[Efficacy against Malaria Vectors of Long-Lasting Insecticide-Treated Mosquito Nets after 17 Months of Use in the City of Bouaké, in the Center of Côte d'Ivoire]

Malaria remains a public health problem in Côte d'Ivoire despite a 95% coverage rate with long-lasting insecticide-treated mosquito nets (LLINs) since 2015. A study was conducted in Bouaké to determine the residual efficacy of LLINs used during 17 months (N = 30) and of stored and unpacked LLINs (N = 8). The physical integrity of LLINs and their biological efficacy on populations of Anopheles gambiae (sensitive and resistant to insecticides) were assessed. The study revealed that 57% (17/30) of LLINs recovered from households have a hole. The proportional hole indices indicated that 70% (21/30) of LLINs were in good condition, 20% (6/30) were repairable and 10% (3/30) were unusable.This study also showed that the LLINs used and stored were effective on the sensitive Kisumu strain but ineffective on resistant populations of An. gambiae of Bouaké. Average knock-down and mortality rates induced by LLINs used on the populations of An. gambiae from Bouaké were 23% and 9%, respectively. The resistance of mosquitoes to insecticides and the alteration of the physil integrity of mosquito nets are probably major factors to be taken into account in estimating the efficacy of LLINs in the field.

Efficacy of mosquito nets treated with a pyrethroid-organophosphorous mixture againstKdr- andKdr+ malaria vectors (Anopheles gambiae)

In order to prevent the resistance of Anopheles gambiae s.l. to pyrethroids from spreading too quickly and to lengthen the effectiveness of insecticide impregnated mosquito nets, it has recently been suggested to use mixtures of insecticides that have different modes of action. This study presents the results obtained with tulle mosquito nets treated with bifenthrin (a pyrethroid) and chlorpyrifos-methyl (an organophosphorous) both separately and in mixture on two strains of An. gambiae, one sensitive to all insecticides, and the other resistant to pyrethroids. The values of KDt50 and KDt95 and the mortality induced with the mixture of bifenthrin (25 mg/m2) and chlorpyrifos-methyl (4.5 mg/m2) show a significant synergistic effect on the strain of An. gambiae susceptible to insecticides. However, the tested combination does not induce any synergistic effect on the VKPR strain selected with permethrin, but only enhances the effectiveness of the two insecticides taken separately.

[Guidelines for phase III evaluation of vector control methods against malaria]

Most new vector control methods against malaria involve the use of pesticides. Prior to release of these products for general use, their efficacy, persistence, and cross-resistance must be tested on mosquito colonies raised in the laboratory (phase I) then on wild mosquitoes in the field (small-scale), individual dwellings, or experimental huts (phase II). The goal of phase III studies is to evaluate the efficacy and effectiveness of the vector-control product or method against malaria in a population at regular risk for transmission. The main objective of phase III tests is to measure the epidemiologic impact, e.g. on the incidence or prevalence of malaria in humans. This article presents guidelines for carrying out phase III tests of vector-control methods against malaria (e.g. home insecticide spraying or insecticide-impregnated bednet use). It was written by participants in a workgroup formed to define recommendations for the WHOPES (WHO Pesticide Evaluation Scheme).

Characterization of insensitive acetylcholinesterase (ace-1R) in Anopheles gambiae (Diptera: Culicidae): resistance levels and dominance

Characterization of insecticide resistance provides data on the evolutionary processes involved in the adaptation of insects to environmental changes. Studying the dominance status and resistance level represents a great interest, in terms of understanding resistance evolution in the field to eventually adapt vector control. Resistance and dominance levels conferred by the G119S mutation of acetylcholinesterase (ace-1R) of the mosquito Anopheles gambiae s.s. (Diptera: Culicidae) were studied for various insecticides belonging to different classes, using strains sharing the same genetic background. Our survey shows that the homozygote resistant strain AcerKis displayed a very high resistance level to various carbamates (range 3,000- to 5,000-fold) compared with that of various organophosphates (range 12- to 30-fold). Furthermore, the dominance status varied between semi-recessivity with fenitrothion and chlorpyrifos methyl insecticides to semidominance with temephos, carbosulfan, and propoxur. These results indicate that this resistance mechanism could spread rapidly in the field and then compromise the use of organophosphate and carbamate compounds in public health. This study underlines the necessity to monitor the ace-1R mutation in natural populations before planning and implementing malaria control programs based on the use of these insecticides.

Multiple insecticide resistance mechanisms in Anopheles gambiae and Culex quinquefasciatus from Benin, West Africa

Because free-insecticide treated net distribution is planned in Benin (West Africa) during the next few years, we investigated the type, frequency and distribution of insecticide resistance mechanisms in Anopheles gambiae and Culex quinquefasciatus mosquitoes in four localities selected on the basis of contrasting agricultural practices, use of insecticides and environment. Bioassays with WHO diagnostic test kits were carried out using pyrethroid, carbamate, organophosphate and organochlorine insecticides. An. gambiae mosquitoes were identified to species and to M or S molecular forms using PCR techniques. Molecular and biochemical assays were carried out to identify kdr and Ace.1 mutations in individual mosquitoes and to detect any increase in the activity of enzymes typically involved in insecticide metabolism (oxidase, esterase and glutathion-S-transférases). WHO diagnostic tests showed high frequency of resistance in An. gambiae and Cx. quinquefasciatus to permethrin and DDT in three areas. This was consistent with the presence of target site insensitivity due to kdr mutation and to increased metabolism through enzymatic activity. Kdr was expressed in both M and S forms. However, less than 1% of An. gambiae or Cx. quiqnuefasciatus showed the presence of the Ace.1(R) mutation. Carbamate/OP resistance was present at higher frequency in Culex than in An. gambiae. Dieldrin resistance was present in both species at all four localities. A higher frequency of pyrethroid-resistance was found in An. gambiae mosquitoes collected in urban areas compared to those collected in rice growing areas. The expansion of vegetable growing within urban areas probably contributed to selection pressure on mosquitoes. The detection of multiple resistance mechanisms in both An. gambiae and Cx. quinquefasciatus in Benin may represent a threat for the efficacy of ITNs and other forms of vector control such as indoor residual spraying in the future.

