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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

Références des articles présentés sur le cédérom / Papers references presented on CD-ROM

Focus. Insecticide Resistance in the Onchocerciasis Control Programme

D.C. Kurtak et C. Fairhurst

Texte intégral

1The Onchocerciasis Control Programme (OCP) in West Africa aims to reduce the transmission of onchocerciasis to the point where the disease is no longer a serious public health problem1,2. The Programme relies on weekly applications of larvicides to the riverine breeding sites of the immature stages of Simulium damnosum s.l., the only vector in West Africa. As a result transmission has been virtually eliminated over about 80% of the Programme area. But suppression of the vector populations must continue for about 15 years in order to eliminate the disease, because the parasite is long-lived in the human host and is not amenable to mass treatment with currently-available drugs.

2From the beginning of treatments in early 1975 until early 1980, temephos was the only larvicide used in the OCP, and it was entirely effective. During most of this period, treatments were directed mainly against the ‘savanna’ species of the Simulium damnosum vector complex (S. damnosum s.s. and S. sirbanum). In late 1978 and early 1979, larvicide treatments were extended to the south, to include the large-river breeding sites of two ‘forest’ species –S. soubrense and S. sanctipauli. This was done primarily to eliminate savanna species in the same breeding sites which invaded areas further north, but it also implied a commitment to control the forest species.

Treatment Failures

  • * Recent cytotaxonomic révision of the sanctipauli group suggests that all temephos resistant populat (...)

3In April 1980 a series of treatment failures on the lower Bandama River in Ivory Coast were shown to be due to resistance to temephos3. Resistance was limited to S, soubrense/sanctipauli* but it spread to include all the previously-known distribution of these species in the treated part of the Ivory Coast the extreme south of Burkina Faso (mid-1981), and western Ghana (early 1982). The resistance rendered temephos virtually useless, even at several times the routine dose. It produced a 50-100 x increase in the LC 100 in susceptibility tests.

4When temephos resistance was first detected, the role of S. soubrense/sanctipauli as a vector of the blinding form of the disease was uncertain. It was therefore judged necessary to maintain a high level of control. The first response was to apply chlorophoxim, another organophosphate, which was the only alternative that had successfully completed screening at the time. But within one year, the same population of S. soubrense sanctipauli developed full resistance to chlorophoxim4. It was then replaced by a formulation of the biological insecticide Bacillus thuringiensis (Bt) serotype H14. Although this product could be effective, it was impossible to maintain control in the wet season because of poor dispersal and the relatively large amount of which had to be added per cubic metre of river discharge (3 - 20 x the quantity of temephos). Only a few small areas could be treated year-round, and that required considerable extra expense by the use of large aircraft Even at moderate river discharge, the use of Bt is several times more expensive than temephos, taking into account the additional flying time and ground transport of insecticide. Only at very low discharge are the two types of treatment similar in cost

Reversion to Susceptibility

5In 1982, it was found that some populations resistant to chlorophoxim reverted to a normal susceptibility some months after use of the product stopped. This reversion is strongest at the western edge of the treated area and is probably due to immigration of susceptible individuals. In 1983 and 1984, reasonable year-round control of S. soubrense/sanctipauli was achieved by alternating B. thuringiensis at low discharge and chlorophoxim in the rainy season.

6In mid-1984, the completion of longterm species-specific transmission studies on S. soubrense/sanctipauli in the resistance zone combined with the earlier observation that blinding onchocerciasis was never found when only these species were present5-6, led to the decision to abandon attempts to control this species pair in the forest zone. This policy is now being implemented and tested. If it is proved that not controlling S. soubrense/sanctipauli has little influence on transmission of blinding onchocerciasis, the significance of the insecticide resistance will be considerably reduced.

7At the same time, however, evidence appeared that the much more dangerous savanna species (S. damnosum s.s. and S. sirbanum) can also develop resistance. In late 1982 and early 1983, an isolated population of savanna species on the lower Bandama River in southern Ivory Coast displayed a progressive regression of susceptibility to temephos, Before this reached the stage of full resistance, blanket Bt H14 treatments were introduced in the zone and continued until the end of 1984. The treatments were effective and savanna species virtually disappeared. Savanna species reappeared in April 1985, after 4 months’ suspension, They were tested and found to have a normal level of susceptibility However, it was judged prudent to avoid the use of organophosphate compounds in that area.

8Due to the long pre-patent period of onchocerciasis, it is still too early to evaluate the impact of insecticide resistance on the epidemiological results. As control had only been going on for one year when resistance was noted, it may not even be possible to see the effects. There was certainly a large increase in biting rates of the resistant forest flies, although often this was not accompanied by much transmission. Even where transmission did occur, it may have been transmission of the more benign form of the disease, which unfortunately can only be distinguished from the blinding form by its developed clinical features in the human host.

Elimination of Savanna Species

9The resistant species became predominant in areas in the northern part of its range where they were previously in the minority, but they never surpassed their original distribution. Their predominance was due to the elimination of the savanna species by the temephos treatments which continued. However, as S. soubrense/sanctipauli moved away from its normal forest habitat, restricted numbers and increased zoophily reduced its vectorial capacity which can be nil at the northem part of its range.

