The epidemiology and clinical features of rickettsial diseases in North Queensland, Australia: Implications for patient identification and management.

Background Rickettsial infections are a common cause of hospitalization in tropical settings, although early diagnosis is challenging in the rural locations where these infections are usually seen. Methods This retrospective, clinical audit of microbiologically-confirmed cases of scrub typhus or spotted fever group (SFG) rickettsial infection between 1997 and 2016 was performed a tertiary referral hospital in tropical Australia. Clinical, laboratory and radiological findings at presentation were correlated with the patients' subsequent clinical course. Results There were 135 locally-acquired cases (95 scrub typhus, 37 SFG, 3 undifferentiated). There were nine hospitalizations during the first 5 years of the study period and 81 in the last 5 years (p for trend = 0.003). Eighteen (13%) of the 135 cases required ICU admission, all of whom were adults. A greater proportion of patients with SFG infection required ICU support (8/37 (22%) compared with 10/95 (11%) scrub typhus cases), although this difference did not reach statistical significance (p = 0.10). Three (8%) of the 37 patients with SFG infection had severe disease (1 died, 2 developed permanent disability) versus 0/95 scrub typhus patients (p = 0.02). Adults with a high admission qSOFA score (>= 2) had an odds ratio (OR) of 19 (95% CI:4.8-74.5) for subsequent ICU admission (p<0.001); adults with a high NEWS2 score (>= 7) had an OR of 14.3 (95% CI:4.5-45.32) for ICU admission (p<0.001). A patient's respiratory rate at presentation had strong prognostic utility: if an adult had an admission respiratory rate <22 breaths/minute, the negative predictive value for subsequent ICU admission was 95% (95% CI 88-99). Conclusions In the well-resourced Australian health system outcomes are excellent, but the local burden of rickettsial disease appears to be increasing and the clinical phenotype of SFG infections may be more severe than previously believed. Simple, clinical assessment on admission has prognostic utility and may be used to guide management. Author summary Rickettsial infections are a common cause of hospitalization in tropical settings, although early, definitive diagnosis is challenging in the rural and remote locations where they are usually seen. It is important to recognise rickettsial infections early in their disease course as they can lead to life-threatening multi-organ failure if specific anti-rickettsial antimicrobial therapy is not prescribed promptly. In tropical Australia, scrub typhus and spotted fever group (SFG) rickettsiae are the dominant rickettsial pathogens and this twenty-year retrospective series examines the clinical and laboratory findings which might facilitate their recognition. The study highlights the infections' increasing local clinical burden and reports that over 20% of the SFG cases in the series required Intensive Care Unit (ICU) admission, suggesting that severe SFG disease may be more common than previously believed. Simple, clinical prediction scores-calculated at presentation-identified patients who would subsequently require ICU admission. Importantly, they were also able to identify patients at low risk of disease progression. These entirely clinical scores-which can be calculated rapidly at the bedside-have the potential to facilitate the management of patients with scrub typhus and SFG infection, particularly in resource-limited settings which have the greatest burden of disease.