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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 3. Who expert committee on onchocerciasis extracts from the second reporta

Texte intégral

1. PROCEDURES FOR FIELD LARVICIDING

  • a Wld Hlth Org. techn. Rep. Ser., 1966, 335.

1"The group recommends that:

  1. Simulium-control campaigns be directed by a qualified entomologist, if possible one with previous experience of Simulium control.
  2. Control measures should be directed against the larvae.1
  3. A thorough survey should be made for all actual and potential breeding sites for a whole year prior to treatment. This study should include an aerial survey to spot possible breeding sites and access routes.
  4. Length of larval life in the control area should be determined.
  5. DDT as an emulsifiable concentrate should be used and the dosage calculated and expressed as the number of parts of p, p’-isomer per million parts of water.
  6. Dosage be at a rate not exceeding 1.0 ppm and not less than 0.1 ppm maintained over 30 minutes. In the absence of hydrological data, discharge can be calculated by measurements of width, depth and speed of flow of the river (for the last, a simple float method is sufficiently accurate).
  7. The season chosen for larviciding should be determined from results of pre-treatment observations on vector population dynamics, fluctuations in river discharge, State of access roads at different times of the year, etc,
  8. Selection of dosage points must be made on basis of pre-treatment observations and on distance over which larvicide is fully effective. In a new control area, effective distance will not be known initially, and experience elsewhere may be used to decide distance apart of dosing points. Within control areas, the position of the dosing points may need modification-as a result of direct observation on effectiveness of the first and subsequent larvicidings; and it is-recommended that, in any control programme, continuous observations be made to assess the effective carry of larvicide, the dosing points being moved as need be.
  9. The dosings at each point should be made every seventh day, unless local observations on length of larval life indicate that less-frequent applications can be made safely.
  10. The initial treatment should be maintained for at least six weeks.
  11. The emulsifiable concentrate should be diluted to at least 1/3 with water and run into the main flow of the river over the 30-minute dosing time, using simple ad hoc devices.

2. SURAMIN

2"Suramin (used at a dosage similar to that described under individual treatment) has been tried in mass campaigns on several occasions but, in retrospect, it has never been a success, not on account of failure of its filaricidal properties, but because its use requires very careful medical supervision, regular examination of the urine, intravenous injection, and assembly of the treated population at weekly intervals for about two months. Also, the occurrence of serious toxic side-effects (some of which are undoubtedly associated with the death of the parasites) are too frequent to justify its general use on a large scale. It should be noted that recent experiments have shown that the toxic reactions encountered with suramin cannot be related to unsatisfactory quality of the drug used. Preparations supplied by three different manufacturers were found to be equally stable chemically, equally effective in killing macro-and micro-filariae and equally prone to produce albuminuria and other toxic reactions.

3The use of suramin on any considerable scale can perhaps only be envisaged at present in certain small savannah villages in Africa in which treatment is urgently needed on account of a very high incidence of blindness. In such circumstances, the drug may be recommended for individual treatments on a village scale, under careful, whole-time medical supervision."

3. SKIN SNIPS

4"It is generally agreed that skin snips are better for detecting microfilariae than is scarification of the skin.

5Skin snips are normally taken by the insertion of a needle into the skin to elevate a cone of skin, which is then cut off with a razor blade or a sharp-bladed scalpel. The area snipped should be approximately 3 mm in diameter. Antiseptic precautions should be taken.

6The portion of tissue removed should-be placed on a microscope slide in a drop of saline or water, covered with a coverslip, and allowed to stand for 30 minutes before being examined. If desired, the specimen may be teased and allowed to stand 10-15 minutes before examination without a coverslip.

7Examination is made with a magnification of not more than x 100. If it is desired to stain the preparation, it should be allowed to dry thoroughly, and for this purpose, preliminary covering with a coverslip is-undesirable. The specimen should then be fixed by immersion in methanol for 30 seconds and stained with Mayer’s haemalum or with Giemsa’s stain.

8In prevalence surveys, at least two snips should be taken from each person; this number represents a compromise between accuracy and acceptability in a population. The snips should be taken from the sites most likely to be heavily infected. In Africa and Venezuela, these are the buttocks and the lower legs; in Mexico and Guatemala, they are the shoulders and buttocks. In areas where there is doubt as to the best sites to use, preliminary investigations should be carried out, taking skin snips at multiple sites.

