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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


Texte intégral

What is Onchocerciasis ?

A public health disease of socio-economic importance

1Onchocerciasis is a dermal filariasis caused by the development in human of the filarial worm Onchocerca volvulus (Duke, 1990).The main clinical manifestations of the disease are skin lesions and visual impairment which may result in blindness. O. volvulus is necessarily transmitted to man by the bites of a blackfly vector of the genus Simulium in West Africa. This fly breeds exclusively in fast-flowing stretches of rivers. Thus the disease is essentially concentrated in foci alongside watercourses, especially in dry savanna areas, hence its name “river blindness”.

2Onchocerciasis is prevalent in 30 African tropical countries from Senegal to Ethiopia, between 12-15° North and 12° South.The disease is also prevalent in small isolated foci in six countries of Central and Northern South America, (transmitted by local blackfly vector species) and South-Eastern Yemen. More than 99 % of the estimated 18 million onchocerciasis patients live in Africa.

3Onchocerciasis main clinical manifestations include skin lesions (skin atrophies or thickenings, depigmentations :“leopard skin, caiman skin...”), which are associated with intolerable itching and scratching ; however the main ones are eye lesions, which may range from mild reversible anterior lesions to severe, irreversible and ultimately blinding anterior or posterior lesions.

4Onchocerciasis is a cumulative disease : in a given bioclimatic area the severity of eye lesions are related to the mean intensity of infection of the human communities by O. volvulus, itself dependent on the density of infective biting vector Onchocerciasis is a disabling and debilitating disease which affect more severely small rural communities. In savannah African valleys, onchocerciasis blinding rates may affect up to 10 % of the total population, and 25 % of the adult active populations.

5Onchocerciasis, and above all its effects on the eyes, leads to distress in the affected populations, prompting young people to abandon the most fertile riverside lands which lead to the breakdown of the social relationships in the villages. While onchocerciasis has not always been the unique cause of depopulation of the valleys in Africa, it is none the less the main obstacle to their development and to the settlement of communities in those regions (Marchal, 1978).

A vector-borne parasitic disease

O. volvulus in man and vector

6The adult O. volvulus female (macrofilaria, about half a meter long) lives in the human subcutaneous tissues, free or more often tangled in fibrous nodules (Schulz-Key, 1990). It is sexually active for 9 to 14 years. During its fertile life span it produces millions of very small embryos (250-330 microns) called microfilariae which can survive more than two years in the human skin. The microfilariae are the pathogenic stages which invade dermal tissues and cause severe disorder, in ocular tissues inclusively. They have to be carried from one person to the other by a female blackfly belonging to the Simulium damnosum complex in West Africa. Only females of S. damnosum bite man, and sometimes animals, as they need blood for the maturation of each batch of eggs laid. There is no known animal reservoir.

7Once the blood arrives in the blackfly stomach most of the microfilariae are digested, but a few of them pass through the intestinal wall and reach the abdominal cavity, and the thoracic muscles where they transform (Bain, 1971): the microfilariae give rise to infective larvae, measuring 650 micros, which find their way in the mouth parts and may thus be transmitted to man during subsequent blood meal. The maturation cycle of the larvae in the blackfly takes about seven days at 27-30 °C. The number of infective larvae in blackfly is generally less than 10 and in most cases from one to three, while one female S. damnosum may ingest hundreds of microfilariae during one blood meal. When infective blackflies bite man they deposit the infective larvae in the skin. The larvae penetrate the skin superficial layers, moult, mate and the nodules appear between seven to twelve months, sometimes up to three years.

The vector : S. damnosum

8As in all the Simuliids, only S. damnosum females are hematophagous. Male adult blackflies feed solely on plant juices and play no direct role in onchocerciasis transmission. The females are largely anthropophilic and may absorb I mg of blood at each meal. Studies carried out on behalf of OCP showed that there are no preferred locations for resting sites and that the resting blackflies occupy the entire gallery forest, hence the difficulties for an efficient adulticidal coverage (Bellec and Hebrard, 1980). The female S. damnosum lives for up to four weeks. This female seeks a sugar meal (plant juices or nectars), then a bloodmeal and may thus ingest microfilariae if the meal is taken from a person infested with O. volvulus. S. damnosum females lay their eggs in fast flowing parts of rivers from which larvae hatch and develop to pupae then to the adult stage in eight to twelve days. The pre-imaginal forms (eggs, larvae and pupae - Elsen, 1979) are all aquatic and strongly rheophilic. After hatching, young larvae stay attached to the substrates but may also drift with the current. They feed by using their unfolded mandibular fans indiscriminately to catch particles suspended in the flowing water ; including the nutrients as they pass through the digestive tube. If an insecticide has been adsorbed on the water suspended particles, or is itself particulate, it is ingested by the larvae in the same way as food.

A serious clinical disease

9The clinical manifestations of onchocerciasis are primarily due to the microfilariae, which are the pathogenic stages of the parasite. They provoke itching and, in case of infections, onchodermatitis (Murdoch et al., 1993). The microfilariae may invade the eye, causing severe eye disorders culminating in blindness. In the case of intense and prolonged invasion of the eye by microfilariae, permanent lesions appear : keratisis which opacifies the cornea, iridocyclisis causing glaucoma, and inflammation of the retina and the optic nerve. Since onchocerciasis is an accumulative disease, the ocular complications appear after an accumulation of infections over several years. In certain hyperendemic areas, blindness occurs between the ages of 30-40 but may occur earlier in some subjects (Remme et al., 1989). Onchocercal nodules are an encysting reaction to the parasite O. volvulus by the human host. They are located mainly where bones are superficial, near the hips, the rib cage and quite often on the head and the legs. They are generally small, 1-2 cm, but may sometimes exceeded 5 cm. The frequency and severity of the symptoms are often closely correlated with the number of microfilariae which, in turn, depends on the number of adult filariae. This number of adult worms is governed by the number of infective larvae received by the subject, and therefore by the number of bites received from infected blackflies in relation to the length of time spent in an endemic area.

Fig. 1
The life cycle of Onchocerca volvulus

A disease controlled through larviciding and chemotherapy

10As most of vector-borne diseases, onchocerciasis can be combated by appropriate vector control operations. According to the geographical context and vector biology, the vector control strategy can have different objectives.

11The strategy of definite elimination of the vector has been envisaged, especially against the S. neavei complex, the East African vector In the case of the S. damnosum complex, because of the important flight potential of those flies, attempts must be restricted to exceptional situations of small isolated foci, through actions limited in time and space. This strategy, which is a little component of the African Programme for Onchocerciasis Control (APOC) (Dadzie, 1997), is being implemented by this programme in four limited foci in Central and East Africa.

