128 research outputs found
Ecologie des arbovirus au Sénégal : exemple du virus de la fièvre West Nile dans le Delta du fleuve Sénégal et le Ferlo
No full textLa fièvre West Nile est une arbovirose émergente endémique au Sénégal, surtout dans la région du fleuve Sénégal et dans le Ferlo. Les oiseaux sont les principaux hôtes-réservoirs et assurent la diffusion du virus des régions infectées vers des régions indemnes à travers les flux migratoires d'oiseaux. La transmission du virus d'un hôte infecté à un hôte sain se fait par le biais de vecteurs qui sont généralement des moustiques. Au Sénégal, bien que plusieurs études aient été faites sur la maladie, beaucoup de questions restent à être élucidées dans les cycles de transmission complexes qui dépendent largement des conditions climatiques et environnementales locales. La région du Ferlo et le Delta du fleuve Sénégal sont deux régions aux faciès écologiques différents. La région agropastorale du Ferlo est constituée de vallées fossiles inondables en saison des pluies et forme des mares temporaires qui disparaissent progressivement avec l'installation de la saison sèche. Tandis que le Delta du fleuve est constitué de vastes étendues aménagées pour la riziculture irriguée dont la principale source en eau est le fleuve Sénégal. Dans ces deux régions aux contextes géographiques et environnementaux différents, l'exploitation des terres par les populations humaines et animales est fortement liée à l'eau qui malheureusement permet le développement de vecteurs d'arbovirus comme ceux de la West Nile. La transmission du virus West Nile est complexe dans ces régions, car elle implique plusieurs intervenants dont des vecteurs (Aedes et/ou Culex) d'écologies différentes, des hôtes résidents ou migrateurs autour d'un environnement favorable (mare temporaire, fleuve). Il en est de même de l'endémisation du virus car les réservoirs sont peu étudiés. Afin de mieux comprendre l'écologie du virus de la West Nile dans ces écosystèmes, nous avons effectué des enquêtes entomologiques par différentes techniques (piège lumineux avec CO2, piège à appât poulet, piège à appât pigeon, piège à appât cheval, aspiration) et des enquêtes sérologiques sur les poulets sentinelles pendant la période à fort risque de transmission. Nous avons aussi, à partir d'observations faites sur le terrain, prélevé des échantillons de sol par raclage au niveau de potentiels gîtes de repos et de ponte du moustique Aedes vexans arabiensis que nous avons comparés avec des expériences effectuées au laboratoire. Les résultats obtenus sont assez encourageants et contribuent à une meilleure connaissance de l'écologie de ce virus dans ces écosystèmes et par conséquent de l'épidémiologie de la maladie. L'intérêt de ce travail est de mettre en évidence le rôle de chacun des acteurs étudiés impliqués dans le cycle épidémiologique de la maladie et les interactions qui peuvent exister entre ces différents intervenants et le milieu. Le schéma de surveillance que nous avons proposé pourra être utilisé pour la détection précoce de la circulation du virus au Sénégal dans le cadre d'un système de surveillance de la West Nile. Cependant, les efforts de recherche doivent être poursuivis pour mieux cerner son épidémiologie et développer des méthodes de prévention et de lutte efficientes
African horse sickness virus: history, transmission, and current status
No full textAfrican horse sickness virus (AHSV) is a lethal arbovirus of equids that is transmitted between hosts primarily by biting midges of the genus Culicoides (Diptera: Ceratopogonidae). AHSV affects draft, thoroughbred, and companion horses and donkeys in Africa, Asia, and Europe. In this review, we examine the impact of AHSV critically and discuss entomological studies that have been conducted to improve understanding of its epidemiology and control. The transmission of AHSV remains a major research focus and we critically review studies that have implicated both Culicoides and other blood-feeding arthropods in this process. We explore AHSV both as an epidemic pathogen and within its endemic range as a barrier to development, an area of interest that has been underrepresented in studies of the virus to date. By discussing AHSV transmission in the African republics of South Africa and Senegal, we provide a more balanced view of the virus as a threat to equids in a diverse range of settings, thus leading to a discussion of key areas in which our knowledge of transmission could be improved. The use of entomological data to detect, predict and control AHSV is also examined, including reference to existing studies carried out during unprecedented outbreaks of bluetongue virus in Europe, an arbovirus of wild and domestic ruminants also transmitted by Culicoides
It’s risky to wander in September: Modelling the epidemic potential of Rift Valley fever in a Sahelian setting
