Version classiqueVersion mobile

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

Sélection d’articles du programme OCP / Selected OCP papers

The Onchocerciasis Control Programme and the monitoring of its effect on the riverine biology of the Volta River Basin

C. Lévêque, M. Odei et M. Pugh Thomas

Texte intégral

1This paper outlines the problems both logistic and scientific, which arose when designing and setting up the World Health Organization’s monitoring programme in connection with the control of river blindness in the Volta River Basin. At present Simulium damnosum Theo. is being controlled by the application of the organo-phosphorous pesticide ‘abate’ to the larval breeding sites. The monitoring programme, using simple but effective techniques, has now been running for more than 18 months and the techniques of monitoring are described and early results are discussed.


2Onchocerciasis is caused by a thread like filarial worm which amongst other symptoms produces lesions in the eye which may eventually lead to total blindness.

3The disease occurs in Africa, the Yemen and in parts of Central and South America, affecting in all some 20 million people. The largest endemic areas occur in Africa and one of the most serious of these is located in the savanna region of the Volta River Basin. In an area of approximately 700,000 km2 about one million people have the disease, and of these some 70,000 are either totally or partially blind. The location of the programme area is shown in Fig. 1.

Figure 1. The location of the World Health Organisation, Onchocerciasis Control Programme in the Volta River Basin of West Africa.

4The filarial worm is carried from person to person by the female adult of the black fly Simulium damnosum Theo. The larvae of these Aies occur only in fast flowing water, requiring a minimum flow of about 50 cm/second for survival. Because of this association with fast flowing waters, the disease has the common name of river blindness.

5In addition to causing blindness, the disease produces other symptoms including hanging groin and intense irritation of the skin. Because of these symptoms and of the threat of blindness, people have tended to leave the river valleys and move to the uplands. Here, they have increased the pressure on already over-taxed soils and this led to soil erosion and loss of fertility. In 1970, the United Nations Development Programme funded the preparation of a strategy for the control of onchocerciasis on behalf of the governments of Benin, Ghana, Ivory Coast, Mali, Niger, Togo and Upper Volta (P.A.G., 1973). The preparation of this report was completed in 1973 and control operations commenced in 1974. The programme is run by the World Health Organisation and is to continue for some twenty years.

6Because of the large area of West Africa to be covered, the poor roads, the general difficulties of the terrain and the restricted and often localised distribution of the fly breeding sites, the disease is being attacked by controlling the numbers of Simulium larvae by aerial spraying. At present the pesticide used is ‘abate’, (a proprietory formulation of‘temephos’, C16H20O6P2S3, 000’0-tetramethyl 00’-thiodip-phenylene diphosphorothiote, an insecticide with a low toxicity to mammals, birds and aquatic organisais) but other pesticides are being considered. Because of the ecological implications of the heavy use of pesticides over so large a part of West Africa, an Ecological Panel was appointed to check on the ecological effects of the work. The Monitoring Programme was set up at the suggestion of the Panel and reports to them. The Panel contains observers from the World Health but no staff of the Organisation sit on it as members.


7The ecological monitoring seeks to evaluate the effect of abate on the non-target organisms. The effect of the pesticide on Simulium damnosum is watched over by the Vector Control staff of the Onchocerciasis Control Programme itself. When setting up the monitoring programme several important considerations had to be kept in mind.

8(i) Would the monitoring aim principally at investigating the long term effects of the abate on the river ecology or would it look only at the short term effects. The techniques for each approach being different. In effect, the monitoring work is divided into two parts. A long term regular sampling programme aimed at investigating the ecological effects of treatment over the duration of the programme; combined with shorter duration research programmes looking at specific short term problems.

9(ii) The best possible use has to be made of the available manpower and of local facilities. The monitoring is based on three teams; one in each of the following countries: Ivory Coast, Ghana and Upper Volta. In order to ensure reasonable comparability of results, all three teams had to use the same methods.

10(iii) The periodicity of sampling, the sites selected for monitoring, and the field methods used had to combine reliability of sampling technique with reliability of access in both wet and dry seasons over many kilometres of roads or tracks which are not yet hard surfaced.

11(iv) The monitoring techniques had to work equally well in shallow slow flowing rivers in the dry season and in the same rivers flowing fast and deep in the wet season.