12Another strategy consists of bringing the transmission of the parasite to an end. The objective is to arrest transmission of the O. volvulus by eliminating vector population for the duration of the lifespan of the adult worm from the human reservoir which at present is calculated to be around 14 years. This has been applied and demonstrated in parts of the original area of the Onchocerciasis Control Programme in West Africa (OCP) where vector control alone has been carried out for more than 14 consecutive years and where the disease is no longer a public health problem (Hougard et al., 2001 – see figure 2). As blackfly adults are difficult to target, the vector control operations consist of treating with appropriate insecticides the breeding sites of rivers where the reophilic larval stages develop. As far as S. damnosum is concerned, the development of the aquatic stage from egg to pupae is around one week, hence the insecticide application is undertaken weekly.

13Onchocerciasis can also be controlled by chemotherapy. However despite the importance of the research effort, only ivermectin has proven to be effective, well tolerated and accepted, and without side effects. Today, it is the only molecule used to control onchocerdasis morbidity (Abiose et al., 2000). In contrast to diethylcarbamazine (DEC), ivermectin is a microfilaricide which is effective at a single dose without causing any side effects (particularly Mazzotti reaction) or serious aggravation of ocular troubles even when the dose and the parasitic load are high. It reduces the microfilarial load by 90 % and this reduction is maintained for six months at least. The objective of the use of ivermectin in APOC and OCP is to control onchocercal morbidity and therefore to prevent onchocercal ocular disease and blindness. To get maximum benefit from the drug, the treatment has to be regularly administrated over a long period of time, the duration of which has not yet been determined and depends on the level of endemicity. This is due to the limited effect of the drug on the adult worm and on the interruption of the transmission (Winnen et al., 2002). Ivermectin is now extensively delivered within OCP and APOC through an institutionalized strategy of Community Directed Treatment with Ivermectin (CDTI). Its principle rests on the distribution of ivermectin by community workers selected by the population itself and specially trained in the various activities related to the treatment (Diarra, 1998).

What’s new about the West African onchocerciasis vector and parasite ?

A complex of blackfly species

14Blackflies are small hematophagous flies of the genus Simulium. There are over 2000 species around the world, only a few of which transmit onchocerciasis. S. damnosum is distributed throughout most of tropical Africa, not as a single taxonomic entity but as a complex of sibling species with own ecological characteristics (Le Berre, 1966). Nine species are present in the OCP area (Boakye, 1993). S. damnosum s.s., S. sirbanum and S. dieguerense are found in the savannah zone until the northern limit of the endemic area for onchocerciasis. S. soubrense, S. sanctipauli, S. konkourense and S. leonense are great dense forest-dwelling ; they may also be found in the zone of light west forests and, in places, even reaches savannah zones. S. yahense is limited to small forest watercourses and S. squamosum probably covers several different entities and is widespread in both forest and savannah zones. Species differentiation is based on the examination of the polytene chromosomes of the salivary glands of the larvae. The morphological and morphometric criteria that enable us to separate most species or groups of females adults have also been identified. Since 1995, a technique based upon mitochondrial encoded gene sequences has been developed and makes it possible to identify most species of the complex (Tang et al., 1995).

Several parasite strains

15Considering the geographical differences in the clinical pattern and focalization of onchocerciasis, the severity of ocular lesions and prevalence of blindness, the existence of several O. volvulus strains differing in their pathogenicity to man and their adaptation to various species of the S. damnosum complex, had been suspected since the years 60's (Duke et al., 1966). For many years OCP has been faced to the problem of parasite identification because the available taxonomic tools could not allow for a reliable identification of the parasite found in the blackfly females during dissections. At the infective stage, O. ochengi, the main cattle filaria which can be transmitted by S. damnosum s.l., cannot be morphologically differentiated from O. volvulus which is responsible for human onchocerciasis. As a result, the measure of the transmission could be overestimated since it took into account all the larvae found during dissections (Philippon, 1977). That was the reason to develop a method for differentiating, on one hand, O. volvulus from the parasites of animal origin and, on the other hand, the O. volvulus strains that are the most pathogenic. The discovery of a family of repetitive sequences composed of 150 basic pairs in the genotype of Onchocerca sp. made possible the development of DNA probes for parasite identification (Zimmerman et al., 1993). Several probes were isolated. OCH probe is able to differentiate O. volvulus from O. ochengi. PFSI and pSS-IBT probes made it possible to differentiate the savannah and forest strains of 0. volvulus. They have been commonly used by OCP from 1992.

Fig. 2
Epidemiological trends from 1975 to 2000 in a village of the core area of OCP (from Hougard et al., 2001)

How was devised the Onchocerciasis Control Programme ?

An overall integrated strategy

16OCP ceased rts activites on December 31, 2002, after 29 years of life. No other public health programme ever benefited for so long from the logistical and financial support of the international community, the reason for this support being that the results obtained have always convinced the donors of the effectiveness of the control strategies used: vector control from 1975 to 1989, then vector control and/or therapeutic control until 2002 (Molyneux, 1995). OCP precisely began its activities in January 1974. Its objective was to eliminate onchocerciasis as a disease of public health importance and an obstacle to socio-economic development (WHO, 1969). The basic strategy of the Programme consisted in interrupting the transmission of the blinding strain of O. volvulus by destroying S. damnosum s.l. at its larval stage through aerial application of selective insecticides on the rivers infested (Hougard et al., 1993). The first aerial treatments began at the very end of 1974 in areas where the incidence of blindness was highest. They were later gradually extended to cover by the end of 1977 an area of 654 000 km2 spread over seven countries (Burkina Faso, south-eastern Mali, south-western Niger ; the northern parts of Côte d'Ivoire, Benin, Ghana and Togo). However ; it was very soon clearly established that the border of this area was affected by infective blackflies originating from regions outside the Programme area. In order to protect permanently the reinvaded area and also clean the basins which were a source of reinvasions, the incriminated hyperendemic regions were identified and then put under larvicidal treatment. To the west of the original area (western extension), these were the basins of western Mali, south-eastern Guinea and northern Sierra Leone. To the south and east of the original area (south-eastern extension), these were the Southern basins of Côte d'Ivoire, Bénin, Ghana and Togo (Fig. 3).

