In summer and autumn 2023, as often, orbiviruses did not behave as what was forecasted:
1. EHDV Early July 2023, EHD serotype 8 started to spread amongst deer and cattle farms in Southern Spain progressing northward across the Iberic peninsula during July and August and crossing the Pyrenees moutains. The first cattle farms infected in France were confirmed by ANSES on the 21 st of September 2023. More than 3,700 farms are infected as of early January 2024 in France and nearly half of the French metropolitan territory is impacted by surveillance measures and trade restrictions on cattle. 2.BTV-8 Meanwhile, France faced an unusual BTV-8 outbreak starting early August 2023, clinical signs on sheep and cattle were much more severe than those reported previously. The French National Reference Lab for BTV confirmed an outbreak caused by a new BTV-8 strain quite different from the strains reported previously. This outbreak can be controlled by the current BTV-8 vaccine (BTVPUR) available in France. Trials performed by the Reference Lab confirmed the good protection induced by this vaccine. 3.BTV-3 Regarding BTV-3, an unpredictable scenario started in the Netherlands on the 3 rd of September 2023. The BTV free country (since 2012) witnessed its first clinical cases in sheep. BTV serotype 3 was confirmed; a strain closely related to the ones circulating in Tunisia, Sicily and Southern Sardinia. The outbreak progressed fast in sheep and cattle farms, from 4 sheep farms infected initially to the total number of cases increasing to 324 in sheep flocks and 61 in cattle herds by the third week. As of early January 2024, nearly 6,000 outbreaks have been recorded in The Netherlands and a few cases have been reported in Belgium, Germany and the UK. The outbreak is estimated to progress by 20 km per week and shows a faster spread and more severe incidence than the 2006 BTV-8 outbreak. The mortality in sheep is reported to reach up to 50%.
These orbiviral diseases have been monitored because of the significant risk of their emergence in Europe. Their historical vector has a repartition area in typical hot Mediterranean climate and until 2023 it was only observed in the Southern fringes of Europe. Their expansion was considered likely to be gradual and slow. Their sudden emergence is likely linked to the adaptation of BTV-3 and EHDV-8 to other midge vectors from the Culicoides obsoletus complex. These midges are well adapted to continental European climate and remain active at temperatures as low as 10°C. This explains the active transmission period we observed for these viruses, which lasted until December 2023, followed by a quieter period during the coldest part of the winter and, most likely, a resurgence linked to the increase of midges’ activity starting from next spring. EHDV and BTV-3 are likely to expand further in the UK and in Continental Europe in 2024.
Dr Guillaume Convert VPH Technical Service Director Veterinary Public Health Center Boehringer Ingelheim Animal Health
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