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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

Onchocerciasis Control

Moving towards the Millennium

D.H. Molyneux et J.B. Davies

Résumé

The recognition of onchocerciasis as a major public health problem in the savanna belts of West Africa resulted in the establishment of the Onchocerciasis Control Programme (OCP) in 1974. Control was initially based on vector control by weekly larviciding. The OCP is now in transition towards its final phase in which repeated treatment with ivermectin, a safe and effective microfilaricide, is incorporated with vector control, or in certain circumstances is used alone. Ivermectin distribution hingeing on sustainable community Systems is the basis of a new programme in endemic African countries outside the OCP and in the Americas. David Molyneux and John Davies describe the latest trends and developments related to onchocerciasis control.

Texte intégral

1Onchocerciasis in This Wormy World’ of Stoll, 1947 (Ref. 1) attracts a single paragraph of seven lines! In 1947, foci of infection in mainland South America were unknown, and Stoll only refers to the ’Zone in Africa eastward from Sierra Leone… contains 57 million inhabitants’. His estimate of 19.8 million for the world total of Onchocerca infections in 1947 was probably an overestimate at that time. The most recent estimate of the number of infected people, provided by the World Health Organization (WHO) in 1995 (Ref. 2), is 17.516 million in Africa and Yemen and 140455 in the Americas. Since 1947, onchocerciasis has been recognized as a disease of significant public health importance and an impediment to socio-economic development. It is endemic in 37 countries; in Africa (30), in the Americas (6) and in Yemen. The disease and its vector are the target of the largest successful and sustained vector-control programme the world has known, the Onchocerciasis Control Programme (OCP), which commenced operations in 1974 in seven countries, has since been extended to encompass a total of 11 countries, and is still continuing.

2Recent years have seen considerable progress in onchocerciasis control and a change of emphasis. The WHO Expert Committee Report2 emphasized the opportunity presented by the registration of ivermectin (Mectizan®) as a safe and effective microfilaricide for use in humans. The commitment by the manufacturera, Merck & Co., via the Mectizan Donation Programme, to provide the drug free of charge to the port of entry for as long as necessary to control the public health problem of onchocerciasis was the incentive for the development of a partnership of Non-governmental Development Organizations (NGDOs) involved in blindness prevention, international organizations (WHO and the World Bank), governments of endemic countries and a donor consortium to establish the African Programme for Onchocerciasis Control (APOC), based on the efficacy, safety and availability of ivermectin3. In parallel with APOC, the programme for the elimination of onchocerciasis in the Americas (OEPA) is supporting the distribution of ivermectin in the endemic countries of Mexico, Guatemala, Venezuela, Colombia, Ecuador and Brazil2. In the OCP area of 11 West African countries, the original strategy of vector control alone has been modified and refined to include the distribution of ivermectin to reduce morbidity more rapidly, while maintaining vector control for 12 years to stop transmission in areas endemic for blinding onchocerciasis4.

3The number of ivermectin treatments administered globally has increased rapidly over the period 1988-1996: in 1996, over 19 million treatments were provided through the Mectizan Donation Programme in 25 countries in Africa (including Yemen) and in six countries in the Americas5. The OCP will continue control operations until the end of 2002 (Ref. 6). Boatin et al. 4 reviewed the most recent approaches to control, the definition and rationale of control in areas that require special interventions (owing to, for example, infiltration from non-controlled countries in border areas, inadequate vector control or premature cessation of larviciding, the indicators used and the current epidemiological stratification of onchocerciasis in West Africa). In 1995, Molyneux summarized the status of the OCP after a review of the programme in 1994 (Ref. 6).

