Modelling the distribution of Culicoides bluetongue vectors in north wales : [Abstract 101]

Bluetongue (BT) serotype-8 emerged in northern Europe in 2006 and the UK initiated a vaccination campaign in 2007. With Wales accounting for 15% of ail sheep within the EU community, incursions of BT could cause devastating production losses. To provide insight into the distribution and density of BT vectors in Wales, satellite-derived climate environmental and soil variables were analysed within a GIS. Surveillance of #Culicoides biting# midges (CBMs) was carried out on 25 farms in Baia, north Wales, over 12 nights in July 2008. Environmental variables were assessed using a farm questionnaire soil data was obtained from the National Soil Resources 'Institute. MOD imagery from the NASA Terra satellite, including day/night-time land surface temperature, middle infrared reflectance, NDVI and EVI, was also obtained. High "1km) spatial regression models were built to investigate explanatory parameters for the CBM variation between farms. Models were produced for each midge trapped, as well as a model for ail species combined. The #C.obsoletus# group, present on farms, represented 62% of individuals trapped. The between-farm variation in was up to 200%. Models produced explain up to 88% of catch variation (species dependent). Only one model explained less than 500/0 of the variation (#C. festivipennis#; 23.28%). The #C. obsoletus# species (main BT vector in UK) model accounts for 85.5% of the variation. It includes the number of sheep on a farm and use of insecticides, alongside climatic variables. Freely accessible climate and environmental data has proved beneficial in modelling CBM distribution. Quickbird satellite imagery and ordinance survey data are being assessed in terms of producing similar models for Bala and will be compared to the current models. Trapping at multiple sites per farm is being undertaken to produce higher resolution models of CBM density in relation to on-farm environmental variables. (Texte intégral

Measuring between-farm variation in Culicoides (Diptera: Ceratopogonidae) density, the vectors of bluetongue virus

Culicoides midges are ubiquitous on farms in the UK, but little research has explored their abundance, an important determinant of disease risk. Models to explain and predict variation in their abundance are needed for effective targeting of control methods against bluetongue (BT) and other Culicoides-borne diseases. Midge abundances were estimated using light traps on 35 farms in north Wales. Culicoides catches were combined with remotely-sensed ecological correlates, and on-farm host and environmental data, within a general linear model. Local-scale variation in abundance exhibited an almost 500-fold difference between farms in maximum Obsoletus Group catches. The variance explained was consistently high for the BT vector species (81% for Obsoletus Group; 80% for Pulicaris Group, 73% for C. pulicaris, 74% for C. punctatus). The abundance of all vector species increased with the number of sheep on farms, but this relationship was missing from any of the non-vector models. At a large spatial scale, there is significant variation in Culicoides Obsoletus Group abundance, which undermines attempts to record their nationwide distribution in larger scale models. A prior survey should be undertaken for farms with high Culicoides catches within a sampling area and stability in catch size should be checked between seasons and years. (Résumé d'auteur

Local dispersal of palaearctic Culicoides biting midges estimated by mark-release-recapture

Background: Farm to farm movement of Culicoides midges is believed to play a critical role in the spread of bluetongue (BT), Schmallenberg and other midge-borne diseases. To help understand and predict the spread of diseases carried by midges, there is a need to determine their dispersal patterns, and to identify factors contributing to the direction taken and distance travelled. Methods: The dispersal of Obsoletus Group members was studied on 19 farms around Bala, north Wales. Field-collected Culicoides were trapped in a black-light (OVI) trap and self-marked in the collecting vessel, using micronized fluorescent dust. Culicoides were released at a central farm and OVI traps set on 18 surrounding farms, at distances of 1 to 4 km. The study was repeated using six colours of fluorescent dust over an 18 day period. Results: An estimated 61,062 (95% CI = 56,298-65,830) marked Culicoides were released during the study and 12 (0.02%) Culicoides were recaptured. Of the females recaptured, six were C. obsoletus/scoticus, two C. dewulfi, two C. pulicaris and one C. festivipennis. The male was C. obsoletus. Recaptures occurred 1–2.5 km from the release site, with greatest numbers at 2.5 km. Most recaptures were 2 nights post-release; none were more than 3 nights post-release. Two females were recovered at 1.5 km on the night of release and one male at 1 km two nights post-release. The mean distance travelled (MDT) for males was 1 km, females was 2.21 km, and all recaptured Culicoides was 2.15 km. Recaptures were made both downwind and upwind of the prevailing wind direction during the trapping periods, highlighting possible passive and active dispersal of Culicoides between farms. Conclusions: This is the first study to demonstrate farm to farm movement of the main Palaearctic BT vector species, the Obsoletus Group. Such movement has disease control implications in terms of the vectoral movement of disease between farms. The results suggest that Culicoides control measures applied at an infected farm (trapping or killing Culicoides) will reduce risk of spread to neighbouring farms by lessening the number of Culicoides dispersing from that farm, as well as reducing transmission at the source farm itself

