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Disease ecology

This section covers vector-borne diseases. Ecological and demographic changes resulting from the introduction of irrigation may create new or more favourable habitats for disease vectors. There are subtle differences in the ecological requirements of a range of disease vectors and there are intricate transmission patterns in different parts of the world. Local health authorities will have this information at hand. An interdisciplinary dialogue should guide planners in the incorporation of engineering and environmental management measures in the design, construction and rehabilitation of irrigation schemes. In general terms, two key determinants can be influenced: vector density (which is, up to a saturation point, linearly related to the transmission level) and vector longevity (the longer the lifespan of an individual mosquito, the greater the chance it transmits a disease to one or more humans).

The vector-transmitted diseases in question are listed below in order of global importance. Any disease may have major importance locally.

TABLE 9 Main infective diseases in relation to water supplies (Adapted from Feacham et al., 1977)

TABLE 10 A broad indication of the vector-borne diseases naturally transmitted in each zoogeographical region (Source: Birley, 1989)

Malaria: infective larvae of the Plasmodium parasite are injected into the bloodstream when an infected anopheline mosquito takes a bloodmeal. Only female mosquitoes take blood meals. Temperature, humidity and availability of clear water bodies (standing or slow moving) are key to mosquito bionomics. They determine the spatial (North and South longitudes; altitude; desert areas) and temporal (seasonal) limits of the disease. Not all anopheline mosquitoes transmit malaria, but as a general rule irrigation development results in fauna simplification which favours vector species. Details of the breeding requirements of local vector species are needed before the effect of environmental change can be predicted and specific design and operational interventions devised (WHO, 1982).

Schistosomiasis is caused by parasitic trematode worms which in their adult form live in the blood stream of human hosts and which, to complete their lifecycle, need to pass a larval stage in certain species of aquatic or amphibious snails. The ecological requirements of these so-called intermediate host snails are a key determinant in the distribution of the disease. Aquatic weeds provide an important substrate for the snails. Unlike mosquitoes, snails do not actively carry the disease-causing organism from one human to another; completion of the lifecycle depends on hygiene (defecation/urinating) and water contact patterns. Human behaviour is, therefore, the other key determining factor.

Japanese encephalitis: a limited number of culicine species transmit Japanese encephalitis, the most important ones, Culex tritaeniorrhychus and Culex gelidus, breed specifically in irrigated rice agro-ecosystems. Pigs are the main amplifying host of the virus and migratory birds are suspected to play a role in the distribution of the virus over large distances. The mosquitoes prefer to take blood meals from animals (a characteristic called zoophily) and disease outbreaks are usually triggered by climatic conditions that favour rapid build-up of vector population densities to the level where a critical threshold is passed and increased human blood meals facilitate the infection to spill over into the human population.

Lymphatic filariasis is caused by one of two species of parasitic worms: Wuchereria bancrofti, transmitted by either culicine or anopheline mosquitoes and Brugia malayi, transmitted by mosquitoes of the genus Mansonia. The association with the irrigated environment only exists where anophelines are the vectors, i.e. in Central Africa and where Mansonia mosquito larvae can develop attached to the roots of aquatic weeds, in South and South East Asia.

Onchocerciasis: this infection with a filarial worm leads, in the long-term, to blindness, and its vector, the Simulium blackfly, needs fast-flowing, highly oxygenated water for its larval development. There is only one documented case of an irrigation scheme in West Africa where a steep canal gradient created a favourable condition for blackfly breeding. Spillways of dams are well known to create this risk, but, on the other hand, impoundments will eliminate any breeding in the inundated parts.

Specific risks and counter measures

This section looks at the human health risks as a discussion of the environmental impacts. Details of interventions are contained in WHO (1982) and also in Pike (1987). Environmental Action Plans and Environmental Management Plans should give clear proposals for interventions to reduce health risks.

Hydrology: a low-flow regime may lead to ponding in the riverbed providing suitable breeding sites for malaria vectors, for instance Anopheles culifacies in Sri Lanka. Where water availability permits, periodic flushing has been successful in eliminating the risk. Periodic flushing can also be effective in dislodging aquatic snails but this is only useful if transmission sites are few in number and not more than a few hundred metres from where the water is released. Where low flow leads to salt intrusion in estuaries, anophelines breeding in brackish water may flourish, such as Anopheles sundaicus (South-East Asia), Anopheles melas (west coast of Africa) and Anopheles merus (east coast of Africa); or, temporary sandbars may be formed, creating coastal lagoons, as happened along the Pacific coast of Central America (Anopheles albimanus). Hydraulic structures with standing water in them may become foci for schistosomiasis transmission. Experience in Zimbabwe shows that their re-design to make them self-draining can contribute significantly to reducing this risk (Chimbale et al., 1993).

