Martin S. Fritsch
Swiss Federal Institute of Technology, Zurich, Switzerland
The interactions between drainage, water management and health
Water related diseases and their vectors
Water-borne excreta related infections
Health risks and chemical pollution
Integrated control of transmission of vector-borne diseases
Environmental management measures in drainage water management
Development of control strategies
Proper surface and subsurface drainage to remove excess water in a safe and timely manner plays an important role in controlling water related diseases. Careful control and appropriate reuse of drainage water can help protect the environment and optimize the use of water resources.
The health issues related to drainage water management can be grouped in three categories:
i. water related vector-borne diseases;
ii. faecal/orally transmitted diseases; and
iii. chronic health issues related to exposure to residues of agrochemicals.
In tropical and subtropical regions there is a close link between the presence of excess water (due to lack of adequate drainage) and the transmission of water related vector-borne diseases. Malaria, schistosomiasis (bilharziasis) and lymphatic filariasis are important water related vector-borne diseases. Despite control programmes, health services and available treatments, these diseases today represent a growing health problem.
Water related vector-borne diseases are caused by bacteria, viruses and parasites (protozoa and helminths) transmitted by water related disease transmitting agents, also called vectors or intermediate hosts. A vector is an animal, often an insect, that transmits an infection from one person to another person or from infected animals to humans (Cairncross and Feachem, 1983). Most infections can only be transmitted by a particular, disease-specific vector, e.g., malaria by Anopheles mosquitoes. An intermediate host has a similar role to a vector. However, such an organism does not actively transmit a pathogen, like freshwater snails in the case of schistosomiasis. Vectors and intermediate hosts represent critical elements in various disease transmission cycles of parasitic water related diseases. In general, they live in or near aquatic environments.
Direct pathogen transfer and the transmission by vectors and intermediate hosts require specific environmental and socio-economic conditions. The conditions are defined by:
i. quality and quantity of water;
ii. type and frequency of human-water contacts;
iii. number and distribution of vector or intermediate host breeding sites; and
iv. exposure of humans to vector and intermediate host populations.
Consequently, the above-mentioned diseases can also be associated either directly or indirectly with the design and management of treatment and disposal plants for the re-use, treatment or disposal of drainage water. The key criteria for such a health risk are:
i. introduction of temporary or permanent open water surfaces bodies, e.g., constructed wetland, stabilization ponds, or evaporation ponds;
ii. suitability of such water for vector breeding;
iii. accessibility for the local population;
iv. location in relation to human settlements and transport links (e.g., roads); and
v. pollution by organic or inorganic substances.
Misuse and lack of maintenance are the two main reasons why drainage structures (road drainage ditches, culverts, dam site drainage or drainage canals in irrigation schemes, and also drainage water treatment and disposal facilities) are often associated with environmental health problems. Farmers, associations or national agencies generally conduct regular maintenance on irrigation canals. Water quality and flow velocity are relatively high. However, in drainage facilities the opposite conditions are frequent. Silting, uncontrolled aquatic weed growth, slow water flow or stagnant pools associated with the resulting wetlands offer ideal breeding conditions for mosquitoes and aquatic snails. Farmers seem to concentrate on irrigation water management rather than on drainage management.
Moreover, there is often a lack of adequate domestic water supplies and sanitation facilities. Thus, drainage canals or drainage water treatment and disposal facilities are often misused for washing, drinking and uncontrolled disposal of human excreta or other waste by the poorest and, thus, most vulnerable social groups. In this way, drainage water contributes to disease transmission.
Incidence of diseases - cases and mortality
Vector-borne diseases: transmission by insects
Water-based diseases: transmission by aquatic and semi-aquatic snails
Although the eradication of diseases such as malaria has long been an objective, the problem is far from being solved. According to the most recent information (WHO, 1997) the malaria situation is serious and deteriorating. Global malaria mortality is estimated at 1.5-2.7 million and global malaria cases at 300-500 million. Malaria is one of the most serious health problems facing African countries and a major limitation on their socio-economic progress. Children under the age of five and pregnant women are most at risk. WHO (1997) reports that 90 percent of the global burden of this disease can be attributed to environmental factors, including land and water management.
Schistosomiasis is almost as widespread as malaria but rarely causes immediate death. An estimated 200 million people are infected and transmission occurs in about 74 countries. The infection is primarily common in children who play in water inhabited by the snail intermediate host. Water development projects, especially those associated with the irrigation of large areas, have often been associated with an increased incidence of schistosomiasis. Intestinal schistosomiasis was unknown or infrequent in the Nile, Senegal and Volta deltas before the construction of the Aswan, Diama and Alosombo dams.
