Plenary 2
Peri-domestic Transmission of Scrub Typhus in South India

The Indian subcontinent is emerging as a region with a particularly high burden of scrub typhus. While in many endemic countries scrub typhus is most common in remote populations living in mountainous areas, scrub typhus in India also affects people living in low land regions with a high population density. Traditional risk factors for scrub typhus such as agricultural activities seem to be of lesser importance in India. To better understand reasons for the high incidence of scrub typhus in South India, we conducted a human cohort study and a vector study in a highly endemic rural setting in Tamil Nadu. The cohort study aimed at identifying risk factors for sero-incidence of scrub typhus in 2206 rural residents from 37 villages in Tamil Nadu over two years. The vector study was done in 25 of these villages, where we trapped small mammals in three types of habitat: village centre, village edge and in fields surrounding villages. We collected trombiculid mite larvae from trapped hosts. Mites and host spleen tissue were tested for Orientia infection. We identified 270 cases of seroconversions (incidence rate 78/1000, 95%CI 67, 91). In multivariable analysis, agricultural and related outdoor activities were only weakly associated with scrub typhus. However, agricultural activities were strongly associated with scrub typhus if local sero-prevalence was low, but not if it was high. Females were at a higher risk than males in high prevalence areas but not in low prevalence areas. Living in a house that was surrounded by other houses tended to increase the risk of scrub typhus, while living close to the village edge did not increase risk. In the vector study we identified 12,431 mite larvae on 883 small mammals, 32% of which were Bandicota indica, 31% Suncus murinus and 31% Rattus rattus. Leptotrombidium imphalum was the most common mite species, and the only species associated with O. tsutsugamushi infection (prevalence of infection 3.6%). Small mammal numbers and the L. imphalum index were considerably higher within human settlements than in surrounding fields. Further, the prevalence of Orientia infection in chiggers was higher in the village centre (4.5%) and village edge (4.2%) than in the fields (1.9%). The findings from our studies suggest that in South India, most human scrub typhus infection may occur within human settlements rather than during agricultural work in the fields or near forest edges.