Morphometric discrimination of two sympatric sibling species in the Palaearctic region, Culicoides obsoletus Meigen and C-scoticus Downes & Kettle (Diptera: Ceratopogonidae), vectors of bluetongue and Schmallenberg viruses

Background Some Palaearctic biting midge species (subgenus Avaritia) have been implicated as vectors of bluetongue virus in northern Europe. Separation of two species (C. obsoletus and C. scoticus) is considered difficult morphologically and, often, these female specimens are grouped in entomological studies. However, species-specific identification is desirable to understand their life history characteristics, assess their roles in disease transmission or measure their abundance during arboviral outbreaks. This study aims to investigate whether morphometric identification techniques can be applied to female C. obsoletus and C. scoticus individuals trapped at different geographical regions and time periods during the vector season. Methods C. obsoletus and C. scoticus were collected using light-suction traps from the UK, France and Spain, with two geographical locations sampled per country. A total of 759 C. obsoletus/C. scoticus individuals were identified using a molecular assay based on the cytochrome c oxidase subunit I gene. Fifteen morphometric measurements were taken from the head, wings and abdomen of slide-mounted specimens, and ratios calculated between these measurements. Multivariate analyses explored whether a combination of morphometric variables could lead to accurate species identification. Finally, Culicoides spp. collected in France at the start, middle and end of the adult vector season were compared, to determine whether seasonal variation exists in any of the morphometric measurements. Results The principal component analyses revealed that abdominal characteristics: length and width of the smaller and larger spermathecae, and the length of the chitinous plates and width between them, are the most reliable morphometric characteristics to differentiate between the species. Seasonal variation in the size of each species was observed for head and wing measurements, but not abdominal measurements. Geographical variation in the size of Culicoides spp. was also observed and is likely to be related to temperature at the trapping sites, with smaller individuals trapped at more southern latitudes. Conclusions Our results suggest that female C. obsoletus and C. scoticus individuals can be separated under a stereomicroscope using abdominal measurements. Although we show the length and width of the spermathecae can be used to differentiate between the species, this can be time-consuming, so we recommend undertaking this using standardized subsampling of catches. (Résumé d'auteur

Seasonal and spatial heterogeneities in host and vector abundances impact the spatiotemporal spread of bluetongue

Bluetongue (BT) can cause severe livestock losses and large direct and indirect costs for farmers. To propose targeted control strategies as alternative to massive vaccination, there is a need to better understand how BT virus spread in space and time according to local characteristics of host and vector populations. Our objective was to assess, using a modelling approach, how spatiotemporal heterogeneities in abundance and distribution of hosts and vectors impact the occurrence and amplitude of local and regional BT epidemics. We built a reaction–diffusion model accounting for the seasonality in vector abundance and the active dispersal of vectors. Because of the scale chosen, and movement restrictions imposed during epidemics, host movements and wind-induced passive vector movements were neglected. Four levels of complexity were addressed using a theoretical approach, from a homogeneous to a heterogeneous environment in abundance and distribution of hosts and vectors. These scenarios were illustrated using data on abundance and distribution of hosts and vectors in a real geographical area. We have shown that local epidemics can occur earlier and be larger in scale far from the primary case rather than close to it. Moreover, spatial heterogeneities in hosts and vectors delay the epidemic peak and decrease the infection prevalence. The results obtained on a real area confirmed those obtained on a theoretical domain. Although developed to represent BTV spatiotemporal spread, our model can be used to study other vector-borne diseases of animals with a local to regional spread by vector diffusion