4Several studies relevant to the future of the OCP have been completed since 1994. (1) Amocarzine7 has been assessed as a potential macrofilaricide in Africa, following studies in Ecuador that indicated its efficacy against adult worms8. (2) The Onchocerca ochengi model has been developed for the evaluation of macrofilaricides9, and has been used to demonstrate a prophylactic activity of monthly doses of ivermectin in cattle exposed to natural challenge with O. ochengi (V. Wood et al., unpublished). (3) The diethylcarbamazine patch test has been developed and evaluated as a non-invasive tool for the diagnosis of onchocerciasis (WHO, OCP in West Africa, Expert Advisory Committee Report, OCP/EAC/18.2, June 1997) to detect recrudescence at the community level within the context of a post-OCP environment. (4) Isolated foci of onchocerciasis have been detected within the original OCP area owing to a variety of circumstances including inadequate larvicide treatment and too early cessation of larviciding4. (5) A new class of insecticide, a pseudopyrethroid, etofenprox (Vectron), has been introduced into the OCP vector-control strategy. Etofenprox fulfils a key operational need in the rotational insecticide application strategy, as it can be used at river discharges of 15-70 m3 s-1, where until now only organophosphate compounds have been effective, thus averting the danger of an operational gap should resistance develop to the existing organophosphates, temephos, phoxim and pyraclofos (see Hougard et al., this issue). (6) Rapid epidemiological mapping of onchocerciasis (REMO) and rapid epidemiological assessment (REA), together with géographic information Systems, have been used extensively for targeting control in APOC countries10.

APOC _

5Effective control within the APOC countries (the 19 endemic countries in Africa outside the OCP) will depend on the sustainable delivery of ivermectin throügh community-directed treatment, the philosophy being to ensure adequate community involvement to sustain delivery after the support to projects has ceased after five years. The WHO Special Programme for Research and Training in Tropical Diseases has undertaken preparatory operational research for APOC through a Task Force, which has focused on developing community-directed approaches11, evaluating the impact of ivermectin on onchocercal skin disease12, determining the social and economic consequences of severe skïn disease and developing the use of REMO3-10. Recent studies13 compared the number of spontaneous abortions in hyperendemic foci in Ecuador before and after ivermectin treatment, providing evidence that ivermectin reduces the frequency of abortions in patients with onchocerciasis. In future, research on operational issues will be directed towards improving the efficiency of communitydirected treatments, ensuring that Systems are appropriate to developing countries and encouraging sustainability of the cost-effective delivery of ivermectin. The challenge facing health Systems in the Americas, in APOC countries and in some OCP countries is the delivery of a free drug to communities in need; if the health community cannot achieve adequate annual coverage with free ivermectin,-it is unlikely that any health intervention in resource-poor rural settings can be cost-effective. The target of cost per treatment to be reached at the end of the five-year APOC funding cycle should be around US$0.20; this figure should represent costs of any national and local (regional or district) structures required, and would largely represent the distribution cost from the port of entry, through a rapid and effective chain, to the periphery. The cost of the distribution System and of reporting on treatment will be borne by national and community structures after the five-year APOC funded period.

6APOC3 is based on the concept of partnerships between NGDOs and governments in the organization of the National Onchocerciasis Task Force and, at the local level, the development through NGDOs and regional or district (or equivalent) govemment of the appropriate community-directed delivery Systems. Different NGDOs are involved in different countries, but all major NGDOs with an interest in blindness prevention participate: Sightsavers, Christofel Blinden Mission, Helen Keller International, Africare, United Nations Children’s Fund, River Blindness Foundation (now incorporated into the Carter Centre Global 2000 programme), International Eye Foundation and the Organisation pour la Prevention de la Cécité.

7APOC has indicated that initial support costs to finance programme establishment can be relatively high, but should not exceed around US$2 per person treated; treatment should be confined to hyperendemic and meso-endemic villages identified by REA and mapped through REMO11. Country maps are required before funding is provided through APOC. The NGDO and govemment contribution should be at least 25% of the funding requested from APOC, whether in cash or in kind. Projects would be funded against a background of a comprehensive national plan; each project should demonstrate a trend to sustainability based on the graduai reduction in cost per person treated over the five-year period to a level of around US$0.20. The concept of sustainability in this development process used by APOC refers to the ability of communities following initial external involvement to maintain the viability and continuity of the ivermectin treatment process in-the absence of extemal support. The indicators of sustainability currently used by APOC are listed in Box 1.