A biological test to quantify pyrethroid in impregnated nets

First instar larvae of Aedes aegypti (L.) (Diptera: Culicidae) are very susceptible to deltamethrin, which kills all larvae at a very low dose (0.1 microg/l). Thus the sensitivity of this insect to detect that pyrethroid is in parts per billion, as obtained with most common chromatographic methods of analysis. Here we describe a biological test (BT) to quantify deltamethrin in long-lasting insecticidal nets (LLINs) by exposing first instar larvae of Ae. aegypti to serial extracted insecticide solutions from net samples. The deltamethrin concentration in the net was calculated at the doses killing 50% of larva, from the LC(50) of deltamethrin (6.5 x 10(-5) mg/l) and the dilution factor (DF(50)) of the extracted net solution. The pyrethroid quantification in LLINs after 0-25 washes with this BT was correlated with those obtained by direct chromatographic analysis (r(2) = 0.84). This BT did not require sophisticated equipment and could be extended to other molecules and materials. It appeared accurate, robust, cheap and well adapted to the national malaria programmes as the eggs of Ae. aegypti might be used for some months. This method was adapted to provide an easy to use kit test for the quality control of LLINs in the field.

Dyeing process may alter the efficacy of insecticide-treated nets

The biological efficacy as judged by mortality against Anopheles gambiae s.s. of three net samples, one white and two colored, treated with deltamethrin by conventional dipping was compared. Efficacy as well as chemical analysis results showed that uptake of insecticide by white net material was much higher than by colored nets. After a single wash, efficacy of colored nets was reduced significantly below 80% mortality, which is the minimum acceptable level for a field application. This unexpected result could be because of either low uptake of insecticide by colored fibers or high alkalinity on the fibers, resulting from the dyeing process (reduction clearing). alpha-Cyanopyrethroids, such as deltamethrin, have a low stability in alkaline conditions and rapidly degrade. Practical implications of this finding are of primary importance because the majority of nets currently purchased by institutional buyers for malaria prevention are colored.

Efficacy of mosquito netting for sustainable small holders' cabbage production in Africa

The efficacy of a mosquito netting to protect cabbages, Brassica oleracea L., against pests was investigated in field trials in Benin, West Africa. A polyester net covered the plants at night by using a wood armature. The net was removed during the day to prevent overheating and excessive shade, both problems of insect-proof screens used under tropical conditions. The number of all lepidopteran larvae with netting protection and foliar insecticide sprays was significantly lower than the unprotected control. The number of diamondback moth, Plutella xylostella (L.), was significantly lower with netting protection compared with foliar insecticide sprays and control. Netting treated with deltamethrin gave total protection of young plants against the aphid Lipaphis erysimi (Kaltenbach). At harvest, the number of marketable cabbages protected with untreated netting was significantly higher compared with the production with foliar insecticide sprays. The protection of cabbages with netting can be an economically viable method. Considering the price of cabbages on local markets (US dollars 1/unit), the net returns per 100 m2 were US dollars 247 by using netting, US dollars 149 by using insecticides, and US dollars 117 for controls. The net returns for using netting are based on replacing the netting each crop cycle. But netting can be reused several times, depending upon conditions, increasing the profit margin. The netting protection may be an alternative to the growing unsustainable practices of vegetable cropping in peri-urban areas of tropical countries.

[Effect of pyrethoid and organophosphate bi-treated nets on the behaviour of Kdr+ Anopheles gambiae mosquitoes]

The efficacy of mosquito nets treated with bifenthrin (pyrethroid) and chlorpyrifos-methyl (organophosphate), alone and in mixture, was evaluated against the pyrethroid-resistant strain (VKPR) of Anopheles gambiae using the tunnel test technique. Results first showed that bifenthrin and chlorpyrifos-methyl kept a good excito-repellency effect against resistant mosquitoes despite the low doses applied on nets. With the mixture, irritancy significantly increased, which consequently limited the duration of exposure of females to impregnated surface. This phenomenon most probably explained the absence of synergism between bifenthrin and chlorpyrifos-methyl concerning the insecticidal activity of this mixture. This study demonstrated the importance of the mosquito behaviour in the global effectiveness of insecticide treated materials and in the development of alternative strategies for vector control. Concerning the prospective insecticide mixtures, not only will we have to consider the insecticidal properties of each insecticide selected, but also their excito-repellency which affects the time of duration of mosquitoes on the treated surface.

Perspectives on the bacterial control of vectors in the tropics

Biological agents have not found wide application in the control of insects that transmit agents of human disease. Fungal pathogens may have a potential for field use, but environmental constraints, lack of knowledge of their life cycle, or dependence on production in vivo, have prevented their widespread use. In fact, sporogenous bacteria are the only biological agents used on a large scale for the control of mosquitoes and blackflies that transmit diseases in the tropics. Two bacteria, Bacillus thuringiensis sp. israelensis and B. sphaericus are very effective for the control of these vectors. In this article, Jean Marc Hougard and Christian Back discuss various factors associated with the use of bacteria for vector control, and the prospects for further developments.