Dams and impoundments can create a variety of health risks, in part because of ecological change (mosquito and snail propagation along shallow shorelines, associated with aquatic weeds, and blackfly breeding on spillways), and in part because of demographic changes. Depending on the ecological requirements of local vector species any of a range of interventions may be successfully applied; periodic reservoir fluctuation, steepening of the shorelines, controlling aquatic weeds, siting settlements away from the reservoir and, for the blackfly problem, constructing dams with two spillways that can be used alternately.

A rise in the water table resulting in waterlogging creates conditions in which many mosquito vector species thrive. Proper drainage is the first thing to attempt, but better water management is another possible solution. Certain types of irrigation (surface, contour and furrow irrigation) carry greater health risks than others (sprinkler, central pivot or drip irrigation). In the case of surface irrigation, canal lining benefits environmental and health concerns alike. Water availability allowing alternate wetting and drying of paddy fields and synchronized cropping of rice may also be effective against vector-borne diseases. A fall in the water table may, in some parts of the world, favour Phlebotomine sandflies which live in semi-arid conditions and transmit leishmaniasis, in its visceral form a fatal illness. A fall in the water table may also force people to revert to polluted or infective sources of drinking water and change water contact patterns, to the detriment of their health.

Water quality: organic pollution of surface waters may create favourable conditions for the breeding of culicine vectors of filariasis. Pesticide residues, a long-term environmental and health risk, may also lead to a rapid induction of resistance in disease vectors, thus rendering future emergency applications of pesticides in the fight against disease outbreaks less effective.

Groundwater may be polluted with pesticide residues and fertilizers. As a consequence, high levels of nitrates may end up in drinking water which may lead to severe illness or even death for some bottle-fed infants.

The eggs of intestinal helminths (roundworm, tapeworm - the latter requiring passage through cattle or pigs) are the most persistent risks of waste water for use in irrigation. They require quality control even where treatment is sufficient to eliminate bacterial risks of pathogens.

Salinity effects: as for the fall in the water table, saline intrusion of groundwater may force people to use unsafe drinking water and change their water contact patterns. If such effects cannot be prevented or are considered an acceptable trade-off, then proper water supplies should be installed to counter the health risks involved.

Ecological imbalances: the emergence of new agricultural pests following irrigation development will trigger pest control activities that can range from simple applications of pesticides to complex integrated pest management strategies. Such activities should be carefully assessed for their human health risks: pesticide poisoning of farm workers (to be countered by standard labelling, strict handling procedures and protective clothing); and, effects on insect populations that may favour a rapid build-up of vector densities. Managers of Integrated Pest Management programmes should attempt to include vectors in their monitoring activities and liaise with health authorities on early warning mechanisms for disease outbreaks.

Aquatic weeds provide a refuge or even an essential habitat element for some vectors and their clearance is crucial in reducing health risks.

Animal husbandry may imply human health risks in two ways: firstly domestic animals may act as reservoirs for human infections. The notorious pig-virus-man combination in the irrigated rice ecosystems of South and South East Asia, in connection with Japanese encephalitis has already been referred to. In the Philippines, the water buffalo is a reservoir-host for the japonicum form of schistosomiasis. Secondly, the presence of cattle may tip the balance either in favour or against disease transmission by its mere presence: with an expanded source of blood meals, vector densities may rise, but where local vectors prefer animals to humans as a source of blood, vectors may actually be diverted away from their human hosts. Strategic siting of cattle between breeding places and human settlements may enhance the latter phenomenon and is referred to as zooprophylaxis.

Health opportunities

Irrigation projects offer ample opportunities for health promotional measures as an integral part of development. Up to a certain level their cost may be absorbed in the overall budget, but for larger health components additional loans or bilateral grants may have to be sought.

The provision of drinking water supply and sanitation is the single largest health promotional component that should be pursued in any irrigation project. As more water becomes available at the household level, the incidence of water washed diseases (several skin and eye diseases) will be reduced. Safe water supply, preferably in combination with adequate sanitary facilities, will reduce the risk of water-borne diseases dramatically. These include many gastro-intestinal infections which contribute significantly to infant mortality, including cholera.

Guinea worm infection (dracunculiasis) has the special attention of the international donor community in the 1990s. The parasitic worm can only enter the human body in its larval form inside the water flea (cyclops). Safe, clean drinking water (or at least filtered drinking water) is the key to elimination of this disease.

Strengthening of national health services, in particular primary health care capacity in the affected area, should ensure that the health risks associated with the demographic change described in the section Socio-economic impacts are dealt with effectively. Special attention is needed for new migration patterns, for instance related to the cropping cycle, and unplanned resettlement. The introduction of new infections or increased incidence of existing ones due to non-immunity of incoming groups are two likely scenarios.

As none of the health safeguards included in project design and operation is likely to be 100% effective, and predictions have a level of uncertainty, health services should prepare to cope with the new conditions. The health sector should take responsibility for the monitoring of the health status during project construction and early operation, and for the adjustment of the health component in the Environmental Action Plan.

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