Worldwide parasitic and infectious communicable diseases (including all water related diseases) caused 32% of all deaths in 1993, resulting in the loss of 16.5 million lives; 99% of reported cases occurred in developing countries. In these countries, parasitic and infectious communicable diseases cause 41.5% of all deaths (WHO, 1995).
Insect vectors represent the largest group of disease transmitting agents. In most cases and for the most widespread diseases, mosquitoes are the main vectors. Among a wide range of vector-borne diseases, two diseases, namely, malaria and lymphatic filariasis stand out as serious health hazards in the context of poor drainage.
Malaria is caused by a protozoan parasite of the genus Plasmodium. Malaria is a complex disease causing fever, anaemia and an enlargement of the spleen. This causes additional cerebral complications, especially for children. Correspondingly, child mortality rates for P. falciparum, one of the four types of malaria affecting humans, are very high with approximately one million children below the age of five dying in 1993 (WHO, 1995; WHO, 1996).
Malaria is transmitted by the bite of a mosquito of the genus Anopheles. The transmission cycle is only between man and mosquitoes. Man acts as the intermediate host or reservoir and the mosquitoes as the vector. Protozoan parasites of the genus Plasmodium have to undergo complex development and multiplication processes both in man and mosquito before they can be further transmitted. Only the female mosquitoes are of importance for transmission, as they need a blood meal for oviposition.
Malaria covers not only all developing countries, but is present on almost the entire land surface between the latitudes 40°N and 60°S. However, the distribution is not uniform and depends mainly on climate, altitude, population density and the specific environmental requirements of the mosquitoes species.
Highly endemic areas are sub-Saharan Africa, Central America and the northern part of South America, the Indian subcontinent and Southeast Asia.
In the last two decades, a growing number of malaria cases have been observed (WHO, 1996). However, this cannot only be explained by the increasing population. To a large extent, this is also due to the increasing number of WRDPs such as irrigation and drainage schemes or hydro-electric dams. With the introduction of new open water surfaces in the form of canals, ponds and artificial lakes, new mosquito breeding sites have been created.
However, the persistence of the disease is also due to the absence of effective long-lasting vaccines, and the growing resistance of malaria pathogens and mosquitoes to treatment and insecticides, respectively. By the end of 1985, 50 of the 150 potential malaria transmitting Anopheles species were already recorded to be resistant to one or more pesticides (including DDT). At least 11 of those 50 species are known to be important and dangerous malaria transmitters (WHO, 1989).
Furthermore, malaria transmission is not only related to WRDPs. Deforestation, mining, road construction and all the negative consequences of rapid and uncontrolled urbanization are also contributing to the creation of mosquito breeding sites. In this context, urban drainage plays two key roles. On the one hand, it is an essential and effective tool for reducing and eliminating mosquito breeding sites by controlling surface water and waterlogging and by eliminating unnecessary open water surfaces. However, on the other hand poorly maintained drainage canals can represent potential breeding sites for various mosquito species if they are permanently flooded and aquatic weeds are not cleared.
Finally, a fresh risk might result from climatic change. First reports suggest that global warming can change the geographical distribution of mosquito breeding and shift the malaria transmission border line to the north (WHO, 1996).
Mosquito-borne filariasis includes a group of diseases which lead to inflammations and obstructive lesions of the lymphatic system. Filarial parasites are nematode worms, which also need an insect to complete their life cycle. The main disease is Wuchereria bancrofti or elephantiasis and is transmitted by mosquitoes of the genera Culex. During a bite, a mosquito might take up a number of microfilariae which then undergo a development cycle in the mosquito. When the infected mosquito bites again, the infectious larvae enter first the skin and then the lymph vessels and lymph glands. Unlike malaria, filarial infection requires repeated and lengthy exposure to the vector due to the low pathogen load per bite. For W. bancrofti, long exposure to repeated infection will finally lead to severe and disfiguring deformities of the legs, arms and genitals. Lymphatic filariasis affected about 120 million people in 1994; 80% of cases were reported either from tropical Africa or India (WHO, 1996).
