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Trente ans de lutte contre l’onchocercose en Afrique de l’Ouest. Traitements larvicides et protection de l’environnement

Laurent Yaméogo
Christian Levêque
Jean-Marc Hougard

Environmental Impact Assessment of Settlement and Development in the Upper Léraba Basin

1. Introduction

Texte intégral

  • 1 See Samba 1994 for a recent Overview of the OCP.

1Since its inception in 1974, the Onchocerciasis Control Programme in West Africa (OCP) has implemented a strategy of onchocerciasis (river blindness) control which has been aimed at the long-term interruption of the transmission of the causative agent (the filarial worm Onchocerca volvulus), in all areas where there was a human reservoir of infection of the severe blinding form of the parasite. In operational terms, this has required the weekly application of insecticides (mainly by aircraft) to the fast-flowing stretches of rivers which constitute the habitats of the larval stages of the blackflies which are the onchocerciasis vectors.1

  • 2 With the appearance of blackfly resistance to temephos in the early 1980s, the OCP arsenal of oper (...)

2Although the OCP commenced its vector control operations with only a single Chemical larvicide, the organophosphate compound temephos,2 it was recognized from the outset that the possible impact of larviciding operations on the aquatic fauna would need to be carefully monitored. During the early years of the OCP a small Ecological Panel fulfilled this function. In 1980, when the OCP structure was reorganized, the Ecological Panel became the Ecological Group of the OCP Expert Advisory Committee, with a mandate to study “the ecological impact on the environment of the use of insecticides in the Programme” (Samba 1994).

3Repeatedly, the Ecological Panel and the Ecological Group were able to demonstrate to the outside world that there had been no significant disturbances of the aquatic environments of the OCP area which could be attributed to OCP larviciding operations. In fact, the environmental soundness of the OCP strategy was repeatedly demonstrated by routine aquatic monitoring and assessment activities; the results of which were published in scientific literature (see, for example, Lévêque et al. 1988, Yaméogo et al. 1993).

4Despite the proven environmental soundness of the OCP control strategy, the last few years have witnessed growing concern that human settlement and development activities in onchocerciasis-controlled areas could impose new threats to both aquatic and terrestrial ecosystems. Increases in agrochemical use and in other potentially pollutant loads, and physical change to the environment could all, directly or indirectly, have adverse effects on the quality and productivity of the valleys concerned.

5From the analysis of aquatic monitoring data collected in the original OCP area during the early 1990s, the Ecological Group concluded that strong scientific evidence was available to substantiate these concerns. For example, environmental studies of some watercourses which were no longer larvicided by OCP showed that faunal recolonization was in progress, but that in certain cases, human activities had clearly contributed to a degradation of the environment, resulting in an impoverishment of the fauna. The Upper Léraba Basin (covering parts of Burkina Faso, Côte d’Ivoire and Mali) was considered to be a good example of where this type of situation existed (OCP 1992).

6In 1991 these matters were discussed by the OCP Committee of Sponsoring Agencies (CSA) and later by the OCP’s governing body, the Joint Programme Committee (JPC), in the context of the CSA’s future work programme of support in socioeconomic development to the OCP Participating Counties. At its 12th session (December 1991) the JPC endorsed a CSA work programme which, inter alia, would provide for the CSA to address some of the broader environmental aspects of settlement and development in the OCP area, and in the first instance, to conduct an environmental impact assessment of agrochemicals. Members of the Ecological Group who attended that JPC session welcomed this new initiative, and stressed the importance of taking a broader view of environmental issues, especially as they related to water resource conservation and management.

