Accelerating towards malaria elimination in south East Asia: identifying and delineating residual pockets of transmission
Author: Sandfort, Mirco
Under the direction of: Ivo Mueller
Sorbonne University
English text
Keywords: Epidemiology, Southeast Asia, Malaria, Epidemiology, Elimination.
Abstract
After decades of global malaria reduction, some countries are now aiming to eliminate this disease. The countries of the Greater Mekong Subregion committed to elimination by 2030. With intensified elimination efforts, malaria transmission becomes fragmented, and high incidence is restricted to remote areas. Some groups are particularly exposed, e.g. forest goers in Cambodia. Risk patterns must be well understood to tailor interventions to the local context. Missed milestones indicate the need for more efficient elimination programmes. This thesis set out to delineate risk heterogeneity in a remote, high-incidence part of Cambodia. Across villages along a gradient of proximity to the forest, we stratified the risk of PCR-detectable infections by gender, age, forest-going, and residence inside or outside the forest. We applied established tools for spatial cluster detection and introduced the spatial signature method to malaria epidemiology, i.e. determining the prevalence within increasing distance to index infections. This way, we also assessed spatial and spatiotemporal clustering of infections in observational studies from Brazil, Thailand, Solomon Islands, and Senegal. In Cambodia, we demonstrated that pockets of high prevalence of Plasmodium spp. infections persist in the country, predominated by P. vivax and asymptomatic, sub-patent infections. This burden can decrease to near zero within a few kilometres of forested areas. While forest work by male adolescents and adults is the strongest behavioural driver of infection risk, in villages inside the forest the whole population is affected. Particularly in Cambodia, Brazil, and Thailand, infections increasingly clustered closer to other infections in space and time. Clustering tended to intensify at lower overall prevalence in the study area. Given remaining pockets of high transmission, Cambodia has scaled up its village and mobile malaria worker programme. The country has also finally started implementing radical cure against P. vivax relapses. However, rapid diagnostic tests and passive case detection will continue to miss most asymptomatic, sub-patent infections. The more homogeneous risk in forest goers and villages could justify mass drug administration or testing and treatment. The clustering of infections in a few hundred metres argues for reactive approaches that include neighbouring households. In summary, this thesis improved the understanding of persisting malaria and offered new tools that hopefully help to accelerate towards malaria elimination.