Fig. 3
The Onchocerciasis Control Programme area

17The set up of vector control operations in the extension area was completed towards the end of the 80's, while all the basins of the original area were still under treatment. The larviciding coverage then reached its peak with more than 40,000 km of river stretches treated, corresponding to a million square kilometres, spread over nine countries of the Programme. Because of the success of the larval control strategy, larviciding operations progressively stopped from 1989 in the basins of the original area. In the extension areas, larviciding was going on satisfactorily in combination with ivermectin. To date, human onchocerciasis is no longer a problem of public health importance nor an obstacle to socio-economic development in all of the treated area. The control of this filariasis is however not over since OCP never aimed at eradication, neither of the parasite nor of its vector In 2003, the eleven participating countries of OCP have taken over the responsibility of carrying out the residual activities of monitoring and control of this disease. This task is of great importance because any recrudescence of the transmission would lead in the long run to the reappearance of the clinical signs of onchocerciasis, if not its most serious manifestations.

A pre-eminent operational structure

18OCP included up to 11 participating countries : Bénin, Burkina Faso, Côte d'Ivoire, Ghana, Guinea, Guinea Bissau, Mali, Niger Senegal, Sierra Leone and Togo. It was sponsored by the United Development Programme (UNDP), the Food and Agricultural Organizaron (FAO), the World Bank and the World Health Organizaron (WHO) and has received since its inception a financial support from 28 donors. An operational agreement signed in 1973 between the Participating Countries and WHO determined the scope, objectives, consultation and management structures of OCP and the means by which control operations and the evaluation procedures were to be conducted (Anonymous, 1973).The uppermost level of the current OCP structure was the Joint Programme Committee (JPC) which has exercised full directional powers as regards overall programme policy, strategy development and budgetary matters. Next came the “advisory level” represented by the Expert Advisory Committee (EAC). The third level was that of "support and collaboration", consisting of the Committee of Sponsoring Agencies, the World Bank for the mobillzation of funds and the WHO headquarters and regional office for Africa for the administration of financial resources and the administrative support. This level also included collaboration with the participating countries and donors. Finally, the last level consisted in planning, programming and implementing the field operations developed on the basis of EAC recommendations approved by JPC.

19In 2002, OCP was composed of two technical units, the Planning, Evaluation and Transfer unit (PET) and the Vector Control Unit (VCU). PET was involved in the epidemiological evaluation and surveillance, biostatistical analysis and information Systems support, training and transfer. VCU, by far the most important unit in terms of staff and budget, had four main functions: (i) to carry out entomological surveillance with a view to guiding aerial operations according to the observed presence or absence of blackfly larvae at breeding sites and of infective adults, (ii) to conduct aerial larviciding for the purpose of interrupting transmission, (iii) to monitor the environmental effect, if any, of OCP operations and, if required, to adjust operations to avoid damage to the non-target fauna, and (iv) to conduct research on the vector aimed at increasing the effectiveness of control measures, and on insecticide compounds, their formulations and their strategies of use, with a view improving the Programme’s ability to deal with development of resistance to insecticide and of maximizing safety and cost-effectiveness of larviciding.

A permanent challenge: a search for new larvicides

Fig. 4
From laboratory
tests to operational uses: the different steps of the insecticide screening process in OCP

The need to overcome the organophosphate resistance

20The strategy of larval control, which was currently practised by OCP, was closely related to the appearance of blackfly resistance to temephos, the only insecticide used by the Programme from l974 to 1979. The identification in 1980 of a focus of resistance on Lower Bandama in Côte d’Ivoire (Guillet et al., 1980) urged the OCP to abandon the “all-temephos” strategy to the benefit of an alternate use of several insecticides that was intended to contain this resistance. The choice of new anti-blackfly larvicides became a major concern, especially after the appearance of resistance to chlorphoxim, approximately a year after its use had started on the same stretch of river (Kurtak et al., 1982): From 1980 to 1997, the Programme invested heavily in operational research on insecticides. First of all it was necessary to select compounds, which were cost-effective, and not toxic for mammals and non-target aquatic fauna. It was then essential to optimize their use so as to manage the eventual emergence of resistance, while preserving the aquatic environment and maintaining reasonable application costs (Calamari et al., 1998).

21The programme initially intensified its support for research involving laboratories such as the OCCGE “Institut Pierre Richet” (IPR) at Bouaké in Côte d'Ivoire, and then developed by 1986 its own research capacities, by creating an insecticide research structure in Bouaké (Fig. 4).

An intensive screening of new larvicides

22In order to make it usable by the Programme, an anti-blackfly larvicide had to meet a number of criteria, the most important of which was efficacy, selectivity and harmlessness (HOUGARD et al., 1993). At sufficiently low concentrations, the larvicide compound might guarantee the total control of blackflies larvae as far as possible downstream the spraying point, in order to reduce the number of aerial applications, on a given stretch of river thus saving flight hours. At the concentrations lethal for blackfly larvae, the insecticide might have a minimum impact on non-target aquatic fauna (aquatic insects, fish, shellfish), both in the short term (no acute toxicity) and in the long term (absence of bio-accumulation).

23From the point of view of selectivity, certain operational larvicides in OCP had a low safety margin, which restricted their use a few applications per year only In the watercourses where the high flow rate guaranteed a high degree of accuracy of dosage. Of course, all the operational larvicides might have a low toxicity for mammals, in order to minimize the risks of handling by operators and affecting the health of men and animal using rivers as sources of drinkable water Apart from these three criteria, an anti-blackfly larvicide had to be correctly formulated so as to ensure that the insecticide had a uniform coverage of the larval breeding sites, a maximum carry downstream from the spraying point. Technically, the selected formulation might also guarantee sufficient fluidity to allow an easy pumping in the tank of the aircraft as well as a uniform application by the spraying tubes. A blackfly larvicide might finally have a good stability under the usual conditions of warehousing in a tropical environment. Operational insecticide stocks might indeed be stored close to the rivers, in open air deposits, for periods that could exceed one year It was thus critical that the larvicides had a sufficient stability to preserve their efficacy and their physical properties, prior to application by helicopter.

24There are several steps to meet in the selection of a new compound. This process relates on a close cooperation with the manufacturer In charge of the development of new compounds and formulations. The first stage consisted in selecting, from among the compounds provided by the manufacturer and approved by the WHO Pesticide Evaluation Scheme (WHOPES), the products that had, at the same time, a significant impact on mosquitoes larvae and a low toxicity for mammals. These compounds might belong, if possible, to families different from that of operational Insecticides, or at least to different groups within the same family, In order to minimize the risk of crossed resistance among insecticides. They were then proposed for evaluation by OCP for screening on blackflies in a closed System, without any risk to the environment (Kurtak et al., 1987). Insofar as it was impossible to maintain permanent colonies of blackflies in the laboratory, the efficacy of the new compounds was evaluated in the field, near larval breeding sites. For this purpose, a laboratory was established in situ by the Programme in the south-western part of Côte d’Ivoire, close to a river located outside the rivers subject to larviciding. Alongside these tests, standard susceptibillty tests were carried out with the active ingredient of the tested compounds in order to assess their basic susceptibility. The following stage consisted in realizing at a small scale some tests on non-target fauna. This stage implied the collaboration of the hydrobiology team of VCU, in charge of monitoring the aquatic environment for the entire Programme. Tests in gutters on non-target aquatic fauna (insects mainly) made it possible to evaluate the safety margin between the estimated operational amount and the dose likely to cause an undesirable impact (Yaméogo et al., 1991). When this margin was acceptable, compared to the already operational larvicldes, a technical report was submitted to the Ecological Group, the authority allowed to authorize the follow-up of the evaluation at a larger scale. If the Ecological Group agreed, large-scale tests might be carried out in river to check the efficacy of the product and its carry, and to measure its impact on non-target fauna under field conditions. Should results-promising, large-scale treatments were carried out.