Although filariasis is more of a serious urban health problem, it can cause significant health hazard in rural settlements and farming communities where poor surface drainage, lack of sanitation facilities and environmental pollution through uncontrolled waste and excreta disposal are prevalent. The principal vector, Culex quinquefasciatus (fatigans), breeds in polluted water in ditches, drains, tanks, barrels, tins and any kind of water accumulation container.
There are more than 3 000 known mosquito species. However, of the 150 species that are potential vectors, only 30 are considered dangerous. The three genera Anopheles, Aedes and Culex, from three sub-families, are disease relevant.
The epidemiology and the life cycle stages demonstrate the importance of climatic factors and that water is the essential environmental component for mosquitoes. The immature stages such as eggs, larvae and pupae require an aquatic environment, whereas adult mosquitoes live in terrestrial ecosystems. The quality and quantity of water, whether it is running or standing, shallow or deep, clean or polluted, sweet or brackish, shaded or sunlit, permanent or seasonal, and finally the climate will determine which particular species can breed. A summary of the physical and biological factors is given below (WHO, 1982).
Physical factors: In general mosquitoes prefer higher air humidity and average water temperatures between 23 °C and 33°C in order to complete their aquatic stages within two weeks. Rainfall can be a limiting as well as a positive factor. Rain will fill ditches, rivers, ponds, etc., but heavy rain can have a flooding effect and flush out breeding places. Mosquitoes do not normally reproduce where excess water is quickly removed. Sunlight or shade can also be positive or negative depending on the species. All these factors also determine the resting and biting habits. Some species rest and feed indoors and/or outdoors. Peak biting activity is usually about one hour before dawn. However, Aedes is a daytime biter, and many Anopheles bite throughout the night.
Biological factors: The presence of vegetation and floating plants are important for optimal breeding conditions. First, the plants are larval food and, more importantly, they provide shelter from predators and protection against wave movement. Therefore, mosquito larvae are not found on the open surfaces of large water bodies. The abundance of a number of species is linked to the presence of specific plants.
As a group, mosquitoes breed in an almost infinite variety of sizes, types and qualities of water bodies and each species requires specific environmental breeding and living conditions. However, most of the mosquito vectors breed in a rather restricted and narrow range of habitats. It is the number of potential transmitting species and their population dynamics which makes control efforts difficult. A comprehensive classification by species, country and habitat, including potential environmental management measures, is presented in the Manual on Environmental Management for Mosquito Control (WHO, 1982).
Natural biological control of mosquitoes can be accomplished with larvivorous fish. Over 250 different species of such fish are known to consume mosquito larvae. The most common mosquito fish is Gambusia affinis which is found in about 60 countries (Gerberich and Laird, 1985). Mosquito fish can be propagated commercially and introduced into drainage water. The fish are most effective during the warmer months of the year.
There are three major types of schistosomiasis that affect man: S. haematobium, S. mansoni and 5. japonicum. The disease is caused by female and male trematode worms inhabiting the blood vessels of the urinary bladder (S. haematobium, S. japonicum) or the portal and mesenteric veins (S. mansoni). Typical visible symptoms of schistosomiasis infection are blood in the urine in the case of S. haematobium, and intermittent diarrhoea and faeces containing blood in the case of S. mansoni (Jordan and Webbe, 1982). Although effective chemotherapy is available, the costs per treatment and per caput make it too expensive for many developing countries.
The transmission of all three species is based on a complex four-phase cycle which includes the presence of freshwater snails (Figure 9). Eggs expelled in the urine or faeces of an infected person may reach water, where they hatch rapidly into miracidia, a free-swimming brief larval stage. Depending on the species, the miracidia have to find a specific aquatic or semi-aquatic freshwater snail as intermediate host within 24 hours.
S. haematobium, S. mansoni and S. japonicum require snails of the genera Bulinus, Biomphalaria and Oncomelania, respectively. Oncomelania are semi-aquatic snails that live part of the time outside water in moist soil or mud. In the snails, the parasites develop within 4 to 6 weeks into cercariae, a second free-swimming larval stage. It is at this stage that they are infectious for man. Snails shed numerous cercariae into the water. These have to find human beings in contact with water in order to penetrate their skin. Having penetrated the skin of the host, the parasites find their way through the veins, heart and lungs to the final organ, there recommencing the cycle with egg production (Jordan and Webbe, 1982).