9In a properly taken skin snip, contamination with blood-circulating microfilariae is very rare. In some areas of Africa, infections with Dipetalonema streptocerca will be encountered but, with a little experience, the microfilariae of this species can readily be distinguished from those of O. volvulus, both in wet–and in stained preparations. In cases where there is doubt as to the identification of microfilariae in wet preparations, those from skin snips should be stained. Likewise, if difficulty is encountered in examining wet films in regions of very low humidity, the preparations should also be examined after staining."

4. INDICES OF ENDEMICITY AND MORBIDITY

10"The Committee decided that three levels of endemicity should be established, on the basis of the percentage prevalence of O. volvulus microfilariae in the skins of the population sampled, namely: high endemicity, 67% and more; medium endemicity, 34%-66%; and low endemicity, 33% or less.

11In areas where the prevalence is less than 5%, microfilarial carriers may well have acquired their infections elsewhere, and the disease should not be regarded as endemic unless clear proof is obtained that transmission is occurring on the spot (i.e., finding of infected Simulium or of infections in persons who have never left the area)."

5. MAZZOTTI TEST

12"This test has been much used as a diagnostic aid in the field, but it is, of course, of qualitative value. It should be remembered that, once it has been used, definite diagnosis and quantitative assessment become impossible for an appreciably long period of time. There is also a considerable subjective element in its interpretation."

6. RESEARCH NEEDS

13"The problems on which research is needed are almost unlimited; the ones mentioned here are chiefly those that have an important bearing on the control of the disease.

4.1 Vectors

4.1.1 Biology of adults

14As current sampling methods used in evaluation are based on one aspect of the vector’s activities, namely, its biting contact with the human host, it is recommended that additional methods of trapping or collection currently used in Europe and North America-should be explored and developed with regard to the vector Simulium. Promising methods include the use of mechanical traps, light traps, sticky traps, etc. Further studies on Visual attractants and on Chemical attractants should also be pursued. Special methods or-combinations.of methods may have to be developed in order to find out more about the resting places and concentration sites of engorged and gravid adult females.

15A wider range of sampling techniques is essential for a fuller understanding of vector biology, flight range, survival, etc. and would be particularly helpful, for example, in providing unbiased samples of engorged females for blood-meal identification. More accurate knowledge about blood-feeding activities of vectors on hosts other than man is required.

16Methods for establishing self-perpetuating colonies of the vector Simulium are urgently needed. Research on this important aspect is being vigorously pursued in several centres at present, and should be encouraged.

4.1.2 Ecology of breeding places

17Long-term studies on the ecology of breeding places, particularly with regard to factors determining the suitability or otherwise of different types of stream for vector species, should be encouraged. Such studies would also assist in evaluating the general biological effect of control (larvicide) measures on associated fauna and microflora of the habitat.

18"There is need for more accurate information about the length of larval life in different habitats and in different areas. A bioassay method, already developed in one country for studying this question, might be tested with advantage under other conditions. This technique has an added advantage in that it can be used to study the effect of larvicidal treatment on larvae under natural conditions.

19The question of larval drift, migration and re-attachment has been studied in the European and North American species. There is need for similar investigations on vector species in Africa and in Latin America.

20Further studies on oviposition habits under various conditions, e.g., with regard to different types of substrate and different-types of spillway, should be encouraged. Nothing is as yet known about the ovipositing habits or ovipositing sites of the S. neavei group.

4.1.3 Insecticides and other methods of control

21The evaluation and testing of new Simulium larvicides has recently been incorporated in the long-established WHO-coordinated programme for evaluating and testing of new insecticides in general. Continued research on these lines is to be encouraged, together with the further improvement of techniques for testing new Simulium larvicides and the development of a standard susceptibility test. There is need for further work on the evaluation and testing of recommended insecticides in relation to changes in flora and fauna and to their accumulation in animals used as food.

22The evaluation and testing of repellents and attractants requires more investigation particularly in relation to future possibilities in the use of chemosterilants as a supplementary control measure against adult Simulium vector species.

