Tick-borne encephalitis virus arrives in the UK

Tick-borne encephalitis (TBE) is caused by the TBE virus (TBEV) and is primarily transmitted to humans through a bite from an infected tick. TBEV (a flavivirus) has three genetically and antigenically closely related subtypes: the European, Siberian and Far-Eastern. TBE is endemic in mainland Europe, Scandinavia and Asia, and has recently been found for the first time in a small number of ticks in two parts of the UK: in Thetford Forest, Norfolk, and in the Hampshire–Dorset border area (Holding et al. Tick-borne encephalitis virus, United Kingdom. Emerg Infect Dis. 2020; 26(1): doi: 10.3201/ eid2601.191085). The epidemiology differs substantially between low- and high-risk areas. In different endemic areas, the risk of infection for humans after a single tick bite varies between 1 : 200 and 1 :1000. It is estimated that one third of infections result in clinical symp-toms, which typically follow a biphasic course. This starts with non-specific flu-like symptoms, followed by an asymptomatic interval. The second phase then develops where the central nervous system is affected, ranging from mild meningitis to severe encephalitis with or without myelitis and spinal paralysis. The European subtype is associated with more mild disease, with mortality rates from 0.5 to 2 % and severe neurological conse-quences in up to 10% of patients. There is no known treatment but vaccination is very effective at disease prevention. Because of the vague clinical symptoms, TBE diagnosis has to be confirmed by clinical lab detection of specific anti-TBEV antibodies in blood or cerebrospinal fluid by enzyme-linked immunosorbent assay, immunofluorescence assay or hemagglutination inhibition. TBE antibodies appear 0–6 days after onset and are usually detected when neurological symptoms are present. Care is needed in interpretation of results; specific IgM antibodies can persist for up to 10 months in vaccinees or individuals who acquired the infection naturally; IgG antibody cross-reaction is possibly observed with other flaviviruses (such as louping ill virus, which is already present in the UK). Detection of genetic material by PCR methods could be valuable for an early differential diagnosis of TBE but depends on timely analysis in the short window of viremia. The risk of acquiring TBE in the UK is very low – a greater to risk to UK nationals is holidaying in high-risk areas, such as Austria, Germany, Switzerland and central Europe. However, now we know it is here and we know we can test for it, we need awareness in the public and GPs to make the association between the tick bite and the vague symptoms and the test to be requested – which as we know from Lyme disease is a challenge.