A selection of seven operational compounds

25Several hundreds of compounds and/or formulations have been evaluated by OCP, under an intensive screening programme. This research led to the selection of seven operational insecticides, six Chemicals and one biological control agent (table 1).

Table I
Main characteristics of the blackfly larvicides used by OCP

Table IMain characteristics of the blackfly larvicides used by OCP

1 Chlorphoxim up to 1991; 2 Emulsifiate concentrate; 3 Water dispersible; 4 according to the WHO classification of active ingredient: II, quite hazardous; III, slightly hazardous; 5 toxicity against non aquatic fauna according to the criteria of the Ecological Groupe; 6 in ml of formulation per cumecs; 7 300 ml in clear water.

26Temephos, introduced from the very beginning of OCP, was an exceptional larvicide. It is an organophosphorous compound, with very low toxicity for vertebrates. It has a good selectivity for blackflies, resulting in a very low impact on nontarget invertebrates. Moreover, the carry can reach 50 kilometres in favourable conditions. The amount normally applied is 300 ml of an Emulsifiable Concentrate formulation (EC 20%) per cumecs of flow in the river Given its excellent carry and its greater efficacy in high turbid water the amount can drop to 150 ml per cumecs in the rainy season. Unfortunately its use was limited since 1980 as a result of the appearance of resistance among Simulium populations.

27Chlorphoxim, the only alternative compound operationally available in 1980, was introduced to replace temephos in the areas of resistance. As it is also an organophosphate, a cross resistance soon appeared in some species in the Southern basins of OCP, but this resistance did not extend. Chlorphoxim, used at 120 ml per cumecs (EC 20%), is less selective than temephos and has a lower carry. Because its industrial production was abandoned it was replaced in 1991 by phoxim, at 150 ml per cumecs (EC 50%). Its use was as limited as chlorphoxim.

28Pyraclofos was introduced in 1990. Its carry is comparable to temephos. It was used at 120 ml per cumecs (EC 50%). It is potentially more toxic for fish than temephos or phoxim, and it was thus restricted to discharge higher than 15 cumecs. Although pyraclofos does not have spontaneous cross resistance with temephos or phoxim, it had been recommended not to use pyraclofos beyond eight consecutive weekly cycles. Thanks to the strategy of rotation, the susceptibility to pyraclofos remained unchanged in the OCP area, in spite of an intensive use on several basins.

29Permethrin, like most of pyrethroids, is a very effective insecticide against blackflies, used at no more than 45 ml per cumecs (EC 20%). While not being very toxic for hot-blooded vertebrates, it is less selective than organophosphates for the non-target invertebrates fauna (insects, shellfish) and fish. Its carry is low, compared to that of temephos and pyraclofos (lower than 10 km even in favourable conditions). The Ecological Group recommended no more than six consecutive cycles per year on the same river stretch and never below 70 cumecs. In spite of these constraints, permethrin played a significant role in the strategy of rotation of insecticides applied to the OCP. It proved indeed very active against blackflies resistant to organophosphates. Moreover its low cost and low operational dosage allowed river treatments over 500 cumecs discharge. No decrease of Simulium susceptibility to permethrin had been detected in spite of the many cases of resistance recorded here and there in some crop rodents and other insect vectors, such as mosquitoes.

30Etofenprox is a “pseudo-pyrethroid” which is much less toxic for fish than permethrin. It was used operationally since 1994 at 60 ml per cumecs (EC 30%) for discharge above 15 cumecs.

31Carbosulfan is a carbamate insecticide. It was introduced in 1985 in order to serve with permethrin as an alternative to organophosphates for the treatment of large rivers. It has been used at 120 ml per cumecs (EC 25%). Its relatively low selectivity and its risk of impact on fish raised the limits of use, as for permethrin, to discharge higher than 70 cumecs with a maximum of six consecutive weekly treatments per year and per river stretch. Its low carry and relatively high cost limited its use to a low range of river discharges. No Simulium resistance was detected since its introduction.

32Bacillus Thuringiensis H-14 (B.t. H-14) is a biological control agent. Discovered in 1977, this bacterium produces protein crystals toxic for Simulium larvae and several other diptera. Commercial formulations were used by OCP on a large scale, since 1982 (Guillet et al., 1982).The toxin of B.t. H-14 is indeed extremely selective for blackfly larvae, and operational spraying does not practically have any effect on non-target fauna. Its mode of action is completely unique and no crossed resistance with Chemical insecticides has ever been recorded. In fact, several rivers in the OCP area have been treated by B.t. H-14 for nearly 20 years without any decrease of Simulium susceptibility. This made B.t. H-14 the insecticide of choice to counter resistance to organophosphates. However its operational dose was relatively high, limiting its use at relatively low discharges. By 1985, the improvement in the commercial formulations made it possible to treat rivers at a discharge of 75 to 100 cumecs.

An adapted operational control strategy

Managing vector resistance to insecticides

33Alongside the programme of development of Simulium larvicides, OCP developed a strategy of use of these compounds which allowed, on the one hand, to contain resistance to temephos, chlorphoxim and phoxim and, on the other hand, to avoid the development of blackfly population resistance to the other insecticide families. Among the possible strategies of management of resistance, one consisted ¡n alternating, in time, the insecticides belonging to different families. This rotation made it possible to reduce the insecticide pressure on a given Simulium population and, thus, to decrease the chances of development of genes of resistance in this population. This strategy would have been relatively simple to implement if all the products of replacement had shown the same characteristics as temephos. The choice of insecticides would then have been limited only to the considerations of management of resistance, other than any other factors, such as efficacy, cost, physical properties and toxicity. Unfortunately, it was not the case, which made the implementation of this strategy even more complex (Guillet et al., 1991).

