Malaria: One of Major Public Health Problem in Southeast Asia

Asia experiences a notably high diversity of infectious diseases and vector species that carry them. According to the World Health Organization (WHO, 2018), malaria is a major public health problem in Southeast Asia (SEA), in which over 1.3 billion people are at risk in ten malaria endemic countries. This paper aims to explore and understand the distribution of malaria in SEA and Thailand by gaining a better understanding of the major vector, the mosquito. By understanding the mosquito’s behavior, preferences and distribution, vulnerable human populations can be identified. This can potentially provide important knowledge to help with control and elimination efforts in the area.

Background

Malaria is a serious and potentially deadly disease caused by a protozoan parasite known as Plasmodium. This parasite is transmitted among humans by mosquitoes of the genus Anopheles. Not all species of Anopheles are equally effective vectors for transmitting Malaria. Some species are biologically unable to carry the parasite, while others are readily infected (CDC, 2015). Female mosquitoes pick up the Plasmodium parasite when they take a blood meal from an infected person. Once inside, the parasites carry out fertilization and eventually produces sporozoites. These sporozoites travel through the mosquito’s salivary glands and is then transmitted to humans when they feed (WHO, 2018).

There is a plethora of mosquito species in the world found all over the globe, with many being vectors for different diseases. Mosquitoes of the Anopheles genus are the only know species to carry the malarial parasite (Morgan et al., 2013). Some species in the genus are exclusive to specific regions and often have different habits and behaviors of one another. Species are often grouped into species complexes, such as the Anopheles dirus complex (An. dirus s.l.), which encompasses a group of closely related, morphologically similar species, that may occur in the same geographic region, yet still behave differently (Sinka et al., 2011).

Different Anopheles species can differ in behavior and location, which has an important effect on their abilities as malaria vectors. The major vectors, Anopheles dirus s.l., An. minimus s.l., are responsible for most malaria cases in SEA (Sinka et al., 2011; Obsomer et al., 2013). These two species are not the only vectors in the region, but for the sake of this paper we will focus mostly one these two species as they are the most commonly found. According to Sinka et al. (2011) correct classification of any vector implicated in malaria transmission is key to successful control. Because most of mainland Southeast Asia is so geographically similar, this paper will focus on this region.

The mosquito goes through four stage in their life cycle: egg, larva, pupa, and adult. Because the first three stages are aquatic, they are most commonly found near a water source (CDC, 2015). Adult female mosquitoes can produce 50-200 eggs every time they lay eggs and are typically laid directly on water (Ahmad et al., 2011). Hatching can take up to three weeks in colder climates but can hatch in two to three days in warmer climates. During the larvae stage they must on the surface of the water as they feed on microorganisms found on the surface such as algae or bacteria (Marasri et al., 2017). The period from egg to adult can vary depending on ambient temperature, and usually takes 10-14 days in tropical climates. Adult male Anopheles only live for a week and typically feed on nectar because they do not require blood meals. Unlike the males, females require a blood meal for the development of eggs and can live up to a month depending on temperature and humidity (CDC, 2015).

Anopheles larval habitats are most often found in or near bodies of water, forests, vegetation and shaded areas. Larvae from the An. dirus complex were found to typically inhabit small, shallow, shaded, and usually temporary bodies of fresh and stagnant bodies of water. This included streams, wells, deciduous forests, bamboo forests and fruit plantations. The An. minimus larvae are generally found in small to moderate sized streams and canals with slow running, clear water, partially shaded and with grassy field margins where females like to lay eggs (Sinka et al., 2011).

Female Anopheles typically lay eggs soon after taking a blood meal meaning larval habitats are located close to human settlements. A survey conducted of potential breeding sites along rivers in Malaysia found that only those close to settlements were positive for larvae (Ahmad et al., 2011).

Adult female mosquitoes can be found in a number of different habitats, but most commonly in forests and forest foothills. Forests have favorable environmental conditions because of the dense vegetation, humid soil, high relative humidity and shade, along with suitable larval habitats. Obsomer (2007) found that Anopheles mosquito distribution in mainland SEA was limited by three main factors; temperature, landcover and rainfall. Thailand’s warm, humid climate, seasonal rains and large forested areas are the ideal habitat for the Anopheles all year round (Obsomer et al., 2012). There is a large range in habitat preference among different species, making habitat identification complicated. Anopheles species can also have varying biting habits, such as the time of day and location. Most biting activity takes place from dusk to dawn but can occur both inside and outside the home (Obsomer et al., 2007). The behavior among different species is so diverse because the An. dirus and An. minimus species are highly opportunistic in their habits, exhibiting significant behavioral and ecological plasticity (Garros et al., 2006).

Human activities play a huge role in the malaria epidemiology of SEA. Colonization of new lands for agriculture, deforestation, population movements and other activities expose people to high transmission risks (Obsomer et al., 2013). Sirwichai (2016) believes that deforestation is one of the most important factors at work in emerging and re-emerging diseases because of the significant change it causes to the ecology of local flora and fauna including disease vectors. Drastic deforestation in recent years has reduced the number of suitable habitats for mosquitoes. This has forced them to adapt to different habits such as orchards and croplands increasing human to vector contact (Yasuoka & Levins, 2007). In some areas of SEA agricultural development has also rapidly altered the composition of local mosquito fauna which has required a change in vector control methods (Sinka et al., 2011).

According to Tisgratog (2012) persistence of disease transmission in Thailand has primarily been associated with small scale agriculture activity, uncontrolled tribal movements and periodic civil unrest that have produced displaced people living in transient conditions that are more conducive for malaria transmission. The highest malaria transmission rates in Thailand occur in the border regions with Myanmar and Laos. Both the Thailand-Myanmar and Thailand-Laos region can be characterized by forest and forest fringe areas where many local populations live in remote, hard to reach villages (Kwansomboon et al., 2017 & Marasri et al., 2017). International borders in SEA often see clusters of cases with the Thailand-Myanmar border being one of them. This is a point of convergence for different countries with differing economic development, public health infrastructures and policy, and sociocultural and political situations (Parker et al., 2015).

The international border between Myanmar and Thailand is almost 2000 km. The landscape includes river basins, watersheds and valleys that are filled with primary and secondary or tertiary forests, agriculture fields, plantations and sometimes dense pockets of human settlements (Parker et al., 2015). Parts of the border can be hard to traverse causing urbanization to be slow compared to other more populated regions. The environment and landscape in this region have ecological and behavioral implications for parasites, mosquitoes and humans.

These human populations can range from refugee camps, agricultural villages to trading towns and most often are found clustered together. The majority of the populations are made up of different minority groups with a wide range of cultural decent. Major population centers occur near rivers which historically have been the best and most reliable source for year-round movement and transportation.