Spatial and temporal variation in the abundance of Culicoides biting midges (Diptera: Ceratopogonidae) in nine European countries

Abstract

Background: Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) are vectors of bluetongue virus (BTV), African horse sickness virus and Schmallenberg virus (SBV). Outbreaks of both BTV and SBV have affected large parts of Europe. The spread of these diseases depends largely on vector distribution and abundance. The aim of this analysis was to identify and quantify major spatial patterns and temporal trends in the distribution and seasonal variation of observed Culicoides abundance in nine countries in Europe.

Methods: We gathered existing Culicoides data from Spain, France, Germany, Switzerland, Austria, Denmark, Sweden, Norway and Poland. In total, 31,429 Culicoides trap collections were available from 904 ruminant farms across these countries between 2007 and 2013.

Results: The Obsoletus ensemble was distributed widely in Europe and accounted for 83% of all 8,842,998 Culicoides specimens in the dataset, with the highest mean monthly abundance recorded in France, Germany and southern Norway. The Pulicaris ensemble accounted for only 12% of the specimens and had a relatively southerly and easterly spatial distribution compared to the Obsoletus ensemble. Culicoides imicola Kieffer was only found in Spain and the southernmost part of France. There was a clear spatial trend in the accumulated annual abundance from southern to northern Europe, with the Obsoletus ensemble steadily increasing from 4000 per year in southern Europe to 500,000 in Scandinavia. The Pulicaris ensemble showed a very different pattern, with an increase in the accumulated annual abundance from 1600 in Spain, peaking at 41,000 in northern Germany and then decreasing again toward northern latitudes. For the two species ensembles and C. imicola, the season began between January and April, with later start dates and increasingly shorter vector seasons at more northerly latitudes.

Conclusion: We present the first maps of seasonal Culicoides abundance in large parts of Europe covering a gradient from southern Spain to northern Scandinavia. The identified temporal trends and spatial patterns are useful for planning the allocation of resources for international prevention and surveillance programmes in the European Union.

Keywords: Culicoides abundance; Culicoides distribution; Europe; Seasonal abundance; Spatial pattern; Temporal trend; Vector season; Vector-borne disease.

Fig. 1

Available data from sampled farms in Europe during entomological surveys from 2007 to 2013. Latitudinal ranges were defined for every 5 degrees of latitude. From south to north, latitudinal ranges were named A, B, C, D, E, F and G

Fig. 2

Left column: Obsoletus ensemble weekly average (log scale) with 10th and 90th percentiles for an average year per latitudinal zone (A-F). Right column: cumulative weekly number of vectors per year, by latitudinal zone. The latitudinal zones ranged from southern Spain (A) to the northern Scandinavia (F)

Fig. 3

Left column: Pulicaris ensemble weekly average (log scale) with 10th and 90th percentiles for an average year per latitudinal zone (A-F). Right column: cumulative weekly number of vectors per year, by latitudinal zone. The latitudinal zones ranged from southern Spain (A) to the northern Scandinavia (F)

Fig. 4

Left column: C. imicola weekly average (log scale) with 10th and 90th percentiles for an average year per latitudinal zone (A-B). Right column: number of vectors per year, calculated as the cumulative sum of the weekly average multiplied by 7, by latitudinal range

Fig. 5

Obsoletus ensemble monthly mean abundance. Dots show observed monthly mean abundance at sampled farms. Spatially interpolated abundance is shown in color. Interpolation values are displayed on the same scale as the observed abundance

Fig. 6

Pulicaris ensemble monthly mean abundance. Dots indicate observed monthly mean abundance in sampled farms. Spatially interpolated abundance is shown in color. Interpolation values are displayed on the same scale as the observed abundance

Fig. 7

C. imicola monthly mean abundance. Dots indicate observed monthly mean abundance in sampled farms. Spatially interpolated abundance is shown in color. Interpolation values are displayed on the same scale as the observed abundance

Fig. 8

Start of the vector season for the Obsoletus ensemble by NUTS 3 polygons and by month

Fig. 9

Start of the vector season for the Pulicaris ensemble by NUTS 3 polygons and by month

Fig. 10

Start of the vector season for C. imicola by NUTS 3 polygons and by months