34In order to monitor the susceptibility of Simulium to Insecticides, the Programme quickly developed simple and reliable methods of evaluation of the susceptibility of S. damnosum larvae to insecticides. These tests, the guiding principles of which were described by Mouchet et al. (1977) for chemical insecticides and Guillet et al. (1985) for B.t. H-14, are easily achievable in the field. They helped to determine, for each insecticide, the diagnostic doses, so as to quickly detect the least fall in susceptibility of the blackflies to a given insecticide. A considerable improvement in the situation of resistance to temephos has, however been noted these last years, since the resistance now persisted only on the lower-Bandama and the lower-Comoé in Côte d’Ivoire, on a relatively low level. Temephos could, thus, be used again in 90% of the Programme area, in rotation with other operational Insecticides. The same goes for phoxim, which had replaced chlorphoxim. With regard to pyraclofos, only one case of resistance was reported on the Marahoué (Côte d’Ivoire), following a succession of 16 consecutive weekly cycles, carried out on an experimental basis. This resistance, fortunately, quickly proved reversible in the absence of pressure of selection. No resistance has, never; been detected with the other insecticide families (Fig. 5).

Fig 5
Operational range of larvicides used by OCP

Less pressuring the aquatic environment

35The weekly periodicity of treatments, and the importance of the hydrological network to be treated, constituted the first factor of complexity. A decision to treat should indeed be taken each week on several thousands of kilometres of rivers, that is to say, several hundreds of spraying points (fig. 6). The first step among the decision-making process was to determine, for a watercourse or a portion of it, if larviciding must take place. This decision depended on a number of factors among which were the results of the weekly entomological evaluation or the level of endemicity of the zone under study. Once the decision for treatment was taken, the choice of insecticide depended, not only on the dynamics of resistance at the local level, but also on the characteristics of each insecticide as described previously.

36Below I cumecs, only B.t. H-14 might be used. The use of temephos was not excluded, but its use in little agitated water courses increases the time of contact of the insecticide with substrates, and the risks of under-dosages, favourable to the survival of the individual heterozygotes, in the case of recessive or semi-recessive resistance gene. Between 1 and 15 cumecs, three of seven insecticides available could be used: temephos, phoxim and B.t. H-14. From 15 to 70 cumecs, two compounds could be added, pyraclofos and etofenprox, bringing the number of available larvicides up to five. Between 70 and 150 cumecs, the use of B.t. H-14 became expensive, but this range of flow allowed the greatest choice and the greatest diversity of compounds with three organophosphates (temephos, phoxim and pyraclofos), a pyrethroid (permethrin), a pseudo-pyrethroid (etofenprox) and a carbamate (carbosulfan).This diversity was reduced between 150 and 300 cumecs because phoxim and carbosulfan become too expensive for use. Between 300 and 450 cumecs, only temephos and permethrin were profitable. Above 450 cumecs, only permethrin was used operationally. By 1999, the gradual reduction in larvicide coverage has made it possible to decrease the number of insecticides, by limiting to the five most effective ones : temephos, pyraclofos, permethrin, etofenprox and B.t H-14.

37The spraying of an insecticide requires precise knowledge in real time of the discharge of rivers to be treated. A mis appreciation of the quantities of insecticides to be sprayed can indeed have several effects on the success of treatment. An overdosage can have significant financial and ecological consequences, if the product used is expensive or relatively toxic. This is why an hydrological surveillance network was set up from the very start of the programme on the entire treated rivers. It included, at the height of larviciding, up to 185 river gauges, 103 of which were equipped with hydrological beacons, allowing a transmission by satellites of recorded water levels (Servat and Lapetite, 1990). In the same time, the aircraft company in charge of treatments, along with OCP, developed a treatment device allowing precise dosage (Hougard et al., 1996).

Fig. 6
An example of rotational use of larvicides in a standard river close to OCP area

Environmental protection: a need to deal with

A strong concern about the fate of the environment

38The mere fact of regularly using insecticides for many years raised the concern of the potential risk such operations could have for the aquatic environment. Indeed, at the time OCP was launched, there was much evidence on biological and ecological consequences of DDT With the awareness of the "DDT syndrom" of the international community, the Participating countries, as well as the Donors that support the Programme (28 countries and foundations), had reasons to fear that 20 years' repeated applications of insecticides in the watercourses would cause serious disturbances of the freshwater ecosystems.

39In 1974, just before the beginning of operational activities, OCP set up an aquatic monitoring programme of rivers planned to be regularly treated with insecticides (Lévêque et al., 1979). It was implemented to satisfy three major concerns:

  • to provide early warning to those carrying out treatments, should toxic effects be noted on the short term and to ensure that the insecticide release did not excessively disturb the functioning of the treated ecosystems on a long term basis (the expected duration of OCP);

  • to avoid the widespread use of Chemicals which may have adverse effects on human populations near the river Systems and/or might accumulate in the food chain as DDT has been known to do;

  • to prevent the irreversible loss of aquatic biodiversity in West Africa both because freshwater fish are a major source of food as well as an economic activity for West African populations, and to meet the objective of the Convention on Biodiversity that stipulates that countries are responsible for the conservation of their biodiversity.

40At the beginning of OCP, the knowledge of riverine Systems and their associated flora and fauna was still very poor In such a situation, the aquatic monitoring activities were devised both to collect basic knowledge on the structure and functioning of the rivers, and to investigate the potential impact of larvicides on the aquatic fauna. In particular ; it was necessary to identify (and sometimes describe) the different species collected, to investigate the biology and seasonal dynamics of the non-target fauna, to understand the long-term trends in river water discharges and their relations to the dynamics of aquatic communities. This basic research, highly critical for the interpretation of the monitoring data, was progressively achieved by several teams, and particularly by the Orstom hydrobiological team based first in Bouaké, and then in Bamako.

The implementation of an efficient organization

41The monitoring of the aquatic environment was made possible through the implementation of a specific organization devoted to laboratory and field studies, as well as to the periodical analysis and interpretation of collected data. Currently, the entire monitoring activity is catered for by the participating Countries using a standard protocol, but is backed by financial and technical support to the Programme. Special studies and independent analysis of data are carried out periodically in collaboration with consultants or specialized institutions to supplement or support studies conducted by the Programme and National hydrobiology teams. The Ecological Group evaluates the different results, assesses the level of toxicity of proposed new Chemicals, and approves or rejects their operational use under the Programme (see figure 6).

The Ecological Group

42Before the Programme was launched, the Sponsoring Agencies set up an independent advisory body, the Ecological Panel, which later became the Ecological Group (E.G.).The Group always consists at maximum of five independent scientists and reports to the Expert Advisory Committee of OCP. Its role is to ensure that vector control carried out by OCP does not endanger the environment and to make recommendations to the Programme for effective protection of the environment. More specifically, the objectives and mandate of the EG are to:

  • organize and evaluate a long-term monitoring programme of the aquatic fauna;

  • assess the level of toxicity of new products or formulations and approve or reject their operational use under the Programme;

  • review the nature and magnitude of ecological problems connected with the programme and with associated economic development projects, proposed in areas freed from onchocerciasis, in order to identify the environmental and human ecological implications of such developments.