Ivermectin-delivery systems

8Ivermectin has been an important intervention within the OCP area since 1989 after early trials in 1987 in Asubende, Ghana14,15. Initially, distribution within the OCP was based on mobile national teams, particularly in the extension areas (those that were not in the OCP initially), with local health sector personnel as well as community members also being involved. Clearly, over the coming years, OCP will move to incorporate community-directed approaches to increase sustainability, particularly in areas designated for special interventions3.

9As the success of ivermectin distribution requires that at least 65% of the exposed population is treated annually3, evaluation of distribution Systems is necessary. Recent studies16 have been undertaken in four OCP countries, Benin, Côte d’Ivoire, Ghana and Togo, to investigate coverage and operational aspects of the distribution System. This study revealed that in 130 selected villages, 97 had received treatment and 67% of the population had received ivermectin. Nearly 30% had taken the drug in all treatment rounds and the main reasons for non-treatment were: absence (54.5%); non-eligibility (young children or pregnant women) (12.2%); refusai (2.6%); and shortage of drugs (only 1.9%). In general, there was community approval for treatment, despite 26% of villages preferring the mobile delivery method. The authors of the study suggested that communities themselves could accept responsibility for distribution, after appropriate instruction and communication.

10An extensive study in five countries (Mali, Ghana, Nigeria, Cameroon and Uganda) was carried out to identify and develop simple, acceptable and sustainable methods for community-directed treatment with ivermectin. A study was designed to ascertain whether programme-directed approaches (ie. those developed in collaboration with the programme and proposed to the community) or community-designed methods (ie. those identified by the communities themselves) could be more effective, taking account of the multiplicity of different communities involved11. The guidelines employed to assess the success of these two approaches were related to the process, structure and changes of methods of drug delivery; ivermectin procurement; adherence to treatment protocols (exclusion criteria and referral of adverse reactions); supervisory strategies; and issues of coverage and reporting. The study was also intended to identify factors relevant to the sustainability of drug-delivery Systems and to provide information applicable on a continent-wide scale in Africa. Similar studies in Nigeria17 suggested that, where appropriate human resources are available at the village level, such community-based workers need to receive continued external and internai support. These studies also demonstrated that data collection is limited by literacy standards. Local leaders considered that there was a need to set payments to compensate for time and effort involved, although local resources (both human and financial) are a prerequisite for true ownership and hence sustainability in these studies. The WHO multicountry study concluded that community-directed treatment is feasible and effective, and is successful in a range of diverse settings, suggesting it is likely to be replicable. Distribution Systems designed by communities achieved better coverage than programme-designed approaches, and the distribution performance was adequate in terms of coverage achieved, adherence to exclusion criteria and dosing level11. The key factor in limiting sustainability was the non-availability of ivermectin at pick-up points when expected. However, better performance was associated with supervision by health service staff; cost-recovery Systems, an obligatory part of the health Systems in Cameroon, had a detrimental effect on coverage and communities had difficulty in reporting.

Box 1. Indicators of Sustainability of Community-directed Treatment
Commitment of the partners
(1) Percentage share of all costs is borne by the Ministry of Health, local NGDO(s), NGDO partner and APOC Trust Fund.
(2) The Ministry of Health budgets an amount for onchocerciasis control in the current year.
(3) Ivermectin is made available to the community.
Integration into primary health care
(1) There is a a number of targeted endemic communities in which the distribution is part of the Primary Health Care System and supervised by the Primary Health Care System.
(2) There is also a number of targeted endemic communities in which supervision is done by the Primary Health Care personnel.
Community involvement
(1) Many targeted endemic communities involve the community in decision-making on mode of distribution; selection of community-based distributors (CBDs); procurement and collection of ivermectin from central collection point; reporting and referral of cases of severe adverse reactions; decision-making on incentives and/or remuneration to community-directed distributors; and change in mode of distribution System.
(2) There are plans to increase community participation.
Estimated cost per person treated
This includes the cost of: drug delivery from port of entry to community; collecting drug from central point by the community; training CBDs; supervising CBDs; monitoring community-directed treatment with ivermectin (CDTI); and remuneration or incentives paid to CBDs by the community.
Indicators of coverage of CDTI
These include targeted communities treated with ivermectin; targeted persons treated with ivermectin; targeted communities treated the previous year; targeted persons treated the previous year; and reasons for refusals.

