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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

Joint US-AID/OCCGE/WHO technical-meeting on the feasibility of onchocerciasis control

Annex 1. Research problems involved in vector control methods1

Texte intégral

1. INTRODUCTION

  • 1 Prepared for the meeting by Dr M. Ovazza, Office de la Recherche scientifique et technique outre-m (...)
  • 2 The meeting was, in part, a follow-up of the Informal Meeting of WHO Advisers on Entomological Nee (...)

1The purpose of the meeting was to review the present situation of onchocerciasis control, to discuss its feasibility in large-scale projects and to indicate the immediate steps that need to be taken in the future.2 Viewed in this light, the most important research requirements are those which could help: simplify present techniques, lower their cost, increase the efficacy of control campaigns in coming years and, finally, allow the treatment both of larger areas and of foci where present control methods are but little effective due to local conditions.

  • 3 List of contributors appended.

2WHO asked a number of experts3 for their opinions on these research requirements, and the intent of the present annex is to relate main points expressed in their answers.

3As far as possible, the experts are quoted in their own words unless more than one proposed the same study, in which case their common opinion is summarized. This exposé is divided into two parts: (a) urgent research aiming to perfect present techniques of control and methods for assessing their effects; (b) longer-term research aiming to develop new techniques and increase present knowledge.

4Both parts are sub-divided in the same way, except for certain subjects which are not included in the second part.

5Concerning the general feasibility of onchocerciasis control, it should be recalled that eradication of S. neavei has been achieved in some foci of Kenya and that control of S. damnosum is proving to be a success at least in one area in Mali and in two others in the Ivory Coast (OCCGE reports). The following opinions may also be quoted: "In some areas control is economically practicable, and is being, or will be, carried out. Experience gained may enable control to be extended to large areas where the problem is quite untouched" D. J. Lewis. "There is, however, no reason why control schemes should not be undertaken in the near future. Indeed, the only way to acquire the necessary experience is in the field; research can proceed at the same time and in the same areas, as well as in adjacent, uncontrolled areas" (McMahon). "Briefly it has been shown that insecticide applied to breeding waters can lead to perfect control and even eradication. What better situation can possibly exist than where the larvae are confined to a breeding-place whose very natural attributes lead to spreading and mixing of the insecticide so that it needs only to be dosed in by very simple methods here and there" (de Meillon). This last author recommends at the same time the establishment of pilot zones where techniques and knowledge could be perfected during the campaigns themselves.

2. URGENT RESEARCH AIMING TO PERFECT PRESENT TECHNIQUES OF CONTROL AND METHODS FOR ASSESSING THEIR EFFECTS

2.1 Pre-adult stages

6In Africa all present schemes and projects are based on the use of larvicides. The reports on these campaigns seem to show that few of the difficulties encountered are due to a lack of precise knowledge of the ecology of pre-adult stages, at least as far as S. damnosum is concerned. At the same time, however, a majority of the experts consulted is of the opinion that research in such a direction might lead to better control with present means and to the possibility of extending the campaigns to areas which seem for the moment difficult to treat.

2.1.1 Eggs

7Since they are known to be unaffected by all available larvicides, research on them will mostly be discussed in section 3.1. To perfect present dosing techniques, however, Burton proposes to study: "...to what extent are eggs laid above the water-line on overhanging grass or leaves, or on the bank, and kept viable by the spray? Do such egg-masses become desiccated by the sun? How many viable larvae result from this position?".

2.1.2 Larvae

8Several experts insist on the importance of studying migration during the larval stages and the influence it may have on control measures (Burton, Duke, Hocking, Le Berre. McCrae, Marr, Noamesi, Ovazza). For most, the main point to determine is over what distance the larvae are able to drift downstream to then re-attach themselves. Hocking, however, requests that studies be made on their ability to creep upstream or across the rivers. For his part, McCrae would like more knowledge on the possibility of larvae of the S. neavei group to drift downstream in the first stage, and afterwards to change host (i.e. the crab). Duke stresses yet another point: the possible difference in location between larvae and pupae sites and the ovipositing sites. Lewis, Marr and Noamesi wish that more precise data be acquired on the length of larval life in different conditions. Le Berre and most of the others admit that for S. damnosum a larval cycle lasts about eight days, but can be shorter under exceptional conditions; and Lewis points out that there is need of "further knowledge of minimum periods".

