Scrub typhus and depression: a nationwide cohort analysis

BACKGROUND: Studies on the relationship between depression and scrub typhus are limited. We conducted a retrospective cohort study to investigate whether scrub typhus is a risk factor for depression. METHODS: Using Taiwan’s National Health Insurance Research Database, this study investigated the incidence of depression, and its risk factors, in patients diagnosed with scrub typhus between 2000 and 2010. Scrub typhus patients who did not have a history of depression before the index date were enrolled. For each patient with scrub typhus, four controls without a history of scrub typhus and depression were randomly selected and frequency matched by sex, age, year of the index date, and comorbidities. The follow-up period was from the time of initial scrub typhus diagnosis to the date of diagnosis of depression, censoring, or December 31, 2010. Cox proportional hazards regression models were used to analyze the risk of depression according to sex, age, and comorbidities. RESULTS: The study comprised a 5238-patient scrub typhus group and a 20,952-patient non-scrub typhus group with similar sex and age distributions. During the follow-up period, the cumulative incidence of depression was higher in the scrub typhus than the non-scrub typhus group (log-rank test P < 0.001). In the scrub typhus group, 45 patients developed depression, yielding an incidence rate of 1.67 per 1000 person-years, and in the non-scrub typhus group, 117 patients developed depression, yielding an incidence rate of 1.08 per 1000 person-years. This yielded a crude hazard ratio (HR) of 1.55 (95% confidence interval [CI] 1.41–1.70) and adjusted HR (aHR) of 1.56 (95% CI 1.42–1.71). Compared with the non-scrub typhus group, the risk of depression in the scrub typhus group was higher in patients of both sexes (men: aHR = 1.46, 95% CI 1.29–1.64; women: aHR = 1.68, 95% CI 1.45–1.96), in patients aged younger than 65 (≤ 49 years: aHR = 1.95, 50–64 years: aHR = 1.73), and in patients without comorbidities (aHR = 2.06, 95% CI 1.85–2.29). CONCLUSIONS: The risk of depression was 1.56-fold higher in patients with scrub typhus than in the general population.