12(v) The sequence of sampling had to be sufficiently frequent to make it possible to at least attempt to differentiate between naturally and artificially induced changes in the populations. In most cases a monthly sequence was established.

13In summary a monitoring programme had to be devised which would not only give the desired levei of scientific information, but which would be reliable and consistent when applied to the conditions pertaining in the Programme Area. It is, therefore, worth expanding some of the points listed above. There is no long term monitoring of the effect of pesticide on the phytoplankton and other river algae. The bulk of the monitoring is being done by the Institute of Aquatic Biology in Ghana and the Laboratoire d’Hydrobiologie at Bouaké in Ivory Coast. Neither team had the full facilities, or more importantly, the fully trained algologists needed to monitor the algae in the rivers in the savanna region of the Volta River Basin. In addition, this effectively would have required an extensive preliminary research programme on the phyto-plankton of African rivers. The absence of a vast fund of knowledge on African river biology, similar to that available for temperate rivers, is one of the major difficulties in interpreting data collected in the monitoring programme. It was, therefore, not possible to include phytoplankton monitoring as part of the long term work. A research programme into the phytoplankton of one of the sampling sites in Ghana is under way and preliminary studies into the effects of various insecticides on phytoplankton reproduction is in progress at the University of Salford.

Figure 2. The location of the monitoring stations. —, National boundaries; —, rivers. 1. Bandama River at Niakaramandougou (treated river). Fish and invertebrate samples. 2. Bandama River at Marabadiassa (treated river). Fish samples only. 3. River Maraoué at Danangaro (untreated river). Invertebrate samples only. 4. River N’zi at the Bridge of Dabakala (treated river). Fish and invertebrate samples. 5. River Comoe at the Gansé Ferry (treated river). Fish and invertebrate samples. 6. River Léraba at the frontier bridge (treated river). Fish and invertebrate samples. 7. River Bagoé at Kouto (to be treated in 1977). Fish and invertebrate samples. 8. River Sassandra at Semien (untreated river). Invertebrate samples only. 9. Red Volta River at Po Bridge (to be treated in 1977). Invertebrate samples only. 10. White Volta River at Poura (to be treated in 1977). Invertebrate samples only. 11. Black Volta River at Bamboi (treated river). Fish and invertebrate samples. 12. White Volta River at Daboya (treated river). Fish and invertebrate samples. 13. River Oti at Sabari near Yendi (to be treated in 1977). Fish and invertebrate samples.

14The selection of sites was based on a preliminary field investigation carried out by Dr Stanley Frost from Salford University, Dr Christian Lévêque from Bouaké and Dr S. K. Prah and Mr J. Samman from Accra. The importance of this survey, by competent freshwater biologists, cannot be over-emphasised. It was possible to select sampling stations on the basis of practical knowledge of the sites rather than solely on the basis of theoretical desirability of map location. The sites were selected to do two main things. To sample a wide range of river types in Upper Volta and Ivory Coast and to sample as near as possible to Volta Lake, with its important fishery, in Ghana. In all seven sampling sites were selected in Ivory Coast, two in Upper Volta and three in Ghana. These are shown in Fig. 2. Of these rivers the Maraoué and Sassandra will not be treated and act as permanent Controls. The Rivers Oti and Bagoë will not be treated until 1977 and the Red Volta, a temporary river, was studied for one year/season, before treatment commenced. (Studied 1975, treatment commenced 1976.) Thus, there will have been precontrol studies for one season on the Red Volta and for 20 months on the Oti and Bagoë. When treatment of the latter rivers commences the data from them will be of great interest as there will be the possibility of a before and after study on the effect of abate on the river faunas.


15As already stated, the methods of sampling were selected to be usable, as far as possible, in both wet and dry seasons. The apparatus was also selected for ease of maintenance and robustness, an important consideration because of the difficulty of replacement in the African bush.

16Five sampling methods are used: drift net sampling for invertebrates; fish drift; surber samples; artificial substrates and fish of edible size sampled using commercial gill nets.