9To obtain better results in larviciding campaigns Fallis, Hocking, and Jamnback ask for a study on the size of the particles which compose the food of S. damnosum larvae.

10Fallis proposes to investigate: "the water velocities that cause larvae to release from substrate". This may have a direct bearing on the measures which can be taken as far as artificial breeding sites such as dams are concerned. The same expert proposes research which would attempt to throw light on the existence of biological rhythms as this might lead to the possibility of determining the hours of the day when larvae might prove to be more susceptible to insecticide.

2.2 Adult flies

11This section refers to studies concerning the biology and ecology of the adult fly. All research related to recent knowledge on the existence of a S. damnosum complex, and its possible consequences on ecology and transmission will be discussed in section 2.3.

12A general preoccupation amongst the experts is the lack of knowledge regarding the migration, dispersion and, generally speaking, the flight of females (Burton, Duke, Fallis, Le Berre, Lewis, McCrae, Noamesi, Ovazza).

13Burton proposes the utilization of gold-dust marking to study both dispersal and possible aestivation of adult flies. Noamesi underlines the importance of this study and draws attention to the fact that research on the subject is in progress. For Lewis it is necessary to study such questions in different climatic conditions. In this connexion, Duke's remark is of interest: "A great deal more information is needed on the dispersai and flight range of S. damnosum in different environments. The lack of knowledge here is not a reason for postponing attempts at control, indeed it may well be that from such attempts we shall learn much of what we want to know about these insects' dispersal. Nevertheless without further knowledge on the subject it may be difficult to prevent reinvasion of treated areas. Studies of the decline of biting density at different distances from known breeding sites can give at best only approximate results on dispersal which are liable to grave errors. Large-scale critical experiments with marked flies are really needed, but a prerequisite for these is the development of reliable techniques for marking and recapture. In connexion with the use of coloured dyes and dusting powder it should be remembered that at certain times a considerable number of wild-caught parous S. damnosum show ovaries tinted with a pinkish-purple colour and that this colour may sometimes spread to all the organs of the body."

14(Page 44 manquant)

15right into the forest.". Ovazza also attempts the following explanation to account for the differences between Le Berre's findings and those of Duke: "It should merely be observed that Le Berre was able to study the same 'variety' in different environments, as confirmed by Lewis' and Duke's above-mentioned map, whilst the two latter authors were led by circumstances to study two populations which they consider to be different. The intervention of biological factors inherent to both these populations consequently made it impossible for Lewis and Duke to put in evidence the influence of extrinsic factors." Ovazza therefore advocates that the taxonomical status of these two "varieties" be more precisely defined; a study be made of their hybridization potentialities; more precise information be given on their distribution (here Ovazza's opinion joins that of Lewis and Duke).

2.4 Insecticides and other methods of control

16All experts insist that studies to improve control techniques should receive first attention The following improvements are mainly needed: a longer effective carry of larvicide, development of a compound with a shorter persistence than DDT, dosing techniques providing a better dispersion of the larvicide in streams. Several experts emphasize that substitute insecticides should now be available in case vector strains develop resistance to the insecticides used. On these points, Jamnback's views are quoted:

"(a) Particularly interesting are larvicides such as carbamates that are fairly soluble in water (40 ppm) and so have some potential for remaining effective for long distances downstream, especially through areas of slow water. Stomach poisons such as GS 10133 that would be selective for filter feeders in streams also require further investigations as do non-persistent organo-phosphorous compounds such as sumithion and Dursban that have shown promise in laboratory testing.

"(b) Formulations, especially finely particulate, low-density dusts or wettable powders that would be ingested or concentrated by the blackfly larvae when applied in low concentrations to the water with resulting high selectivity in killing blackfly larvae should be investigated. Another possibility might be blocks of carbamyl placed in streams that would gradually dissolve over a period of several months providing control over a long period.

"(c) While the drip can method of application is simple and effective in applying emulsions which tend to disperse themselves in streams, it is less effective with formulations that tend to settle in the can, as is the case with suspensions. Solutions are emitted in large droplets which are less effective than fine droplets. A simple apparatus to disperse these formulations should be developed."

17In addition, all experts agree there is need for field tests permitting continuous observations to be made on the susceptibility level of larvae.

