Effect of repeated treatments with ivermectin on the incidence of onchocerciasis in northern Cameroon.

Mass treatments with ivermectin have been undertaken each year since 1987 in an area hyperendemic for onchocerciasis in northern Cameroon. The impact of these successive treatments on the incidence of infection in humans was evaluated by comparing the prevalence of skin microfilariae (PMF) and the mean microfilarial skin densities (MFD) observed in 1987 and 1992 in 5-7-year-old children who had never taken the drug but who were members of the treated communities. In 1992, the PMF and the MFD in children in this age group who never received ivermectin were reduced by 55% and 77%, respectively, in comparison with the values observed in 1987, before the first treatment round. These results reflect a pronounced reduction in the intensity of the transmission of Onchocerca volvulus in the treatment zone. The influence of the ivermectin treatment coverage in the human population, as well as the vectorial capacity and the dispersal of the vector blackflies, on the transmission of onchocerciasis is discussed.

Impact of combined large-scale ivermectin distribution and vector control on transmission of Onchocerca volvulus in the Niger basin, Guinea.

As part of the WHO Onchocerciasis Control Programme in West Africa (OCP), the attack phase of operations in the Niger basin in Guinea began in 1989 with the simultaneous use of ivermectin and vector control. Larvicide applications coupled with annual large-scale ivermectin distribution have greatly reduced blackfly infectivity (by 78.8% for the number of infective larvae per 1000 parous flies). The combination of vector control and ivermectin has permitted excellent control of transmission. In the original OCP area, it took 6-8 years of vector control alone to obtain an equivalent decrease in blackfly infectivity. For the same number of flies caught, transmission was much higher in areas where ivermectin had not been distributed. The combined use of ivermectin and vector control has opened up new prospects for carrying out OCP operations with, notably, the possibility of reducing larviciding operations.

PIP: As part of the World Health Organization Onchocerciasis Control Programme in West Africa (OCP), the attack phase of operations in the Niger basin in Guinea began in 1989 with the simultaneous use of ivermectin and vector control. All the 16 catching points were in holoendemic foci: 8 in the Niger basin in Guinea and 8 in the original OCP area (Mali, Ivory Coast, Ghana, and Burkina Faso). The data were analyzed according to prevalence of microfilariae in the skin and the mean community microfilarial load (CMFL). Between 1990 and 1992 the number of people in the villages treated increased by a factor of 6. In 1992 a total of 91,840 persons were treated in 550 villages. The study covered 10 years, during which 34,492 blackflies were caught at the 8 sites, 87.8% of which were parous. Larvicide applications coupled with annual large-scale ivermectin distribution had greatly reduced blackfly infectivity (by 78.8% for the number of infective larvae per 1000 parous flies; the number infective larvae in the head fell by 75.7% compared with the 1986-87 data before treatment began). After 2 years of large-scale ivermectin treatment, the reduction was 64.6%. In February and March of 1992 a defective larvicide worsened the situation. The average transmission potential during this period in Guinea was 7.3 compared with 93.7 for the original area. For the same number of blackflies caught, transmission in the original area was 5.6 times higher. The combination of vector control and ivermectin permitted excellent control of transmission. In the original OCP area, it took 6-8 years of vector control alone to obtain an equivalent decrease in blackfly infectivity. For the same number of flies caught, transmission was much higher in areas where ivermectin had not been distributed. The combined use of ivermectin and vector control has opened up new prospects for carrying out OCP operations with the possibility of reducing larviciding operations.

[The necessity of conducting to the end the activities of vector control in the onchocerciasis control program in west Africa: recall of the stakes and proposal of a minimum budget for the period 1998-2002]. / De la nécessité de mener à terme les activités de lutte antivectorielle du programme de lutte contre l'onchocercose en Afrique de l'Ouest: rappel des enjeux et proposition d'un budget a minima pour la période 1998-2002.

After a brief presentation of the Onchocerciasis Control Programme in West Africa (OCP), the authors realize the health and socioeconomic consequences that could follow a cessation of larvicide treatment before 2002 in the south-eastern and western extensions of the Programme. Taking into account that OCP activities are theoretically supported until 1997, but aware of financial constraints that will probably increase from now to 2002, this paper proposes an a minima estimation of the residual vector control activities for a "phasing out" spread out for five years (1998-2002). These estimations essentially concern the larvicide coverage, the insecticides used, the entomological surveillance, the logistical support and their financial aspects. As far as 48 U.S. $ million amount for 5 years are concerned, the budget allocated for vector control activities should not exceed the third of the global amount allocated to OCP for the actual fourth financial phase of the Programme (1992-1997).