Twenty-two years of blackfly control in the onchocerciasis control programme in West Africa

Twenty-two years after the launch of the Onchocerciasis Control Programme in West Africa (OCP), Jean-Marc Hougard and colleagues critically review the vector-control strategy adopted. They go on to identify the few hydrological basins where transmission of the infection remains difficult to control, to analyse the causes and to propose appropriate corrective measures on a case-by-case basis. Most of these measures, which are mainly based on ivermectin chemotherapy, will continue to be applied after the end of the OCP in 2002, under the control of the countries concerned.

The spread of the Leu-Phe kdr mutation through Anopheles gambiae complex in Burkina Faso: genetic introgression and de novo phenomena

During extensive sampling in Burkina Faso and other African countries, the Leu-Phe mutation producing the kdr pyrethroid resistance phenotype was reported in both Anopheles gambiae ss and A. arabiensis. This mutation was widely distributed at high frequency in the molecular S form of A. gambiae while it has been observed at a very low frequency in both the molecular M form and A. arabiensis in Burkina Faso. While the mutation in the M form is inherited through an introgression from the S form, its occurrence is a new and independent mutation event in A. arabiensis. Three nucleotides in the upstream intron of the kdr mutation differentiated A. arabiensis from A. gambiae ss and these specific nucleotides were associated with kdr mutation in A. arabiensis. Ecological divergences which facilitated the spread of the kdr mutation within the complex of A. gambiae ss in West Africa, are discussed.

First report of a kdr mutation in Anopheles arabiensis from Burkina Faso, West Africa

The leu-phe kdr mutation was detected in a specimen of Anopheles arabiensis during an extensive survey of pyrethroid resistance in An. gambiae s.l. in Burkina Faso. The detection of this mutation in An. arabiensis, which had so far been observed only in An. gambiae s.s., is important at both epidemiologic and fundamental levels. It can be useful to understand the history of this gene throughout the range of An. gambiae complex.

Efficacy of mosquito nets treated with insecticide mixtures or mosaics against insecticide resistant Anopheles gambiae and Culex quinquefasciatus (Diptera: Culicidae) in Côte d'Ivoire

Only pyrethroid insecticides have so far been recommended for the treatment of mosquito nets for malaria control. Increasing resistance of malaria vectors to pyrethroids threatens to reduce the potency of this important method of vector control. Among the strategies proposed for resistance management is to use a pyrethroid and a non-pyrethroid insecticide in combination on the same mosquito net, either separately or as a mixture. Mixtures are particularly promising if there is potentiation between the two insecticides as this would make it possible to lower the dosage of each, as has been demonstrated under laboratory conditions for a mixture of bifenthrin (pyrethroid) and carbosulfan (carbamate). The effect of these types of treatment were compared in experimental huts on wild populations of Anopheles gambiae Giles and the nuisance mosquito Culex quinquefasciatus Say, both of which are multi-resistant. Four treatments were evaluated in experimental huts over six months: the recommended dosage of 50 mg m(-2) bifenthrin, 300 mg m(-2) carbosulfan, a mosaic of 300 mg m(-2) carbosulfan on the ceiling and 50 mg m(-2) bifenthrin on the sides, and a mixture of 6.25 mg m(-2) carbosulfan and 25 mg m(-2) bifenthrin. The mixture and mosaic treatments did not differ significantly in effectiveness from carbosulfan and bifenthrin alone against anophelines in terms of deterrency, induced exophily, blood feeding inhibition and overall mortality, but were more effective than in earlier tests with deltamethrin. These results are considered encouraging, as the combination of different classes of insecticides might be a potential tool for resistance management. The mixture might have an advantage in terms of lower cost and toxicity.

Natural swarming behaviour of the molecular M form of Anopheles gambiae

In Anopheles gambiae, as in most species of mosquitoes, mating is initiated in flight. The males aggregate in aerial swarms and conspecific females individually fly to these swarms where they mate with males. In this study, we investigated the swarming behaviour of A. gambiae and conducted 2 surveys in the rice field area of the Vallée du Kou in Burkina Faso in 1999 and 2002. A high number of anopheline mosquitoes were observed in this area and both molecular M and S forms of A. gambiae were found in sympatry. Swarms formed a few minutes after sunset in different places and no obvious markers were associated with their occurrence. However, swarms occurred close to cow herds generally in open flat areas, 2-3 m above the ground. Overall, 2829 anopheline mosquitoes were collected from 21 swarms composed primarily of males. A few specimens of Culex quinquefasciatus were collected from 3 swarms. Although both molecular M and S forms were found in sympatry in the village, swarms were composed almost exclusively of the molecular M form. This suggests that there are alternative swarming habits for both molecular M and S forms of A. gambiae in nature.

First report of the kdr mutation in Anopheles gambiae M form from Burkina Faso, west Africa

The kdr mutation, conferring resistance to pyrethroid insecticides, has been reported in several West-African populations of Anopheles gambiae S form and in the M form populations from tropical forest of Benin. We report the finding of a single M specimen collected in the rice-field area of Vallée du Kou (Burkina Faso) showing the mutation at the heterozygous state. The monitoring of kdr mutation in An. gambiae forms/species is of paramount importance to implement effective malaria control tools and may greatly improve the knowledge of the relationship between and within An. gambiae populations.