18A number of experts also suggest that studies be carried out on the following: size of the particles ingested by the larvae (see 2.1.2); behaviour of the larvicide in flowing water (Burton, Le Berre, McCrae, Ovazza); the precise maximum depth at which larvae are found, in relation to that reached by the larvicide (Burton, Le Berre); possible variations in larval susceptibility according to extrinsic and intrinsic factors (Burton, Le Berre, Hocking).

19A study concerning the fate of larvae once they leave their substrate under the effect of sub-lethal doses is also proposed, but opinions differ on this subject: Hocking, Le Berre and Marr believe that the larvae die because according to Hocking, they are exposed longer to the larvicide, and also because, in the natural conditions of the Sudan savanna during a dry-season campaign, they are unable to find another attachment point (Le Berre, Marr). Hocking proposes that a releasing agent be found; Marr asks for a larvicide which would have mostly a knock-down effect. While sharing also the opinion that once detached the larvae will remain in longer contact with the larvicide, Muirhead-Thompson seems to fear that part of them at least will be able to survive. He is the only expert to propose the development of techniques requiring the application of very small doses over much longer periods.

20Concerning artificial breeding sites, Fallis (as already mentioned in 2.1.2) wishes to study the feasibility of eliminating the larvae in such sites by "flushing", requiring the use of water velocities higher than those which the larvae are able to withstand. Le Berre draws attention to the fact that large dams will concentrate breeding in one point, viz: the discharge canal. He therefore asks for studies regarding definitive construction modifications which would prove unfavourable to the species; these studies should be carried out whenever hydrological projects permit it.

2.5 Methods of study

21This section refers to research proposed with a view to improving methods in order to meet ecological and epidemiological needs as well as to provide a better assessment of results.

22For the most part, the experts agree as to the choice of goals to be set, but they sometimes differ as to the selection of methods to be used. Regarding the dispersal of adult flies and the assessment of the effect of campaigns on the female fly population, several experts request the development of better trapping techniques: Davies thinks that an effective trapping device is necessary for two reasons: (1) after a campaign capture on human bait provides few data because of the small number of remaining female flies; (2) he finds this method inaccurate. Jamnback, also, has not much faith in the human bait method. McCrae, though he admits that the traps he tried did not give him the expected results, finds that capture by seated immobile-people is unnatural.

23It is to be noted that these objections were already raised at the time of the Informal Meeting of WHO Advisers (July 1964). The opinions expressed by other experts are either opposed or, at any rate, less radical.

24On this subject, Le Berre's remarks: "Its (capture on human bait) advantages: it allows the capture of female flies at a very precise time of their life, and knowledge of the physiological age of the females captured makes it possible to follow the campaign's progress with great precision; ... when the adult fly population is greatly reduced, the human collector method provides excellent results. Its inconveniences: these arise from the collector himself." Concerning the use of traps, the same author underlines that none up to now has given valid results in Africa with S. damnosum. Another obstacle, to his mind, is the fact that trap collections must be sorted out in order to separate females of S. damnosum from a high number of other insects. A third point is that a trap will take females at different stages of their physiological cycle and results will be very difficult to analyse. His conclusion is that capture by trapping techniques does not present an advantage for the assessment of campaign results but these techniques must all the same be perfected as they are useful for studying the biology of adults and perhaps as a complementary method for assessing campaign results.

25Marr considers that capture on human baits is still the basic method; nevertheless, he is in favour of the development of better traps which, although possibly less effective, would be more accurate.

26De Meillon underlines that main interest lies in the degree of residual transmission after a campaign. Therefore, capture on human bait is necessary as it affords information on just those flies which are implicated in transmission.

27Opinions also widely differ as to what capture methods should be studied and as to the advantages of those presently in use or being developed. Burton proposes trying sugar-baited traps, blackflies being nectar feeders and consequently likely to be attracted by sugar. Davies favours a trap which would offer bait more attractive than human bait, would be able to work continuously and keep its collection for three to seven days. Fallis wants to promote catching with visual attractants + CO2 and carried out tests with such a trap in Ghana where these are presently being continued by Noamesi. Jamnback suggests experimenting the sticky trap designed by West. McCrae, as mentioned above, has tried several types of traps, most of which proved disappointing; however, he has had interesting results with a modified Fredden's trap allowing the use of human bait and of bait-animals, even large-sized ones; he also asks that a combination of amino-acids + hormones + CO2 be tried as attractant.