Vector control in Africa

11OCP countries. The extensive vector-control activities in the OCP area have been regularly reported and reviewed18,19. The rationale for continuing vector control in key areas of the OCP area until the end of 2002 is the need: (1) to maintain vector control to prevent transmission for a period equivalent to the maximum duration of adult worm life, and (2) to reduce the risk of re-invasion of the core OCP area from infected blackflies carrying third-stage larvae (L3s) of savanna Onchocerca volvulus. Vector control ensures the eventual absence of a human reservoir, the only feasible approach in the absence of a safe macrofilaricide that could be delivered to the communities. Continuous effective larviciding is effectively the best available’macrofilaricide’and protects more people than other interventions by totally blocking transmission. Hougard et al. (this issue) review recent developments and approaches in vector control used by the OCP.

12The change in prevalence and annual transmission potential within the OCP area between 1974 and 1996 has been reviewed by Boatin et al. 4. Figures 1a and 1b demonstrate the change in prevalence during the programme’s activées. Table 1 shows the change in estimated numbers of cases of infection and of onchocerciasis-related blindness between 1970 and 1995. Considerable epidemiological variation in prevalence exists over the 11 countries of the OCP. The different areas of the programme and the pre-and post-control prevalences are provided in Table 2.

13In the whole OCP area, the numbers of people prevented from going blind between 1974 and 1995 are estimated at 125 000–200 000; 30 million people in 11 countries are protected from damaging ocular and skin lesions, 10 million children born since 1974 are at no risk of blédness and 1.5 million people originally infected are no longer infected.

14Elsewhere in Africa. Table 3 provides estimates of the number of infected and blind individuals (where known) in countries in Africa outside the OCP area.

15Vector control has been successful in the elimination of populations of Simulium damnosum at the Owen Falls Dam, Jinja, in Uganda, while Simulium neavei was eradicated from a focus in North Nyanza, Kenya using only seven treatments of DDT at 0.5-2.0 ppm at intervals of 10 days19. Combined ivermectin treatment and vector control have been used with great effect in the Itwara focus, in the Kabarole District of Uganda20. Annual ivermectin treatments since 1991 immediately reduced infection rates in the vector S. neavei, but after two months, rates began to increase, indicating that transmission was still continuing. Consequently, in 1994-1995, treatments with temephos (Abate®) were applied at 29 dosing points every one or two months. Annual biting rates immediately fell from 3600 to 65 bites per person per year and the last S. neavei was caught on 3 August 1995, signalling the end of transmission. The ability to eradicate S. neavei and S. damnosum s.l. populations in East Africa has led APOC to include vector elimination as a part of its strategy. Clearly, if (1) Simulium populations are isolated and susceptible to temephos (or other insecticides), (2) treatment would cause limited environmental and ecological consequences, and (3) the vector is a specific local non-migratory cytoform that could be eliminated, then control is logistically feasible.

Fig. I. Prevalence of positive skin-snips in villages of the OCP before control (a) and in 1993-1996 (b) (redrawn after Ref. 4).

