Tick-borne encephalitis: an increasing threat in Europe

Abstract
Tick-borne encephalitis, caused by the tick-borne encephalitis virus, is a rapidly emerging disease in Western, Central and Northern Europe, affecting dogs and people with potentially fatal consequences. This rapid spread, alongside the presence of the Ixodes ricinus vector throughout the UK, had led to concerns that it may become endemic through introduction of infected ticks on imported animals or on migratory birds. This was realised last year when evidence of endemic foci in the UK was demonstrated, particularly in Thetford Forest. This article reviews current information on tick-borne encephalitis, its distribution in Europe and the risk it poses to UK dogs and their owners.
Tick-borne encephalitis virus (TBEV) is a flavivirus closely related to, but distinct from louping ill virus (LIV) in sheep. Although TBEV is known to be less virulent than LIV in sheep (Gritsun et al, 2003), it can cause a neurological disease known as tick-borne encephalitis (TBE) in humans, and less commonly, dogs. There are three subtypes of TBEV currently recognised (European, Siberian, and Far Eastern), although Baikalian and Himalayan subtypes have also been proposed (Dai et al, 2018).
The European subtype has rapidly spread across Central and Western Europe in recent years, where it is primarily transmitted by Ixodes ricinus ticks and maintained in endemic foci. In endemic areas, the prevalence of TBEV in questing ticks rarely exceeds 1%, even where human incidence of disease is high (Imhoff et al, 2015). The risk of human and canine infection from short visits to endemic areas with limited tick exposure is therefore low. People and pets living, frequently visiting or working in endemic areas, however, would be at significantly greater risk of exposure over time. This has made the possibility of TBEV establishing in the UK in the face of an increasing European distribution, increased pet travel and pet importation a concern, as endemic domestic foci would put the UK human and canine population at greater risk. That concern now appears to be a reality, since evidence has emerged that endemic foci are present in the UK, particularly in the Thetford Forest.
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