[ Onchocerciasis control program in West Africa: socioeconomic development and risk of recrudescence of transmission. 1. Experimental study of the transmission of Onchocerca volvulus strains from Southwestern Sierra Leone by Simulium sirbanum]. / Le programme de lutte contre l'onchocercose en Afrique de l'ouest: développement socio-économique et risque de recrudescence de la transmission. 1. Etude éxperimentale de la transmission des souches d'Onchocerca volvulus du sud-ouest de la Sierra Leone par Simulium sirbanum.

As part of the return of savanna migrants installed since a long time in forest regions, in the south of Sierra Leone, we carried out an experimental study about a cross-transmission between Simulium sirbanum from Missira (West-Mali) and the forest strain of Onchocerca volvulus in the south-west of Sierra Leone. This study will allow to know if there is a risk of onchocerciasis transmission recrudescence in relation to the reinstallation of these migrants in their native region. Because of the very high limitation to the forest strain of O. volvulus microfilariae output of the peritrophic membrane reduction with savanna black-flies and according to the very low mature parasite out put of S. sirbanum with this strain observed along this experimentation, the forest strain of O. volvulus from the south Sierra Leone appears maladjusted to S. sirbanum, the main vector of onchocerciasis in savanna regions. This observation implicates a very low intensity of transmission for this forest strain by savanna onchocerciasis vectors. The return of savanna migrants in their native region, installed in the south Sierra Leone since several decades, could not be, in a short time, an origin of onchocerciasis recrudescence in savanna regions of the Onchocerciasis Control Programme area cleaned by an effective vector control carried out since 1975 sustained now by a chemotherapeutic treatment reducing the human parasite reservoir. However, the preservation of this acquired necessitates an epidemiological supervision increased, because the interactions between the vector and the parasite for a long time could carry away a mutual adaptation and a sickness recrudescence.

[Importance and role of spreading larvicides on the soil in river beds for the control program against onchocerciasis in west Africa]. / L'importance et le rôle des épandages larvicides au sol dans l'aire du Programme de lutte contre l'onchocercose en Afrique de l'Ouest.

The control of blackfly is based on larvicide spraying in rivers where the insects breed and their larval stages are vulnerable. The first technique for large-scale operations, consists of using aircraft in the same way as for mosquito control and crop protection operations. The second technique, which will be discussed in this paper, consists of ground treatment of rivers, either directly with a container or with a spraying pump or by boat spraying insecticide in cross strip. In areas of West Africa where onchocerciasis is still prevalent, ground treatment is done to support aerial operations and to a lesser extent to control nuisance in areas where the endemicity of the disease is low. In onchocerciasis-freed zones, control of blackfly aims only at suppressing the nuisance in order to enhance the socio-economic development in areas at unfair disadvantage. This distinction is important for determining the responsibilities of various groups. Vector control to interrupt the transmission of onchocerciasis is the mandate of OCP while the control of nuisance is the responsibility, depending on the circumstances, of the private sector, the government and/or village communities. In this paper, we have attempted to highlight the objectives of such treatments, the responsibilities of various groups and the prospects in West African countries located in the Onchocerciasis Control Programme area. We also present the situation of ground larviciding in countries outside the Programme, in temperate as well as tropical conditions, in order to provide some basis for the development of ground treatment strategies in the OCP area.

Criteria for the selection of larvicides by the Onchocerciasis Control Programme in west Africa.

One of the weekly decisions the Onchocerciasis Control Programme has to make, in its operations in 11 west African countries, is the selection of one insecticide out of the six used that is most appropriate to the river stretches to be treated. This decision depends on several criteria, linked not only to the compounds themselves but also the hydrological conditions and blackfly populations involved. Given the great number of breeding sites (gites) to be treated, in 23,000 km of rivers at the height of the rainy season, this paper identifies the parameters needed to facilitate the choice of insecticide and to optimize the larviciding in terms of cost effectiveness, management of resistance and minimizing the environmental impact.