Deforestation and the spatio-temporal distribution of savannah and forest members of the Simulium damnosum complex in southern Ghana and south-western Togo

Spatio-temporal data on cytotaxonomic identifications of larvae of different members of the Simulium damnosum complex collected from rivers in southern Ghana and south-western Togo from 1975 until 1997 were analysed. When the data were combined, the percentages of savannah blackflies (S. damnosum sensu stricto and S. sirbanum) in the samples were shown to have been progressively increasing since 1975. The increases were statistically significant (P < 0.001), but the rates of increase were not linear. Further analyses were conducted according to the collection seasons and locations of the samples, to account for possible biases such as savannah flies occurring further south in the dry season or a preponderance of later samples from northern rivers having more savannah flies. These analyses showed that the increasing trend was statistically significant (P < 0.0001) only during the periods April to June and October to December. The presence of adult savannah flies carrying infective larvae (L3) indistinguishable from those of Onchocerca volvulus in the study zone was confirmed by examinations of captured flies. The percentages of savannah flies amongst the human-biting populations and the percentages with L3s in the head were higher during dry seasons than wet seasons and the savannah species were found furthest south (5 degrees 25'N) in the dry season. Comparisons of satellite images taken in 1973 and 1990 over a study area in south-western Ghana encompassing stretches of the Tano and Bia rivers demonstrated that there have been substantial increases in urban and savannah areas, at the expense of forest. This was so not only for the whole images but also for subsamples of the images taken at 1, 2, 4, 8 and 16 km distant from sites alongside the River Tano. At every distance from the river, the percentages of pixels classified as urban or savannah have increased in 1990 compared with 1973, while those classified as degraded or dense forest have decreased. The possibility that the proportionate increases in savannah forms of the vectors of onchocerciasis, and hence in the likelihood of the transmission of savannah strains of the disease in formerly forested areas, were related to the decreases in forest cover is discussed.

[An experimental tool essential for the evaluation of insecticides: the testing huts]

The following study analyses the potentialities of the experimental huts built in M'be Valley (Côte d'Ivoire) where the evaluations of the insecticide products have been carried out for many years in line with the WHOPES protocol on the methodology of stage 2 assays. Starting a testing station first requires a good knowledge of the sensitivity of Anopheles gambiae to the main insecticide families. Then thanks to the experimental huts the efficacy of the various means of treatment can be compared with the one in untreated huts; this study focuses on house spraying using 100 mg a.i./m2 and bednets impregnated with lambda-cyhalothrin at a dose of 15 mg a.i./m2. The fipronil used in house spraying doesn't show any repellent effect, however it does have an irritating effect that increases the natural exophily of An. gambiae females entering the testing huts. The blood-feeding rate recorded in the treated huts was reduced to 24% and to 38% mortality rate consisting mainly of a 24 hours delayed mortality. The bednets treated with lambda-cyhalothrin have greatly reduced the contact between man and vector since the entry rate of An. gambiae females was cut down by 68% compared to the control. The exophily of this anopheles was twofold greater with the impregnated bednets and the blood-feeding rate reduced to 47%. Finally the global mortality rate, two thirds of immediate mortality, one third of delayed mortality, reached 35%. The experimental huts in the M'be Valley therefore provide essential information regarding the selection of the most efficacious insecticides against An. gambiae. This experimental method must be extended to other sites in order to finalize ever more selective and appropriate means of control against nuisance and disease-vector mosquitoes.

Insecticide mixtures for mosquito net impregnation against malaria vectors

Insecticides belonging to the pyrethroid family are the only compounds currently available for the treatment of mosquito nets. Unfortunately, some malaria vector species have developed resistance to pyrethroids and the lack of alternative chemical categories is a great concern. One strategy for resistance management would be to treat mosquito nets with a mixture associating two insecticides having different modes of action. This study presents the results obtained with insecticide mixtures containing several proportions of bifenthrin (a pyrethroid insecticide) and carbosulfan (a carbamate insecticide). The mixtures were sprayed on mosquito net samples and their efficacy were tested against a susceptible strain of Anopheles gambiae, the major malaria vector in Africa. A significant synergism was observed with a mixture containing 25 mg/m2 of bifenthrin (half the recommended dosage for treated nets) and 6.25 mg/m2 of carbosulfan (about 2% of the recommended dosage). The observed mortality was significantly more than expected in the absence of any interaction (80% vs 41%) and the knock-down effect was maintained, providing an effective barrier against susceptible mosquitoes.

Onchocerciasis in West Africa after 2002: a challenge to take up

Initially planned for a 20 year life time, the Onchocerciasis Control Programme in West Africa (OCP) will have finally continued its activities for nearly three decades (vector control alone from 1975 to 1989, then vector control and/or therapeutic treatment until 2002). Although onchocerciasis is no longer a problem of public health importance nor an obstacle to socio-economic development in the OCP area, the control of this filariasis is not over because OCP never aimed at eradication, neither of the parasite (Onchocerca volvulus), nor of its vector (Simulium damnosum s.l.). In 2003, the eleven Participating countries of OCP will take over the responsibility of carrying out the residual activities of monitoring and the control of this disease. This mission is of great importance because any recrudescence of the transmission could lead in the long run to the reappearance of the clinical signs of onchocerciasis, if not its most serious manifestations. For epidemiological and operational reasons, and given the disparity in national health policies and infrastructures, the capacities of the countries to take over the residual activities of monitoring and control of onchocerciasis are very unequal. Indeed, the interventions to be carried out are very different from one country to another and the process of integrating the residual activities into the national health systems is not taking place at the same pace. This inequality among the countries vis-a-vis the challenges to be met does not, however, prejudge the epidemiological situation after 2002 whose evolution will also depend on the effectiveness of the provisions made before that date by OCP, then after 2002, by the Regional Office for Africa of the World Health Organization which is currently setting up a sub-regional multidisease surveillance centre.