National Monitoring Teams

43The monitoring activities have been conducted by National Hydrobiological Teams which were developed and established in most of the Participating Countries where larvicide treatments occurred (Burkina Faso, Côte d'Ivoire, Ghana, Togo, Bénin, Mali, Guinée, Sierra Leone). Most national scientists received OCP grants to be trained in the methodologies used by OCP. Some of them received PhDs. They also received support from OCP to conduct their monitoring and research activities and they meet annually to discuss their results with the Ecological Group. Exchanges between monitoring teams were promoted during the programme.

The Vector Control Unit

44The Vector Control Unit (VCU) in OCP has been one of the major contributors to environmental protection through many of its activities. For example: (i) it has been responsible for the judicious operational use of insecticides in the OCP area; (ii) it has been in charge of the screening of new insecticides; (iii) it reports its activities at the Ecological Group meetings.

The OCP Hydrobiological Section

45The OCP Hydrobiological Section was created in 1981 at the headquarters of OCP in Ouagadougou, as a component of VCU, and under the leadership of a senior scientist. Its main functions were (i) to carry out the ecotoxicological research; (ii) to coordinate monitoring activities, and to assist the national teams in their field work in the OCP area; (iii) to manage the monitoring data provided by the national teams (Fig. 7).

Fig. 7
Structure de l’OCP et position des activités de surveillance de l'environnement

The short-term risk assessment of new larvicides

46A risk assessment was performed for every new larvicide to be used in OCP. After the first trials devised to evaluate the efficacy on blackflies, a review of literature was performed together with laboratory tests on fish and gutter tests on invertebrates (Calamari et al., 1998). Acute toxicological tests have been performed on African fish species according to standard protocols (Yaméogo et al., 1991) to obtain original data on fish toxicology. Short-term impact on non-target invertebrates was also studied using a System of artificial gutters and different concentrations of insecticides (Yaméogo et al., 1993). From these tests, larvicides were classified according to their general toxicity and a typology of the susceptibility of the most common taxa was established (Yaméogo et al., 1991, 1993).

47Quite a lot of information has been collected also from field trials of insecticides : B. t. H-14 (Dejoux, 1983 ; Dejoux et al., 1985); deltamethrin (Dejoux, 1983), Gh 14 (Troubat et Lardeux, 1982), temephos (Dejoux et Elouard, 1977; Elouard et Jestin, 1982) pyrachlofos (Yaméogo et al., 1993) permethrine (Yaméogo et al., 1993) etofenprox (Yaméogo et al., 2001)

48Comparing the different operational larvicides, B. t. H-14 proved to be the least environmental damaging, followed by temephos, chlorphoxym, pyraclofos, etofenprox, permethrin and carbosulfan, in increasing order of toxicity. Among the taxa, the Baetidae (Ephemeroptera) were the most susceptible to the Chemical larvicides while the chironomidae (Diptera) were the least susceptible to most of the insecticides.

49The Ecological Group reviewing the results recommended, in cases of acceptable toxicological results, small-scale pilot studies on the field. For the most toxic Insecticides that have many advantages to their possible use (such as low cost, wide range of application in relation to discharges and long carry distance), large-scale studies below the operational dose were recommended. This allows a complete risk assessment scheme to be obtained before the insecticide is used as an operational larvicide. Permethrine for example was tested this way (Yaméogo et al., 1993; Calamari et al. 1998).

The long-term monitoring of the aquatic environment

50The criteria retained by the Ecological Group for the evaluation of the long-term impact of insecticides on aquatic environment have been the following (Lévêque et al., 1988):

  • the vector control activities should not reduce the number of invertebrate species, or cause a marked shift in the relative abundance of species;

  • the pesticides applied should have a direct impact neither on fish, nor on the life cycle of fish species;

  • bioaccumulation and biomagnification through food webs should be avoided ;

  • human activities in the control area should not be impaired;

  • temporary and seasonal variations in non-target invertebrate populations due to insecticides should be acceptable.

51The monitoring programme was primarily concerned with two major categories of organisms: (i) the benthic invertebrates that abound in the watercourses and that are directly threatened by the insecticide in the same way as Simulium damnosum larvae; (ii) the fishes, by virtue of their economic interest for the people living along the rivers, but also for the psychological reasons to show the villagers occupied in fishing that care was taken to avoid the risks of pollution.

Methods and protocols

52In order to evaluate the magnitude of the environmental risk, hydrobiologists have used consistent methods and protocols to monitor potential long-term effects of continuous use of larvicides on aquatic populations (Yaméogo et al., 2001; Crosa et al., 1998; Lévêque et al. l979). When setting up the monitoring protocol, several important considerations had to be kept in mind:

  • the monitoring had to deal with a long-term regular sampling 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;

  • the periodicity of sampling, the sites selected for monitoring, and the field methods used had to combine reliability of sampling techniques with reliability of access in both wet and dry seasons, over many kilometres of road or tracks which are not yet hard surfaced;

  • the monitoring techniques had to work equally well in shallow, slow-flowing rivers in the dry season, and in the same rivers, deep and flowing fast in the wet season;

  • in order to ensure reasonable comparability of results, all teams had to use the same methods.

53A network of sampling stations throughout the Programme area was established (Fig. 8). Forty sampling sites were used at the start of the Programme, but as the programme evolved together with the treatment strategy, the number has been reduced recently to ten for invertebrates and ten for fish, after an evaluation was made on the impact of the first ten years.

54For invertebrates, three main sampling methods were used (Yaméogo et al., 2001):

  • Drift net sampling using 2 m long nets, 20 x 20 cm aperture, 300 mm mesh size. The basic techniques of drift sampling used in the Programme were standardized.

  • Surber samples using 15 x 15 cm Surber sampler. This simple method, which allows rocky substrates to be sampled, cannot be used in deep waters and was therefore limited to the low-water period.

  • Artificial substrates: special apparatus were designed and used, from concrete blocks which were left immersed on the bottom, to floating substrate made of a bunch of plastic fibres.

55For fishes, the monitoring programme mainly concerned (Lévêque et al, 1988):

56The study of changes in the catch (expressed in weight or number of individuals) and species composition of experimental fishing carried out at regular intervals (usually 2 or 3 months) with a standardized set of gill nets.