7During 1992, after discussions between the Ecological Group, the CSA and staff of the OCP, a detailed “Draft Proposal for a Pilot Project on Environmental Impact Assessment in the OCP Area” was prepared (OCP 1992), and submitted to the 13th session of the JPC (December. 1992) for approval. This proposal made provision for pilot studies in two river basins, the Upper Léraba Basin and the White Bandama Basin (in Côte d’Ivoire). However, it was decided that, in the first instance, it would be prudent to embark upon a study in only one of the basins, with preference being given to the smaller of the two; the Upper Léraba. Accordingly, in early 1993, The World Bank, on behalf of the CSA, launched a Pilot Project for an environmental impact assessment in the Upper Léraba Basin (hereafter referred to as the Basin), in accordance with the detailed terms of reference that were prepared and approved at the end of 1992 (OCP 1992).

Objectives of the Pilot Project

8The primary objectives of the Pilot Project were:

  • assessment of the present environmental situation in the Basin, with a view to determining potential sources of impact on the aquatic environment,
  • quantification of Chemical loads, and assessment of modification to the physical environment,
  • identification of simple study methods which would be applicable to other areas, and which could be offered as examples to predict environmental impacts of settlement and development, and to furnish guidelines to minimize those impacts.

9Although several methodologies for regional environmental impact assessments have been proposed (see, for example, proposals for thematic maps for regional ecotoxicological risk assessment by Herrchen 1994), very few impact assessments have actually been conducted worldwide (one of the few examples being that described by Di Guardo et al. 1993a). One of the few assessments so far made in sub-Saharan Africa, a study conducted by Calamari et al. (1992) in the Winam Gulf Basin of Kenya, had some relevance to the Léraba investigation. Although the ecological differences between the two basins were such that direct comparisons could not be made, some elements of the Kenyan experience provided guidance during the planning of the Léraba Pilot Project. Thus, to the best of the present authors’ knowledge, the Léraba environmental impact assessment conducted by them is unique for West Africa, if not for sub-Saharan Africa as a whole. The authors have therefore prepared this account in such a way as to present the Léraba investigation as a model which is applicable to assessments that may be required in other parts of Africa, where environmental conditions are the same or similar.

Project activities

10The Pilot Project commenced in January 1993, with the collection of geographical, meteorological and hydrometric data, with the assembly of quantitative information on agricultural practices, fertilizers and pesticides, and with the preparation of ecotoxicological profiles of the pesticides known to be used in the Basin. In May 1993, an OCP helicopter was used to make aerial reconnaissance and photographic flights over the Basin, and some adjacent areas. The aerial photographs that were taken were then used to make maps of pre-selected parts of the Basin, and to construct vegetation profiles. Details of these latter activities are given in Annex 1.

11During the latter months of 1993 and during early 1994, the main Project activities were:

  • quantification and assessment of organic loads, nutrients and fertilizers;
  • pesticide risk assessment, employing the SoilFug model to predict pesticide runoff;
  • computerization and interpretation of cartographic land utilisation data;
  • localized vegetation mapping around Léraba Bridge, and assessment of the status of riverine forests;
  • sampling of fish and aquatic invertebrates at Léraba Bridge and at two other nearby sites;
  • overall assessment of the environmental situation in the Basin, and reporting.

Other sources of information

12Information on the general geography and climatology of the Basin, as well as technical information on agrochemicals, were taken from the published literature and sources are acknowledged accordingly in the text. Quantitative data on rainfall, hydrometry, soil properties, human and livestock populations, crops, fertilizers and pesticides, were obtained from the appropriate Government offices in Burkina Faso.

13Historical data on land utilization (especially two maps: “Carte II6-B-24. Situation en 1972. OCP/ECO. J. C. Clanet & P. H. Somé, 1985”, and “Carte II6-B-25. Occupation du sol 1983. OCP/ECO. J. P. Hervouet, 1985”) were provided by OCP Headquarters in Ouagadougou.


1 See Samba 1994 for a recent Overview of the OCP.

2 With the appearance of blackfly resistance to temephos in the early 1980s, the OCP arsenal of operational insecticides was progressively increased. By 1994, six compounds were in regular, rotational use; temephos, Bacillus thuringiensis (B.t. H-14), permethrin, carbosulfan, phoxim and pyraclofos (Samba 1994).

© IRD Éditions, 2003

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