Morphometric discrimination of two sympatric sibling species, Culicoides obsoletus" and C. scoticus (Diptera: Ceratopogonidae), vectors of bluetongue and Schmallenberg. [P105]

Purpose: Certain Palaearctic biting midges have been implicated as vectors of bluetongue virus in northern Europe. Separation of two species (Culicoides obsoletus and C. scoticus) is considered difficult morphologically, with females often grouped together in entomological studies. Species specific identification is desirable to assess their roles in disease transmission or measure abundance during arboviral outbreaks. Our aim is to investigate whether morphometric identification techniques can be applied to female C. obsoletus and C. scoticus individuals trapped in different geographical regions and time periods during the year. Methods: Using light-suction traps, female C. obsoletus and C. scoticus were sampled from two locations in the UK, France and Spain. A total of 759 individuals were identified with a molecular assay using the cytochrome oxidase I gene. Fifteen morphometric measurements were then taken from the head, wings and abdomen of slide-mounted specimens. Multivariate analyses investigated whether a combination of these could lead to accurate species identification. Results: Principal component analyses revealed that the length and width of the smaller and larger spermathecae, and the length of, and width between, the chitinous plates can differentiate the species. These are all abdominal characteristics. Seasonal and geographic variation was observed for head and wing measurements, but not for those from the abdomen. Conclusions: Our results suggest that female C. obsoletus and C. scoticus individuals can be separated under a stereomicroscope using abdominal measurements. Although we show that morphometrics can be used to differentiate the species, this can be time-consuming and we recommend undertaking this using standardized subsampling of large catches. Relevance: This work highlights a new morphometric method of discriminating two of the main vector species of bluetongue virus. Such separations generally rely on molecular techniques, which can be expensive. Morphometric identifications may prove useful in outbreak situations when they can be quickly undertaken on a subsample of individuals to determine the proportions of each species present. (Texte intégral

Modelling the spatial distribution of Culicoides biting midges at the local scale. [237]

Purpose: The Culicoides midge vectors of bluetongue (BT) are ubiquitous on farms in the UK, but little research has explored their spatial abundance, an important determinant of disease risk. Models to explain and predict variation in their abundance are needed for effective targeting of BT control methods. Although epidemiological models are commonplace at the national scale, no investigations have taken place at a finer spatial scale. Our aim is to identify determinants of midge abundance at a local 1 km scale. Methods: Midge abundances were estimated using light traps on 35 farms in north Wales. Culicoides catches were combined with remotely-sensed ecological correlates, and on-farm host and environmental data, within a general linear model. Drivers of local scale variation were determined at the 1 km resolution. Results: Local-scale variation in Obsoletus Group abundance exhibited an almost 500-fold difference (74 to 33,720) between farms, but the Obsoletus Group model explained 81% of this variance. The variance explained was consistently high for the Pulicaris Group, C. pulicaris and C. punctatus (80%, 73%, and 74%), the other possible BTV vector species in the UK. The abundance of all vector species increased with the number of sheep on farms, but this relationship was missing from any of the non-vector models. Performance of the non-vector models was also high (65-87% variance explained), but species differed in their associations with satellite variables. Conclusions: At a large spatial scale, there is significant variation in Culicoides Obsoletus Group abundance, undermining attempts to record their nationwide distribution in larger scale models, which have historically explained the abundance of these vectors poorly. Satellite data can be used to explain a high proportion of this variation and may produce effective predictive models of disease vector abundance. Relevance: This work highlights how novel local-scale modelling of disease vectors can explain a large degree of spatial variation that national-scale models fail to explain. This should be of note to policy makers when deciding upon guidelines for entomological surveys before, during and after disease outbreaks. (Texte intégral