16J.F. Walsh (pers. commun.) has undertaken several detailed analyses of S. neavei foci in Uganda, Tanzania and Malawi to evaluate the potential for elimination of Simulium as an alternative to long-term ivermectin delivery. These studies suggest that only a limited number of such foci can be expected to match the criteria for a vector-elimination programme, and those that do will require appropriate advice on treatment, monitoring and operational issues, provided adequate assistance from local people is available. Clearly, vector elimination at a focus of onchocerciasis would reduce the duration of ivermectin distribution necessary; calculations suggest that, in the absence of vector control, ivermectin distribution would need to be carried out for around 15 years. The impact of ivermectin in such situations is to reduce the morbidity of ocular and dermal symptoms and, in East Africa, if confirmed, those of epilepsy, goitre and dwarfism (Nakalanga syndrome).

17The Americas. Onchocerciasis in the Americas is endemic in Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela. In Mexico, Guatemala and probably Colombia, the onchocerciasis-endemic areas are well delineated, but elsewhere, because they lie in extremely inaccessible regions, the full extent of the endemic area is still unknown. In Brazil, a new focus has been discovered at Minaçu on the Tocantins river 500 km north of Brasilia and 2500 km south of the previously known foci near the Venezuelan border. It is believed that the focus is a resuit of an influx of miners who had previously been working in onchocerciasis-endemic areas. The vector is probably Simulium guianense s.l.21 A new westward extension to the Southern Venezuelan focus has been found in the Unturan mountains22, and the Ecuadorian focus is intensifying and expanding owing to the extreme efficiency of the vector in that area, Simulium exiguum 23.

Table I. Estimated numbers of cases of onchocerciasis and onchocercal blindness in the seven original OCP countriesa

Year

Total population (thousands)

No. infected (thousands)

No. blind (thousands)

1970–1971

33093

1706

35.0

1984–1985

48250

1463

39.7

1995

61000

1219

29.3

Change since 1970

+84.0%

-28.5%

-16.3%

a Data from Ref. 2.

  • a Data from Ref. 4.

Table 2. Prevalences of onchocerciasis in OCP areas before and after control strategies implementeda

OCP areas

Control method and commencement date

Pre-control prevalence (%)

Post-control prevalence (%)

Area 1: western extension: Senegal, western Mali, northern Guinea and Guinea Bissau

Ivermectin only, 1988

18.8–81.9

0.0–17.0

Area 2: western extension: northern Sierra Leone

Temporarily suspended, 1987–1990

65.6–87.6

30.4-49.0

Area 3: western extension: Southern Sierra Leone

Ivermectin only, 1987–1990

60.2-88.5

39.0-74.3

Area 4: western extension: Mali, Southern Guinea

Vector control and ivermectin, 1987–1990

61.2–82.6

5.0-23.8

Area 5: Southern extension: Côte d’Ivoire

Larviciding, 1979; ivermectin, 1992

49.4-87.4

0.0-50.6

Area 6: Southern extension: Benin, Togo, Ghana

Larviciding, 1979; ivermectin distribution, 1992

60.4–85.1

14.0–64.5

Area 7: original area

1974

60.0-84.2

0.0-6.0

18The availability of the safe microfilaricide ivermectin resulted in the establishment of the OEPA following a Pan-American Health Organization resolution in 1991, with the objective of eliminating the severe manifestations of onchocerciasis in the Americas through mass distribution. In contrast to the position in Africa, vector control has not been regarded as a credible option because the vectors inhabit either myriads of small streams or, as in Brazil and Venezuela, rivers with huge discharges. In South America, the logistics and costs of vector control preclude its consideration as an approach to reducing onchocerciasis transmission. The one exception is in the San Vicente Pacaya focus in Guatemala, where temephos in various formulations, including briquettes, was applied to control Simulium ochraceum, leading to Virtual eradication of the vector and cessation of transmission24,25 . Simulium ochraceum has just begun to retum eight years after the end of control (J O. Ochoa, pers. commun.). In Central America, vector control is still an option in a few hyperendemic coffee estates where blackfly biting is also a serious nuisance. Vector control was also undertaken in the adjoining focus in Chiapas, in Mexico in the 1950s19.