Drift net sampling for invertebrates

17At dusk many riverine invertebrates leave the river bottom and drift downstream. These are sampled by placing nets across the flow of the river. The apertures of the nets were 25 x 25 cm, and the mesh aperture was 300 μm. In order to allow a large filtering surface the nets were 2 m long. The nets are arranged in several positions across the river and located so that the top of the frame is 2 cm below the surface. Samples are taken approximately one and one half hours before and after sunset. For the day drift samples, three nets are used for half an hour. For night samples a total of six nettings are used, the nets remaining in the river for three minutes. The river flow is measured at the time of sampling and the number of animais represented as the number of organisms per cubic metre filtered. In terms of actual numbers of animais per cubic metre, the numbers are only approximate but the data collected are comparable both within one site over successive months and for the different sites in the monitoring programme. The basic techniques of drift sampling are well established, but those in use in the programme are as described by Elouard & Lévêque (1975).

Fish drift

18Small fish drift at night in a similar way to the invertebrates and as they are not caught by gill or other commercial nets, fish drift nets are used to catch small fish either as fry or as small species. These nets have a rectangular opening 40 x 70 cm with a mesh size of 1.5 mm. They are 3 m long. Two fish drift nets are used, the nets being placed in position at sunset and retrieved two hours later. Sampling of the fish drift usually occurs every two months. There are at present considerable taxonomic problems concerning the identification of the small fish, but these are being overcome by the fish biologists on the staff of the two hydrobiological institutes and by Dr B. Roman, a World Health Consultant based at Ouagadougou in Upper Volta. The details of this method were worked out by Dr C. Lévêque from whom information may be obtained.

Surber samples (Fig. 3)

19A modified form of surber sampler developed by the O.R.S.T.O.M. team is used to sample known areas of river bed. Again, this is a simple method using robust apparatus. In this case, however, it cannot be used during the wet season. The apparatus cannot be used in water deeper than about half arms length. For comparative work the figures are represented as number of animais per square metre. (Although in fact the method is unlikely to sample every animal in the area enclosed by the apparatus.) As the surber samples are standardised for all monitoring groups, it is possible to compare the data not only on successive months, but also for different stations. The area of river bottom covered by the sampler is 15 x 15 cm, and a metal mesh is placed across the upstream opening of the apparatus. This grid reduces the possibility of drifting organisms entering the sampler.

Figure 3. Surber sampler as used by the monitoring teams. This form of sampler was devised by the O.R.S.T.O.M., team at Bouaké and is more robust and reliable than the traditional pattern. The grill from the upstream aperture of the apparatus has been removed in this photograph.

Artificial substrates

20The team at Bouaké have developed a method of securing small concrete blocks in the rivers. These are colonised over three months and the animals counted. The blocks are colonised by some genera which are not caught in the drift. This method can be used during both low and high flow periods. Several artificial substrate methods were tried but this proved the best, principally because the blocks are less likely to be stolen than the bags and metal baskets used in the other trial methods. The apparatus as used in the monitoring programme consists of a metal rod 90 cm long. On this rod there are five hooks. At each end of the rod are large concrete weights. In use, small cernent blocks 7 x 7 x 4 cm in size are secured to the hooks and the whole apparatus is then securely anchored in the river. The blocks are left immersed for one month, after which the organisms are carefully removed and counted. The blocks are easily cleaned and are of a definite size, making it possible to represent the fauna as numbers of animals per square metre. This method, together with other artificial substrate techniques investigated for the monitoring programme are described in a paper by Dejoux & Venard (1976).

Fish of edible size sampled using commercial gill nets

21These nets are 25 m long and 2 m deep. They are divided into five blocks of mesh size, 15, 20, 25, 30 and 40 mm. Each block is 5 m long. Such nets are used by the teams in Ghana and Ivory Coast, but labour is not available for fish sampling by this method in Upper Volta. The nets are left in position over night. The fish catch is represented as fish per unit effort, which is the number of fish caught per 100 m of net per night. Only that area of net which is in a position to fish is considered in the calculation. For instance in shallow water those parts of the net which are not usable are excluded from the calculation.

22Finally in Upper Volta, Roman is making fish collections on the basis of hand net sampling for a fixed time per site. From these data species percentage composition is worked out.


