Investigation into possible influence of urban irrigated agriculture on malaria transmission in Kumasi - Ghana

Urban irrigated agriculture contributes to food security, employment generation and poverty alleviation in cities of West Africa. However, it might lead to the creation of permanent breeding sites for the principal vector of malaria parasites, Anopheles gambiae, which in turn could also lead to the increase in the transmission of malaria in a city. The objective of this study was to investigate whether, and to what extent, urban irrigated agriculture affects the transmission of malaria in Kumasi, located in the rain forest zone of Ghana. Fifteen locations were selected for the study. Five locations in the urban area with irrigated open-space vegetable production (UA), another five in the urban area without agriculture (UW) and five sites in the pen-urban area (PU) with rainfed agriculture. At all locations, an inventory of mosquito breeding sites was done. Night catching of adult mosquitoes was carried out in both the rainy and dry seasons in the three areas on a weekly basis over a period of 10 weeks for each season. The species of Anopheles gambiae complex present in the study area were identified by Polymerase Chain Reaction (PCR). Household questionnaires were also issued to a total of 1000 respondent households, in both seasons at all selected locations. . This represented roughly 330 households per area. The questionnaire was to collect information on household characteristics, malaria incidence and preventive measures adopted to reduce the incidence. In both seasons, in the UA areas, the larvae of Anopheles spp. were found breeding in sites on the informal smallholder irrigation facilities on vegetable farms. In the rainy season, temporary roadside pools became additional breeding sites. Anopheline breeding sites in the UW were found in such temporary roadside pools in the rainy season; but hardly any of these sites were found in the dry season. In PU areas anophelines were found breeding in pools in sand winning pits, hoof prints of cattle round streams and at edges of streams in the rainy season. In the dry season, the larvae were found in the larger sand winning pits with high water table and in isolated pools of drying riverbeds. Night catches of adult mosquitoes in both seasons revealed that the highest numbers of Anopheles spp. were caught in the PU areas, relatively less in the UA areas and the least numbers in the UW areas. The difference in numbers between UW on one hand and UA and PU on the other were significant in both seasons (p<0.05). The differences between UA and PU however, were not significant in any season. PCR analysis of Anopheles gambiae spp. involved in the study revealed that all specimens processed were Anopheles gambiae sensu stricto. Individuals living close to urban irrigated agricultural areas and in the pen-urban areas reported significantly (p<0.05) more episodes of malaria and subsequent labour days lost than individuals living in city areas without agriculture. Living in the vicinity of irrigated urban agriculture added estimated malaria-related costs of about 150,000 cedis (18-24 USD as of March, 2003) per year to the average household expenditure. Though significant the differences and subsequent financial losses were marginal.

Source: KNUST