Bionomics of snail intermediate hosts
Although individual snail species require a specific physical environment, the variety of aquatic habitats is almost infinite. Marshes and swamps, permanent or temporary ponds and pools, natural or man-made freshwater lakes or reservoirs, seasonal or permanent or slow flowing river streams, irrigation or drainage canals, rice fields and all other types of standing, slow flowing or impounded water are potential snail breeding sites. Generally, the water needs to be shallow, clean or brackish, with little turbidity. The duration of the life cycle depends on water temperatures. The ideal water temperature ranges from 26° C to 28 °C.
Snail intermediate hosts of both S. haematobium and S. mansoni show great tolerance regarding pH values (5.3-9), mean water temperatures (18°-30°C) and salinity. Snails are found in shaded water bodies but are also known to be active when exposed to direct sunlight. The shedding of cercariae correlates with day-time and thus the intensity of sunlight. Furthermore, snails have a remarkable capability to survive long periods (5-8 months) in moist sand or mud (aestivation). Their main source of food is organic matter originating from decaying submerged or emerged vegetation, different species of algae, bacteria and fungi.
However, both of these aquatic snail species are sensitive to water velocity and water table fluctuations. Tolerable average current speed ranges between 0.0-0.3 m/s (Jobin and Ippen, 1964; Jobin et al., 1984). In natural rivers, snails are generally dislodged due to turbulence and fleeting shear stresses along the transition zone between the embankment vegetation and the river bed sediments, which are their actual habitat zone (Fritsch, 1993).
FIGURE 9 Transmission cycle of schistosomiasis
Most of the aquatic vector or intermediate host habitats correspond to sites where humans access water for washing, bathing, swimming (mainly children) or fishing.
Cairncross and Feachem (1983) proposed a classification of water-borne diseases in order to more fully understand the effects of excreta disposal:
i. Faecal-oral diseases (non-bacterial): Infections transmitted from person to person due to domestic contamination and lack of personal hygiene. Examples: hepatitis A, giardiasis.
ii. Faecal-oral (bacterial): Person-to-person transmission with longer transmission cycles either through contaminated food, crops or water contaminated with faecal material. Examples: Various diarrhoeas and dysenteries like cholera, E. coli diarrhoea or salmonellosis, enteric fevers such as typhoid.
iii. Soil transmitted helminths: Eggs of parasitic worms are expelled in faeces and require a development stage in moist soils. They reach the human host either by being ingested on vegetables or by penetrating the soles of the feet. Transmission takes place in communal defecation areas or around dirty latrines without clean concrete floors. Examples: ascariasis, trichuriasis, hookworm.
iv. Beef and pork tapeworms: Transmission cycle includes an intermediate development stage in an animal and infection of man occurs when the meat is eaten without sufficient cooking. Transmission can be triggered through the application of sewage sludge as fertilizer on grazing land. Examples: taeniasis.
v. Water-based helminths: The most typical example is that presented above in the section on schistosomiasis. As indicated, eggs in faeces must reach water in order to undergo the next development stage in an aquatic snail. The guinea worm (fasciolopsiasis) follows a similar cycle.
vi. Excreta related insect vectors: Filariasis transmitting Culex mosquitoes prefer to breed in highly polluted water. Badly maintained, unventilated latrines or uncovered septic tanks offer the best breeding conditions, mainly in urban areas. However, filariasis may also reach man simply by being carried by flies or cockroaches.
An important characteristic is the persistence of a specific pathogen, i.e., its ability to survive in the environment and whether animals, either in series or parallel, are part of the transmission cycle in view of the above classification, water quality and the environmental conditions around water bodies can be seen to play an essential role in the transmission.
Thus, unlined open drains, stabilization and evaporation ponds or wetlands might become particularly dangerous transmission sites. It is at such sites that excreta disposal, animal feeding and human water contacts might occur in a concentrated and uncontrolled form, so leading to the necessary pathogen-man/animal contacts.
Health risks related to water quality, primarily due to agrochemicals, represent a growing problem. This is mainly due to the intensified production and the expansion of irrigated agriculture, often in association with food processing industries (FAO, 1993; Hespanhol, 1996). In this context, both subsurface and open drains can have an impact on human health on four levels:
i. Drainage water re-use (see Chapter 3). Leaching or surface runoff from fields treated with pesticides, e.g., herbicides, fungicides, insecticides or molluscicides (Cairncross and Feachem, 1983), represents a non-point source of toxic organic substances. If re-used as irrigation water, toxic substances can be ingested by individuals directly or indirectly in the case of drainage water being mixed with drinking water resources. Another source of direct water contamination with agrochemicals is the washing of spraying equipment in open drains or irrigation canals or the spraying of irrigation canals and reservoirs against aquatic weeds (Hespanhol, 1996). Leaching also includes the transport of inorganic substances like salts, nitrate, phosphorus or heavy metals. Such substances can lead to health problems if consumed in high enough doses (FAO, 1993). However, salt will be less of a health problem as a salty taste will prohibit individuals from drinking too much salt polluted water. Trace elements are more of a concern in the case of subsurface drainage water.