23Six-monthly meetings of the monitoring groups have been held to discuss field methods and to try to ensure that the monitoring groups keep to the standard methods. To aid in the standardisation of methods they have been written up in the form of a ‘Protocol for Aquatic Monitoring’which defines the various methods of sampling. Copies of this Protocol are held by the monitoring teams from Bouaké, from the Institute of Aquatic Biology and by the workers in Upper Volta. The field sampling programme has been running for twenty months and appears to have worked well. There has been no breakdown in the sampling programme and once standard equipment was available, identical collecting methods were used by all monitoring groups.


24The data collected from the field is discussed at the six-monthly meetings and it is also sent regularly to WHO, at Geneva. Recent work at Salford by Drs Stanley Frost and Colin Fairhurst (1976) has indicated that similar river types have similar faunas and that data from them can be compared one with the other. Some of the rivers are not duplicated in type and can only be compared with themselves on previous occasions and at periods of similar river flow, the latter being a very important consideration.

25Analysis of the data is as yet at a very early stage but we hope to look for changes in the species diversity index and changes in the proportion of pollution tolerant to pollution resistant species. This is based on the concept that abate is essentially a pollutant and will therefore affect pollution tolerant species less than resistant ones.

Figure 4A-D. Variation in the coefficient of condition for Alestes baremose in the treated Rivers Bandama (A, B) and Comoé (C) and the untreated River Sassandra (D). There is no evidence to show that there is a change in the coefficient of condition due to treatment. A. barcmose is a general feeder. E, F. Coefficient of condition, for the cat-fish Eutropius mentalis in the treated Bandama. There is no indication that treatment is affecting the fish. Data are available for both male and female fish. Only data for female fish are presented in this figure. (From Lévêque, 1976.)

26A literatura survey on the effects of organic and toxic Chemical pollution on the fauna of tropical and particularly of African rivers is at present being undertaken. Information gained from this survey will, it is hoped, help in thc interpretation of monitoring data.

27With regards to the fish; in addition to data on the relative abundance of the various species, the Coefficient of Condition is being calculated by the formula:

Figure 5. Variation in the number of fish caught per unit effort in the treated River Bandama. The major fluctuations occur because of fluctuations in river flow. A. Fish caught per unit effort for all fish. B. Catches for Alestes baremose. (From Lévêque, 1976.) Large mesh net; ○, medium mesh net; ●, small mesh net.

28Where P = weight in grammes, L = length in millimetres of fish from tip of head to end of caudal peduncle

29The coefficient of condition is an indication of the general health of a fish.

30The analysis and interpretation of data from the monitoring is only now being developed and one of the largest problems of interpreting results is ignorance of the biology of African rivers and of the behaviour of tropical riverine organisms to pollution. Ease of the interpretation is not aided by the absence of long-term control rivers in the programme area. There are considerable differences in the topography of the rivers outside the programme area to the south, as they are in rain-forest and not in savanna. To the north east most of the larger rivers drain into the Niger System and not into the Volta River Basin. The Niger differs both in catchment and water regime from rivers in the Volta System. This part of the work is, therefore, being developed on a progressive basis combining knowledge gained from the actual monitoring, from general field observations and from a search of the relevant literature. We have only now completed a full 12 months of sampling and thus have only just obtained complete coverage of an annual cycle of river conditions in the Programme Area. The analysis of data and it nterpretation is, therefore, now being given active consideration.

Figure 6. Numbers of fish caught in the untreated River Sassandra. A. Total number of fish caught per unit effort. B. Number of Alestes baremose caught per unit effort. In general, the same pattern of fish catches can be seen as in the treated River Bandama. (Fig. 5). Large mesh net;, medium mesh net;, small mesh net.

31At this stage it is, however, possible to say that there are no discernable effects on the fish in the programme area on the basis of fish caught per unit effort, species composition of the fish fauna, or coefficient of condition, see Figs 4, 5 and 6.

32For the invertebrates the situation is more difficult to interpret because of the rapid turnover of many invertebrate species. At present it is not possible to demonstrate any clear long term effects of abate on the invertebrates in treated rivers. Such changes as have occurred, could well be within the natural variations of the river faunas. When the Oti and Bagouë are treated in Phase III of the Control Programme, it will be possible to compare the two rivers before and after treatment. This comparison will be very interesting.

