Epidemiology and risk factors in a scrub typhus cluster in Odisha, India, 2023: Investigating with a One Health approach.

INTRODUCTION: Scrub Typhus accounts for approximately one-fourth of all acute undifferentiated febrile illness cases in India. In September 2023, Odisha, an eastern Indian state, reported several scrub typhus deaths. We investigated a scrub typhus cluster in Block-A, Odisha, to describe the epidemiology and assess risk factors. METHODOLOGY: We conducted an unmatched (1:1) case-control investigation. Line-list of scrub typhus IgM-ELISA tested individuals was collected from the district laboratory. Cases were defined as scrub typhus IgM-ELISA positive residents of Block-A between July1 and October30, 2023 and controls were test-negative block-A residents. We interviewed cases and controls from list of tested individuals for socio-demographic, clinical and exposure histories. We present adjusted-odds ratio and 95% confidence intervals. We conducted an entomological survey in Block-A, isolated mites to detect pathogen and strain (47kDa Real Time PCR) and report entomological indices. RESULTS: Total 151 cases were reported in 2023 in Block-A, with case-fatality rate of 2.6%, median age 30 years, and inter-quartile range 15–46 years. Among the 61 interviewed cases, 46% were hospitalised, 13% admitted in ICU, 16% visited health-facility in second week of symptom onset and 30% were tested in second week of symptom-onset. We enrolled 61 cases and 61 controls, among whom recent farm/forest visit was significantly associated [adjusted OR = 6.1, 95%CI: 2.0-18.4] with illness on multivariate analysis. Chigger-index for block-A was 18.94(322/17); 36%(4/11) of tested mite-pools were positive for Orientia tsutsugamushi and sequences were identified to be Karp-like. CONCLUSIONS: Our investigation confirmed a lab-verified scrub typhus cluster in Block-A, identifying farm/forest visits without protective clothing as the most significant factor for acquiring infection. Detection of infected vector, hosts, and pathogen completed the epidemiological triad of scrub typhus, establishing local transmission in Block-A. We sensitised frontline-healthcare workers on early identification and timely referral, recommended community education on limiting mite exposure and seeking timely medical attention.