Environmental factors linked to reporting of active malaria foci in Thailand

Abstract

Background: Thailand has made substantial progress toward malaria elimination, with 46 of the country's 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. Methods: Routine geolocated case- and foci-level epidemiological and case-level demographic data were extracted from Thailand's 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. Results: Active foci are highly concentrated along international borders, particularly Thailand's 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. Conclusion: These results confirm that Thailand's 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.

Mapping malaria transmission foci in Northeast Thailand from 2011-2021: approaching elimination in a hypoendemic area

“…Changes in human processes have presented challenges to control strategies in the past and the possibility that they may do so in the future should be prepared for. Human processes in areas covered by plantations and tropical forests may lead to malaria importation and subsequently local transmission (20). Community members in Si Sa Ket and Ubon participate in different agricultural activities in rice elds (21), rubber plantations (21,22), cassava plantations (22), and forest-going activities (21).…”

The contribution of active case detection to malaria elimination in Thailand

“…Rather than a national policy, PACD could be alternatively implemented to maximise yield and reduce wastage, with individuals targeted based on identifying the networks to which they are affiliated. For example, imported cases could trigger screening of fellow travellers from malaria-endemic areas using snowball sampling or a variation on time-location sampling that would be safe and appropriate for this context 5 36. Using Thailand’s extensive VHV network could support higher quality PACD implementation by offering more flexible times and locations to reach potential patients per their convenience.…”

“…Malaria cases have become increasingly clustered into three distinct transmission zones in Thailand: the east (along the border with Cambodia), south (adjacent to Malaysia) and west (along the border with Myanmar). The drivers of this success are thought to be a combination of human and demographic factors rather than environmental factors alone 5…”

Malaria Stratification Mapping in Thailand to Support Prevention of Re-establishment

“…Malaria incidence in Thailand exhibits substantial heterogeneity, with malaria-free areas predominantly located in the central part of the country and remaining transmission areas along the borders with Cambodia, Myanmar, and Malaysia. 6 To account for this heterogeneity, the DVBD implements malaria strategies at a focus level 7 , necessitating a stratification approach to support targeted POR interventions with increased resolution. This study presents a novel method for performing malaria stratification mapping in Thailand by using routine malaria surveillance data and environmental information to support POR planning.…”

“…However, even malaria-free provinces remain vulnerable to the reintroduction of malaria parasites and the re-establishment of indigenous transmission, threatening national progress toward elimination. Thus, subnational prevention of re-establishment (POR) planning is an essential component of a malaria elimination program 6 to maintain the status of malaria-free areas while addressing remaining hotpots.…”