The spatial distribution and epidemiology of trachoma: application and evaluation of geographical information in defining disease burden and planning control

The last decade has seen significant progress towards the elimination of blinding trachoma as a health problem. However, gaps in our understanding of the epidemiology of trachoma at large scales are increasingly important in the context of programmatic scale up. This thesis therefore aimed to define the current distribution and burden of trachoma, in addition to investigating the spatial heterogeneity of trachoma and underlying risk factors at different scales. A systematic review of trachoma prevalence data was used to generate the Global Atlas of Trachoma, a unique spatially-referenced global database. In addition to highlighting important regional differences in the geographic distribution of trachoma, this database was used to quantify the disease burden in Africa; estimating nearly 153,000 disabilityadjusted life years (DALYs) attributed to trachomatous vision loss and 155,500 additional DALYs to trichiasis. Detailed analyses of individual and cluster-level risk factors underlying the distribution of trichiasis in Nigeria and active trachoma in Kenya identified a number of key socio-demographic and environmental factors. Both analyses suggested that spatial dependency (generated by underlying associations with shared risk factors at larger scales) may vary in endemic areas. These findings emphasise the importance of local epidemiology and the need for robust and well-designed survey methodologies to identify areas of high risk. Computerised simulations were used to evaluate the performance of Integrated Threshold Mapping (ITM) in comparison to the accepted gold standard trachoma survey design. The results found that ITM tended to underestimate the prevalence of trachoma across a range of epidemiological contexts where attendance was low and/or the risk of disease was lower in school-going children. This thesis provides the first systematic investigation into the geography of trachoma; highlighting heterogeneities at different scales and their potential programmatic implications. In particular, the findings and methods from this thesis may help to inform future survey design. 3