Impact sanitaire de l'utilisation d'eaux polluées en agriculture urbaine. Cas du maraîchage à Ouagadougou (Burkina Faso)

In Ouagadougou and in many similar Sahelian cities, urban agriculture uses very often polluted water for irrigation of vegetables, including those that are likely to be eaten uncooked. As the pathogenic germs can survive long enough in water, soil and plants, this practice entails potential sanitary risks, particularly for farmers and their families. The potential of risks related to the use of sewage in agriculture is well known and sanitary instructions exist. However, there is no assessment of real risks within the context of sewage use on a small scale in small size plots, as it is the case in Ouagadougou, and common in Sahelian countries. The aim of the current thesis is to contribute to a better understanding of interactions between the environment and health. The specific objectives are: (i) - to assess incidence rates for diarrhoea In a period of two weeks and to assess prevalence rates for parasitic infections; (ii)- to assess relative risks and prevalence ratios among exposed groups and those of the population in general; (iii) - to assess levels of microbiological pollution of irrigation water, soils and vegetables; (iv) - to assess sociocultural factors and behavioural attitudes of the various actors; (v) - to assess environmental pollution at the level of residential areas; (vi)- to propose possible measures in order to reduce risks and to discuss their feasibility. The general approach that has been adopted is transdisciplinary, particularly with an epidemiological perspective associating microbiology, socio-anthropology, cartography and geomatics. The cartographical approach, with the assistance of geomatics made it possible (i) - to characterize residential areas in Ouagadougou according to pollution by refuse dumps and throwing out of sewage, (ii) - to locate market gardening sites in urban fabric, and (iii)- to assess the importance of exploited surfaces during the dry season, the cool season and the rainy season. Peripheral sectors are more polluted than central sectors. Variation in the total exploited surfaces between the cool season (December) and the dry season (April) is almost 85%. Six areas of market gardening represent in all seasons more than 55% of the total exploited surfaces. The majority of fanners in these 6 areas of market gardening live in 4 to 6 peripheral sectors (79% in 1992, 90% in 1995). The microbiological approach made it possible to monitor bacteriological (fecal coliforms) and parasitological (helminth and protozoan eggs) pollution in water, soils and plants. Water and soil samples have been taken in 4 market gardening areas which represent in all seasons between 49 and 56% of the exploited surfaces in the city; plant samples have been taken in the 4 areas of market gardening and in the central market of Ouagadougou. 668 samples of irrigation water, 247 samples of lettuce, 48 samples of carrots and 218 samples of soils have been analyzed between October 1993 and December 1995. The levels of bacteriologi