4.2 Epidemiology

23The contrasting epidemiology of the disease in the Americas and in different climatic zones of Africa should be studied so as to provide dues to some of the-paradoxes of onchocerciasis. Indices of endemicity should also be determined.

24Further development of epidemiological methods for assessing the results of-control schemes is desirable, and further studies on the variations of microfilarial numbers in the skin from day to day and hour to hour are needed.

4.3 Clinical and ophthalmological investigations

25Longitudinal studies should be made on patients with onchocerciasis to determine the order of appearance of clinical signs and the effects on life expectancy and health status. The pathology of onchocerciasis is still poorly understood, partly because the patients studied suffer from or are exposed to several other diseases concomitantly. The State of nutrition in relation to the appearance and intensity of onchocercal infection must be studied, for example, by investigating vitamin A and carotenoid blood levels and minimum light thresholds. Attempts should be made to correlate the localization of nodules with ocular symptomatology and to investigate the effects of denodulization on the relief of certain ocular lesions. The use of intravital staining techniques, and possibly also of radioactive tagging, were mentioned as means by which the movements of microfilariae in the body and in the eye might be traced. The whole question of the etiology and pathogenesis of posterior-segment lesions requires further study and might include observations as to whether their progress is halted by treatment.

4.4 Chemotherapy

26Improved macrofilaricidal drugs are required. Present drugs need further investigation for possible chemoprophylactic action. The differences in the reactions to drugs of patients with onchocerciasis and patients with trypanosomiasis needs elucidation. For chemotherapeutic control purposes, more knowledge is needed on the minimum dosage of diethylcarbamazine that will clear the skin of microfilariae, and much further work must be done to determine how Mel W may best be employed as a single-dose macrofilaricide in the field.

4.5 Immunology

27Studies on the isolation, purification and Chemical characterization of specific antigens from various stages of the parasite, as well as the essential components, should be encouraged. Those antigens that come in contact with the host would probably serve as the best possible testing and immunizing agents. It is essential for the standardization of the antigen to know the biochemical constitution and the dose response. For the complement fixation test, as well as other bio-immunological tests, it is important to ascertain both the specificity and the sensitivity of the antigen since, in long-standing infection, the test may become negative although microfilariae may remain in the skin. The use of exo-antigens rather than somatic antigens and their comparable immunogenic activity should be investigated. In order to compare results, a standard complement fixation test should be used uniformly in several areas where large-scale surveys are being conducted. A correlation between the results obtained by intradermal tests and complement fixation tests should be sought. The use of fluorescent-antibody techniques should be studied, and the relationship of immune reactions to eye lesions observed in onchocerciasis should be investigated.

4.6 Helminthology

28Studies on possible differences between strains of O. volvulus from different areas should be encouraged. The use of the chimpanzee as a laboratory host for the infection opens the way for many basic studies on the parasite in the vertebrate host, and possibly later for studies on chemotherapy.

29There is an urgent need for a laboratory culture of some species of Onchocerca that can be maintained by cyclical transmission in an alternative, more easily obtainable definitive host, and further efforts should also be made to culture the parasite in vitro. In the meantime, much could be learned from a study of species such as O. cervicalis of the horse, O. gutturosa of the cow and O. flexuosa of the deer, onchocercae that are prevalent in Europe. It has recently been shown that the microfilariae of O. guttorosa will survive in the skin of rats for several weeks. This finding provides an opportunity for assessing diagnostic techniques, for testing microfilaricidal drugs, and for studying various aspects of the pathology of onchocerciasis.

30The possibility of a natural animal reservoir host for O. volvulus must be borne in mind, and basic research into the physiology of filarial worms should be encouraged."

7. INDIVIDUAL ONCHOCERCIASIS RECORD CARD

* Indicate whether positive or negative.

•• Delete where inapplicable.

Notes

1 The possibility of adult control as distinct from larval control is stressed (Vargas), particularly by measures against the older parous flies (Davies). The ovipositing female may represent a vulnerable phase for attack, and more knowledge about behaviour, concentration sites, etc., at this stage is needed (Davies)."

Notes de fin

a Wld Hlth Org. techn. Rep. Ser., 1966, 335.

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