A new algorithm quantifies the roles of wind and midge flight activity in the bluetongue epizootic in North-West Europe

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Luigi Sedda; Heidi Brown, Beth Purse, Laura Burgin, John Gloster, David Rogers

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Proc Biol Sci. 2012 Feb 8. [Epub ahead of print]



Data description: 

Model, Midge movements, farm level blue tongue occurrence


directional correlation, stochastic simulation, wind speed and direction, disease speed and direction, 3D-geometric vectors, trajectory analysis, insect flight, incubation period, Culicoides spp., spatio-temporal epidemiology


The 2006 bluetongue outbreak in North-Western Europe had devastating effects on cattle and sheep in that intensively farmed area. The role of wind in disease spread, through its effect on Culicoides dispersal, is still uncertain, and remains unquantified. We examine here the relationship between farm-level infection dates and wind speed and direction within the framework of a novel model involving both mechanistic and stochastic steps. We consider wind as both a carrier of host semio-chemicals, to which midges might respond by up-wind flight, and as a transporter of the midges themselves, in a more or less down-wind direction. For completeness, we also consider midge movement independent of wind and various combinations of up-wind, down-wind and random movements. Using stochastic simulation we are able to explain infection onset at 94% of the 2,025 affected farms. We conclude that 54% of outbreaks occurred through (presumably midge) movement of infections over 28 distances of no more than 5km, 92% over distances of no more than 31 km and only 2% over distances greater than this figure. The modal value for all infections combined is less than 1 km. Our analysis suggests that previous claims for a higher frequency of long distance infections are unfounded. We suggest that many apparent long distance infections resulted from sequences of shorter range infections; a ‘stepping stone’ effect. Our analysis also found that down-wind movement (the only sort so far considered in explanations of bluetongue epidemics) is responsible for only 39% of all infections, and highlights the effective contribution to disease spread of upwind midge movement, which accounted for 38% of all infections. The importance of midge flight speed is also investigated. Within the same model framework, lower midge active flight speed (of 0.13 rather than 0.5 m/s), reduced virtually to zero the role of upwind movement, mainly because modelled wind speeds in the area concerned were usually greater than such flight speeds. Our analysis therefore highlights the need to improve our knowledge of midge flight speed in field situations, which is still very poorly known. Finally, the model returned an intrinsic incubation period of 8 days, in accordance with the values reported in the literature. We argue that better understanding of the movement of infected insect vectors is an important ingredient in the management of future outbreaks of bluetongue in Europe, and other devastating vector-borne diseases elsewhere.