19Ivermectin has thus provided a major new opportunity to reduce the morbidity associated with onchocerciasis, but the length of time that treatments will need to be continued has yet to be determined. Ivermectin distribution in the Americas is currently based on the use of communities, local structures and existing health Systems. Mexico has been using twiceyearly treatments since 1988-1989 (pre-dating OEPA) in an attempt to reduce not only morbidity but also, eventually, transmission26. In the Oaxaca focus, treatments have been so successful that there have been no new cases for several years. National plans in Brazil, Colombia, Ecuador and Venezuela seek to integrate ivermectin distribution into other interventions, such as hepatitis B vaccination, through the primary health care System. Although, in Guatemala, the stated intention was for distribution to be made twice a year in hyperendemic communities, in practice, this has hardly been attained, most receiving ivermectin only once a year. In these localities, it is unlikely that transmission will have been halted (R. Lujân and B. Morales, pers. commun.).

Surveillance and epidemiological assessment

20A workshop convened by OEPA and held in Guatemala in June 1995 to examine the transmission cycle of O. volvulus by S. ochraceum s.l. in Guatemala and Mexico concluded that (1) the low infection rates in this vector (<0.2%) rendered indexes such as the annual transmission potential and infective biting rate of questionable statistical value; however, the infection rates of all Onchocerca stages in host-seeking flies were still a feasible parameter; (2) entomological surveillance should be an integral component of the ivermectin-distribution programme, which should be carried out in meso-endemic and hyperendemic sentinel communities; and (3) research should continue into the use of DNA probes, other alternatives to fly dissections, and the development of computer models.

21For areas where skin-snipping (the removal of a portion of skin for parasitological diagnosis and collection of prevalence and incidence data) is no longer acceptable, a proxy measure of skin microfilarial density, the vector microfilarial uptake (numbers of microfilaria ingested by vectors) has been developed27. In separate studies, DNA probes on pooled samples of S. ochraceum 28 have been used in Guatemala to demonstrate a decrease in vector infection rates after ivermectin treatments (J.B. Davies et al., unpublished). Similar techniques for use with S. damnosum have been described by Katholi et al. 29 The classical parameters of epidemiological assessment used by the OCP are increasingly being re-evaluated because of (1) the variety of complex epidemiological situations within the programme; (2) the problems posed by the impact of vector control and of ivermectin treatment, which complicates the assessment of prevalence because it has reduced the number of microfilaria in the skin (microfilidermia); (3) the need for robust parameters to inform decisions on when to stop larviciding; (4) the need to provide countries with cost-effective, non-invasive methods of community diagnosis for detecting recrudescence; and (5) the need to determine how cost-effective evaluation of transmission can be achieved using crushed Simulium to replace dissection.

New approaches to chemotherapy

22Macrofilaricides. Vector-control strategies in the OCP area have been based on the need to prevent transmission for sufficiently long to allow the human reservoir of adult O. volvulus to die out because new adults are not recruited. The only intervention that could short-circuit this approach would require a safe, effective and deliverable macrofilaricide, and intensive research to identify candidate compounds with appropriate qualities has been undertaken. In recent years, the most promising target compounds have been developed through a variety of partnerships with drug companies, studied in several screens and evaluated at the Onchocerciasis Chemotherapy Centre at Hohoe, in Ghana.

23In the early 1990s30,31, the efficacy of amocarzine (CGP6140) was investigated in studies in Ecuador and Guatemala. The optimal dosages were established in different racial groups in the context of the relationships of food intake to the efficacy and side effects of the drug. These studies showed amocarzine to be an orally active drug. Four months after treatment with a regime of 3 mgkg-1 twice daily for three days (ie. a total of 18 mgkg-1), 81-88% of adult female worms and 69-82% of male worms were dead or moribund, and all intrauterine stages were degenerate. Although side effects were recorded they were regarded as acceptable31.

