Incidence of Inflammatory Bowel Disease in Canterbury, New Zealand, 2018-2023.

Background: New diagnoses of inflammatory bowel diseases (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), can highlight health system burden and potentially give clues to disease aetiology. This population-based study aimed to measure the annual incidence of IBD over six years (2018-2023) in the Canterbury region of New Zealand.

Methods: The medical records from public and private gastroenterology clinics were examined for new patients with a confirmed diagnosis of IBD from 1 January 2018 to 31 December 2023. Demographic and disease information (including the Montreal phenotype) was gathered in a secure database. Information was collected prospectively from 2021 to 2023 and retrospectively from 2018 to 2020.

Results: The incidence of IBD in 2023 was 30.1 per 100,000 person-years (95% CI 25.9-34.7), CD: 14.8 (95% CI 11.9-18.1), and UC: 14.5 (95% CI 11.7-17.7). The average age was 35.0 years (SD 17.2) and New Zealand European was the predominant ethnicity (83.4%), with smaller proportions of Māori (7.7%) and Pacific Islanders (1.7%). Between 2018 and 2023, the annual incidence of IBD was 32.4 per 100,000 person-years. The proportion of CD diagnosed each year decreased, and in 2023, the proportion of CD was equivalent (49%) to the proportion with UC.

Conclusions: The incidence of IBD in Canterbury remains high, and the rates appeared stable over the six years 2018-2023, consistent with the stabilizing incidence stage of the 4-stage epidemiological model of IBD. Māori and Pacific Island ethnicities contributed a small number of cases of IBD, but the rates of IBD in these groups were higher than in previous studies.