Ecological niche modelling to estimate the distribution of Culicoides, potential vectors of bluetongue virus in Senegal

Abstract

BackgroundVector-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.MethodsA 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.ResultsThe 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.ConclusionWe 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.

Assessing the Risk of Occurrence of Bluetongue in Senegal

“…imicola and C. oxystoma species have been identified as the main BTV vectors in several entomological studies conducted in South Africa, North Africa, southern Europe, and India [18,19]. The roles of C. kingi, C. enderleini, and C. miombo as potential BTV vectors in Senegal due to their abundances, distribution, or feeding behaviors have been reported by several authors [19][20][21]. In the case of enderleini, some BTV seropositive specimens were collected in Sudan.…”

“…Culicoides species were identified, as explained in Reference [29]. A maximum abundance of two nights trapping was considered as the best estimate of the population present, as abundance can decrease rapidly in sub-optimal trapping conditions [21]. The geographical coordinates of each site were recorded with a Garmin© hand-held global positioning system receiver (accurate to within 10 m) and projected in UTM Zone 28 N. The species distribution is shown in Figure 1; each box shows the distribution of the species, the size of each box is proportional to the number of Culioides collected.…”

“…Climatic, environmental, and livestock variables characterizing favorable habitats for Culicoides were selected based on a literature review of presence and abundance models [20,21,25]. A total of 26 variables were selected belonging to four categories: Eleven bio-climatic variables related to temperature (Bio01-Bio11); eight bio-climatic variables concerning rainfall (Bio12-Bio19); elevation (one variable), and animal density (six variables).…”

“…Elevation data (digital elevation model) were extrapolated from the Moderate Resolution Imaging Spectroradiometer (MODIS) with a spatial resolution of 30 arc-seconds (~1 km). The complete list of all data used in the analysis can be found in Supplementary Information Table S2 [20,21,25]. Finally, livestock data (density of cattle, small ruminants, horses, and donkeys) was obtained from the global distribution of livestock maps produced by FAO (http://www.fao.org/ag/againfo/resources/en/glw/GLW_dens.html).…”

“…The specific objectives of this work were to (i) model the spatial abundance of Culicoides of veterinary interest, (ii) estimate the epidemic potential of BT in Senegal, and (iii) assess the probability of an epidemic of BT being triggered in the area. Following previous studies in the area [20,21], we predicted vector abundance using bioclimatic and environmental variables to train random forest models. We used the basic reproduction number (R 0 ) as a measure of BT epidemic potential and mapped its values.…”