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Deforestation inhibits malaria transmission in Lao PDR: a spatial epidemiology using Earth observation satellites | Tropical Medicine and Health

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The present study attempted to answer the academic question of how deforestation impacts malaria distribution in Lao PDR, with consideration of climate change. Deforestation had been steadily increasing in Lao PDR, particularly in the five southern provinces where the majority of the country’s malaria cases were reported. In fact, the present study indicated a decrease in P. falciparum cases alongside an increase in P. vivax cases, and there may be a connection with deforestation. Some researchers already noted that P. falciparum cases exhibited a stronger correlation with deforestation, whereas the relationship between deforestation and P. vivax malaria cases is less pronounced. This could be attributed to the fact that P. vivax can relapse months or even years after the initial infection [12, 13]. Therefore, it is possible that deforestation is contributing to the decline in P. falciparum cases.

With global warming steadily progressing, forested land was expected to play an important role in malaria incidence in Lao PDR. In the present study, SEM identified two factors that were independently associated with API: area and the proportion of forested land. Specifically, most malaria cases occurred in the southern region, and the malaria incidence increased as the proportion of forested land increased (both p < 0.01). In Lao PDR, Anopheles (A.) dirus and A. minimus, which were found in forested zones, were known to be the primary malaria vector mosquitoes [14, 15]. Their geographical distribution overlapped with areas of high malaria prevalence [14, 16]. In addition, forest ecosystems would provide favorable conditions for primary vectors, and livelihood of malaria patients in Asian countries including Lao PDR [17, 18].

The results of this study, conducted in the Asian nation of Lao PDR, are consistent with the established epidemiological theory that deforestation reduces the risk of malaria in Southeast Asia [2, 18]. In Southeast Asia, malaria transmission is particularly pronounced in forests because the vector mosquitoes are adapted to the forest ecosystem, thus malaria cases are closely associated with forested areas [19]. Already in 1990, the World Health Organization (WHO) reported the land area of malaria-endemic countries in Southeast Asia. Although forests occupied only 20% of the total land area, 40% of all malaria cases and 60% of P. falciparum malaria cases in the region were reported from forested areas [20]. Although progressive deforestation may reduce the risk of malaria in Asia, there might also be a risk that other vector mosquitoes (e.g., Anopheles darling), which prefer forest edges and agricultural lands, may enter into the new ecosystem causing the re-emergence of malaria.

The present study’s analysis of forest coverage and satellite images suggested that the areas of increased forest coverage were actually plantations divided into 0.5 km squares (data not shown). A major challenge in Asia is the lack of information on at-risk populations, which include individuals living or working in or near forests where malaria vectors are present [21, 22]. In addition, a fifth human parasite, P. knowlesi [23], is essentially a primate malaria species and has been reported to be widely prevalent from forested areas in Southeast Asian countries, including Lao PDR [24]. With the accelerated development of deforestation and plantations, there is a risk that P. knowlesi should become endemic among people in Lao PDR.

Additionally, in the malaria-endemic areas of Lao PDR, the average surface temperature was higher than 22.5 °C, a temperature suitable for the growth and survival of vector mosquitoes. In fact, the LST was higher in the capital city, where malaria was not endemic. One possible explanation is that in addition to climate factors such as temperature, vegetation influences the distribution of malaria. In the present study, this was demonstrated by SEM.

Furthermore, Lao PDR has set a goal to eliminate the lethal malaria parasite, P. falciparum, from the country by 2024. Since 2004, the first-line treatment for P. falciparum in Lao PDR has been artemisinin-based combination therapy (ACT) [25]. The current drug regimen for P. vivax cases in Lao PDR involves a 3-day course of ACT along with a single low dose of primaquine only. This is because G6PD testing and the 14-day primaquine treatment to prevent relapses in P. vivax cases are available only in a limited number of hospitals in the county. Therefore, it can be inferred that the introduction of the ACT alone will not reduce the number of P. vivax cases to the same extent as it does for P. falciparum cases.

Although global warming in the southern region is more moderate than that in the capital city and Lao PDR as a whole, global warming in the southern region may increase malaria endemicity at the boundaries of malaria-endemic areas, such as Sekong, where surface temperatures are lower than in the southern region [26]. While the number of malaria cases in Lao PDR is decreasing, the risk of re-emergence of malaria in the places with increased forest coverage (e.g., plantations) should be considered. Surveillance and monitoring of malaria prevalence in these areas are critical to attain malaria elimination in Lao PDR, and Earth observation satellite data can provide rapid and appropriate assistance.

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