Even though the largest part of livestock in the Netherlands has become immune to Schmallenberg virus (SBV), the virus is still circulating. A recent study by Central Veterinary Institute (CVI), part of Wageningen UR, shows a large decrease of the percentage of SBV infected Culicoides biting Midges in the Netherlands.
In the Netherlands, biting midges of the Obsoletus complex were implicated to act as the main field vectors for SBV in 2011. A major part of livestock in Europa became infected with SBV; in the Netherlands 80 - 100%. The proportion of SBV-infected Culicoides of the Obsoletus complex in 2011 was 0.5%. In 2012 this percentage decreased by a factor 5: 0,1%.
Circulation SBV despite very high herd immunity
In dairy farms where 96% resp. 100% of the adult animals had seroconverted, and therefore were not susceptible for SBV, Culicoides midges were found with a low viral load. This may indicate that SBV is being transmitted very efficiently. However, the percentage of SBV-infected Culicoides midges is significant lower than the percentages found in 2011.
Prediction SBV epidemic in Europa
One can only speculate about the further spread of the epidemic. In countries earlier affected, the remaining susceptible hosts may become infected and in the long run the epidemic could fade-out in these areas. However, while expanding geographically the number of young susceptible animals in the first affected countries at a certain stage might also increase as a result of growth of newborns. In such a scenario, whereas the frontline of the epidemic is still moving, infections might reoccur in the primary outbreak regions and it is to be seen where it will end.
Reference: Elbers ARW, Meiswinkel R, van Weezep E, Kooi EA, van der Poel WHM. Schmallenberg virus in Culicoides biting Midges in the Netherlands in 2012: a decreased level of circulation compared to 2011. Transboundary and Emerging Diseases 2013 (accepted)