The Culicoides 'snapshot': a novel approach used to assess vector densities widely and rapidly during the 2006 outbreak of bluetongue in The Netherlands

A novel method was developed and implemented during the recent outbreak of bluetongue (BT) in sheep and cattle in The Netherlands to obtain rapidly a `snapshot¿ of Culicoides vector densities at the national level. The country was divided into 110 raster cells, each measuring 20 km × 20 km; within 106 of these cells, a farm was selected with a minimum of 10 cattle and sampled for Culicoides for one night only using the Onderstepoort-type blacklight trap. Prior to deployment of the light traps in the field, local veterinarians were trained in their use and in the preservation of captured Culicoides. The collections were despatched daily by courier to a field laboratory where the Culicoides were counted and identified. The `snapshot¿ commenced on 12 September 2006 and was completed on 28 September coinciding with the 5¿7 weeks of BT virus (BTV) activity in The Netherlands and when the number of weekly cases of disease was on the rise. Analysis of the 106 collections was completed on 5 October. The number of grid cells in which a taxon occurred is represented by the index 202 gFR (=20 km × 20 km grid Frequency Rate); this index essentially reflects the percentage of examined raster cells found to contain the potential vector in question. The `snapshot¿ results can be summarised as follows: