Could drought conditions trigger Schmallenberg virus and other arboviruses circulation?

Abstract

Background: In 2011, a new orthobunyavirus, named the Schmallenberg virus (SBV), was discovered in Europe. Like the related Shamonda virus, SBV is an arbovirus (arthropod-borne virus). After its discovery, the virus was detected in a wide area in north-western Europe, an unexpected finding in a territory where climatic conditions would not seem ideal for arbovirus transmission. This sudden expansion suggests the effect of 2011 drought as a key factor that may have triggered SBV circulation. The possible influence of drought, recorded in north-western Europe in early 2011, on virus circulation was evaluated.

Methods and results: The locations of SBV detections in Europe until April 2012 were obtained, and area of virus circulation was evaluated by kernel density estimation. Precipitation data in SBV circulation area, summarized by the 3 month precipitation indexes of May, were compared with precipitation data outside that area, confirming driest conditions in that area.

Conclusions: The onset of drought conditions recorded in the SBV detection area in early 2011 may have promoted the circulation of this virus. A correlation between circulation of some arboviruses and drought has been reported elsewhere. This was mainly explained by an effect of water deficit on the environment, which altered the relationships between vectors and reservoirs, but this correlation might be also the result of unknown effects of drought on the vectors. The effect of drought conditions on arbovirus circulation is most likely underestimated and should be considered, since it could promote expansion of arboviruses into new areas in a global warming scenario.

Figure 1

Drought conditions in Schmallenberg virus (SBV) circulation area in 2011. The sites of SBV detection until April 2012 are shown on Europe map (a). These sites were utilized to evaluate the area of virus circulation by kernel density estimation (KDE) (b). Then (c) 95% and 50% volume contours of KDE (yellow line and red line respectively) were overlaid to the map showing the interpolated 3 month standardized precipitation indices (SPI3) of May 2011. SPI3 value: red <-2, orange >-2 <-1, green >-1 <1, azure >1 <2 , blue>2. SPI3 indices were provided by European Drought Observatory [24], due to the improvement in measurements, future SPI values, referring to the same period, may be slightly different.