MALARIA
Malaria is a life-threatening parasitic disease transmitted by mosquitoes. In 1880, scientists discovered that the cause of malaria is a one-cell parasite called plasmodium. Later they discovered that the parasite is transmitted from person to person through the bite of a female Anopheles mosquito, which requires blood to nurture her eggs.
Malaria, together with HIV/AIDS and TB, is one of the major public health challenges undermining development in the poorest countries in the world. Today malaria is found throughout the tropical and sub-tropical regions of the world and causes more than 300 million acute illnesses and approximately 1 and 1.5 million people die from it every year (University of Leicester, 2003). According to the World Health Organization, approximately 40% of the world’s population mostly those living in the world’s poorest countries is at risk of malaria.
Ninety per cent of deaths due to malaria occur in Africa south of the Sahara mostly among young children; those who survive an episode of severe malaria may suffer from learning impairments or brain damage (WHO, 2003). Here, malaria accounts for 10% to 30% of all hospital admissions and is responsible for 15% to 25% of all deaths of children under the age of five. Around 800,000 children under the age of five die from malaria every year, making this disease one of the major causes of infant and juvenile mortality. Pregnant women are also vulnerable to malaria. They are at risk since the disease is responsible for a substantial number of perinatal mortality, low birth weight and maternal anemia.
World Health Organization (2003) Fact Sheet No. 94. http://www.rbm.who.int/cmc_upload/0/000/015/372/RBMInfosheet_1.htm
There are four types of human malaria Plasmodium vivax, P. malariae, P. ovale and P. falciparum. The first two are the most common and falciparum the most deadly type of malaria infection, which is most common in Africa, south of the Sahara (WHO, 2003). This type of human malaria is spreading into new regions of the world and its reappearance in areas where it had been eliminated.
The malaria parasite enters the human host when an infected Anopheles mosquito takes a blood meal. Inside the human host, the parasite undergoes a series of changes as part of its complex life-cycle. Its various stages allow plasmodia to evade the immune system, infect the liver and red blood cells, and finally develop into a form that is able to infect a mosquito again when it bites an infected person. Inside the mosquito, the parasite matures until it reaches the sexual stage where it can again infect a human host when the mosquito takes her next blood meal, 10 to 14 or more days later.
http://www.malaria.org/LIFECYCL.HTM
Malaria symptoms appear about 9 to 14 days after the infectious mosquito bite, although this varies with different plasmodium species. Typically, malaria produces fever, headache, vomiting and other flu-like symptoms. If drugs are not available for treatment or the parasites are resistant to them, the infection can progress rapidly to become life-threatening. Malaria can kill by infecting and destroying red blood cells and by clogging the capillaries that carry blood to the brain or other vital organs.
Malaria parasites are developing unacceptable levels of resistance to one drug after another and many insecticides are no longer useful against mosquitoes transmitting the disease. Years of vaccine research have produced few hopeful candidates and although scientists are redoubling the search, an effective vaccine is at best years away.
All species of mosquitoes require standing water to complete their growth cycle; therefore, any body of standing water represents a potential mosquito breeding site. Because areas that pond surface water that are flushed by daily tides are not stagnant for periods sufficient for mosquito larvae to mature, such areas are not mosquito production sources. Water quality affects the productivity of a potential mosquito breeding site (Public Health, 1998). Typically, greater numbers of mosquitoes are produced in water bodies with poor circulation, higher temperatures, and higher organic content (and therefore with poor water quality) than in water bodies having good circulation, lower temperatures, and lower organic content.
On the other hand, permanent bodies of open water that have good water quality (good circulation, low temperatures, and low organic content) typically sustain stable nutrient content and support rich floral and faunal species diversity, including mosquito predators and pathogens. Wave action across larger bodies of water physically retards mosquito production by inhibiting egg laying and larval survival.
Irrigation and flooding practices also influence the level of mosquito production associated with a water body. Typically, greater numbers of mosquitoes are produced in water bodies with water levels that slowly increase or recede than in water bodies with water levels that are stable or that rapidly fluctuate. Mosquito larvae prefer stagnant water and the protected microhabitats provided by stems of emergent vegetation (Public Health, 1998). If not properly maintained, ditches can be major producers of mosquitoes.
