Leptotrombidium imphalum Chiggers as Vector for Scrub Typhus in Human Settlements, India, 2022-2023

Scrub typhus is a common bacterial infection in many parts of Asia. The causative agent, Orientia tsutsugamushi, is transmitted by trombiculid mite (chigger) larvae that require small mammals as maintaining hosts. We studied the prevalence of O. tsutsugamushi infection in mites and small mammals in villages and land surrounding them in South India to determine high-risk settings. We identified 12,431 mite larvae on 883 small mammals, 32% of which were bandicoot rats, 31% black rats, and 31% Asian house shrews. Leptotrombidium imphalum was the most common mite species and the only species associated with O. tsutsugamushi infection (prevalence 3.6%). Orientia infection increased with mite population size on a host. Host numbers, the L. imphalum index, and the prevalence of Orientia infection in chiggers were considerably higher within human settlements than in surrounding fields, suggesting that most human scrub typhus infection occurs inside villages rather than during agricultural work.