The costs of controlling disease vectors or hosts, either through chemical and/or mechanical means, are often so high that regular eradication in irrigation canals is not performed, and consequently many chronic diseases persist in the poorer parts of the world. The vectors of many of these diseases are organisms which depend, to a greater or lesser degree, on the aquatic environment to complete their life cycle, and they are therefore commonly associated with irrigation waters.
The most important of these diseases include malaria, filariasis, yellow fever, equine encephalitis, and dengue fever (transmitted by mosquitos). Diseases borne by aquatic snails include schistosomiasis, paragonimiasis, distomatosis, clonorchiasis, those by the simulium fly, onchoceraiasis (river blindness) and by the tse-tse fly, trypanosomiasis (sleeping sickness).
The abundant growth of aquatic vegetation found in many irrigation schemes provides an ideal habitat for many of these organisms, and as a consequence their numbers rapidly increase, as does the incidence of the diseases associated with their presence. The manual clearance of weeds in many schemes puts the workers at constant risk of exposure to diseases, especially schistosomiasis.
Chemical control involves the use of pesticides and herbicides which may affect the crops or non target fauna. The results are only temporary and in recent times the emergence of resistant strains of disease organism or host has rendered many forms of chemical control useless.
Biological control is always a preferable alternative. However, the amount of research required before using such control measures is often too lengthy to be considered in areas where disease problems are acute and immediate control measures are necessary.
In the Mahi-Right Bank Canal Project (MRBC) in India poor drainage and increased vegetation cover has been suggested as the cause of the increase in malaria (Michael, 1987). The main vector Anopheles culicifacies can be found in the small ponds formed by seepage water, the borrow pits alongside the canals, in stagnant water in the canals and ditches and in the rice fields. Table 14 shows how the rise in malaria can be correlated to the increase in irrigation (Michael, 1987). The increase in irrigation canals alone may not be the sole cause of the increased incidence of malaria, as there tends to be an increase in the growth of aquatic vegetation accompanying the construction of canals, which in turn will provide a suitable breeding site for mosquitoes.
In the irrigation canals of the Gezira Irrigation Scheme in Sudan the use of the mosquito fish Gambusia affinis has been shown to be effective in controlling mosquito larvae. Early experiments in 1973 prompted the Sudanese government to adopt the stocking of Gambusia affinis into canals as a supplementary measure of control. However later experiments have shown that there are limitations to using this fish as the only control agent. The seasonal fluctuation of water in the canals causes changes in the habitat which is detrimental to populations of the fish (Mahmoud, 1985). In addition the brevity of the conditions favouring the breeding of Gambusia are a limitation to population growth and hence effective control.
The control of water flow by gates prevents the movement of Gambusia to the smaller field canals where the mosquito larvae are prevalent (Mahmoud, 1985). It was concluded that the fish, whilst proving an effective control measure in the dry season were not so effective in the peak transmission period during the rains.
In Korea the stocking of a native larvivorous fish, Aplocheilus latipes, resulted in a 60% reduction of malaria-carrying mosquitoes in simulated rice paddies. However the reduction lessened when the amount of vegetation cover (aquatic macrophytes and algae) increased (Yu, 1986). There is a need to explore alternative stocking strategies for Gambusia affinis to those already practised. In the Korean study fish were stocked into the larger canals at a density of 12 000 fish per canal (approx 5 km in length), which seems rather low. Alternative species, in waters in which G. affinis is not indigenous, should be investigated.
Schistosomiasis is reputed to affect well over 200 million people in Asia, Africa, the Caribbean and Latin America (Obeng, 1976). It spreads via faecal contamination of communal waters, often canals in irrigation areas. Despite the knowledge of the disease and its pathology there is still no effective prophylactic treatment.
|Water Hyacinth||Artificially fertilised ponds||75.6–191.1||Yount and Crossman (1970)|
|Fertilised with sewage effluent||219–657||Wolverton and McDonald (1979)|
|Grown in irrigation canals in China||400–750||Coche (1980)|
|Duckweeds||Average figure under tropical conditions||22.1||Edwards(1980)|
|Lemna, Wolffia Azolla||Grown in irrigation canals in China||150–187||Coche (1980)|
|Year potential||Incidence of malaria||Gross irrigation|
The aquatic snails Bulinus spp. and Biomphalaria spp. are the main intermediate hosts of Schistosoma haematobium and Schistosoma mansoni, the disease organism. These snails inhabit waters of slower flow and where weed cover in the canals is substantial (Redding-Coates and Coates, 1981; Wolff, 1988, Brabben and Bolton, 1988; Sadek, 1988).
In both the Gezira Irrigation Scheme, Sudan, and the South Chad Irrigation System in Nigeria, snail vectors were found associated with aquatic vegetation in the slower flowing distribution and field canals (Betterton, 1984). Many areas where increased irrigation has occurred has also seen the increased transmission of schistosomiasis. In the Gezira irrigation scheme it is thought that 90% of the population living on the scheme may have contracted the disease during their life (Redding Coates and Coates, 1981). In China the increase in irrigation canals may have been the cause of the increase in the incidence of schistosomiasis seen in the Jiangsu province and other irrigated areas (Dakang, 1987).
Snails were associated in Nigeria with plants such as Typha, Phragmites, Ipomoea in the larger canals, and Pistia stratiotes, Cyperus spp. and Utricularia inflexa in the field canals. Betterton (1984) found four species of bulinid and one biomphalarial snail within the Nigerian irrigation system. Bulinus rohlfsi, B. globosus and B. senegalensis are known hosts of Schistosoma haematobium in West Africa. These species were found predominantly in shallow, clay-lined canals with abundant aquatic vegetation growing in the slow-moving waters (Betterton, 1984).
Snails prefer a habitat where the flow rates are slow and aquatic weeds are abundant, conditions found in most field and distribution canals (Betterton, 1984; Redding-Coates and Coates, 1981). To alleviate this problem the most effective method would be either to destroy the habitat, i.e. the aquatic weeds, or remove the snail hosts directly. The efforts to control weeds using various fish species is discussed in section 4.
Studies on the removal of the snails by molluscivorous fish have been few. In 1975, 20 species of snail eating cichlids were caught in Lake Victoria, of which only 50% had been described. This work prompted a recent study on the use of the cichlids Astatoreochromis allaudi, Haplochromis ishmaeli and Macropleurodis bicoloras as snail control agents in irrigation canals in the Benouie River irrigation scheme, in the Cameroun (FFI, 1988).
In Africa it is known that other species of fish are also malacophagous, e.g. Synodontis sp., Clarias sp., Haplochromis mellaudi, and Chrysichthys mabusi (Bont and Hers, 1952). Despite this the control of snail hosts still remains a problem, especially in areas of dense vegetation growth. Experiments in western Kenyan dams using A. allaudi to control various species of snail met with some success. However this species was less effective in controlling snails where the vegetation was dense (McMahon et al., 1977). Coates (1984) reported the presence of snails in the gut contents of catfish species in the Gezira irrigation system, in Sudan, and the presence of a molluscivorous species of fish (Tetraodon sp.) in small numbers, in the canals.
Whilst there is certainly considerable potential for the control of numerous water-borne diseases through the establishment of suitable fish species in irrigation canal systems, poor management strategies have, so far, often limited their effectiveness. The main problem is that of finding a suitable integrated approach. This would involve cooperation from the engineering, public health, administrative and financial sectors in the initial stages of the project. Biological control using various species of fish could provide a suitable solution, since encouraging results have been obtained in several areas.