Transmission de l'onchocercose par Simulium squamosum (Diptera : Simuliidae) dans le foyer urbain de Kinsuka a Kinshasa/Republique democratique du Congo

Contexte. Les etudes realisees dans le foyer de Kinsuka entre 1985 et 2001 ont montre une tendance a la reemergence de la transmission de l'onchocercose dans ce foyer. Le but de la presente etude exploratoire etait de determiner le niveau actuel de la transmission de cette parasitose; en vue d'apprehender des facteurs explicatifs du niveau d'endemicite constate. Methodes. Les simulies ont ete capturees dans deux points de capture (P1 et P2); pour une periode de 12 mois (mars 2008-fevrier 2009); par le procede classique de capture sur appats humains. Dans l'ensemble; 82;1des simulies capturees ont ete dissequees pour la determination de la parite; et toutes les simulies pares ont ete dissequees pour denombrer les larves d'O. volvulus. Les autres especes de simulies ont ete inventoriees; et les supports larvaires ont ete prospectes. Resultats. Au total; 12282 simulies ont ete capturees dont 5018 au S1; et 7264 au S2. Le cycle d'agressivite journaliere a presente une courbe concave avec deux pics; dont un mineur entre 8 et 9 heures; et un majeur entre 17h et 18h. Dans l'ensemble de simulies dissequees; 42;4etaient pares. Seules 25 simulies pares etaient infectees par les larves d'O. volvulus; dont 14 au S1; et 11 au S2. Une seule simulie etait infectante. Les taux annuels de piqures (TAP) etaient de 564453; et 78111 piqures par personne par an respectivement. Le potentiel de transmission est pratiquement nul : 0 au S1; et 28 larves infectantes par personne par an au S2. Conclusion. Le taux d'agressivite est environ 12 fois plus eleve que celui observe il y a 27 ans; dans le meme site. Le taux observe est en partie du a l'influence des activites realisees par la nouvelle entreprise d'exploitation de moellons dans la carriere de Kinsuka. Ces activites ont provoque entre autres; le retrecissement du lit du fleuve pendant la saison seche; qui entraine une augmentation du niveau du fleuve; creant une grande disponibilite des supports larvaires. La transmission de l'onchocercose est presque nulle. Des etudes ulterieures permettront d'en identifier les determinants

Susceptibility of Simulium damnosum complex larvae to temephos in the Tukuyu onchocerciasis focus; southwest Tanzania

Tukuyu onchocerciasis focus was earmarked for vector control using insecticide against larval stages.Susceptibility tests of mature larvae of Simulium damnosum s.l. vectors to temephos insecticide were carried out before and after two years of insecticide treatment of rivers within Tukuyu onchocerciasis focus, south-western Tanzania. The tests were done in 1999/2000 and 2004 using WHO standard methods. Mature larvae were exposed to 9 concentrations of temephos active ingredient, from the weakest 0.00975mg/litre to the strongest of 2.5mg/l. Each test concentration and control was run in duplicates of 25 larvae each, set for three hours in a cool temperature. After incubation, test solution was discarded and larval condition checked. Numbers of larvae in each category were recorded and used to determine mortality rate for each concentration as well as for the LC50 and LC95. A total of 1,666 larvae were tested, 942 during the pre- and 724 post-treatment. Results showed that both pre and post-treatment samples were susceptible, attaining 100% mortality at the diagnostic dose of 1.25mg/l, and LC50 between 0.129-0.34mg/l pre - and 0.144-0.211 mg/l (95% CI, P<0.05) post- treatment. These values fall within the standard diagnostic dose of ≤0.4mg/l for susceptible S. damnosum s.l populations. It was concluded that the endemic S. damnosum population was susceptible to temephos before and after two years of intermittent field application. Temephos was thus recommended for continued use in onchocerciasis vector control in the Tukuyu focus, to complement Community Directed Treatment with Ivermectin, but close monitoring of vector susceptibility should be done

Vector elimination project: Progress report (1999 - 2000)

A combination of factors unique to Bioko (Republic of Equatorial Guinea) renders it possible to stop onchocerciasis transmission pernanently through the elimination of the local vector, a member of the Simulium damnosum complex. These factors include its geographical isolation, its small size, the very few rivers that will require treatment and even for those that do, the short stretches that have to be treated, the close proximity of rivers that need treatment and the very low river discharges which means that relatively small quantities of insecticide will be needed. These facts were established by a study funded by WHO/TDR in 1996 and the subsequent project was adopted for implementation by APOC. Three main activities were initiated in the first one and half years (the period under review) and these were; development of the necessary skills and capacity in Bioko, pre-control entomological data collection and an impact assessment studies of the effect of Temephos on non-target aquatic fauna of Bioko. Six Guineans received training in all aspects of entomological data collection foronchocerciasis. control and are now competent enough to conduct these exercises with very minimum supervision. The data collection was initiated in April 1999 at four sites located on the most important rivers in the northem inhabited part of the island. Regular fly catches and dissections have been carried out since then with each site being visited once each week. A total of 21,713 flies were caught during 199 visits and 16,988 of them were dissected to find Onchocerca volvulus infections. When the data was analysed according to sites, it revealed that biting was intense throughout the period at all sites. The highest annual biting rate (ABR) was 60,698 bites/person/year recorded at Barleycorn on R. Apu and the lowest ABR recorded was 21,849 at Sampaca (R. Sampaca). Periodicity in fly densities were observed with most biting occurring during the rainy season when the rivers are as expected most productive. With regards to transmission the highest recorded index of annual transmission (i.e., ATP) of 1,447 infective larvae/person/year was at Sampaca and the lowest was 385 at Balacha de Riaba. These figures are low if considered that similar values are recorded monthly at sites on mainland West Africa which have had no history ofonchocerciasis control and but have similar biting rates. Bioko has a 10 year history of ivermectin distribution and this was thought to be the reason for the observed low transmission. Seasonal variation in transmission was also observed with most transmission occurring in the dry season, which is also not surprising because of increased proportions of old flies in S. damnosuz populations during this season. The environmental impact assessment studies also provided baseline information of densities and diversity of fish, prawns and macro- invertebrate fauna of Bioko. The main findings were that these were poor if compared to the richness of the fauna of rivers on the mainland. The major conclusions drawn from the studies on the impact of Temephos were that neither mortalities nor harm to fish and prawns should be anticipated at the treatment dosages envisaged and that the overall impact on the macro- invertebrate fauna was not that high as to prevent their recovery after treatment. Ground treatment of rivers is planned for the coming 2000 - 2001 dry season. The main aim is to establish the extent to which it could complement any aerial larviciding operations. The detailed planning has been completed (the plan of action is attached to this report). All the rivers on the island have been coded, the treatment points have been identified, treatment maps have been prepared and other activities, including procurement of spraying equipment, insecticide, etc have been initiated. A strategic meeting at APOC/FIQ is planned after the ground larviciding operation for evaluation and to decide on the future direction of the project.