The Culicoides ‘snapshot’: A novel approach used to assess vector densities widely and rapidly during the 2006 outbreak of bluetongue (BT) in The Netherlands

Abstract

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 20 2 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: • A total of >35,000 Culicoides were captured ( μ = 333 midges/light trap) representing 16 species; • Not a single specimen of C. imicola, the principal Afro-Asiatic vector of BTV in southern Europe, was captured; • The three European species of Culicoides implicated previously as potential vectors in the Mediterranean region and the Balkans also occur in The Netherlands; • The Obsoletus Complex (represented by both potential vectors Culicoides obsoletus and Culicoides scoticus) was the most prevalent taxon (20 2 gFR: 93.4%) followed by the Pulicaris Complex (76.4%); • The Pulicaris Complex comprised at least six species. One of these, Culicoides pulicaris sensu stricto ( ss), a potential vector in southern Europe, had a low 20 2 gFR of 17.9% indicating it played no role in the transmission of BTV; • The next most prevalent taxa were Culicoides dewulfi (70.8%) and Culicoides chiopterus (67.0%) both of which breed in cattle dung. This close association with livestock raises their potential as vectors of BTV; • Culicoides achrayi, Culicoides circumscriptus, C. dewulfi, Culicoides halophilus, Culicoides lupicaris and Culicoides stigma are new species records for The Netherlands and demonstrates that the extant knowledge on the Culicoides fauna of northern Europe is incomplete; • Cooling trends in the weather induced a >50-fold decrease in overall Culicoides numbers but with subsequent warming these would rebound rapidly indicating that more intense cold is required before heightened mortalities are induced amongst adult biting midges. The northward advance of BT in Europe compels the competent authorities in affected and in neighbouring territories to acquire rapidly baseline information around which to plan sound vector surveillance and livestock movement strategies. The Culicoides ‘snapshot’ is a tool well suited to this purpose. It is stressed that a vector surveillance program must be built upon a firm taxonomic base because misidentifications will flaw the mapped seasonal and geographic distribution patterns upon which veterinary authorities depend.