No full textInternational audienceEstimating the epidemic potential of vector-borne diseases, along with the relative contribution of underlying mechanisms, is crucial for animal and human health worldwide. In West African Sahel, several outbreaks of Rift Valley fever (RVF) have occurred over the last decades, but uncertainty remains about the conditions necessary to trigger these outbreaks. We use the basic reproduction number (R0) as a measure of RVF epidemic potential in northern Senegal, and map its value in two distinct ecosystems, namely the Ferlo and the Senegal River delta and valley. We consider three consecutive rainy seasons (July-November 2014, 2015 and 2016) and account for several vector and animal species. We parametrize our model with estimates of Aedes vexans arabiensis, Culex poicilipes, Culex tritaeniorhynchus, cattle, sheep and goat abundances. The impact of RVF virus introduction is assessed every week over northern Senegal. We highlight September as the period of highest epidemic potential in northern Senegal, resulting from distinct dynamics in the two study areas. Spatially, in the seasonal environment of the Ferlo, we observe that high-risk locations vary between years. We show that decreased vector densities do not greatly reduce R0 and that cattle immunity has a greater impact on reducing transmission than small ruminant immunity. The host preferences of vectors and the temperature-dependent time interval between their blood meals are crucial parameters needing further biological investigations
Relations entre la pluviométrie et le risque de transmission virale par les moustiques : cas du virus de la Rift Valley Fever (RVF) dans le Ferlo (Sénégal)
No full textII existe, dans les mares du Ferlo, deux espèces de vecteurs potentiels du virus de la Rift Valley Fever possédant des modalités de développement différentes: Aedes vexans arabiensis et Culex poicilipes. De fortes pluies au début de la saison entraînent l'apparition soudaine d'une importante population d'Aedes, car les oeufs de cette espèce, pondus l'année précédente, sont présents en grand nombre sur les parois des mares, prêts à éclore dès qu'ils sont immergés. Pour peu que le virus soit présent dans la nature (la transmission transovarienne de virus chez les Aedes est un phénomène courant), une amplification virale suivie d'un début de diffusion est alors possible parmi des hôtes vertébrés non immuns. Des pluies légères, régulières, comme c'est souvent la règle en milieu de saison, auront tendance à favoriser d'autres espèces que les Aedes, en particulier Culex poicilipes dont la dynamique est liée à la stabilité du milieu aquatique, leurs oeufs ne supportant pas la dessiccation. Puis, en fin de saison, les pluies deviennent rares et les mares commencent à tarir. Les Aedes ont pratiquement disparu, leurs oeufs ne pouvant plus être recouverts par les rares pluies tombant alors. Mais des pluies tardives peuvent encore apparaître, de manière assez exceptionnelle. Si elles sont conséquentes, elles permettent alors aux Culex de maintenir leurs populations à un niveau élevé et aux oeufs d'Aedes de retrouver les conditions nécessaires pour éclore. Il y aura donc maintien des populations de Culex et apparition de femelles d'Aedes, donnant au virus la possibilité d'être à nouveau disséminé et amplifié. Après les pluies, les femelles de Culex vont disparaître et les oeufs d'Aedes - dont certains sont susceptibles d'héberger le virus RVF - entrer en diapause, jusqu'à l'année suivante
Seasonal dynamics of Culicoides (Diptera: Ceratopogonidae) biting midges, potential vectors of African horse sickness and bluetongue viruses in the Niayes area of Senegal
No full textBackground The African horse sickness epizootic in Senegal in 2007 caused considerable mortality in the equine population and hence major economic losses. The vectors involved in the transmission of this arbovirus have never been studied specifically in Senegal. This first study of the spatial and temporal dynamics of the Culicoides (Diptera: Ceratopogonidae) species, potential vectors of African horse sickness in Senegal, was conducted at five sites (Mbao, Parc Hann, Niague, Pout and Thies) in the Niayes area, which was affected by the outbreak. Methods Two Onderstepoort light traps were used at each site for three nights of consecutive collection per month over one year to measure the apparent abundance of the Culicoides midges. Results In total, 224,665 specimens belonging to at least 24 different species (distributed among 11 groups of species) of the Culicoides genus were captured in 354 individual collections. Culicoides oxystoma, Culicoides kingi, Culicoides imicola, Culicoides enderleini and Culicoides nivosus were the most abundant and most frequent species at the collection sites. Peaks of abundance coincide with the rainy season in September and October. Conclusions In addition to C. imicola, considered a major vector for the African horse sickness virus, C. oxystoma may also be involved in the transmission of this virus in Senegal given its abundance in the vicinity of horses and its suspected competence for other arboviruses including bluetongue virus. This study depicted a site-dependent spatial variability in the dynamics of the populations of the five major species in relation to the eco-climatic conditions at each site
West nile virus transmission in sentinel chickens and potential mosquito vectors, Senegal River Delta, 2008-2009