33As stated above, data from the field is sent regularly to the World Health Organisation Headquarters in Geneva. Provision has been made there for the storage, analysis and print out of the field information. Expert statistical advise is also available on how the accumulated material may best be handled in the future.


34The biological monitoring programme of the Onchocerciasis Control Programme is aimed at identifying major ecological change in the biology of the rivers in the control programme area. To achieve this, sites and methods have been selected which will allow the collection of data on a regular and standarised basis. These data can then be used to try to estímate changes in the river fauna due to the use of pesticide in the area.

35There are 12 sampling sites in the monitoring programme. Two of these are permanently outside the Simulium control area and two of them have not yet been treated with abate (December 1976). By looking at data from all the sites it should be possible both to see and, therefore, discard natural changes in the fauna on one or two of the individual river catchments and at the same time check on long term changes in the fauna of the Onchocerciasis Control Programme region of the Volta River Basin as a whole. The size and difficulty of this task is not to be underestimated. There are 700,000 km2 of land in the treatment area. Little is known about the biology of African Rivers and even less about their biology when polluted. The monitoring programme is rapidly increasing this knowledge. Even in temperate rivers the concept of significant ecological change is difficult to comprehend and define. The ecology of the fauna of the rivers in the Programme Area will certainly change if only because of the control of Simulium damnosum larvae. It is also unlikely that during the whole of the control period the non-target organisms will be totally unaffected.

36The use of biological indicators of pollution to show changes in water quality as outlined by Wilhm (1975) may help to indícate ecological change in a theoretical or academic sense. In terms of the Onchocerciasis Control Programme, significant ecological change musí be related to 70,000 severely disabled people, to upland soil erosion and to the general deprivation of the population in the Volta River Basin. The concept of ‘significant ecological change’is, therefore, being actively considered by the Ecological Panel.


37The following have contributed to the setting up and running of the Aquatic Biology Monitoring Programme in the Onchocerciasis Control Programme Area: E. K. Abban, J. J. Albaret, J. S. Amakye, H. N. Appler, R. Bigorne, C. A. Biney, C. Dejoux, J. Duppenthaler, J. M. Elouard, C. P. Fairhurst, P. Forge, S. Frost, P. Herbinet, H. B. N. Hynes, A. litis, J. D. M. Marr, B. de Merona, D. Paugy, S. K. Prah, B. Roman, J. Samman, P. Venard, Frank Walsh and Brenda Walsh. They have all contributed, therefore, to the writing of this paper. The World Health Organisation finance the monitoring programme by contracts with the Institute of Aquatic Biology, Accra, Ghana, and the Laboratoire d’Hydrobiologie, Bouaké, Ivory Coast.



DEJOUX, C. & VENARD, P., 1976. Efficacité comparée de deux types de substrats artificiels. Rapport No. 3. Laboratoire d’Hydrobiologie, Bouaké, Cote d’ivoire. B.P. 1434.

ELOUARD, J. M. & LÉVÊQUE, C., 1975. Observations préliminaires sur la dérive des invertebrates et des poissons dans quelques riviéres de Cote d'Ivoire. RepportNo.03 94/ORSTOM/75. Laboratoire d’Hydrobiologie. Bouaké, Cote d’ivoire. B.P. 1434.

FROST, S. & FAIRHURST, C. P., 1976. The Drift of Rivers in the Volta River Basin Part 1. Environmental Resources Report 28. Department of Biology, University of Salford.

LÉVÉQUE, C., 1976. Convention O.R.S.T.O.M., -O.M.S., Surveillance du Milieu Aquatique, Rapport Annuel d'Activities, 1975. Laboratoire d’Hydrobiologie, Bouaké, Cote d’ivoire, B.P. 1434.

P.A.G., 1973. Onchocerciasis Control in the Volta River Basin. Paper No. OCP/73.1. Geneva: The World Health Organisation.

WILHM, J. M., 1975. Biological indicators of pollution. In B. A. Whitton (Ed.), River Ecology. Berkeley, Los Angeles: University of California Press.

© IRD Éditions, 2003

Conditions d’utilisation :

Cette publication numérique est issue d’un traitement automatique par reconnaissance optique de caractères.
Rechercher dans OpenEdition Search

Vous allez être redirigé vers OpenEdition Search