Abstract
Inflammatory bowel diseases (IBD) are chronic immune-mediated conditions that can cause disabling symptoms for patients and can come with high costs for the health system. Understanding the epidemiology of IBD in terms of incidence, prevalence, and mortality can support research and health services planning and may give clues to disease aetiology. This thesis describes the current descriptive epidemiology of IBD in Canterbury and uses this, along with data from previous periods, to position the region within the 4-stage epidemiological model of IBD. Disease outcomes, including medication use, hospitalisation, surgery and mortality, were evaluated. In addition, an alternative administrative data-collection approach is considered for descriptive epidemiological studies. The major findings of this research include:
- The high incidence and prevalence rates of IBD seen in Oceania are driven by studies from Australia and New Zealand, as there is very little data from the Pacific Islands. There is heterogeneity between the studies, with recent population-based research reporting higher incidence and prevalence rates. Crohn’s disease (CD) is the predominant disease type in children and there were more male than female children with IBD.
- The incidence of IBD in Canterbury has stabilised, with the current rate being higher than it was in 2004 but lower than in 2014. New Zealand European remains the largest ethnic group with small, but increased, proportions of Māori and Pacific Peoples being diagnosed with IBD. The ratio of new cases with CD has equalised with ulcerative colitis (UC).
- Canterbury currently has the highest reported prevalence in Oceania, with 1 in 150 people having IBD. This is nearly 3 times the prevalence observed in 2005 and indicates Canterbury is in the compounding prevalence stage of the epidemiologic model of IBD. This high rate of disease also likely represents a significant burden for the health system.
- In the first 10 years of diagnosis, most patients with IBD experience disease progression or treatment escalation. High rates of immunomodulator use, hospitalisation and surgery occurred within the Canterbury incident cohort. Younger age at diagnosis, CD and perianal disease were associated with immunomodulator or biologic use, and male gender was associated with surgery.
- Patients with IBD were at increased risk of mortality compared to the general population over 20 years of follow-up. Older age and primary sclerosing cholangitis (PSC) were associated with increased risk of death, and common causes of death included cancer, heart disease and pneumonia. This was one of the first population-based mortality studies from the Southern Hemisphere, and these findings highlight the negative impact IBD has on life expectancy even under contemporary medical treatment.
- Administrative health data within the integrated data infrastructure (IDI) was used for the first time to define an IBD population and estimate incidence, prevalence and mortality over a 16-year period. This alternative approach has the potential to reduce data collection costs associated with medical record review studies, although the limitations associated with hospital admissions and International Classification of Diseases (ICD)-coded data, need to be recognised.
In summary, the data outlined in this thesis positions New Zealand alongside other Western countries in the compounding prevalence stage of the IBD epidemiological model. The high rates of IBD observed represent a significant burden to the health system and patients, demonstrated by the high proportion of disease progression and treatment escalation observed in patients following diagnosis. Furthermore, this research presents current mortality information showing individuals with IBD are at increased risk of dying. The IDI is highlighted as an IBD data collection approach that may be explored as an alternative to labour-intensive medical record review.