Bifenthrin: a useful pyrethroid insecticide for treatment of mosquito nets

Bifenthrin, a pyrethroid insecticide already used in agriculture was evaluated in laboratory conditions against susceptible and pyrethroid resistant mosquitoes, as a potential insecticide for treatment of mosquito nets. Two laboratory strains of Anopheles gambiae s.s. Giles, the major malaria vector in Africa, and two of Culex quinquefasciatus Say, a major pest mosquito in urban areas, were used. Compared with other pyrethroids such as permethrin and deltamethrin, the intrinsic toxicity of bifenthrin, measured by topical application with susceptible strains, was intermediate. By forced tarsal contact on filter papers (cylinder tests) or on netting materials (cone tests), bifenthrin was found slightly more effective against A. gambiae than against C. quinquefasciatus, in terms of mortality and knock-down effect. With free flying mosquitoes (tunnel tests), bifenthrin was very efficient in killing mosquitoes and inhibiting blood feeding. Against the two pyrethroid resistant strains, bifenthrin was relatively efficient against A. gambiae but the impact of resistance was greater with C. quinquefasciatus. In tunnel tests, blood feeding remained almost entirely inhibited with the two species despite resistance. The high mortality of susceptible mosquitoes and excellent blood feeding inhibition of susceptible and resistant strains makes bifenthrin a good candidate for treatment of netting materials, particularly in areas where C. quinquefasciatus, the main nuisance in urban areas, is resistant to pyrethroids. The slower knock-down and lower irritant effect also makes this insecticide especially attractive when a mass killing effect on mosquito populations is expected.

Eliminating onchocerciasis after 14 years of vector control: a proved strategy

From 1976 through 1989, weekly aerial spraying operations against blackflies were carried out along the rivers of a wide savanna area of West Africa (approximately 700,000 km(2)) where onchocerciasis was hyperendemic. The level of endemicity began to decrease significantly after 4 years of vector control and became very low in 1989. This situation has been maintained without any vector control activity or chemotherapy, and no incidence of any new cases has been detected. An ophthalmological study carried out in 2000 has confirmed these good results, showing only cicatricial ocular lesions in the examined population. These results led to the conclusion that 14 years of vector control may achieve long-term elimination of onchocerciasis, even in the absence of chemotherapy, provided that the treated areas are not subjected to any contamination by exogenous parasites carried in infected humans or flies.

Risk assessment of etofenprox (Vectron) on non-target aquatic fauna compared with other pesticides used as Simulium larvicide in a tropical environment

Within the rotational scheme developed by the Programme to fight the resistance of Simulium damnosum to chemical larvicides, there was an operational gap at discharges between 5 and 70 m3 s(-1) for the treatment of rivers where resistance to organophosphates was present. The use of permethrin and carbosulfan was precluded because of risk of environmental impact and, Bacillus thuringiensis ser. H-14 treatments were not envisageable due to cost and logistics constraints. Among the possible complementary groups of larvicides tested, the pseudo-pyrethroids, held promise, because of a mode of action similar to that of pyrethroids, but along with a usually lower toxicity for fish. Etofenprox, one of the pseudo-pyrethroids tested, shows a global detachment of non-target insects in 24 h close to that of pyraclofos, an organo-phosphorus compound (27 against 23%). In laboratory conditions, six times the operational dose which is 0.03 mg l(-1) 10 min, is needed to cause 50% mortality of Caridina sp. (a small shrimps species) and 30 times this same dose for 95% mortality. For fish species, a safety margin of 400-800 times the operational dose is observed for Oreochromis niloticus and 200-400 times for Tilapia zillii.

[The campaign against onchocerciasis in Africa: update]

Two methods are being used to control onchocerciasis. The first has a delayed effect and consists in reducing or interrupting transmission of Onchocerca volvulus by eradication of the vector at its most vulnerable developmental stage, i.e. the larval stage. The second method has more immediate effects and consists in mass treatment using ivermectin, the only widely available drug, to reduce the density of microfilariae (the pathogenic stage of the parasite) in the population. Both strategies have been implemented within the framework of two international programs: the Onchocerciasis Control Program (OCP) in West Africa, which started in 1974 and will continue until the end of 2002, and the African Program for Onchocerciasis Control (APOC), which was launched in 1995 and will last for 12 years. This article presents an overview of the efficacy of available control tools, as well as the objectives, strategies, organization, and results of the two ongoing control programs. Also dealt with are future perspectives of onchocerciasis control including monitoring techniques to maintain OCP gains, and research to develop new control tools and optimize the program efficacy.

[A case of cross infection by Onchocerca volvulus and Onchocerca ochengi in Simulium damnosum S.L]

During a routine entomological survey conducted within the framework of the Program to Control Onchocerciasis in West Africa, a female simulium forest fly was found to be contaminated by 13 Onchocerca volvulus larvae and 7 Onchocerca ochengi larvae. The two Onchocerca species were identified using specific DNA probes. We speculate that cross infection could be related either to behavioral factors, e.g. interruption of blood meals on two different hosts, or developmental factors, e.g. asynchronous development of parasites of the same species or specific differences in the duration of parasite cycles. Further study will be needed to determine the incidence and scope of cross infection in areas where accurate assessment of the impact of vector control on transmission of onchocerciasis in man is required.

[Systemic action of ivermectin on Culex quinquefasciatus and Simulium squamosum]

The mortality of Culex quinquefasciatus mosquitoes having fed ten days earlier on a chicken treated by 2,000 micrograms/kg of ivermectin (about ten times the therapeutic dosage) is about 25% higher than the mortality recorded on mosquitoes having fed on a non treated chicken. This systemic effect occurs only if the blood meal takes place between 7 and 31 hours after the administration of the drug. However, no systemic effect is recorded either with the C. quinquefasciatus mosquito or with the Simulium squamosum black fly when they are fed on human beings treated at the therapeutic dose.

