Background: Acute undifferentiated febrile illness (AUFI) may have similar clinical presentation, and the etiology is varied and region specific. Materials and Methods: This prospective observational study was conducted in a tertiary hospital in South India. All adult patients presenting with AUFI of 3-14 days duration were evaluated for etiology, and the differences in presentation and outcome were analyzed. Results: The study cohort included 1258 patients. A microbiological cause was identified in 82.5% of our patients. Scrub typhus was the most common cause of AUFI (35.9%) followed by dengue (30.6%), malaria (10.4%), enteric fever (3.7%), and leptospirosis (0.6%). Both scrub typhus and dengue fever peaked during the monsoon season and the cooler months, whereas no seasonality was observed with enteric fever and malaria. The mean time to presentation was longer in enteric fever (9.9 [4.7] days) and scrub typhus (8.2 [3.2] days). Bleeding manifestations were seen in 7.7% of patients, mostly associated with dengue (14%), scrub typhus (4.2%), and malaria (4.6%). The requirement of supplemental oxygen, invasive ventilation, and inotropes was higher in scrub typhus, leptospirosis, and malaria. The overall mortality rate was 3.3% and was highest with scrub typhus (4.6%) followed by dengue fever (2.3%). Significant clinical predictors of scrub typhus were breathlessness (odds ratio [OR]: 4.96; 95% confidence interval [CI]: 3.38-7.3), total whole blood cell count >10,000 cells/mm 3 (OR: 2.31; 95% CI: 1.64-3.24), serum albumin <3.5 g % (OR: 2.32; 95% CI: 1.68-3.2). Overt bleeding manifestations (OR: 2.98; 95% CI: 1.84-4.84), and a platelet count of <150,000 cells/mm 3 (OR: 2.09; 95% CI: 1.47-2.98) were independent predictors of dengue fever. Conclusion: The similarity in clinical presentation and diversity of etiological agents demonstrates the complexity of diagnosis and treatment of AUFI in South India. The etiological profile will be of use in the development of rational guidelines for control and treatment of AUFI.
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