28As already mentioned, many authors ask that the drifting possibilities of larvae be studied (2.1.2) and also that the effects of larvicides on them be closely followed during the campaigns. With regard to these two related subjects of study, certain methods are proposed. Fallis suggests the destruction of existing supports at a given point of the river and replacing them by green strips of plastic or mops of string, for instance. De Meillon's technique is somewhat similar, but he would not destroy the existing supports and merely put floating pieces of wood a week before larviciding. Larvae would attach themselves on these floats allowing an assessment of their susceptibility. As a reminder, it must be mentioned that, in Ghana, Burton and Noamesi used plastic tubes closed by gauze in which they placed larvae with a naturel support such as grass or leaves. To study the drift of larvae, Hocking recalls that white plastic cones trailed in the water have given good results in Canada. For carrying out sensitivity tests in the field, Jamnback speaks of a technique in which the larvae would be tested against the concentration used in plastic bags, these bags being afterwards placed in the river during the time of observation.

29Some experts have already expressed doubts as to the efficacy of marking and recapture techniques for studying dispersal of the adult fly (2.2). Nevertheless, Duke believes that their development ought to be promoted. Fallis, if it were first possible to improve quantitative methods of sampling which would provide large samples of adults and larvae, would prefer marking with radio-isotopes. Gold-dusting gave good results in the studies carried out by Noamesi.

30Lewis and McCrae would like research to continue on a method which would permit complete laboratory rearing of S. damnosum. Both believe that the main point of interest resides in the feasibility of forced mating; and McCrae recalls that McMahon has an effective technique of membrane feeding. Muirhead-Thompson is also pursuing research in the same direction.

31McMahon believes that the effects of reinvasion on the one hand, and possible dry-season survival on the other, ought to be assessed by campaigns in a completely isolated S. damnosum focus. For this purpose, he suggests the island of Fernando Po.

2.6 Operational problems

32De Meillon points to the necessity of improving present methods for reaching rivers to which access is required as well as of obtaining better results with the larvicides actually in use.

33With the same aim in view, Duke, Jamnback, Le Berre and Marr insist on the various possible uses of aircraft, be it for preliminary surveys, for larviciding in certain conditions, or for providing greater transport facilities, mainly for the entomologist in charge. They consider it urgent that the limits and difficulties of larviciding by aircraft be estimated and that this technique be adapted to conditions in Africa.

34Lewis, McCrae and McMahon suggest that the possibility to effect control by long-term application of larvicides - a method used in the Budongo forest against S. neavei - be studied for use in other limited foci.

35Hocking requests the development of simple techniques of flow measurement in order to ensure correct dosing.

2.7 Methods of assessing results

36All the experts recognize that the present techniques of entomological evaluation are sound, at least in their principle. But Duke and Jamnback underline that assessment based only on an entomological evaluation of the reduction of the vector population does not, at present, give a true idea of the actual degree of reduction in transmission. Davies believes that persisting transmission in the Abuja area, where very few simuliids are still to be found, signifies that transmission can be maintained with a very low number of female simuliids. According to these experts, a particular effort should be made to increase present knowledge on that question.

37From a practical point of view, those experts who, for entomological surveys, prefer the use of traps to human bait as sampling techniques, also give preference to trapping techniques for the assessment of results.

3. LONGER-TERM RESEARCH AIMING TO DEVELOP NEW TECHNIQUES AND INCREASE PRESENT KNOWLEDGE

38The aims of long-term research are not the same in West Africa as in East Africa. In the first instance, the experts are interested in studies that will in the long run, not only improve but also, most probably, radically change present methods which nevertheless could be put into immediate practice. In East Africa, owing to the existence of numerous "varieties" of the damnosum complex, some of which probably sympatric, improvement of fundamental knowledge of the vector is a prerequisite to successful control.

3.1 Pre-adult stages

39Burton asks for detailed studies on the mechanisms by which eggs and pupae develop resistance to insecticides.