  • The study of biological parameters, more especially the coefficient of condition which is a measure of the health of fishes. Complementary research was also conducted on the analysis of stomach contents of selected species, spawning periods and fecundity, as well as the impact of organophosphorous compounds on brain acetylcholinesterase activity.

57Some methods and protocols for the monitoring and risk assessment programmes have evolved since 1974, the Programme being dynamic in the face of continually changing situations (e.g. insect reinvasion, resistance to pesticides, variability in hydrology, etc.). However the basic concepts and issues have remained.

Fig. 8
Monitoring sites location in OCP area

Twenty five years aquatic monitoring

58The major concern of OCP regarding the aquatic environments exposed to insecticides has been to avoid long-term or lasting changes in aquatic biodiversity. Major groups of organisms which have been monitored during over twenty years of OCP operation for any indications of undue changes are fishes and non-target invertebrates (Fig. 9).

Main fish monitoring results

59Potential impacts of larviciding on fish have been evaluated by assessment of changes in species richness of catch, catch per unit effort (CPUE) of fishing and coefficient of condition of fish species (Lévêque et al., 1988 ; Paugy et al., 1999). In relation to species richness of experimental catch (which is the number of species caught in a standard set of experimental gill nets during two nights’n fishing), long-term trends observed in three major areas of the Programme (ie. rivers in Côte d'Ivoire, Volta basin rivers in Ghana and Niger basin rivers in Guinea) have been different. However after a period of declines especially in Côte d’Ivoire and Volta basin rivers, recovery and improvements in species richness of catch in all rivers have been observed since 1994 and l996. Thus, after several years of larviciding with several insecticides, up to twenty years in the original Programme area, there is no evidence of reduction in fish species diversity in treated rivers. A similar observation was made after the initial ten years of monitoring during which only three larvicides had been used. Comparison of species richness changes with hydrological trends suggests that the observed trends in species richness might be attributed to climatic factors and the long period of drought that occurred for many years in West Africa (Fig. 10 and 11).

Fig. 9
Long-term changes in the annual discharge for some of the monitored rivers. Low discharges are characteristics of the early 1980s and the early 1990s in many rivers of the OCP area

60Overall trends of catch per unit effort (CPUE) in relation to larviciding during 20 years also indicate various scenarios in the different major Programme areas/basins and sometimes among rivers of the same basin in spite of a generally similar larviciding regime in the Programme area. For example, catches reduced in Côte d'Ivoire rivers up till 1989 and 1993 and increases were observed in all rivers since 1995 although larviciding was stopped on different rivers at different times. In the Niger basin, no decrease in catch has been observed since monitoring began, while increases over the original status have been observed in the three rivers monitored in the basin since 1994. In the Volta basin, different trends in catch are being observed over the years. However, a common seasonal pattern of catch, high at low water periods (Dec./Jan. till Apr/May) and low at high water periods (Jly/Aug. till Oct./Nov.) is observed for all basins and rivers. The influence of hydrological changes on fish catch has been previously suspected.

Fig. 10
Long-term changes in fish species richness per sample in some monitored rivers

Fig. 11
Long-term changes in fish catch per unit effort (CPUE) per sample using a standard set of gill nets sample in some monitored rivers

61Coefficient of condition values (a ratio between weight and length) which express the “well being” of fish have also been monitored to assess direct effect of larvicides on fishes (acute toxicity) and/or indirect effects through larvicide impacts on their food sources which are in many cases aquatic invertebrates. Over the years, various assessments of trends of ‘condition’ of several fish species in the Programme area indicate only fluctuations around expected means but no significant changes in values. The situation indicates that larviciding has not directly affected fish and suggests that where fish food items have been affected, others have been found and used reasonably well in their place.

62Bioaccumulation of pesticides in fish was a major concern with DDT. Actually, the effects of organophospates in laboratory experiments showed that fish were able to accumulate temephos (Matthiessen and Johnson, 1978). But this accumulation seems to be limited and does not increase to a point observed with DDT Field data in OCP area confirmed that temephos did not accumulate in fish (Quélennec et al., 1977). Moreover, in field conditions, the acetylcholinesterase activity in the fish brain does not seem to be significantly different in the rivers treated with temephos or untreated (Antwi, 1985; Scheringa et al., 1981).

Main monitoring results for invertebrates

63The specific concern of OCP with regard to non-target invertebrates and larviciding of rivers has been to prevent loss of faunal diversity and to maintain the quality of biomass available for higher levels of the aquatic ecosystem food web. Impact assessment of larvicides has been based on evaluation of two types of data. These are Surber samples data and drift (day and night) data.

64By analysing the invertebrate data which were collected using various sampling strategies between 1977 and 1996, Yaméogo et al. (2001) evaluate the long-term changes of the invertebrate populations with respect of their taxonomic composition as well as their trophic structures. Surber samples provide a qualitative and quantitative assessment of the invertebrate community at the sampling location. They allow a clear examination of the community changes both as taxonomic as well as functional structure. Generally, results indicate that individual larvicides have different impacts on rivers and exhibit a range of effects on various groups of invertebrates. The greatest reduction in the diversity and abundance of the invertebrate assemblages has been detected during phoxim, permethrin, carbosulfan and pyraclofos treatments (Yaméogo et al., 1992). Temephos and B.t H-14 are the less stressing larvicides. The taxonomic units that present the wider changes in relative abundance are Tricorythidae, Leptoceridae, Chironomidae, and Baetidae. From a trophic point of view, all the communities are dominated by the gathering collectors and, to a less extent, by the filtering collectors. The abundance of these feeding groups is a direct evidence of the availability of the fine particulate organic matter that characterizes the food resources within the studied rivers. This dominated structure tends to increase with the application of all insecticides but the B. t. H-14 (Yaméogo et al., 2001). As a whole, results suggest that neither the taxonomic nor the trophic structures are greatly altered from the range of biological, flow-related variation that normally occurs in the studied rivers (Yaméogo et al., 2001, Crosa et al., 2001).

65This allows concluding that the effect of insecticides on the aquatic fauna is usually low but results for invertebrates in changes of species composition and community structure. However this impact does not affect the general functioning of the aquatic system and is therefore ecologically acceptable. To be sure that there were not irreversible losts of species the question of recovery of the aquatic fauna has been raised by the Ecological Group. The basic question was: does the aquatic community return to a structure and species composition more or less similar to the pre-treatment one at the end of the treatment period? Actually, field data provide indications that recolonization by taxa (for example, Neoperla sp. and Caridina sp.) which had been affected during treatment period was observed at a majority of stations after larviciding stopped. The ability of the aquatic fauna to recover in treated rivers has therefore been demonstrated, even if it is at a slow rate.