24Recently, Awadzi et al. 7 reported the outcome of a study of ivermectin and amocarzine treatment in 100 patients in Ghana. A detailed investigation of all clinical parameters together with a nodule biopsy after 120 days were carried out in three groups of patients receiving ivermectin alone, amocarzine alone or a combined dose of ivermectin and amocarzine. Amocarzine alone provoked Mazzoti-type allergic reactions which were more frequent than those observed with ivermectin; ivermectin pre-treatment suppressed the amocarzine-induced Mazzoti reactions. This study demonstrated that: (1) ivermectin at standard dosage produced minor macrofilaricidal effects; (2) amocarzine did not affect male worms and was a less potent macrofilaricide than was ivermectin; (3) the efficacy of a combined dose was similar to that of ivermectin alone. Awadzi and colleagues7 concluded that ’amocarzine has no role in the treatment of onchocerciasis in Africa’.

25Prophylaxis. A prospective study in Cameroon using the O. ochengi-cattle model (V. Wood et al., unpublished) provided evidence that ivermectin given at monthly intervals has a prophylactic effect. Calves from two to eight weeks old were treated monthly with ivermectin at either 200 μgkg-1 or 500 μgkg-1 for 21 months. None of 15 calves treated with ivermectin at either dose developed an O. ochengi infection, whereas five of six control untreated animais maintained in the same herd and exposed to natural challenge of O. ochengi from Simulium became infected. A total of 54 O. ochengi nodules developed and all five animals developed a microfilidermia. This resuit has important implications for the use of ivermectin in the control of human onchocerciasis; if prophylactic ivermectin can be given immediately before the control period there is likely to be a more significant effect on transmission. Ivermectin is currently distributed in the USA by Merck & Co. Inc. as a prophylactic against dog heartworm, Dirofilaria impiritis, under the name Heartgard. Such a strategy might be equally applicable in human onchocerciasis.

  • 1 Data for APOC countries from unpublished APOC Project Document; data for Americas from Ref. 2. The (...)

Table 3. Estimates of the numbers of people infected with Onchocerca volvulus and the numbers of people blind due to onchocerciasis by country in Africa outside the OCP area, and in the Americas1

  • 2 Denotes that onchocercal blindness is not considered a significant public health problem.
  • 3 Denotes that onchocercal blindness is not considered a significant public health problem.

Country

Total population (millions)

No. infected (thousands)2

No. blind (thousands)3

Angola

10.0

100

2

Burundi

5.5

143

?

Cameroon

11.8

1300

26

Central African Republic

3.0

390

19

Chad

5.7

870

20

Congo

2.3

50

0.6

Equatorial Guinea

0.4

60

?

Ethiopia

49.2

929

?

Gabon

1.2

60

?

Kenya

26.0

?

?

Liberia

2.6

600

2.6

Malawi

8.8

150

?

Mozambique

16.6

?

?

Nigeria

99.0

3302

100

Rwanda

7.8

?

7

Sudan

25.8

620

10

Tanzania

27.3

650

?

Uganda

18.8

1200

?

Zaire

35.6

4565

37.5

The Americas

140.5

0.75

Total

357

14 989

217.7

26More effective microfilaricides. The Onchocerca lienalis-cattle-mouse surrogate model32 has been used to compare the ivermectin analogues, doramectin and moxidectin, with ivermectin for their ability to clear O. lienalis microfilaria from the skin; in these studies, moxidectin was up to ten times more effective as a microfilaricide than ivermectin.

Costs

27Although OCP has been subject to probably the most rigorous financial control of any vector programme, very little mention of costs appears in the literature. A cost-benefit analysis was carried out by Benton and Skinner in 1990 (Ref. 33), who estimated that the total cost of OCP from inception in 1974 to completion in 2004 would be US$437 million (1985 dollar values) at an average annual cost of US$14 million or US$0.54 per head of the total population in the 11 OCP countries. A calculation based on the average at-risk population and a period of protection of 50 years (until 2023) yielded an annual cost of US$0.45 per person protected. More recently, Benton (pers. commun.) has calculated the annual costs of OCP to be around US$0.57 per person protected in endemic areas. This figure includes all related activites, covering administration, vector control, ivermectin delivery, training, research and development. The last item includes the search for a macrofilaricide (the’Macrofil’project). In comparison, it has already been stated that the target costs of the ivermectin-based APOC should be about US$0.20 per head per treatment.

