|Agricultural Development and Vector-Borne Diseases (FAO - HABITAT - UNEP - WHO, 1996, 91 p.)|
|Topic C: Vector habitats|
Slide C.15 Malaria vector habitats: irrigated rice fields, Office du Niger, in Mali, where a succession of species breeds
Irrigated rice fields make up the largest man-made wetlands environment in the world. Of the 150 million hectares of the global harvested rice area, about 77 million hectares are irrigated; of the total hectarage, 95% can be found in developing countries.
A flooded rice field is an agroecosystem that is frequently disturbed by farming practices, i.e. tillages, irrigation, fertilization, crop establishment and weeding, as well as by natural phenomena such as rainfall and flooding, which result in extreme instability on a short time scale during the crop cycle, but relative stability on a long time scale.
Flooded rice fields are eutrophic systems with exceedingly high recycling rates of nutrients and energy, as exemplified by the rapid succession of algae. The ecology of rice environments exhibits enormous spatial variation due to extremes in climatic, soil and hydrological conditions under which the crop is grown. The predominant role of water depth and dependability of the flooding regime in delineating rice environments is well recognized. Current terminology considers five dominant environments based on the maximum sustained depth of water in the field:
· irrigated with controlled shallow water depth (5-10 cm)
· rainfed lowland, with uncontrolled shallow water depth (1-50 cm)
· deep water, with maximum sustained depths from 50 to 100 cm
· very deep water, more than 100 cm deep, and
· upland, with no surface flooding
The association between irrigated rice production systems and vector-borne diseases is the subject of extensive literature. In general, Asian rice fields may breed very specific species in well defined areas. These include Anopheles culicifacies in India, A. aconitus in Indonesia and A. maculatus in Nepal. In terms of health impact, however, none of these vectors can compete with the forest breeding A. dirus group. Also, technically, a clear distinction should be made between those vectors that exclusively breed in rice fields and those that breed in irrigation schemes at large, including ancillary structures such as irrigation or drainage canals.
Slide C.14 shows ideal conditions for the breeding of A. maculatus in Nepal, where rice is grown on irrigated hillsides, providing many small pools and seepages. When the paddy is flooded, cattle may be taken to pasture elsewhere, thus reducing the availability of animals as a source of blood meals. This, in turn, may cause the so-called anthropophilic index to rise, i.e. higher proportions of mosquitoes will bite humans instead of cattle. These events will coincide with the time of the year when rainy and warm weather gives rise to higher densities of A. maculatus. Malaria transmission is likely to increase due to a combination of these conditions.
In Africa, the range of breeding habitats of the A. gambiae complex is so broad and transmission so intense that it is hard to determine to which proportion malaria can be attributed to irrigated rice production systems. Extensive research by Lindsay in the Gambia, by Carnevale in Burkina Faso and by Coosemans in Burundi has revealed more detailed information, but has also raised new questions. Unlike the Asian situation, in Africa mosquito density is not linearly correlated to transmission intensity - on the contrary, where irrigation development has led to an increase in the density of mosquito populations, transmission levels have frequently decreased.
In order to further clarify the various phenomena of irrigated rice associated malaria, the West Africa Rice Development Association, together with PEEM and IDRC/Canada, initiated, in 1995, a consortium research project on the association between irrigated rice production and vector-borne diseases in West Africa, with support from IDRC, DANIDA and the Government of Norway. The contact person is Dr Thomas Teuscher at WARDA in BouakCdIvoire (see list of PEEM collaborating centres).
Amerasinghe, F.P., Amerasinghe, P.H., Malik Peiris, J.S. and Wirtz, R.A., 1991. Anopheline ecology and malaria infection during the irrigation development of an area of the Mahaweli Project, Sri Lanka. Am. J. Trop. Med. Hyg. 45(2):226-235
Carnevale, P., Robert, V., Snow, R., Curtis, C., Richard, A. Boudin, C., Pazart, L.-H., Halna, J.M., and Mouchet, J., 1991. Limpact des moustiquaires impr sur la prevalence et la morbiditiau paludisme en Afrique sub-Saharienne. Annales de la Soci belge de Mcine tropicale 71 (suppl.): 127-150.
Coosemans, M., 1985. Comparaison de lende malarienne dans une zone de riziculture et dans une zone de culture de coton dans la Plaine de la Ruzizi (Burundi). Annales de la Soci belge de Mcine tropicale 65 (suppl. 2.): 187-200.
Coosemans, M., and Barutwanayo, M., 1989. Malaria control by antivectorial measures in a rice growing area of the Rusizi Valley (Burundi). Transactions of the Royal Society of Tropical Medicine and Hygiene 83: suppl. 97-98
Lacey, L.A., and Lacey, C.M., 1990. The medical importance of riceland mosquitoes and their control using alternatives to chemical insecticides. Journal of the American Mosquito control association, supplement # 2, June 1990, pages 1-93.
Lindsay, S.W., Wilkins, H.A.. Zieler, H.A., Daley, R.J., Petrarca, V., and Byass, P., 1991. Ability of Anopheles gambiae mosquitoes to transmit malaria during dry and wet seasons in an area of irrigated rice cultivation in the Gambia. J. Trop. Med. Hyg. 94: 313-324
Smits, A. Coosemans, M., van Bortel, W., Barutwanayo, M. and Delacollette, Ch., 1995. Readjustment of the malaria vector control strategy in the Rusizi Valley, Burundi. Bulletin of Entomological research 85: 541-548