ii. Drainage water treatment and disposal. As outlined in Chapters 5 and 6, both lead to an accumulation, a transfer or a transition of pollutants. In addition, treatment facilities such as constructed wetlands or stabilization ponds introduce new and permanent open water surfaces, and aquatic weed growth. Here, the question is whether this will change the human-water contact patterns and create new vector breeding sites. Furthermore, the quality of inadequately treated drainage water might also represent a potential health risk.
iii. The application of insecticides or molluscicides during chemical vector control campaigns. This can lead to short-term high concentrations of toxic chemicals in irrigation or drainage canals. Unprotected workers are exposed to health risks and local populations might consume polluted water without being aware of the risks. Indirect contamination might occur through the ingestion of fish.
iv. Uncontrolled disposal of industrial liquid waste in drainage canals or treatment and disposal systems. Due to the lack of environmental regulations and proper treatment plants, especially in urban areas, drains often serve as liquid waste disposal sites for local industries, thus representing dangerous point sources of toxic substances.
Although health risks arising from chemical pollution are accepted as a major environmental concern, data on medical implications are difficult to obtain and often unavailable for developing countries. Due to the lack of environmental protection and regulation standards as well as the lack of environmental monitoring data, governmental services often neglect the issue of pollution control.
Components of integrated control approaches
Environmental management for vector control
Control of water related diseases always requires the interruption of transmission cycles, which include a number of quite different actors: man, animals, or vectors. These all have different functions, react differently to environmental changes and have different capacities in either transmitting a disease or in resisting infection. There are basically three ways to disrupt transmission:
i. by eliminating or reducing vector densities;
ii. by protecting the susceptible host through immunization, prophylactic drugs, reduced man-vector contacts using repellents, window screening, bed nets, or by installing adequate water supplies and sanitation facilities; and
iii. by reducing the reservoir of infection, by treating infected people or animals and by eliminating the pathogen in the host body.
FIGURE 10 Elements for integrated control of schistosomiasis
Therefore, an effective and integrated disease and vector control strategy has to integrate four basic elements (adapted and extended after WHO, 1988; Hespanhol 1996): chemical vector control, biological vector control, environmental management, and medical treatment. Figure 10 presents an example of an integrated approach to controlling schistosomiasis.
Medical treatment is still considered as most important. In fact most research and financial resources are focused on medical or bio-medical issues. However, it is clear from the magnitude of the present health problems, the increasing number and spread of diseases and the close environmental interactions set forth above, that any intervention can only be successful when all four elements are integrated. This requires all parties involved to have a sound understanding of all environmental, social, economic and bio-medical implications of the epidemiological dynamics of water related disease transmission.
The core of such an approach can be reduced to the formula: transmission and morbidity control. Transmission control combines chemical, biological and environmental management control of vectors, intermediate hosts, or any activity in the field of water supply, sanitation facilities and water treatment. Morbidity control includes chemotherapy or any medical control of the pathogen and the treatment of the disease. Oomen et al. (1990) visualize the environmental management component as a vehicle having hygiene education and community participation as wheels rolling on the road of basic infrastructure.
In 1979, WHO, FAO and UNEP established the Panel of Experts for Environmental Management (PEEM). The aim was to better introduce and develop the environmental management component in the context of vector and disease control. Attached to the panel are 13 collaborating centres. These are research institutes working in the field of either bio-medical or engineering related topics of tropical disease control. PEEM defines environmental management as all technical and managerial interventions which modify and/or manipulate environmental factors and their interaction with man and vectors. The overall objectives are to prevent or minimize vector propagation and to reduce man-vector-pathogen contacts in order to achieve an optimal health status for a target population (WHO, 1982; Bos et al., 1993). In 1979, the WHO Expert Committee on Vector Biology and Control defined three categories of environmental management for vector control as shown in Table 8.
