The Epidemiology of Psychosis in Indigenous Populations in Cape York and the Torres Strait.

Background: The treated prevalence of psychotic disorders in remote communities of Cape York and the Torres Strait, Australia, has been shown to be elevated compared with the Australian population. Our study used a unique dataset to assess treated incidence and prevalence of psychotic disorders and mortality over a 23-year period in the adult Indigenous population of this region.

Methods: Data was collated from a clinical database that contains complete psychiatric records from 1992 to 2015, extracted for all Indigenous patients who received treatment for a psychotic disorder from the Remote Area Mental Health Service, and linked to the Queensland Deaths Registry. We calculated 12-month treated prevalence and incidence for each calendar year. Mortality rates were compared to the overall and Indigenous population death rates in Queensland.

Findings: Between 1992 and 2015, 424 patients were treated for psychosis - an age-standardised 12-month prevalence of 1·7% in 2015, approximately two times higher in men than women, and three times higher in Aboriginal versus Torres Strait Islander populations. The highest treated prevalence was observed in 2015 in Aboriginal men (4.0%). A range of psychotic disorders were detected, including many substance-induced cases (n = 93) and schizophrenia (n = 252). The age-standardised 12-month incidence rate over the study period was 3.61 per 1000 person-years for women and 4.23 per 1000 person-years for men. Treated prevalence increased throughout the study period, largely attributable to increases in incidence of schizophrenia and schizoaffective disorder - in contrast, the incidence and prevalence of bipolar and mood disorders remained low and stable. Increased mortality risk compared to the Queensland Indigenous population (SMR = 1.9; 95% CI 1.4-2.6) was attributable to the elevated risk shown in the Aboriginal population in our study (SMR = 2.6; 95% CI 1.8-3.7).

Interpretation: Our results show extremely high prevalence rates of psychosis; increasing prevalence over time; differences in the distribution of psychosis between Aboriginal and Torres Strait Islander populations; and increased mortality risk for Aboriginal people living with psychosis in this region. These observations strongly suggest an aetiological role of environmental and neurodevelopmental factors, and the contribution of social factors to vulnerability and premature mortality.

Role Of The Funding Source: This study was funded by Queensland Health who are the custodians of this database. The funder had no role in study design, data analysis, data interpretation, writing of the report, or submission for publication. All authors had full access to all the study data. The corresponding author had final responsibility for the decision to submit for publication. FJC is supported by an Australian National Health and Medical Research Council (NHMRC) Early Career Fellowship (APP1138488).