10There may have been some transmission by savanna species during the total treatment suspensions which were undertaken for susceptibility testing or trials of new products as the methods for combating the resistance were developed.

11Development of resistance in savanna species is a serious risk to the OCP, because even if replacement compounds are fully tested, they will undoubtedly be more difficult and more expensive to use than temephos. This risk will increase as the programme is extended because reservoirs’ of susceptible individuals presently unexposed will come under treatment. For this reason, close surveillance of susceptibility is continuing, along with an intensive screening programme for hew compounds. Already, a pyrethroid (permethrin) and a carbamate (carbosulfan) have reached the level of large-scale operational trial. In general, however, the influence of resistance on OCP results is limited. Against savanna species, which are the major vectors of the grave form of the disease, temephos is still completely effective over virtually all of the OCP areas.

Aerial application of insecticides to riverine breeding sites of Simulium damnosum s.l. in the Onchocerciasis Control Programme

Acknowledgements

12The authors would like to thank the many OCP staff members who contributed to the development of this paper.

Environmental Monitoring of Rivers in the Onchocerciasis Control Programme

13The Onchocerciasis Control Programme was established with a strategy that recognized the importance of social, geographical and environmental aspects, as well as the fundamental vector control and epidemiology work. Depopulation of the river valleys in West Africa in favour of neighbouring areas with poorer soils has been profound, and a return following successful control of onchocerciasis requires careful national and international planning and support. Clearly, a control programme designed around a weekly insecticide treatment of rivers in a 700 000 km2 area should bear in mind the longterm effects on non-target organisms in the rivers, safeguarding against substantial changes in productivity leading to a reduction of fish stocks.

14An Ecology Panel, comprised of internationally recognized and independent experts was established to plan the environmental monitoring programme, and progress is reported at Hydrobiologists’ meetings each year. Any changes in protocol have to be agreed or suggested by them.

15The initial monitoring protocol was inevitably a compromise between academic desirability and practical possibility. The selection of sites was carried out by personnel from the ORSTOM laboratory in Bouaké, Cote d’Ivoire, the Institute of Aquatic Biology, Achimota, Ghana, and the Department of Biology, University of Salford. Methods involved were day and night drift net samples, modified Surber substrate samples and artificial substrates for invertebrates, and drift and gill nets of five mesh sizes for fish. Standard forms for ease of computer storage and analysis were introduced. Sites were to be sampled every month where possible, and a hydrobiologist based in Ouagadougou was contracted to work on the Upper Volta sites. No pretreatment samples were available, so as collections in the Phase I treatment zone would follow insecticide applications, further stations outside the control area were sampled for comparison. Subsequently these areas have also been treated because of problems due to reinvasion by Simulium from untreated areas, and they provide an ideal before and after comparison. All data has been regularly sent to the WHO, Geneva for checking and computer entry, and independent analyses are also carried out for the monitoring teams and the Ecological Panel by a team at Salford University. Essential supporting research in the form of short-term toxicity tests and biological work on the taxonomy and life cycles of non-target organisms has been carried out – the subject of many reports by ORSTOM and IAB, who have assisted in the screening of other candidate insecticides.

16Monitoring commenced at the end of 1974, and it was soon apparent that there were no major changes in non-target river fauna that could be attributed to applications of temephos. Therefore, reports in 1980 and 1981 suggested a reduction of monitoring intensity, by concentrating on those sites, methods and times of year which had proved particularly reliable and informative. With extension of the programme area to the west and south-east being discussed, new river stations have been established and training programmes initiated to enable more national teams to be involved.

17A full analysis of the data collected over the ten year period is now being undertaken. The aquatic monitoring programme in the Volta Basin is probably the largest of its type ever carried out and apart from environmental protection, has contributed greatly to our knowledge of tropical riverine ecosystems.

Bibliographie

References

1 Davies, J. Biet of. (1978) Mosq. News 38.466–472

2 Walsh. J.F., Davies. J.B. and Le Berre. R. (1979) Tropenmed. Parasitol. 30.328-344

3 Guillet P. et of. (1980) Cah. O.R.S.T.OM. Ent. Med. Parasitol. 18, 291–299

4 Kurtak, D. et of (1982) WHO unpublished document WHO/VBC/82/850.

5 Prost A. (1980 Am. Parasitol 55, 239-245

6 Prost A. (1980) Am. Parasitol. 55, 347-353

Notes de fin

* Recent cytotaxonomic révision of the sanctipauli group suggests that all temephos resistant populations are in fact S. sanctipauli, in the sense of having a fixed diagnostic inversion which is consistently absent from S. soubrense (Post R.J., Genetica. in press).

Table des illustrations

Légende Aerial application of insecticides to riverine breeding sites of Simulium damnosum s.l. in the Onchocerciasis Control Programme
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Auteurs

Dan Kurtak is at the WHO Ochocerciasis Convoi Programme. Ouagadougou. Burkina Faso.

Colin Fairhurst is at the Department of Biological Sciences, University of Salford, Salford M5 4WT. UK

© IRD Éditions, 2003

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