TABLE 8 WHO definition of environmental management
Long-lasting or permanent transformation of land, water and vegetation to prevent, reduce or eliminate vector or intermediate host breeding habitats (water related, vector-borne diseases) or environmental conditions which favour water-borne and water-washed disease transmission.
Grading, filling, drainage, land levelling, housing, urban drainage.
Changes of environmental conditions to create temporary unfavourable breeding conditions for vector breeding or transmission.
Water level fluctuations, water velocity changes, flushings, weed clearing, salinity changes.
Modification or manipulation of human habitation or behaviour
Any environmental manipulation of modification measures to reduce man-vector and/or man-pathogen contacts.
Bed nets, personal protection, house screening, safe bathing and laundry places, latrines, wastewater treatment, water supply.
Chemical vector control would include the application of repellents, attractants, insecticides, molluscicides and chemosterilants. Biological vector control can be achieved by the release of predators and microbial insecticides and by genetic manipulation. Finally, the medical interventions would focus on morbidity control, such as chemotherapy, vaccines, general treatment, improved nutrition, preventive health safeguards, prophylaxis and health education.
Drainage water treatment, re-use and disposal
Environmental management measures applied to drainage structures
In the context of drainage water re-use, treatment and disposal, the potential health risks can be analysed within a simplified system of drainage water flow in combination with the transfer of pathogens, toxic substances and the occurrence of potential vector breeding sites. This allows one to differentiate whether a health risk is related to water quality, vector breeding or both.
For each of the three drainage water management options, a number of questions are formulated to identify potential health risks. These are followed by a range of recommendations for possible control and preventive measures.
At first glance, re-use may appear to be a water quality problem mainly regarding high salt concentrations which can affect plant growth. However, pesticides in different forms can be washed into surface drains and trace elements can be leached out by subsurface drains. Where drainage water with high loads of pesticides and trace elements is re-used for irrigation, a health risk can occur for irrigation workers or for persons using this water for domestic purposes.
i. What is the quality of the drainage water? Is the drainage water used directly or blended with high quality water? What are the maximum loads of toxic substances after the blending? Are there seasonal changes in concentration according to agricultural management schedules?
ii. How do individuals or social groups come into contact with irrigation water blended with re-used drainage water? Do people use this irrigation water for any other purposes such as drinking, washing or cooking? Are children in frequent contact with the water?
iii. Are there any other risks? Can polluted water enter other hydrological cycles with high quality freshwater (e.g., groundwater), which will then be used for drinking?
Control and prevention:
In the case of direct re-use, there are few opportunities for the application of environmental management measures. Once drainage water has entered an irrigation system, it is probable that individuals will come into contact with this water. Therefore, effective monitoring of the quality standards of drainage effluents is most important. It is necessary to develop safeguard strategies in case of unacceptable or dangerous contamination levels.
Agriculture-forestry system and solar evaporators
This system of drainage water management, which aims at a continuous concentration of salt in progressively smaller volumes of water, is less of a health risk, as saline water is unsuitable for drinking. Here, the question is whether this series of irrigation systems will create new open water surfaces for vector breeding.
i. Does the system create new and permanent open water surfaces (e.g., solar evaporator)?
ii. If so, can they serve as breeding sites for mosquitoes? Do mosquitoes already breed in the area and to what extent are mosquito transmitted diseases prevalent? Are there mosquito species involved which can breed in brackish water (e.g., some Aedes, Culex and Anopheles species)? Can water snails tolerate the salt concentrations?
iii. Can aquatic weeds grow?
iv. How near are settlements, roads or larger urban centres? What diseases are prevalent there?
Control and prevention:
i. Intermittent drying out of ponds or storage tanks for at least 5-7 days.
ii. If possible, lining of all facilities to avoid seepage and minimize aquatic growth. In the case of earth lining: regular weed control. Only restricted or no applications of insecticides or molluscicides.
iii. Restricted accessibility in order to reduce man-water contacts.
iv. Settlement planning: geographical separation of settlements from ponds and tanks.
v. Monitoring of vector breeding activities and water quality.
vi. Evaluation of options.