Current status

28Those planning onchocerciasis-control strategies face the problem that there is still no effective alternative to vector control to reduce transmission by eliminating the adult worms. Studies on repeated and high-dose ivermectin therapy have proved that this strategy has no signifïcant macrofilaricidal effect34-35; thus, there is no effective macrofilaricide available for use in onchocerciasis-control programmes. Planners are therefore left with the necessity of employing treatment with ivermectin through government or local-community delivery Systems, together with the option of additional vector control, where feasible, to reduce transmission and to shorten the overall duration of the programme. At present estimates, ivermectin-delivery programmes that have a limited impact on transmission will need to be sustained for a minimum of 15 years.

Acknowledgements

29We are grateful to the members of the Expert Advisory Committee of the OCP, and the Director, K.Y. Dadzie, for support and encouragement. We also wish to thank our colleagues, mentioned in the text, who provided us with hitherto-unpublished information.

Note added in proof

30Finally, the most recent information on onchocerciasis describes monitoring and ivermectin treatment in the war conditions of South Sudan36 and in the same publication a brief note describes the endemicity and prevalence of ocular lesions amongst the Yanomami Indians of the Brazil-Venezuela border37.

Bibliographie

References

1 Stoll, N.R. (1947) This wonny world. J. Parasitol. 33, 1-18

2 World Health Organization (1995) Onchocerciasis and its control. WHO Tech. Rep. Ser. 852

3 Remme, J.H.F. (1995) The African Programme for Onchocerciasis Control: preparing to launch. Parasitol. Today 11, 403-406

4 Boatin, B. et al. (1997) Patterns of epidemiology and control of onchocerciasis in West Africa. J. Helminthol. 71, 91-101

5 Mectizan Programme Notes (1997) Mectizan Donation Program (Atlanta, USA) 17, 3

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7 Awadzi, K. et al. (1997) The safety and efficacy of amocarzine in African onchocerciasis and the influence of ivermectin on the clinical and parasitological response to treatment. Ann. Trop. Med. Parasitol. 91, 281–296

8 Poltera, A.A. et al. (1991) Onchocercacidal effects of amocarzine (cyp6140) in Latin America. Lancet i, 583-584

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10 Ngoumou, P., Walsh, J.K. and Mace, J.M. (1994) A rapid mapping technique for the prevalence and distribution of onchocerciasis: a Cameroon case study. Ann. Trop. Med. Parasitol. 88, 463-474

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37 Chaves, C. (1997) Ocular onchocerciasis in Brazil. Bull. Trop. Med. Int. Health 5, 6

Notes

1 Data for APOC countries from unpublished APOC Project Document; data for Americas from Ref. 2. The total population at risk in the Americas cannot be accurately estimated.

2 Denotes that onchocercal blindness is not considered a significant public health problem.

3 Denotes that onchocercal blindness is not considered a significant public health problem.

Notes de fin

a Data from Ref. 4.

Table des illustrations

Légende Fig. I. Prevalence of positive skin-snips in villages of the OCP before control (a) and in 1993-1996 (b) (redrawn after Ref. 4).
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Auteurs

David Molyneux and John Davies are at the Liverpool School of Tropical Medicine, Pembroke Place, Liverpool. UK L3 5QA Tel:+44 151 708 9393, Fax: +44 151 708 8733, e-mail: fahy@liv.ac.uk

David Molyneux and John Davies are at the Liverpool School of Tropical Medicine, Pembroke Place, Liverpool. UK L3 5QA Tel:+44 151 708 9393, Fax: +44 151 708 8733, e-mail: fahy@liv.ac.uk

© IRD Éditions, 2003

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