40Burton, Fallis and Hocking consider that physiological and ecological research on larvae must be promoted, particularly on:

  • mechanisms and stimuli governing the looping movements and the use of silk for attachment and drifting;
  • feeding and rate of growth;
  • possibility of competition in the breeding sites between different Simulium species;
  • animal environment in the breeding places, particularly with regard to other arthropods and possible predators on the different stages of blackflies.

3.2 Adults

41Two types of research are proposed:

42The first aims at improving present knowledge of the ecology and behaviour of adults, particularly of the female, e.g. stimuli involved in oviposition, attractants, fat-body formation and utilization, physiological influence of nectar feeding, different aspects of flight and dispersal (Burton, Fallis, Hocking, McCrae).

43The second would consist in a detailed study of the bio-ecological differences that may exist between the "varieties" in the "damnosum complex" and the species in the "neavei group", e.g. anthropophily and zoophily, possible autogeny of some damnosum "varieties", transmission power, behaviour and ecological reaction to the seasonal disappearance of breeding sites, annual cycle, biting cycle (Fallis, Hocking, McCrae).

3.3 Microtaxonomy

44Hocking, Lewis and McCrae insist on the need for more accurate knowledge regarding the composition of the "damnosum complex", the morphology, genetics and degree of speciation of its "varieties".

3.4 Methods of control

45Attention is drawn to the biological, ecological or even integrated methods of control that could be used if present knowledge were improved and enriched. Burton asks that the following possibilities be studied:

  • chemical and mechanical techniques of destruction of eggs and pupae;
  • ways of making breeding sites unacceptable for oviposition;
  • use of attractants in constructing new traps for the destruction of adults.

46Duke proposes a detailed study of the different members of the "damnosum complex", aiming at the introduction of non-anthropophilic "varieties" in onchocerciasis foci.

47Fallis, like Burton, advocates killing traps and suggests also a study of possible repellents to interrupt transmission.

3.5 Methods of study

48Several experts express the need, already mentioned in section 2.5 for a study of the technical possibilities to promote complete laboratory rearing of flies and subsequently a study of cross-mating between the different forms of the "damnosum complex".

LIST OF CONTRIBUTORS

Prof. A. W. A. Brown

Department of Zoology, University College of Arts and Sciences, University of Western Ontario, London, Ontario, Canada

Dr G. J. Burton

National Cancer Institute, National Institute of Health-, Bethesda, Maryland, United States of America

Mr J. B. Davies

Trinidad Regional Virus Laboratory, University of the West Indies, Trinidad

Dr B. O. L. Duke

Helminthiasis Research Unit, Kumba, Western Cameroon

Prof. A. M. Fallis

Department of Parasitology, University of Toronto, Canada

Prof. B. Hocking

Department of Entomology, University of Alberta, Edmonton, Canada

Dr H. A. Jamnback

New York Museum and Science Service, University of the State of New York, Albany, New-York, United States of America

Dr R. Le Berre

Section Onchocercose, Mission ORSTOM auprès de 1'OCCGE, Bobo-Dioulasso, Upper Volta

Dr D. J. Lewis

Medical Research Council External Staff, c/British Museum, London England

Mr A. W. R. McCrae

East African Virus Research Institute, East African Common Services Organization, Entebbe, Uganda

Mr J. P. McMahon

London School of Hygiene and Tropical Medicine, Winches Farm, St. Albans, England

Mr J. D. M. Marr

WHO Entomologist, AFRO-0131 Project

Dr Botha de Meillon

South-east Asia Mosquito Project, Smithsonian Institution, United States National Museum, Washington, D.C., United States of America

Dr R. C. Muirhead-Thompson

London School of Hygiene and Tropical Medicine, London, England

Dr G. K. Noamesi

Onchocerciasis Unit, Ministry of Health, Accra, Ghana

Dr M. Ovazza

Entomologie Médicale, Centre ORSTOM de Tananarive, Madagascar

49World Health Organization Vector Biology and Control unit, Geneva, Switzerland

Notes

1 Prepared for the meeting by Dr M. Ovazza, Office de la Recherche scientifique et technique outre-mer (ORSTOM), Paris, France, WHO Consultant.

2 The meeting was, in part, a follow-up of the Informal Meeting of WHO Advisers on Entomological Needs in Onchocerciasis Control, July 1964, Wld Hlth Org. techn. Rep. Ser., 1966, 335, 51.

3 List of contributors appended.

© IRD Éditions, 2003

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