Eco-serological study of scrub typhus (Orientia tsutsugamushi) among indigenous populations in South India.

Scrub typhus, a vector-borne zoonotic disease, remains a growing public health concern in India, particularly in the Western Ghats. We conducted a sero-prevalence and ectoparasite survey during 2023-2024 among indigenous populations of Kothagiri and Gudalur blocks, Nilgiris district, Tamil Nadu. A total of 240 serum samples from the general population with a mean age of 17 (Range: 5-45) years were screened using IgM and IgG ELISA. In Kothagiri, IgM and IgG positivity rates were 5% and 2.5%, respectively, whereas Gudalur reported 5.83% and 43.33%. Overall, 28% of sera tested positive, with higher rates in females (31.45%) than males (24.58%). Among indigenous communities, Paniyas (56.82%) and Kurumba (58.33%) groups in Gudalur recorded elevated sero-positivity, particularly in rural and forest-fringe areas. Screening of 79 undifferentiated febrile illness cases revealed a 32.9% positivity rate, with the Paniyas and Kattunayakkan groups most affected. Rodent surveillance yielded 42 hosts representing five species, harboring 3516 trombiculid mites. Six species were identified in Kothagiri and nine in Gudalur, including Leptotrombidium langati (new country record), and L. puta and Schoengastiella singularis (first reports from southern India). These findings confirm active transmission of scrub typhus in Nilgiris and demonstrate considerable ecological diversity of vectors. Targeted surveillance, community awareness, and vector control strategies are warranted to mitigate disease risk among vulnerable indigenous populations.