[Long-term effect of a single dose of ivermectin on skin microfilarial density in an endemic onchocerciasis area of North Cameroon]. / Effet à long terme d'une prise unique d'ivermectine sur les charges microfilariennes dermiques dans un foyer d'onchocercose du Nord-Cameroun.

The long-term efficacy of a single dose of ivermectin (150 micrograms/kg) on skin microfilarial densities was evaluated by comparing parasite levels, before and after treatment, in 192 patients living in an area endemic for onchocerciasis in North Cameroon. A mass treatment with ivermectin has been undertaken in 1989 in two villages, where pretreatment community microfilarial loads (CMFL) were 5.5 and 25.1 microfilariae per snip. The parasitological examination carried out 32 months later showed that the microfilarial densities in the adults where only 24 and 51% of the initial values, respectively. In a third village, treated once in 1989 but located in an area where annual mass distributions of ivermectin have been undertaken since 1987, the mean microfilarial density 28 months after treatment was only 12% of the initial value. In 1992, the mean microfilarial load in children had decreased significantly in comparison with those of 1989 only in the village where the level of endemicity was initially the lowest. The rate of increase in the number of skin microfilariae after one dose of ivermectin is related with the pretreatment endemicity level. The impact of large-scale ivermectin treatments on the transmission of onchocerciasis enhances the long-term efficacy of this drug on the skin microfilarial densities.

[Campaign against Culex quinquefasciatus using Bacillus sphaericus: results of a pilot project in a large urban area of equatorial Africa]. / Lutte contre Culex quinquefasciatus par Bacillus sphaericus: résultats d'une campagne pilote dans une grande agglomération urbaine d'Afrique équatoriale.

Culex quinquefasciatus, which is sometimes the vector of Bancroft's filariasis, is a harmful mosquito, the immature stages of which live in collections of waste water resulting from human activity. Larval control, the most appropriate method, is at present carried out with chemical insecticides. But the toxicity of these compounds, together with phenomena of resistance, and the cost of substitute insecticides have turned research towards products of biological origin, and one of the most promising is a liquid concentrate of Bacillus sphaericus strain 2362. This was applied experimentally over an area of 200 hectares in a large city in the south of Cameroon, characterized by a short dry season during which mosquito density is at its highest. Spraying was carried out every three months for a year in a concentration of 10 g/m2 and its efficacy evaluated at the level of adult mosquitos through an indirect system of capture on human baits. The results of this study show, in essence, a reduction by 52.7% in the number of females captured in the overall study area treated, and that the impact of treatment is greater during the period of high mosquito density (55.1%) than in the low density period (40.7%). After analysis of the full set of results, the authors conclude that the pattern of rainfall, the conditions in which the insecticide is applied and reinvasions of mosquitos from untreated areas were the principal factors limiting the efficacy of this control campaign.(ABSTRACT TRUNCATED AT 250 WORDS)

[Parasitological efficacy of repeated treatments with ivermectin in an onchocerciasis focus in north Cameroon]. / Efficacité parasitologique de traitements répétés par l'ivermectine dans un foyer d'onchocercose du Nord-Cameroun.

321 adults living in an hyperendemic onchocerciasis focus in North-Cameroon who received a first dose of ivermectin (150 micrograms/kg) in 1987 and a total of three, four or five doses from 1987 to 1991 were parasitologically examined in 1992. The prevalence of skin microfilariae (PMf) and the geometric mean microfilarial load (GMMf) were respectively reduced by 33 and 97% from the initial values in the group of subjects who received five treatments. In 1992, the PMf and the GMMf were not significantly different in the groups which received three, four or five doses of ivermectin. It is not necessary to achieve an important drug coverage to maintain a tolerable level of infestation in the community.

[National entomological teams of the western extension zone of the Onchocerciasis Control Program (OCP) in west Africa from 1986 to 1990]. / Equipes nationales entomologiques de la zone d'extension ouest du programme de lutte contre l'onchocercose en Afrique de l'ouest (OCP) de 1986 à 1990.