Pool screen polymerase chain reaction for estimating the prevalence of Onchocerca volvulus infection in Simulium damnosum sensu lato: results of a field trial in an area subject to successful vector control

Detection of infective parasites in the vector population can be an early indicator of recrudescence in areas freed of new cases of onchocerciasis. However, dissection of vector black flies is inefficient in areas subject to effective control. Recently, a polymerase chain reaction (PCR)-based assay has been used to detect a single Onchocerca volvulus-infected black fly in pools containing large numbers of uninfected flies. This method had not been validated on wild-caught black flies in an area subject to effective vector control. Here, we report a method of restricting the pool screen PCR assay to infectious parasites and the results of a field test in an area subject to long-term vector control. The prevalence of infection determined by dissection did not differ from that determined by pool screen PCR. The results suggest that the PCR assay may be a useful tool for epidemiologic surveillance for 0. volvulus infection.

Environmental Assessment of Larvicide Use in the Onchocerciasis Control Programme

The objective of the Onchocerciasis Control Programme (OCP) is to eliminate onchocerciasis as a disease of public importance and as an obstacle to socio-economic development. The OCP was initially based solely on the control of the blackfly vector, Simulium damnosum sensu lato, by insecticide spraying of the breeding sites on river systems, where larval stages develop. Results of monitoring the environmental effects and the process of risk assessment for new insecticides are reviewed. The achievements of this strategy are outlined here by Davide Calamari, Laurent Yameogo, Jean-Marc Hougard and Christian Leveque.

Long-term quantitative ecological assessment of insecticides treatments in four African rivers: a methodological approach

In West Africa different insecticides had been applied in selected river areas for the reduction of the blackfly populations vectors of Onchocerca volvulus, a parasite causing blindness. To evaluate the possible long term effects of the larvicides on the non target fauna an aquatic monitoring programme has been up from the initial phase of the project. Addressing the attention to the invertebrates data collected in four countries during a maximum period ranging from 1977 to 1996, this paper shows and discusses the data analysis strategy for the measure and interpretation of the biological variation. In particular the application of quantitative ecological analysis methods: Principal Component Analysis, rank abundance models and the community diversity indexes, is critically discussed and comments are given to the ecological interpretation of the results.

Blackfly control: what choices after onchocerciasis?

Blackflies are reappearing in areas of West Africa where they used to be controlled with insecticides because they were vectors of the parasite Onchocerca volvulus. Even though they no longer transmit onchocerciasis in these areas they can hinder optimal land use through their biting behaviour. The authors discuss the problems associated with resuming the use of insecticides to control the blackfly and recommend that ground treatment be restricted to areas where it is likely to be effective on a continuing basis. In communities lacking technical and financial resources the only alternative consists of individual protection through the use of repellents or protective clothing.

The impact of Mectizan on the transmission of onchocerciasis

For many years there was no suitable drug available for the control of onchocerciasis. The advent of Mectizan (ivermectin, MSD; an effective microfilaricide), its registration in October 1987 for the treatment of human onchocerciasis, and its suitability for large-scale application were major break-throughs in the control of human onchocerciasis via chemotherapy. Several studies, both fly-feeding experiments and community trials, have established that Mectizan treatment causes a significant reduction in the transmission of infection. Although long-term treatment in some isolated foci (such as occur in the New World and in some hypo- and meso-endemic areas elsewhere) appears to interrupt transmission, more prolonged treatment is required to prove if transmission can be stopped. Advantage could be taken of the significant impact of Mectizan on transmission by giving treatment while or just before transmission by blackflies is most intense.

Impact of treatments with Bacillus sphaericus on Anopheles populations and the transmission of malaria in Maroua, a large city in a savannah region of Cameroon

Simultaneously with a control of breeding sites primarily for Culex quinquefasciatus and secondarily for anophelines with Bacillus sphaericus in the town of Maroua (120,000 inhabitants) in North Cameroon, a survey of anopheline populations and of transmission rates of malaria was performed. Monthly night catches in 8 districts of the town emphasized the relation between the biting rate by Anopheles in the districts and two main factors. One factor was the distance of a district from the breeding sites, i.e., natural flooded areas along the periphery of the town or artificial breeding sites (ditches, puddles) filled with rain water during the rainy season and with water from the water network throughout the year. The second factor was the density of the habitation that reduced dispersal of female mosquitoes from the breeding sites and the risk for inhabitants to be injected because of scattered bites. The treatment with B. sphaericus was followed by a delay (2 months) in the beginning of the transmission period and a decrease in the incidence of malaria cases studied in a health facility of the town. It thus seems to be possible to reduce malaria transmission by applying B. sphaericus to the breeding sites, but this requires a good knowledge of the location and dynamics of breeding sites and an improved formulation of the pesticide.

Control of Culex quinquefasciatus (Diptera: Culicidae) with Bacillus sphaericus in Maroua, Cameroon

Two strategies were tested to control Culex quinquefasciatus with Bacillus sphaericus in Maroua (population 130,000), Cameroon. The treatment of all potential breeding sites (27,000) with B. sphaericus during the dry season caused up to a 90% reduction in the adult biting rate. Because of the short persistence of B. sphaericus and the occurrence of new breeding sites, unacceptable levels of adult biting rates were reached again in 5 months. In the second strategy, two treatments per year of the most productive breeding sites (10,000) stopped the biting rate increase during the rainy season. The results were only partially successful because of variations in B. sphaericus toxicity. The first treatment required 1,200 man-days of work vs. 200 for the simplified treatments. The density of breeding sites depends on the rainfall and the presence of a tap-water network. A sustained control program of Cx. quinquefasciatus will depend upon the dynamics of the principal breeding sites and an improved formulation of B. sphaericus.

