Since 1998 bluetongue virus (BTV), which causes bluetongue, a non-contagious, insect-borne infectious disease of ruminants, has expanded northwards in Europe in an unprecedented series of incursions, suggesting that there is a risk to the large and valuable British livestock industry. The basic reproduction number, R 0 , provides a powerful tool with which to assess the level of risk posed by a disease. In this paper, we compute R 0 for BTV in a population comprising two host species, cattle and sheep. Estimates for each parameter which influences R 0 were obtained from the published literature, using those applicable to the UK situation wherever possible. Moreover, explicit temperature dependence was included for those parameters for which it had been quantified. Uncertainty and sensitivity analyses based on Latin hypercube sampling and partial rank correlation coefficients identified temperature, the probability of transmission from host to vector and the vector to host ratio as being most important in determining the magnitude of R 0 . The importance of temperature reflects the fact that it influences many processes involved in the transmission of BTV and, in particular, the biting rate, the extrinsic incubation period and the vector mortality rate.

Control of many infectious diseases relies on the detection of clinical cases and the isolation, removal, or treatment of cases and their contacts. The success of such "reactive" strategies is influenced by the fraction of transmission occurring before signs appear. We performed experimental studies of foot-and-mouth disease transmission in cattle and estimated this fraction at less than half the value expected from detecting virus in body fluids, the standard proxy measure of infectiousness. This is because the infectious period is shorter (mean 1.7 days) than currently realized, and animals are not infectious until, on average, 0.5 days after clinical signs appear. These results imply that controversial preemptive control measures may be unnecessary; instead, efforts should be directed at early detection of infection and rapid intervention.

Summary 1.Bluetongue virus (BTV), a pathogen of ruminants transmitted by Culicoides midges, has emerged dramatically across Europe since 1998. Surveillance of Culicoides is currently carried out in every European country affected by BTV, most commonly using standardized light-suction traps that sample the population of adult Culicoides present. To date, however, it is not clear whether these trap catches accurately reflect the biting population of Culicoides . 2. In our study, we carried out 192 drop-trap catches at dusk on Poll Dorset sheep, drawing comparisons with surveillance samples taken using standard trapping protocol at the same site. A multiplex polymerase chain reaction technique was optimized for high-throughput processing of cryptic species to allow Culicoides collected to be identified to species level, the first time this has been achieved in this region for an ecological study. A mathematical model was then constructed to describe rates of midge biting under the meteorological conditions recorded during trials. 3. Light-trapping surveillance was found to substantially underestimate the numbers of Culicoides chiopterus present on sheep. This finding is extremely significant given that, due to the low numbers of this species caught at light traps across northern Europe, C. chiopterus had not been seriously considered as a potential vector of BTV. 4. The models constructed of successful blood feeding predict that while biting rates on sheep are significantly reduced under conditions adverse to midge flight (wind speeds that exceeded 3 mps, wind turbulence of greater than 40 degrees change in direction during the trial and solar intensity exceeding 200 Wm -2 ), low levels of biting can also occur under sub-optimal meteorological conditions, that have the potential to span entire days in the later part of the adult season. 5. Synthesis and applications. It is clear from these results that light-trapping surveillance does not provide an accurate reflection of the biting population of Culicoides present. Given that the surveillance systems are already in wide employment across Europe, but provide misleading results as we have demonstrated, and considering the huge economic and animal welfare impact of BTV incursions, it is vital that alternative/additional methods of surveillance are explored. Additionally, the vector competence of C. chiopterus as a potential vector of arboviruses requires urgent assessment, along with a clearer understanding of diel periodicity of Culicoides attacks throughout the adult season.

Summary1. Culicoides biting midges are vectors of internationally important arboviruses including bluetongue virus (BTV). The ecological constraints imposed by the small body size of these insects strongly influence the epidemiology of the diseases they can carry. Bluetongue virus recently emerged in northern Europe, and atmospheric dispersion models have subsequently been employed to simulate vector movement (and hence likely spread of BTV). The data underlying such models, however, have hitherto either been obtained from small-scale studies or from outside the northwestern Palaearctic. 2. The effects of seasonality and local meteorological conditions upon the daily presence and abundance of Culicoides vectors were examined using 2760 samples collected across a network of 12 different habitat types in England during 2008. Over 50 000 individuals were estimated to be in the samples with males constituting 62% of the total collection, allowing straightforward comparison between potential vector species in terms of their activity rates and seasonality. Culicoides abundance was linked to livestock density and land use. Farm-associated Culicoides species were recorded at all sites including species thought to be restricted to this ecosystem by larval habitat, suggesting a greater potential for dispersal over land than previously thought. 3. Synthesis and applications. The model developed has already been applied in a functional dispersion model to predict disease risk from wind-borne infected Culicoides incursion into the UK and elsewhere. The study has expounded the long-distance dispersal potential of Culicoides, essential for future prediction of the incursion and spread of Culicoides-borne pathogens. It has additionally contributed to the understanding of the ecology of highly dispersive insect vectors.

BackgroundRecently much attention has been given to developing national-scale micro-simulation models for livestock diseases that can be used to predict spread and assess the impact of control measures. The focus of these models has been on directly transmitted infections with little attention given to vector-borne diseases such as bluetongue, a viral disease of ruminants transmitted by Culicoides biting midges. Yet BT has emerged over the past decade as one of the most important diseases of livestock.Methodology/Principal FindingsWe developed a stochastic, spatially-explicit, farm-level model to describe the spread of bluetongue virus (BTV) within and between farms. Transmission between farms was modeled by a generic kernel, which includes both animal and vector movements. Once a farm acquired infection, the within-farm dynamics were simulated based on the number of cattle and sheep kept on the farm and on local temperatures. Parameter estimates were derived from the published literature and using data from the outbreak of bluetongue in northern Europe in 2006. The model was validated using data on the spread of BTV in Great Britain during 2007. The sensitivity of model predictions to the shape of the transmission kernel was assessed.Conclusions/SignificanceThe model is able to replicate the dynamics of BTV in Great Britain. Although uncertainty remains over the precise shape of the transmission kernel and certain aspects of the vector, the modeling approach we develop constitutes an ideal framework in which to incorporate these aspects as more and better data become available. Moreover, the model provides a tool with which to examine scenarios for the spread and control of BTV in Great Britain.