[Entomologic study on the transmission of onchocerciasis in a forest-savanna transition area of Cameroon]. / Etude entomologique sur la transmission de l'onchocercose dans une zone de transition forêt-savane du Cameroun.

An entomological study was conducted in the basin area of middle Sanaga (Cameroon) in order to document the pattern of onchocerciasis transmission in a region where a high prevalence of infection had been recorded in villages located 30 km from the main rivers. The main vector of O. volvulus was found to be S. squamosum s.s. No breeding site was found in the small tributaries of the Sanaga and Mbam Rivers, and the dispersal of S. damnosum s.l. in the area thus appeared to be particularly high. The highest blackfly population densities were recorded during the long rainy season, and a second peak of density occurred along the Mbam River during the short rainy season. A seasonal variation in dispersal patterns was found at three of the four transects studied. The transmission of Onchocerca volvulus in the area occurred principally between January and May (i.e. at the end of the long dry season and the beginning of the short rainy season). Vector control operations might well reinforce the effect of ivermectin distributions in this onchocerciasis focus.

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.

[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)

An episode of resistance to permethrin in larvae of Simulium squamosum (Diptera: Simuliidae) from Cameroon, after 3 1/2 years of control.

Because of the biting nuisance from females of Simulium squamosum, a 30 km section of the Sanaga River (Cameroon) was treated since 1987 with permethrin for the control of larval populations. In 1990, resistance to permethrin occurred in a small proportion of the larvae, with a resulting 2-4x increase of the LC95 for dead larvae (moribund larvae considered as live). In 1991, after a 6-month interruption of the treatments, susceptibility to permethrin returned to the initial level, and was similar to the susceptibility of S. squamosum larvae from a non-treated section of the Sanaga. In the context of a small-scale control program, resistance to permethrin can be reversible, and it can be avoided by rotation with other types of insecticides such as Bacillus thuringiensis serovar, israelensis.

[Effect of a sublethal dose of an insect growth regulator on the physiology of onchocercosis vectors]. / Etude de l'influence d'une dose sublétale d'un insecticide régulateur de croissance sur la physiologie des vecteurs de l'onchocercose.

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.

[Campaign against the vectors of onchocerciasis in the surroundings of a refugee camp located in a savanna zone of Cameroon]. / Lutte contre les vecteurs de l'onchocercose aux alentours d'un camp de réfugiés situé en zone de savane du Cameroun.

In view of the risks of onchocerciasis facing Chadian refugees who have settled in a camp in a savanna hyperendemic area of northern Cameroon, the authors concluded that a vector control directed against the black fly larvae located on small seasonal tributaries of the main water course of this area should be carried out during the four to five months of the rainy season, i.e., during the whole period of transmission of this disease. The operations were undertaken during two consecutive rainy seasons. The first year consisted in developing ground-based control techniques and evaluating the larvicidal efficacy at the larval level. The second year was, particularly, devoted to the study of the impact of the vector control on the biting rate and on the transmission of onchocerciasis. Every week, three of the tributaries close to the camp were treated with an Emulsifiable Concentrate of temephos. Throughout the rainy season, the larvicide eliminated almost all the pre-adult black fly stages on these water courses. At the point in a high onchocercal endemicity area, it also reduced the biting rate by 60% and the Annual Transmission Potential by 72%, i.e., to values corresponding to the lower meso-endemicity threshold. Since these results were obtained at a very unfavourable point from the standpoint of black fly density and onchocerciasis transmission, the authors consider that vector control carried out under these conditions protects the local populations and the Chadian refugees effectively from black fly bites and a risk of severe onchocerciasis.

Large river treatment with Bacillus thuringiensis (H-14) for the control of Simulium damnosum s.l. in the Onchocerciasis Control Programme.

Complete mortality of Simulium damnosum Theobald s.l. larvae was obtained along a 19 km stretch of the Marahoué River including and downstream of the Danangoro rapids complex in the Bandama Basin of Ivory Coast after treatment with 1.5 ppm/10 min of the Sandoz 402-1-WDC formulation of Bacillus thuringiensis Berliner serotype H-14. Partial control was observed for an additional 15 km. The level of control was especially encouraging considering the low concentration (.8%) of active ingredient in the Sandoz formulation. Additionally, the treated population has demonstrated resistance to temephos (Abate), the larvicide currently utilized in the Onchocerciasis Control Programme. Other species of Simulium were somewhat less affected by the treatment; living larvae were found 4 km downstream of the treatment point. Reinvasion of the Danangoro complex, ostensibly by drifting larvae of Simulium spp., was detected the day after treatment. Non-target organisms, including Ephemeroptera and Chironomid midges, were observed before and after treatment and were apparently not affected. In a second test, 0.8 ppm/10 min of the Sandoz formulation was tested, 0.8 pm/10 min of the Sandoz formulation was tested against S. damnosum s.l. and three other Simulium species in the N'Zi River in Ivory coast utilizing a gutter bioassay apparatus. Six hours after treatment 91% of the S. damnosum s.l. larvae had died. The other species responded with 91--100% mortality.