The western extension area of the Onchocerciasis Control Programme in West Africa (OCP) covers five countries: Guinea, Guinea-Bissau, Mali (western part), Senegal and Sierra Leone. From 1986 to 1990, national teams employed by the respective governments have been regularly collecting entomological data on the vectors of onchocerciasis in these countries. As in the initial programme area of the OCP, the entomological surveillance network was composed of entomological sectors and subsectors (the latter are called "operational bases" in the western extension). In 1990, 308 staff in 47 capture teams were employed for the entomological surveillance activities in seven sectors and twenty-five operational bases. They included a national coordinator as head of the teams for each country, a wide range of technicians, and administrative support staff to assist the national coordinator in the overall management of available resources. The national teams worked under the technical responsibility and supervision of WHO/OCP but with no employee/employer relationship between them and WHO, since they were employed and their salaries were paid by their governments. The OCP, however, paid additional lump sum allowances to each worker, as well as daily subsistence allowances when away from their duty station. Vehicles, entomological equipment, office supplies and furniture, fuel and lubricants were provided by WHO/OCP. Despite the difficulties encountered in the field, which were often great, and their lower salaries (compared with colleagues paid by WHO/OCP), the technical workers in the national teams performed well by OCP standards, with results as satisfactory as those obtained by the WHO/OCP teams in the rest of the programme area. The main reasons for the efficiency and dynamism of the national entomological teams are described. The future of these teams after OCP has ceased its activities is also discussed.

Twenty-five years of blackfly control in a localized moist forest area of Cameroon: a review.

The high number of blackfly bites in a moist forest area of Cameroon, around the Sanaga river, was causing considerable nuisance and had led to a local ground-based larval control campaign. We have reviewed the 25 years of almost uninterrupted weekly larvicide applications in this area and emphasized the problems related to application difficulties, environmental pollution and blackfly resistance to the insecticides. Although the number of larvicidal compounds readily available is now limited, the future of blackfly control at this site does not seem to pose real technical difficulties because of its economic importance (hydroelectric dam) and the progress made in the fields of resistance management, search for new insecticide molecules and development of new control methods.

[Control of onchocerciasis with ivermectin: results of a mass campaign in northern Cameroon]. / Lutte contre l'onchocercose par ivermectine: résultats d'une campagne de masse au Nord-Cameroun.

A mass chemotherapy campaign to treat onchocerciasis with ivermectin was carried out in northern Cameroon, in the North Vina valley, a savanna area that is part of the extended Vina-Pendé-Longone focus. More than 20,000 people aged five years and over were treated at least once and more than 30,000 treatments were distributed. One to four treatments were given at intervals of six months or one year. Clinical and parasitological examinations of representative samples of the treated population and full ophthalmological examinations of males aged 15 to 35 years were carried out before each treatment to assess the efficacy of the different protocols of treatment. Secondary effects were recorded daily during the week after administration of ivermectin in the whole population treated. The evolution of arterial blood pressure was monitored over seven days in about 300 adults living in the hyperendemic region. The impact of mass treatment on natural transmission of the disease was evaluated by daily capture and dissection of blackflies during the month before and the two months after treatment. All levels of onchocerciasis endemicity were represented in the study area (maximum community microfilarial load (CMFL), 300 microfilariae per biopsy). Six months and one year after the first treatment, loads were down by more than 90% and more than 60% respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

[Evaluation of control measures against Simuliidae in the framework of the OCP (Onchocerciasis Control Program)]. / Evaluation des moyens de lutte contre les simulies dans le cadre du programme OCP (Onchocerciasis Control Programme).

To control the vector S. damnosum s.l., the Onchocerciasis Control Programme in West Africa (OCP) uses larvicides which are sprayed mainly by air. Despite the more or less reversible resistance of some species of the S. damnosum complex to the organophosphorus compounds (temephos and chlorphoxim), the Programme has succeeded in finding effective replacement insecticides. These larvicides, which are used in rotation, taking into account the river discharges, blackfly species present, and their susceptibility to the different insecticides, have made it possible to maintain the good results obtained by the Programme. Through vector control alone, OCP has been able to bring the disease under control in more than 90% of the initial area. Resettlement is taking place on the fertile lands which were formerly deserted because of onchocerciasis and many socioeconomic development projects are currently being carried out. The extensions made towards the western and the southern parts of the Programme area and the use of ivermectin, a microfilaricide, will enable these results to be improved further.