Fig. 12
Long-term changes in the density of invertebrates on the rocks, as estimated using Surber samplers.

The goal is achieved, but what about the future?

66The OCP in West Africa closed in December 2002, after 29 years activities. There is no equivalent of a public health programme benefiting for so long a financial support of the international community. One of the reasons for this support is that OCP always convinced the donors of the effectiveness of the control strategies used. The other reason has been the permanent concern of OCP to take care of the aquatic environment with the involvement of national teams and international expertise. The implementation of a long-term monitoring programme to assess the potential effects of larviciding, and the large-scale screening of larvicides to select the most efficient for S. damnosum while the less drastic for the non-target fauna, are unique features in large health control programmes. These efforts had an economic cost in terms of insecticide consumption and operational strategies but they made it possible to preserve the quality of the water used by the riverine populations as well as the fishing resources which constitute a significant part of the foodstuff of these populations.

67The goal is achieved: onchocerciasis has been virtually eliminated from the OCP area as a disease of public health importance, and as an obstacle to socio-economic development. By eliminating the threat of blindness, OCP has made possible the repopulation of those river valleys which had formerly been deserted by fear of the disease. Practically, no new infection is currently recorded in this zone and one of the most significant results is that 18 million children born in the OCP area have no longer been infected by onchocerciasis. However the control of filariasis is not over. Indeed, OCP never aimed at the eradication, neither of the parasite nor of its vector and onchocerciasis will still be present. The capacity to manage the “risk” of onchocerciasis will constitute a new challenge in the future.

68From the environmental point of view, the success of OCP may be jeopardized by an unsustainable use of the freed land. For example, a pilot study conducted in the Léraba area thus showed that 75 % of the original wooded savannah was cleared for agricultural development and the settlement of villages (Baldry et al., 1995).The riverine forests of many small rivers were destroyed and on some of the banks, soil erosion is going on. On the other hand, the bordering forests and the easily flooded plains of the larger rivers did not undergo any disturbance of this scale. It is therefore necessary both to take measures and sensitize the riverine populations on the need for environmental protection and biodiversity management along with the development of agricultural activities. In other words, that is the field application of the principles of sustainable development as developed at the conference on the planet Earth in Rio in 1992, and in Johanesburg in 2002.

Fig. 13
Changes in land use in the vicinity of the Leraba monitoring station

Blackfly nuisance

69In certain areas, the strong decrease of blackfly bites has been a relief for riverine populations whose life and/or working conditions have been improved. That was the most immediate benefit perceived by populations from the activities of OCP at the beginning of the Programme.

70Larviciding has, however progressively ceased from 1990 in the regions where the disease was under control and the blackfly bite rates again reached high levels in some areas. Even though blackflies no longer transmit onchocerciasis their reappearance was perceived by populations who associated these insects with transmission of the disease, as the comeback of onchocerciasis. On the other hand, blackfly bites are also a real nuisance that could hinder the ongoing socio-economic development of the river valleys (Hougard et al., 1998).

71To take care of the public concerns OCP encouraged individual actions against blackflies, through low-cost control techniques on the ground for dealing with breeding sites. Transfer to villages and development units has been achieved in different areas. However; larvicide applications performed by non specialists for an indeterminate period could carry an enormous risk of environmental pollution. The use of two insecticides which present low environmental hazards (B.t. H-14 and temephos) has been recommended by OCP. However, once OCP closed, there is a risk that these communities will use the insecticides that are available locally for agriculture purposes for instance, with the attendant danger of causing resistance and contaminating the environment.

Increasing knowledge and expertise

72Apart from the success of OCP in controlling the disease, there are also several positive consequences which may be stressed.

Improvement of national expertise in the field of river ecology and management

73A significant contribution of OCP is the training of national scientists in relation with the monitoring of aquatic environment and related research programmes. As part of the training, several national scientists received grants to be trained in Africa and in northern countries. The result is in an overall improvement of the expertise in aquatic biology and in environmental sciences.

A better knowledge of the ecology of West African rivers

74Before OCP, the knowledge of the ecology of African rivers was very poor. The implementation of the monitoring programme leads to:

  • A better knowledge of the fauna and ecology of West African rivers, particularly for insects and fish (Dejoux et al., 1981; de Mérona, 1981; Iltis, 1983; Iltis and Lévêque 1982; Gibon and Statzner, 1985; Lévêque et al., 1990, 1992);

  • A better knowledge of the-long term dynamics of aquatic populations in relation with climatic changes and human influences ;

  • The acquisition of a bulk of information on reaction of African aquatic fauna to diverse Chemical products ;

75All this knowledge will be useful for other developmental activities and for conservation of the west African environment.

Establishment of an aquatic monitoring data base : an exceptional property

76All results recorded during the environmental monitoring of the Programme have been constituted into a data base managed and available at the OMS Head-quarters in Ouagadougou. This data base, updated and validated on the ecology of West African rivers generated during a period of more than 25 years, is a unique heritage.

Table des illustrations

Légende Fig. 1The life cycle of Onchocerca volvulus
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Légende Fig. 2Epidemiological trends from 1975 to 2000 in a village of the core area of OCP (from Hougard et al., 2001)
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Légende Fig. 3The Onchocerciasis Control Programme area
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Légende Fig. 4From laboratory tests to operational uses: the different steps of the insecticide screening process in OCP
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Titre Table IMain characteristics of the blackfly larvicides used by OCP
Légende 1 Chlorphoxim up to 1991; 2 Emulsifiate concentrate; 3 Water dispersible; 4 according to the WHO classification of active ingredient: II, quite hazardous; III, slightly hazardous; 5 toxicity against non aquatic fauna according to the criteria of the Ecological Groupe; 6 in ml of formulation per cumecs; 7 300 ml in clear water.
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Légende Fig 5Operational range of larvicides used by OCP
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Légende Fig. 6An example of rotational use of larvicides in a standard river close to OCP area
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Légende Fig. 7Structure de l’OCP et position des activités de surveillance de l'environnement
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Légende Fig. 8Monitoring sites location in OCP area
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Légende Fig. 9Long-term changes in the annual discharge for some of the monitored rivers. Low discharges are characteristics of the early 1980s and the early 1990s in many rivers of the OCP area
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Légende Fig. 10Long-term changes in fish species richness per sample in some monitored rivers
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Légende Fig. 11Long-term changes in fish catch per unit effort (CPUE) per sample using a standard set of gill nets sample in some monitored rivers
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Légende Fig. 12Long-term changes in the density of invertebrates on the rocks, as estimated using Surber samplers.
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Légende BURKINA FASO 'S SIDE OF THE LERABA RIVERFig. 13Changes in land use in the vicinity of the Leraba monitoring station
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