The Epidemiology of Rickettsial Infections in South India: Cohort Study

The aim of the cohort study is to better understand the epidemiology, sero-epidemiology and transmission of scrub typhus, murine typhus and tick-borne spotted fever. The overall focus of the work will be on scrub typhus, being the most important of the three from the public health perspective. We aim to determine the "severity pyramid" of serological, symptomatic and severe infection. The results from this study will help clinicians to understand the natural course of scrub typhus infection, health policy makers to estimate the global burden and the scope for community-based interventions, and immunologists to better understand the immunogenicity of single and repeat infections.

The specific objectives are to:

1. Estimate the incidence of symptomatic and severe scrub typhus infection in the community.
2. Estimate the incidence of serological (asymptomatic) scrub typhus infection.
3. Estimate the effect of previous scrub typhus infection (sero-positive at baseline) on the risk of subsequent infection and disease severity (subclinical vs clinically apparent). This includes the effect of antibodies in mothers on the risk of severe infection in young children via residual maternal antibodies.
4. Estimate the effect of potential risk factors such as age, gender, occupation, water/sanitation access and comorbidity on the risk of scrub typhus infection.
5. Estimate the effect of spatial-temporal risk factors for scrub typhus by identifying high risk areas, accounting for human population density and land use.
6. Compare the rate of rodent trappings and vector parameters of chiggers between high risk and low risk areas identified under 5).
7. Estimate the disease burden associated with scrub typhus and the economic impact of scrub typhus in terms of loss of earnings and health expenditure.
8. Estimate the incidence of asymptomatic and symptomatic murine typhus and spotted fever infection and explore associated socio-demographic, spatial and vector-related risk factors.

STUDY OVERVIEW

The study will be a population-based cohort study over two years (i.e. two scrub typhus seasons) in about 30,000 people living in approximately 40 villages with a sero-prevalence of at least 15% (see flow diagram above). These individuals will be followed-up through active surveillance for recent febrile illnesses (clinical cohort). For passive surveillance, participants will be encouraged to notify the study team in case of fever or come to one of 4 local study clinics. At study clinics we will also enrol fever cases who are not part of the clinical cohort, i.e. have not been enrolled prior to their fever. The study will include a smaller cohort (sero-cohort) for serological surveillance of 4000 people drawn from the 30,000 people of the clinical cohort. Longitudinal serological surveys linked to active surveillance are essential to estimate true attack rates unbiased by disease severity. The sero-cohort will consist of three annual blood testing in the same group of participants over two scrub typhus seasons (baseline, inter-season, endline).

Target population- A pilot study showed that scrub typhus infection is highly focal This makes it difficult to define a target population for the purpose of making inferences. We will define the target population in terms of the village level sero-prevalence. The aim is to correlate the incidence of scrub typhus with the sero-prevalence at village level. This will allow using the results of the study to predict the expected incidence and burden of scrub typhus in other settings with a known sero-prevalence, and eventually estimate the global burden of scrub typhus (not part of the study but a potential extension of the work). The target population for the study is therefore "villages in which the IgG sero-prevalence is at least 15%".