Environmental factors linked to reporting of active malaria foci in Thailand

BackgroundThailand has made substantial progress toward malaria elimination, with 46 of the countrys 77 provinces declared malaria free as part of the subnational verification program. Nonetheless, these areas remain vulnerable to the reintroduction of malaria parasites and the reestablishment of indigenous transmission. As such, prevention of reestablishment (POR) planning is of increasing concern to ensure timely response to increasing cases. A thorough understanding of both risk of parasite importation and receptivity for transmission is essential for successful POR planning. MethodsRoutine geolocated case- and foci-level epidemiological and case-level demographic data were extracted from Thailands national malaria information system for all active foci from October 2012 to September 2020. A spatial analysis examined environmental and climate factors associated with remaining active foci. A logistic regression model collated surveillance data with remote sensing data to investigate associations with the probability of having reported an indigenous case within the previous year. ResultsActive foci are highly concentrated along international borders, particularly Thailands western border with Myanmar. Although there is heterogeneity in the habitats surrounding active foci, land covered by tropical forest and plantation was significantly higher for active foci than other foci. The regression results showed that tropical forest, plantations, forest disturbance, distance from international borders, historical foci classification, percentage of males, and percentage of short-term residents were associated with high probability to report indigenous cases. ConclusionThese results confirm that Thailands emphasis on border areas and forest-going populations is well placed. The results suggest that environmental factors alone are not driving malaria transmission in Thailand; rather, other factors, including demographics and behaviors, may also be contributors. However, these factors are syndemic, so human activities in areas covered by tropical forests and plantations may result in malaria importation and potentially, local transmission, in foci that previously had been cleared. These factors should be addressed in POR planning.