The various physical, chemical and biological treatment processes may require a number of water retention structures such as constructed wetlands. This could lead to new and mostly permanent open water surfaces. Here again, the question is whether vector breeding sites will be created, or whether the purification capacity will determine the effluent quality and thus the water quality for low-end consumers.
i. How many new and permanent water bodies will be created? How large is the area?
ii. What kind of vegetation (e.g., aquatic weed) will grow?
iii. Is the local population already exposed to water related diseases? Where are the transmission foci located? What vector or intermediate host species are involved?
iv. Do their breeding requirements correspond to the environmental conditions created by a constructed wetland (e.g., for mosquitoes)?
v. How near are settlements, urban centres and roads? Are there any migration movements in the areas?
vi. What is the quality and performance of the local health service? What is the perception of the population regarding environmental health issues?
vii. Are there any data on prevalence, incidence, vector population dynamics or breeding habitats?
Control and prevention:
i. Water level fluctuations and intermittent drying out of the wetland area.
ii. Restricted accessibility.
iii. Geographical location outside and separated from human settlements.
iv. Off site: settlement planning and maintenance, housing improvement, personal protection.
v. Health education.
vi. Monitoring: vector populations, water quality, case reporting.
vii. In the case of high community vulnerability and environmental receptivity: evaluation of options.
Drainage systems need to be connected either with subsurface or surface drainage canal outlets. In the case of surface collector canals, additional open water surfaces will be introduced. In warm and hot climatic zones, drainage canals include a number of typical features which favour vector breeding, disease transmission and direct pathogen propagation, such as low and irregular flow velocities, low embankment slopes, high seepage, uncontrolled water access, uncontrolled deposition of excreta and aquatic weed growth.
Modifications to the drainage system environment would include: the change from open to piped or covered drains; canal lining with concrete in order to increase flow velocities and reduce aquatic weed growth; installation of special structures for cattle crossings and drinking; and boat ramps to protect earth embankments.
The key elements of environmental manipulation would be flow and water level management measures. Periodical flushing will help to dislodge snails and mosquito larvae if drag forces or shear stresses due to higher velocities exceed certain limits (Jobin, 1987; Oomen et al., 1990; Fritsch, 1993). Water level fluctuations can have distinct control impacts on both snail and mosquito breeding. If the drop is fast enough, snails, larvae and eggs become stranded and die (Fritsch, 1993). Intermittent flow, drying out of canals in connection with flushing, and water table fluctuations can also be effective tools to control mosquito or snail breeding. However, the approach differs for snails and mosquitoes, according to the locally specific population dynamic and vector bionomics (Oomen et al., 1990). Finally, canal maintenance means weed control and the removal of sediments. Weed control can be done either mechanically, chemically by applying herbicides, or biologically with fish.
Modification and manipulation of human habitation or behaviour
Interventions and environmental changes related to human habitation might not be sufficient if canals are continuously misused for excreta and waste disposal. In this case, health risks have to be minimized with a set of non-drainage related measures. This includes the improvement of sanitation facilities and personal hygiene (e.g., water supply systems or latrines) and the planning and maintenance of settlements.
There are no standard packages of engineering techniques available, nor should environmental management be considered as the ultimate solution for controlling water related parasitic diseases (Fritsch, 1993).
Birley (1995) has introduced a systematic approach to forecast vector-borne disease implications. In this assessment methodology he addresses three main components which contribute to the potential health hazards:
Community vulnerability: This describes the prevalence of specific diseases in social groups such as children, adults, males, females, workers or farmers. The prevalence is brought into relation with the proximity of disease foci, the immune status, previous history of exposure, general health status and the role of migrants. Vulnerability is ranked as high, moderate or low.
Environmental receptivity: This is the receptivity to transmission of the pathogen related to the abundance of the vector, to human contacts with water or vectors and to any other ecological or climatic factors favourable for transmission. The assessment is followed according to possible transmission but not to occurrence, transmission easily resumed, or to high receptivity.
Vigilance of the health services: This describes the quality and performance required of a health service to cope with an increased health hazard. The question is whether a health service is able to support and manage vaccination campaigns, continuous case detection, drug provision and delivery, hospitals, sufficient and skilled staff, health education and information or means for chemical or biological vector control. The ranking includes: very good, effective preventive measures only, effective treatment only, and none.
The assessment of control strategy effectiveness is organized in a sequence of flow charts and worksheets. It includes a comprehensive set of questions which finally lead to the ranking of the three components. The methodological approach forces one to focus on control elements and will structure data and help to set up additional survey or monitoring concepts if there is a lack of data. It will also help in assessing the quality and reliability of data. Overall the final outcome (the total assessment of potential health hazards) will provide a sound basis for identifying the required set of safeguards and preventive intervention measures, including the environmental engineering required to control water management related diseases.