The IBD crude is the highest reported in Australia. Like others, we found a high and low incidence of upper gastrointestinal Crohn's disease and complicated disease at diagnosis respectively, likely reflective of the increased availability and early uptake of endoscopic procedures.