These are diseases spread by insects that either breed in water, or are found near water.
Malaria
Malaria is the most significant of these, having a widespread distribution and severe health effects, causing acute and recurring bouts of fever; it is spread by the Anopheles mosquito that breeds in still, relatively clean water. The problem is most severe in Africa, southern Asia, South-East Asia, and the South-West Pacific.
Central and South America (particularly the Amazon basin) are other problem areas.
• Malaria is responsible for 1.5 to 1.7 million deaths/year, over 90 per cent of these are in Africa.
• There are 300 to 500 million cases of malaria, over 90 per cent of which are in Africa.
Anopheles mosquitoes have a wide variety of possible breeding sites which means that water supplies make relatively little difference to malaria, except in those areas too arid to have any other year-round breeding sites. In such places, water tanks and streams or puddles of spilt water can provide opportunities for
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Figure 7.3Global distribution of malaria in 2003 Source: World Health Organization (2003)
mosquito-breeding throughout the year, creating the risk of falciparam malaria, the more lethal type of the disease.
Filariasis
Filariasis is an infection of parasites spread by urban Culex and some rural Anopheles mosquitoes, which breed in unsanitary conditions, such as flooded latrines and septic tanks, blocked sewage drains, etc. The parasites may block lymph ducts, leading to the swelling of limbs (elephantiasis) or the scrotum (hydrocele). While many people are infected, the number of clinical cases is far smaller, but can build up to 1 per cent or more in the population over 40 years old.
Dengue
Dengue is a viral infection which leads to acute fevers, and repeated infection can lead to the more serious dengue haemorrhagic fever. Dengue is spread by the domestic day-biting mosquito Aedes aegypti which breeds in artificial sources of water, such as that trapped in tin cans and tyres after rainfall, and especially in water storage vessels. Dengue transmission occurs in virtually every low/middle income country (Figure 7.4). As a result, between 50 and 100 million people are infected by the disease. Because dengue cannot be prevented by vaccine or treated with antibiotics, the only available means of control is to prevent mosquito breeding by environmental intervention.
Yellow fever
Yellow fever is another often fatal viral infection transmitted by the same vector as dengue, and consequently having the same links to water and solid waste management as a preventive intervention. It is very much a disease of epidemics;
in 1987 over 120,000 cases occurred in the cities of Western Nigeria; in Senegal in 1965, an epidemic was traced to the practice of storing water in containers buried in the ground. Yellow fever is currently restricted to Latin America and Africa.
Onchocerciasis
Onchocerciasis, more commonly known as ‘river blindness’, is caused by parasites known as microfilariae spread by Simulium blackflies which breed in well-aerated rapids. Ironically, it is the death of the parasite that can lead to blindness. Disease control consists largely of the use of insecticides and the distribution of the drug invermectin, which kills the parasite without consequent damage. The acute- ness of the illness varies, so that large numbers of people can be infected without blindness.
Environmental factors in the spread of vector-borne diseases
A number of environmental factors, including the source of the water supply, waste disposal mechanisms and temperature influence the spread of vector-borne dis- eases. Humidity, vegetation, crop cultivation patterns and housing quality and density are also important. Modification of habitats due to changes in patterns of water use has led to an increase in these diseases. The increasing use of irrigation
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Figure 7.4Global distribution of dengue in 2003 Source: World Health Organization (2003)
and the construction of dams and reservoirs with the related changes in land use have been an important source of vector-borne disease. Poor water management in urban areas has also been implicated. Some of this is the result of the poor planning of projects and lack of communication or cooperation between construction and health sectors in the design of water resources. Table 7.4 outlines some examples of how development can increase the spread of vector-borne disease.
There is a further major environmental influence changing the range of vector- borne diseases – climate change. As the rainfall patterns change and there is a warming of the Earth’s surface, the range of vector-borne diseases, particularly malaria, is changing. This is covered in detail later in Chapter 10 where we deal with climate change.
Activity 7.2
Water-related vector-borne diseases are responsible for millions of lost disability adjusted life years (DALYs) world-wide each year and are an important public health issue in many countries.
What are the main ways that water-related vector-borne disease can be prevented through environmental or other means?
Feedback
The most effective method of disease reduction is improving water quality and decreasing access to poor supplies. This reduces transmission of water-related disease and also of water-borne, water-washed and water-based diseases. Water-based dis- eases that are spread through intermediate hosts can be reduced by decreasing the Table 7.4 Examples of how changes in environmental conditions increases the spread of vector-borne disease
Disease Area Change in environment
Malaria Amazon urban development
Africa, South Asia poor water management
SE Asia deforestation, aforestation,
reforestation
Tropics increase in irrigation
Schistosomiasis Sub-Saharan Africa irrigation schemes and reservoirs Dengue fever Urban (some rural) tropical areas solid waste problems combined with
inadequate storage of household drinking water
Filariasis Urban tropical areas vectors breed in open sewage canals, blocked drains and waste water used for irrigation
Japanese encephalitis
South and SE Asia irrigated rice growing areas where cropping patterns have changed Source: WHO (2004)
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need for contact with the water such as supplying drinking and bathing water through standpipes. Transmission of water-related diseases can additionally be reduced by improving surface water management, thus destroying breeding sites for insects and decreasing the need to visit breeding sites.