Patterns of epidemiology and control of onchocerciasis in west Africa

This paper summarizes the work of the Onchocerciasis Control Programme (OCP) in West Africa, a programme which over a 22 year history has reduced the public health problems of blinding onchocerciasis in eleven countries of West Africa through vector control and, more recently, ivermectin distribution. The paper emphasizes the different approaches to control the programme has developed in the different parts of the programme area which have been determined by the epidemiology of the disease (savanna/forest form), the migratory characteristics of the vectors, intensity of the disease before commencement of treatment, the combined impact of vector control and ivermectin and the likelihood of infiltration of infective blackflies from outside the programme area. The programme has constantly monitored the impact of operations on the trends in prevalence, incidence, annual transmission potential, ocular morbidity and species of fly populations, and as a result, has identified areas where special interventions are required until the programme comes to an end in 2002. The paper illustrates the changes in intensity of infection as measured by community microfilarial load and annual transmission potential over the duration of the programme control activities. The paper also defines and justifies the control strategies in different areas and identifies areas for special interventions.

[Health indicators of a population in an agro-industrial complex of southern Cameroon]

An agro-industrial program involving sugar cane farming was established in Mbandjock (Cameroon) in the 1960's. We studied the impact of this development project on the health of the population by determining the prevalence and distributions of the major parasitic diseases according to district, ethnic origin, age and sex. Three main conclusions can be drawn. First, in the study area, economic development was not associated with deteriorating health conditions. Indeed, the incidence of parasitic disease was lower in Mbandjock than in surrounding areas. Second, imported diseases (loaiasis and schistosomiasis for example) did not develop locally despite the large population concentrations created by the implantation of the agro-industrial complex. Third, endemic parasitic diseases (malaria, onchocerciasis and intestinal infection by helminths or protozoan) were found only in a few districts. Thus, integrated control measures should be taken in these areas as a priority.

Drastic reduction of populations of Simulium sirbanum (Diptera: Simuliidae) in central Sierra Leone after 5 years of larviciding operations by the Onchocerciasis Control Programme

The major vectors of the blinding form of human onchocerciasis in West Africa are two blackfly species, Simulium sirbanum and Simulium damnosum s.s. (Diptera: Simuliidae), identified at the adult stage as the 'savanna group' of the Simulium damnosum complex. In 1988, in the central part of Sierra Leone, the average daily biting rate (females/man/day) by savanna blackflies (mostly S. sirbanum) during the peak of the dry season (April-May) was 59.9, making up 69.1% of total captures on average. There was evidence of a strong long-range immigration of adult females of S. sirbanum through eastern Guinea in the dry season, with a reverse movement towards Guinea in the rainy season. Therefore, in 1989, the World Health Organization's Onchocerciasis Control Programme (OCP) extended its vector control operations from central West Africa to rivers of central and northern Sierra Leone, and to rivers of eastern Guinea. Four years of efficient larviciding drastically reduced adult populations of S. sirbanum in Sierra Leone. In the peak of the dry seasons of 1993 and 1994, the average biting rate by savanna blackflies in central Sierra Leone had dropped to 1.0, making up only 4.3% of total captures on average. Yearly biting rates by S. sirbanum in central Sierra Leone were therefore reduced to 2% of their pre-intervention levels. Based on larval samples, the S. sirbanum has been replaced by two forest species, S. leonense in the south and S. squamosum in the north. Since 1992, it has been possible to calculate accurate transmission rates for blinding onchocerciasis, based on DNA-probe identifications. From 1993, the risk of transmission has not only been reduced by vector control but also by mass distribution of ivermectin to rural communities. In terms of control strategy, the authors conclude that larviciding operations could be alleviated in central Sierra Leone without increasing the risk of blinding onchocerciasis transmission, as long as the migration of S. sirbanum through eastern Guinea and northern Sierra Leone is prevented.

[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]

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).

[Control of onchocerciasis vectors in West Africa: description of the logistics adapted for a large-scale public health program]

The Onchocerciasis Control Programme (OCP) in West Africa, launched in 1974, includes 11 participating countries and covers more than one million square kilometres. The aim of the OCP is to control blinding onchocerciasis (river blindness) which is caused by the savannah strain of Onchocerca volvulus transmitted by the Simulium damnosum complex. There is no effective macrofilaricide, so vector control to prevent the transmission of the parasite remains the method of choice, despite the availability of ivermectin, a drug which controls ocular morbidity. The potential value of vector control has been demonstrated by the original programme: 14 years activity has eliminated the disease as a public health problem in the areas included. This strategy requires adapted logistical support involving (i) widespread insecticide coverage (27,000 km of river are treated by the OCP during the rainy season), (ii) frequent (weekly) application of larvicide and (iii) prolonged intervention due to the life-span of the worm in the human reservoir, estimated to be approximately 14 years. We describe the vector control operations and their organisation 20 years after the initiation of the OCP. The OCP can be divided into 5 areas of logistic activity. The first covers activities involving insecticide and fuel management for the OCP as a whole: assessment of the requirements for the following year, ordering from insecticide and petrol suppliers, stocking fuel and insecticide at the depots covering the area. The second activity is the treatment of rivers with insecticide. This includes treating the ground with larvicide, the aerial operations run by an independent company supplying 12 helicopters on contract to the OCP, and use of satellite beacons for retransmitting of hydrological data. The third activity is monitoring the impact of larvicides on both the target (adult and larval S. damnosum) and on other fauna (fish, crustaceans and other insects). The fourth activity is field data collection and its processing. This involves a data transmission network to facilitate stock management insecticide application and entomological and hydrological surveillance using computer systems. The fifth activity is the coordination of vector control operations, technical and administrative staff and estimations of the funds available to the Vector Control Unit. The logistic aspects of other large-scale-insect-control programmes world-wide are considered, and the possibility of using the OCP as a model for such programmes (both public health and agricultural) is assessed.

