Malaria in the Greater Mekong Subregion: Heterogeneity and complexity

Abstract

The Greater Mekong Subregion (GMS), comprised of six countries including Cambodia, China's Yunnan Province, Lao PDR, Myanmar (Burma), Thailand and Vietnam, is one of the most threatening foci of malaria. Since the initiation of the WHO's Mekong Malaria Program a decade ago, malaria situation in the GMS has greatly improved, reflected in the continuous decline in annual malaria incidence and deaths. However, as many nations are moving towards malaria elimination, the GMS nations still face great challenges. Malaria epidemiology in this region exhibits enormous geographical heterogeneity with Myanmar and Cambodia remaining high-burden countries. Within each country, malaria distribution is also patchy, exemplified by ‘border malaria’ and ‘forest malaria’ with high transmission occurring along international borders and in forests or forest fringes, respectively. ‘Border malaria’ is extremely difficult to monitor, and frequent malaria introductions by migratory human populations constitute a major threat to neighboring, malaria-eliminating countries. Therefore, coordination between neighboring countries is essential for malaria elimination from the entire region. In addition to these operational difficulties, malaria control in the GMS also encounters several technological challenges. Contemporary malaria control measures rely heavily on effective chemotherapy and insecticide control of vector mosquitoes. However, the spread of multidrug resistance and potential emergence of artemisinin resistance in Plasmodium falciparum make resistance management a high priority in the GMS. This situation is further worsened by the circulation of counterfeit and substandard artemisinin-related drugs. In most endemic areas of the GMS, P. falciparum and Plasmodium vivax coexist, and in recent malaria control history, P. vivax has demonstrated remarkable resilience to control measures. Deployment of the only registered drug (primaquine) for the radical cure of vivax malaria is severely undermined due to high prevalence of glucose-6-phosphate dehydrogenase deficiency in target human populations. In the GMS, the dramatically different ecologies, diverse vector systems, and insecticide resistance render traditional mosquito control less efficient. Here we attempt to review the changing malaria epidemiology in the GMS, analyze the vector systems and patterns of malaria transmission, and identify the major challenges the malaria control community faces on its way to malaria elimination.

Introduction

According to World Malaria Report 2010, the estimated annual malaria incidence for 2009 was 225 million cases, resulting in ∼781,000 deaths (WHO, 2010b). While most of the malaria burden is in sub-Saharan Africa, Southeast (SE) Asia accounted for 10% of the global malaria morbidity and 5% of the global mortality in 2008. Since the launch of the Roll Back Malaria Initiative by WHO in 1998, malaria control has intensified in endemic countries, supported by better financial support and technological development. A Global Malaria Action Plan seeks to eliminate malaria using integrated approaches including vaccines, insecticide-treated mosquito nets (ITNs), indoor insecticide residue spray (IRS), and improved drug treatments (http://www.rollbackmalaria.org/gmap/index.html). Once again, malaria eradication is on the agenda of the international community (Feachem and Sabot, 2008, Roberts and Enserink, 2007). Of the 99 malaria endemic countries, 32 have declared a national policy of malaria elimination (Feachem et al., 2010).

Within SE Asia, the Greater Mekong Subregion (GMS) has been one of the most dangerous foci of malaria. The GMS is comprised of Cambodia, China (Yunnan Province), Lao PDR, Myanmar (Burma), Thailand, and Vietnam, which vary in political structure, socio-economic and financial resources, public health system, and disease ecology. The GMS is one of the most densely populated areas with topographical environments ranging from coastal plains to river estuaries and rugged mountainous terrains. Such divergent ecological systems offer diverse breeding habitats for multiple mosquito vector species with distinctive preferences for forest edges, foothills, or agricultural fields. Climates range from temperate conditions with disrupted malaria transmission during the harsh winter to tropical climates where malaria transmission occurs year round. The Mekong River runs through all six countries, and its watershed plays an important role in the transmission of vector-borne diseases. As such, malaria epidemiology in this region is complex, characterized by immense geographical heterogeneity in disease distribution with many areas of high endemicity, differential prevalence of the two most predominant parasites Plasmodium falciparum and Plasmodium vivax, which require different drug treatments, and diverse vector systems with different vectorial capacities for these parasites (Socheat et al., 2003). Furthermore, the GMS harbors the epicenter of multidrug resistant (MDR) P. falciparum in the border area between Cambodia and Thailand, which is gradually encompassing the tropical world (Hastings, 2004, Wongsrichanalai et al., 2002). Recent detection of artemisinin resistance in the same area represents a regional and global emergency (Dondorp et al., 2009, Noedl et al., 2008). Therefore, malaria control in the GMS is not only important for the immediate region, but also for global malaria control (WHO, 2011).