Upward trend in warming and precipitation will increase the incidence and the range of tropical diseases; this is supported by the continued emission of greenhouse gases. Currently, burning of coal, oil and natural gas releases about 6 billion tons of CO2 while forest burning and logging releases another 1-2 billion tons. This has resulted in a 30 % increase in CO2 since 1860. During the same period methane levels have doubled and nitrous oxide levels have risen by 15 %. Atmospheric increase of these gases causes a greenhouse effect by absorbing and re-emitting infrared radiation back to the earth’s surface. The overall emissions of green house gases are growing at about 1 percent per year. This effect impacts upon the hydrological cycle.
Geographic locations contribute towards vulnerability of populations, especially to those living near tropical coastal areas and lakeshores. These populations could be at a higher risk of cholera. Colwell (1996) demonstrated the link between cholera and sea surface temperatures (SST). Upwelling of the sea due increased SST, increases the abundance of phytoplanktons, which in turn support, a large population of zooplanktons, which are reservoirs of cholera bacteria. Besides other epidemiological factors, the effects of SST on the spread of cholera are perhaps the most profound because they affect large areas of the tropical seas and lakes.
During the 1997/98 El Niño, a rise in SST and excessive flooding (WHO 1998) provided two conducive factors for cholera epidemics which were observed in Djibouti, Somalia, Kenya, Tanzania and Mozambique, all lying along the Indian Ocean. Cholera epidemics have also been observed in areas surrounding the Great Lakes in the Great Rift Valley region. Birmingham et al (1997) found a significant association between bathing, drinking water from Lake Tanganyika and the risk of infection with cholera. Shapiro et al (1999) have recently made a similar observation along the shores of Lake Victoria.
Individuals living above an altitude of 1500 m above see level around the tropics will be at high risk of malaria epidemics. For example in a highland area of Rwanda malaria incidence increased by 337% in 1987 and 80% of this variation could be explained by rainfall and temperature (Loevinsohn, 1994). A similar association has recently been reported in Zimbabwe (Freeman and Bradley 1996). Other epidemics in East African highlands have largely been associated with El Niño.
There is a likelihood of an increase in the frequency and intensity of extreme weather events and these will increase the frequency of highland malaria epidemics and also outbreaks in arid areas due to flooding. The same effects is felt at a lower altitude in the low and high latitudes. As the warming increases in the higher latitudes areas such as the southern United States and Europe could be at a higher risk of local malaria transmission. Countries in tropical Africa account for more than 90% of the total malaria incidence and the great majority of malaria deaths.
Africa suffers from a number of diseases that are sensitive to temperature and precipitation. A simulation of malaria transmission using the vectorial capacity model (Macdonald, 1957) indicates that increased warming and precipitation in western Kenya result in a multiple-fold increase in malaria transmission. Results of the simulation are shown on fig 3. It has been assumed that above normal precipitation at least doubles the number of malaria transmitting mosquitoes.
While the principle causes of malaria epidemics in the African Highlands are still a subject of debate in literature (Mouchet et al., 1998) there is increasing evidence that climate change has a significant role (WHO, 1998). For example in a highland area of Rwanda malaria incidence increased by 337% in 1987 and 80% of this variation could be explained by rainfall and temperature (Loevinsohn, 1994). A similar association has recently been reported in Zimbabwe (Freeman and Bradley 1996). Other epidemics in East Africa have largely been associated with El Niño. It can be expected that small changes in temperature and precipitation will support malaria epidemics at the current altitudinal and latitudinal limits of transmission (Lindsay & Martens, 1998). Furthermore flooding could facilitate breeding of malaria vectors and consequently malaria transmission in arid areas (Warsame et al., 1995). The Sahel region, which has suffered from drought in the last 30 years, has experienced a reduction in malaria transmission following the disappearance of suitable breeding habitats. However, there are risks of epidemics if flooding occurs (Faye et al., 1998).
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