[ 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]

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.

[Onchocerciasis control program in West Africa: socioeconomic development and risk of recrudescence of transmission. 2. Experimental study of the transmission of Onchocerca volvulus strains from Southwestern Sierra Leone by Simulium yahense and Simulium squamosum]

The movements of human populations towards the mining wealth of the northern parts of Sierra Leone are favorable to a high contact rate between onchocerciasis patients coming from the south-western area of this country and the vector species Simulium yahense and Simulium squamosum which assume the essential of onchocerciasis transmission in the above-mentioned mining area. In fact, the Onchocerca volvulus strains concerned by this contact seem to be more pathogenic than those locally transmitted. In order to assess the danger it could represent for the Onchocerciasis Control Programme in West Africa, we carried out the experimental study of transmission which may result from this contact when more or less infected onchocerciasis patients are involved. The results indicated that this transmission by S. yahense may reach high proportions only when heavily infected onchocerciasis patients are implicated. We took also notice of the low capacity of S. squamosum to transmit the O. volvulus strains from the south-western Sierra Leone, irrespective of the microfilarial load of patients. Thus, in the most favorable conditions of a high parasite-vector contact of the study, involvement of S. yahense and onchocerciasis patients with high skin microfilarial loads is the only occurrence to which a high risk of intensive transmission may be related. The authors consider that the probability of such a risk occurring will be drastically reduced, due to the considerable decrease of skin microfilarial loads in human communities which regularly have the advantage of ivermectin (Mectizan) mass treatments.

[Importance and role of spreading larvicides on the soil in river beds for the control program against onchocerciasis in west Africa]

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.

[Factors affecting the efficacy of 2 growth regulator compounds, OMS 3010 (phenoxycarb) and OMS 3019 (ethoxypyridine), on larvae of the Simulium damnosum complex]

Two insect growth regulators (IGR'S) acting as juvenile hormone analogs (JHA'S), OMS 3010 (phenoxycarb) and OMS 3019 (ethoxypyridine) were selected after a screening of several compounds proposed by industry on black fly larvae of the Simulium damnosum complex (Diptera: Simuliidae) the vector of onchocerciasis in West Africa. Bioassays were designed to study two parameters: larval age and exposure time. The results showed that these compounds are more efficient on old larvae (6th and 7th instars) than young larvae (3rd, 4th and 5th instars). However, exposure time seems essential. The black fly larvae live in fast running water courses. Thus, it is difficult to insure a prolonged exposure time in natural conditions. Though being active on black fly larvae, three factors are limiting the prospects for operational use of these two compounds: high dosages, limited effectiveness on young instar larvae and exposure time.

[Campaign against Culex quinquefasciatus using Bacillus sphaericus: results of a pilot project in a large urban area of equatorial Africa]

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)

[Small-scale evaluation of the efficacy of growth-regulating insecticides on larvae of the Simulium damnosum complex (Diptera: Simuliidae)]

The efficacy of insect growth regulators was assessed in small scale tests on larvae of the Simulium damnosum complex (Diptera: Simuliidae) in the Ivory Coast. Three compounds [OMS 2015 (triflumuron), OMS 3009 (teflubenzuron), OMS 3013 (chlorfluazuron)] belong to the group of benzoylphenyl-urea substitutes; these IGR's are supposed to inhibit chitin synthesis. Two other compounds are Juvenile Hormone Analogs (JHA's) (OMS 3007 and OMS 3019). The last compound (OMS 3010) is a phenoxycarbamate. The first three compounds had a low efficacy on blackfly larvae, which is consistent with the literature data for another compound of this group: diflubenzuron. The other three compounds (OMS 3007, OMS 3010 and OMS 3019) were much more efficient, OMS 3010 and OMS 3019 showing high activity at low concentrations. These results would justify further studies on the effect of larval age and exposure parameters, and eventually full scale river tests.

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.

[Effect of a sublethal dose of an insect growth regulator on the physiology of onchocercosis vectors]

In most of the large scale vector control programs, the larviciding operations are not fully successful, resulting in a surviving vector population due to the use of sublethal doses. This problem leads the authors to study the effect of such doses upon the future of the residual population in order to know if it represents an epidemiologically dangerous population. One insect growth regulator was selected to study this phenomenon using Onchocerca volvulus as the parasite and Simulium damnosum s.l. as the vector. The experiment was conducted under field conditions by treating a river of a savanna area in north Cameroon where onchocerciasis remains endemic. The sublethal doses yielding 20% in adult blackflies from treated larvae was determined and the effects on physiological and vectorial capacities were assessed. The authors also carried out observations on the peritrophic membrane formation and the passage of microfilariae into the hemocele. The observation of flies fed on Onchocerca after larval treatment showed that this insecticide had a light effect on female longevity but did not induce any modifications in the life cycle of the parasite. An unusual high proportion of infected females, observed by the authors, is probably due to an anomaly in the phenomenon of limitation. However, the histological analysis showed to the evidence that no apparent difference between treated and untreated blackflies was observed although the process of blood meal digestion seemed slightly altered in treated flies.