Water for people.pdf - WHO Thailand Digital Repository

B A S I C N E E D S A N D T H E R I G H T T O H E A LT H / 1 0 7(true vectors, intermediate hosts and rodent reservoirs of pathogens)whose elimination will interrupt disease transmission.These diseases are linked to water through the ecologicalrequirements of the vector species. Mosquito vectors all have an aquaticlarval stage. The distribution of the diseases reflects the differencesbetween genera and individual species. Lymphatic filariasis and malariaprovide an example. The Culex mosquitoes transmitting filariasis breedin organically polluted water and distribution of the disease is thereforelinked to the urban environment, particularly where open sewers andtreatment ponds provide the required breeding habitat. On the otherhand, Anopheline species of mosquitoes transmitting malaria generallyrequire standing or slowly running clean freshwater, a habitat often notavailable in cities. In fact, in many African cities a downward gradient inmalaria transmission intensity can be seen when moving from theoutskirts to the city centre, considered to reflect the increasingpollution of water collections.MalariaMalaria is a life-threatening parasitic disease transmitted from oneperson to another through the bite of female Anophelesmosquitoes. The disease exerts its heaviest toll in Africa south ofthe Sahara, where about 90 percent of the annual global rate ofover 1 million deaths from malaria occur. It is the leading cause ofdeaths in young children.In 2001, the estimated global burden of malaria amounted to42.3 million DALYs out of 359.4 million DALYs for all infectious andparasitic diseases, and 1,467.2 million DALYs for the overall globalburden of disease. The malaria burden constitutes 10 percent ofAfrica’s overall disease burden. Malaria causes at least 396.8 millioncases of acute illness each year. Pregnant women are the mainadult risk group in most endemic areas of the world. As one of themajor public health problems in tropical countries, malariacontributes substantially to the erosion of developmentachievements and hampers poverty alleviation efforts in the world’spoorest countries. It has been claimed that the disease has reducedeconomic growth in African countries by 1.3 percent each year overthe past thirty years (Sachs and Malaney, 2002).There are some 422 species of Anopheles mosquitoes in theworld, but only seventy are vectors of malaria under naturalconditions and of these some forty are of major public healthimportance. A wide range of aquatic ecologies provides the contextfor malaria transmission, including shaded and sunny pools andstreams, rooftop drinking water tanks (in south Asia), rainwaterpools of a certain duration, mangroves and brackish water lagoons,and the canals, ditches, night storage tanks and other componentsof irrigation schemes and hydraulic infrastructure.There is no vaccine available to protect against infection withthe Plasmodium parasites causing malaria. Parasite resistanceagainst prophylactic and curative drugs is a permanent concern, asis poor people’s access to drugs and health services constrained bymany factors. Insecticide-impregnated mosquito nets are effective asa means of personal protection, particularly for children under five,but only where mosquito biting patterns overlap with the use ofnets. Chemical vector control is faced with insecticide resistance aswell as increasing regulations concerning the use of insecticides.The 2001 Stockholm Convention on Persistent Organic Pollutantsrestricts production and use of DDT, with an exemption for thereportable application of this pesticide in disease vector control,while it aims to promote the development and testing ofalternatives to manage vector populations (UNEP, 2001).Depending on the species, malaria vector mosquitoes can relyon a range of aquatic ecologies to propagate. The example below(box 5.2) illustrates the impact of one type of water resourcedevelopment in the Ethiopian highlands.SchistosomiasisSchistosomiasis, also known as bilharzia, is a disease caused byparasitic flatworms called trematodes of the genus Schistosoma forwhich aquatic (Africa and the Americas) or amphibious (westernPacific) snails serve as intermediate hosts. An estimated 246.7million people worldwide are infected, and of these, 20 millionsuffer severe consequences of the infection, while 120 million suffermilder symptoms. An estimated 80 percent of transmission takesplace in Africa south of the Sahara.The estimated global burden of schistosomiasis was establishedas 1.8 million DALYs in 2001, but the parameters and assumptionsused to arrive at this estimate are currently under review.Box 5.2: The compounded malariaimpact of microdams in EthiopiaRecent studies in Ethiopia using community-basedincidence surveys revealed a 7.3-fold increase of malariaincidence associated with the presence of microdams. Thestudy sites were all at altitudes where malaria transmissionis seasonal (in association with the rains). The increasewas more pronounced for dams below 1,900 metres ofaltitude, and less above that altitude. In addition,observed trends in incidence suggest that dams increasethe established pattern of transmission throughout theyear, which leads to greatly increased levels of malaria atthe end of the transmission season.Source: Ghebreyesus et al., 1999.