No full textWest Nile virus (WNV) is an arthropod-borne Flavivirus usually transmitted to wild birds by Culex mosquitoes. Humans and horses are susceptible to WNV but are dead-end hosts. WNV is endemic in Senegal, particularly in the Senegal River Delta. To assess transmission patterns and potential vectors, entomological and sentinel serological was done in Ross Bethio along the River Senegal. Three sentinel henhouses (also used as chicken-baited traps) were set at 100 m, 800 m, and 1,300 m from the river, the latter close to a horse-baited trap. Blood samples were taken from sentinel chickens at 2-week intervals. Seroconversions were observed in sentinel chickens in November and December. Overall, the serological incidence rate was 4.6% with 95% confidence interval (0.9; 8.4) in the sentinel chickens monitored for this study. Based on abundance pattern, Culex neavei was the most likely mosquito vector involved in WNV transmission to sentinel chickens, and a potential bridge vector between birds and mammals
Environmental impact of tsetse eradication in Senegal
No full textThe sterile insect technique is an environment friendly control tactic and is very species specific. It is not a stand-alone technique and has been used mostly in combination with other control tactics within an area-wide integrated pest management strategy. For a period of eight years, the direct impact of a campaign to eradicate a population of the tsetse fly Glossina palpalis gambiensis in Senegal was monitored using a set of fruit-feeding insect species (Cetoniinae and Nymphalidae) that served as ecological indicators of the health of the ecosystem. Here we show that the eradication campaign had very limited impacts on the apparent densities of the most frequent species as well as three diversity indexes during the reduction phase involving insecticides but reverted to pre-intervention levels as soon as the release of the sterile male insects started. These results greatly expand our understanding of the impact of vector eradication campaigns on non-target species
Ecological niche modelling to estimate distribution of Culicoides, potential vectors of bluetongue virus in Senegal
No full textBackground: Vector-borne diseases are among the leading causes of morbidity and mortality in humans and animals. In the Afrotropical region, some are transmitted by Culicoides, such as Akabane, bluetongue, epizootic haemorrhagic fever and African horse sickness viruses. Bluetongue virus infection has an enormous impact on ruminant production, due to its high morbidity and mortality rates. Methods: A nationwide Culicoides trapping campaign was organized at the end of the 2012 rainy season in Senegal. A Maximum Entropy approach (MaxEnt), Boosted Regression Tree (BRT) method and Ecological Niche Factor Analysis (ENFA) were used to develop a predictive spatial model for the distribution of Culicoides, using bio-climatic variables, livestock densities and altitude. Results: The altitude, maximum temperature of the warmest month, precipitation of the warmest quarter, mean temperature of the wettest quarter, temperature seasonality, precipitation of the wettest quarter and livestock density were among the most important factors to predict suitable habitats of Culicoides. Culicoides occurrences were, in most of the cases, positively correlated to precipitation variables and livestock densities; and negatively correlated to the altitude and temperature indices. The Niayes area and the Groundnut basin were the most suitable habitats predicted. Conclusion: We present ecological niche models for different Culicoides species, namely C. imicola, C. oxystoma, C. enderleini and C. miombo, potential vectors of bluetongue virus, on a nationwide scale in Senegal. Through our modelling approach, we were able to determine the effect of bioclimatic variables on Culicoides habitats and were able to generate maps for the occurrence of Culicoides species. This information will be helpful in developing risk maps for disease outbreaks
Host preferences and circadian rhythm of Culicoides (Diptera: Ceratopogonidae), vectors of African horse sickness and bluetongue viruses in Senegal
No full textAfrican horse sickness- and bluetongue virus are orbiviruses transmitted by Culicoides biting midges (Diptera: Ceratopogonidae) to horses and to ruminants, respectively. Since the last epizootic outbreak of African horse sickness in 2007 in Senegal, extensive investigations have been undertaken to improve our knowledge on Culicoides species involved locally in the transmission of the virus. The purpose of this study was to compare and quantify the host preferences of potential vectors of these orbiviruses on horse and sheep and to study their circadian rhythm. We found that Culicoides oxystoma and species of the sub-genus Avaritia (Culicoides imicola, Culicoides bolitinos and Culicoides pseudopallidipennis) had a preference for horse when compared to sheep (the predicted ratio between horse and sheep was 80 for C. oxystoma and 26 for C. imicola), and were mostly crepuscular: C. oxystoma had continuous activity throughout the diel with peaks in numbers collected after sunrise and sunset, while C. imicola was mostly nocturnal with peak after sunset. Unexpectedly, species of the subgenus Lasiohelea was also collected during this study. This diurnal biting species was a nuisance pest for both animal species used as bait
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