Abstract:
Indigenous Culicoides biting midges are suggested to be putative vectors for the recently emerged Schmallenberg virus
(SBV) based on SBV RNA detection in field-caught midges. Furthermore, SBV replication and dissemination has been
evidenced in C. sonorensis under laboratory conditions. After SBV had been detected in Culicoides biting midges from
Belgium in August 2011, it spread all over the country by the end of 2011, as evidenced by very high between-herd
seroprevalence rates in sheep and cattle. This study investigated if a renewed SBV circulation in midges occurred in 2012 in
the context of high seroprevalence in the animal host population and evaluated if a recently proposed realtime RT-PCR
approach that is meant to allow assessing the vector competence of Culicoides for SBV and bluetongue virus under
laboratory conditions was applicable to field-caught midges. Therefore midges caught with 12 OVI traps in four different
regions in Belgium between May and November 2012, were morphologically identified, age graded, pooled and tested for
the presence of SBV RNA by realtime RT-PCR. The results demonstrate that although no SBV could be detected in
nulliparous midges caught in May 2012, a renewed but short lived circulation of SBV in parous midges belonging to the
subgenus Avaritia occured in August 2012 at all four regions. The infection prevalence reached up to 2.86% in the south of
Belgium, the region where a lower seroprevalence was found at the end of 2011 than in the rest of the country.
Furthermore, a frequency analysis of the Ct values obtained for 31 SBV-S segment positive pools of Avaritia midges showed
a clear bimodal distribution with peaks of Ct values between 21–24 and 33–36. This closely resembles the laboratory results
obtained for SBV infection of C. sonorensis and implicates indigenous midges belonging to the subgenus Avaritia as
competent vectors for SBV.
