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Summer 2023 accelerating Orbivirus emergence in Europe
Orbiviruses are a genus of viruses characterized by their segmented double stranded RNA genome, able to mutate and adapt to a changing environment in different species. These viruses are considered as arboviruses i.e., “arthropod borne viruses” meaning that mammalians can be infected through virus inoculation by arthropods such as mosquitoes, ticks, or midges. This genus of viruses is responsible for several groups of diseases, among them Bluetongue (BTV), Epizootic Hemorrhagic disease (EHD) and African Horse Sickness (AHS) are considered major threats. For each of these 3 diseases, there are several circulating serotypes leading to several clinical forms and making immunization a challenge because an animal immunized against one serotype is not protected against the others. Over the past 2 decades Europe has faced several waves of BTV infections by several serotypes coming from Africa and the Middle East. Initially, the vector was an exotic midge (Culicoides imicola which is adapted to hot climate and not to the European temperate climate), and BTV outbreaks were considered to be limited to the Southern part of Europe around the Mediterranean Basin where the exotic midge can survive. Then unexpectedly, in 2006 serotype 8 appeared in Northern Europe - starting from the Netherlands - and quickly spread across all continental Europe. BTV-8 remarkably adapted to a new competent host: a midge from the Culicoides obsoletus complex,
changing dramatically the potential spread of the virus to the ruminant populations in Europe. It took Merial less than two years to get a safe and efficient BTV-8 vaccine, and eventually Europe managed to control the expansion of this disease responsible for severe illness on sheep and cattle thanks to a combination of biosecurity measures, diagnostics and vaccination. For years, Europe has been monitoring other orbiviruses circulating in Northern Africa and making shy incursions in the more southern parts of the Old Continent. In 2022, BTV-3, known to be transmitted by Culicoides imicola was detected in Sicily and southern Sardinia originating from Tunisia. Its northward expansion was predicted to follow the movements of its insect vector. In unison, another disease transmitted by the same vector appeared: Epizootic Hemorrhagic Disease Virus (EHDV) serotype 8. This disease originally affects deers and antelopes, but a few years ago cases in cattle farms were reported in Tunisia. In 2022, cases affecting cattle in Sicily, Sardinia and extreme Southern Spain were reported. Each time the incriminated vector was Culicoides imicola and the disease was forecasted to progress slowly in Europe due to the reduced geographic distribution of this midge.
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