Bulk-Tank Milk Longitudinal Serosurvey Reveals Decreasing Schmallenberg Virus Circulation in a Confined Population of Portuguese Sheep

Introduction: In 2011, Schmallenberg virus (SBV) was first detected in dairy cattle herds in The Netherlands and Germany having since then spread across Europe. Today studies are starting to show a decrease in new SBV infections, a circumstance that raises alerts for possible re-emergence if ideal conditions for vector development occur. To assess the potential decrease in SBV circulation, we performed a 2-year longitudinal serological investigation for SBV infection at the herd level by using bulk-tank milk of a specific sheep breed from central Portugal. Materials and Methods: Bulk-tank milk samples from 68 flocks were collected in both 2015 and 2016, and lactosera were tested for IgG anti-SBV by EIA. Results and Discussion: Results show that in 2015, 92.6% (95% confidence interval [CI]: 83.9–96.8) of the bulk-tank milk samples were positive, whereas in 2016 only 77.9% (95% CI: 66.7–86.1 of the samples from the same flocks were positive. Differences in the 2015/2016 seroprevalences showed to be statistically significant (p = 0.027). This significant decrease at the herd level seems to be in agreement with reported data from other European countries and raise alerts, since increasingly favorable conditions (higher number of susceptible animals) are now present, potentially favoring SBV epidemics if improved conditions for midge replication occur in the future.