In September 2023 the first cases of bluetongue appeared in the Netherlands. The virus spread was rapid. Researchers of Wageningen Bioveterinary Research looked into the spatial transmission characteristics of the 2023 bluetongue epidemic. "We found that the virus dispersed over long distances. In addition to between-farm spread via midges, other transmission routes probably played a role", states the WBVR research team.
A devastating bluetongue (BT) epidemic caused by bluetongue virus serotype 3 (BTV-3) has spread throughout most of the Netherlands in the autumn of 2023. In total, tens of thousands of sheep died or had to be euthanised. By October 2023, more than 2.200 locations with ruminant livestock were officially identified to be infected with BTV-3. Additionally, ruminants from 1.300 locations were showing BTV-associated clinical symptoms (but not laboratory-confirmed BT). Based on the available data from the epidemic, researchers of Wageningen Bioveterinary Research (WBVR, part of Wageningen University & Research) characterized the intensity of the transmission related to the distance.
Between farms
The research team looked into the data of the bluetongue epidemic of 2023 and compared it to the previous bluetongue epidemics of 2006 and 2007 in Northwestern Europe. At that time the epidemic was caused by BTV serotype 8. For the 2006/2007 epidemic, transmission kernels were estimated using the between-farm transmission kernel approach. "This method avoids explicit modelling of different routes of spread, such as midges or by 'over the fence'-transmission and animal movement in the field. Yet by characterizing the distance-dependent probability of transmission it provides clues about the possible roles of different routes."
Analyses
Based on the epidemical analyses, the WBVR research team concludes that the 2023 BTV-3 kernel parameters are in line with those of the transmission kernel estimated previously for the spread of BTV-8 in Europe in 2006 en 2007. "This suggests that the distances over which the virus was transmitted in 2023 followed a comparable distribution to that of the 2006 and 2007 epidemic", according to the researchers.
Long distance
"We found that the 2023 BTV-3 transmission kernel has a long-distance spatial range across tens of kilometres. This finding underlines that in addition to short-distance dispersal of the bluetongue virus via infected midges, other transmission routes probably played an important role." One of the likely alternative transmission routes is via livestock transport.
