Tick-borne encephalitis: an increasing threat in Europe

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.