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 bacteriological pollution for all types of water, most of the time, are above the thresholds recommended by WHO for the watering of vegetables that are likely to be consumed uncooked (1000 FC/100 ml). Water from canals or channels (Abattoir and Canal Central sites) prove to be more polluted, both bacteriologically and parasitologically, than the one from wells and dams (Boulmiougou and Tanghin sites). Lettuce from both market gardening sites and market has a bacteriological pollution that is beyond sanitary thresholds (100 FC/100 g), with higher levels in the market. A proportion of at least 10% of soil samples from paths are contaminated by parasites such as Ankylostomes, Ascaris and Anguillules. The market gardening sites which have the most polluted beds by parasites are those of Abattoir (79% of samples) and Canal Central (75%) which correspond to the sites with irrigation water that is also the most polluted. The socio-anthropological approach made it possible to gather ideas and beliefs of various actors in the chain of market gardening products (from producer to consumer) and to carry out direct observations about environmental factors as well as about practices in the various places in this chain. During the two years, 15 FGD (Focus Group Discussion) have been held with farmers, retailers, caterers and consumers. In 1995, 226 observations sessions have been held from 22 observation posts (5 market gardening sites, 3 markets, 3 caterers and 11 selected households). Whilst the environmental factors on market gardening sites are characterized by a high level of pollution, the behavioural attitudes of farmers during their activities, particularly walking barefoot, and expressed beliefs reveal a lack of awareness. The epidemiological approach made it possible to assess differences in incidence or prevalence rates between climatic seasons, between farmers and the general population, between market gardening sites, and between sexes, for diarrhoea, stomach aches and various parasitic infections. The data have been collected through households surveys and examinations in analysis laboratories of excreta and of households' drinking water. Three cross-sectional surveys with children as targets have been undertaken in 1994 (2 in the dry season and in the rainy season) and in 1995 (one at the end of the rainy season). Sample sizes were respectively 512, 599 and 3040 children aged between 0 and 4 full years and who have been weaned. During the third cross-sectional survey 208 adults, fathers or mothers of the surveyed children, have been also involved and water samples of 2996 households have been analyzed. A fourth cross-sectional survey dealt only with the group of market gardeners, with 191 adults, 97 children of farmers and 186 samples of households' drinking water. Differences are significant between the end of the rainy season and the two other seasons as far the diarrhoea and the main parasitic infections are concerned among the general population's children aged less than 5 years old. The incidence of two weeks' diarrhoea is very important in all seasons among the children of the general population (always > 30%) as well as among the children of market gardeners at the end of the rainy season (37.10 ± 9.61%). On the other hand, the incidence rate for two weeks' diarrhoea is low among adult farmers themselves at the end of the rainy season (6.30 ± 3.45%). The group of farmers, both children and adults, have in a very significant way higher prevalence rates than those of the general population as far as Ankylostoma are concerned (10.80 ± 6.68% against 1.40 ± 0.43% among children; 40.60 ± 7.38% against 14.90 ± 5.09% among adults). The difference between men and women is very significant for Ankylostoma among market gardeners (48.40% against 30.70%, PR= 1.58, p= 0.03). The epidemiological approach also made it possible to carry out a case-control analysis of diarrhoea among children of the general population aged less than 5 years old. The final logistic regression model draws a significant association (p < 0.05) between children's diarrhoea and 11 risk factors. These factors are related to either habitat conditions, or socio-economic conditions, or behavioural attitudes. The geomatic approach made it possible to disintegrate the main results of the epidemiological approach (as it has been done already for those of the cartographical approach) to present them at the sector level as thematic maps. The study leads to the formulation of recommendations in the area of (i)- actions and interventions; and (ii)- possibilities in research perspectives, regarding health and environment.