24 Jun 2025
Ian Wright
Job Title
Image: dm / Adobe Stock
Tick-borne encephalitis virus (TBEV) and louping ill virus (LIV) are both zoonotic flaviviruses transmitted by Ixodes species ticks.
Historically, TBEV has been endemic in Europe, with its distribution having rapidly expanded this century. Conversely, LIV’s geographical distribution is predominantly in the UK.
Recently, however, evidence of TBEV endemic foci have emerged in the UK, complicating the epidemiological picture. LIV is more virulent in sheep than TBEV, but both can cause progressive and sometimes fatal neurological signs in dogs.
The detection of TBEV in the UK is a concern given its zoonotic potential and ability to cause severe disease in dogs, but how concerned should veterinary professionals, pet owners and the wider public be regarding flavivirus exposure?
The European subtype of TBEV has rapidly spread across central and western Europe in the past few years, where it is primarily transmitted by Ixodes ricinus ticks and maintained in endemic foci by rodent reservoir hosts.
Although larger wild animals such as deer are not considered to be competent hosts for the virus, they are important transport hosts for infected ticks, contributing to the maintenance of large tick populations and contributing to their geographical spread. Migratory birds are also thought to be playing an important role in the rapid dissemination of infected ticks to new geographical areas.
The abundance of I ricinus ticks, transport and reservoir hosts across Europe has allowed the virus to become endemic in many European countries; the Netherlands reported its first human case in 2016 (Jahfari et al, 2017).
This increase in European distribution makes it more likely for the movement of infected ticks into UK and for UK dogs travelling abroad to encounter them.
A surveillance programme carried out by Public Health England in 2018 looked for evidence of TBEV in wild animals and ticks. Serum was collected from 1,309 deer culled across England and Scotland.
Four per cent of samples were ELISA-positive for TBEV, with foci in the New Forest and Thetford Forest. The Thetford Forest area had the highest proportion (47.7%) of seropositive samples. Engorged ticks collected from culled deer within seropositive regions were tested for viral RNA, with 5 of 2,041 ticks tested positive by LIV/TBEV real-time reverse transcription PCR. All of the positive ticks were from the Thetford Forest area, with a full-length genomic sequence of TBEV being identified in one tick (Holding et al, 2020).
This is strong evidence for TBEV being endemic in at least one endemic foci in the UK, and possibly more.
LIV, in contrast, has a predominant geographical distribution in UK upland areas (Jeffries et al, 2014), with reservoirs in grouse and sheep. This means that large geographic areas of the UK are likely to be harbouring ticks infected with one or both pathogens, with an unknown overall distribution of TBEV.
The risk of exposure should be kept in perspective for concerned pet owners, however. 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.
Between 2015 and 2023, 21 TBE cases with a clinical disease were diagnosed in the UK, with 12 of these being diagnosed between 2022 and 2023. Three cases acquired the infection in the UK (Callaby et al, 2025). This is a small number, but demonstrates the importance of ongoing surveillance programmes – especially as up to 60% of encephalitis cases in the UK have unknown causes, so this may well be an underestimate (Kennedy et al, 2017).
Using wildlife surveillance data and spatial modelling, scientists from the UK Centre for Ecology and Hydrology, the UK Health Security Agency, University of Liverpool and University of Glasgow have mapped suitability for TBEV in Britain, identifying key drivers linked to exposure. They have created risk maps that will help to guide future surveillance and prevention strategies (Hassall et al, 2025).
Clinical disease occurs in an estimated one-third of human TBEV infections with an incubation period of 3 to 28 days. The severity of neurological signs can vary from mild meningitis to severe encephalitis with or without myelitis and spinal paralysis (Lindquist and Vapalahti, 2008).
Between 1% and 5% of cases are fatal. TBE should, therefore, be considered a serious zoonosis.
In contrast, the incidence of TBE in dogs appears to be relatively low when compared to humans. In Switzerland, a higher seroprevalence exists in dogs than humans and yet the incidence of clinical cases in dogs in Switzerland is significantly lower than human incidence of disease. When disease does occur, however, it is often severe, with a typical incubation period between 7 and 14 days.
Disease in dogs is febrile in nature, with a variety of neurological signs including ataxia, proprioceptive deficits, seizures, tremor, paresis, paralysis, and cranial nerve deficits such as facial paresis. Neurological signs are often progressive, and disease often fatal.
Although the risk of disease in developing in dogs is lower than in humans, its severity means that dogs with risk of tick exposure should have adequate tick prevention if visiting or residing in endemic areas.
An observational retrospective multi-centre case series study was recently carried out involving three UK-based veterinary referral hospitals of six dogs presenting with an initial phase of pyrexia and/or lethargy, followed by progressive signs of spinal cord and/or intracranial disease. MRI showed bilateral and symmetrical lesions affecting the grey matter of the thalamus, pons and medulla oblongata.
Serology for TBEV was positive in five dogs and one dog with negative serology was positive for immunohistochemistry at postmortem examination.
Three dogs survived, but with neurological sequelae, and three dogs were euthanised due to the severity of clinical signs. The viral distinction between flaviviruses could not be achieved, but demonstrated the importance of considering flaviviruses as a differential when presented with dogs with progressive neurological disease – particularly if initially presenting with pyrexia (Gonzalo-Nadal et al, 2023).
A suspected increase in louping ill cases has also been recently highlighted in UK dogs (Dagleish et al, 2024).
Although geographic location and primarily affected species are largely different between both diseases, the presence of both TBEV and LIV in the UK makes clinical and serological differentiation difficult.
This is due to the antibody cross-reactivity between LIV and TBEV (Holding et al, 2020).
A definitive diagnosis relies on isolation of the causative virus from blood or cerebrospinal fluid, but carries poor sensitivity. Negative results, therefore, do not rule out infection.
IgG serology can be used, with a four-fold increase in titers indicative of acute flavivirus infection.
No specific treatment can target either virus; therefore, treatment is supportive and symptomatic. Pain relief and intravenous fluid therapy may be required alongside anticonvulsives, sedatives and muscle relaxants (Pfeffer and Dobler, 2011).
NSAIDs can be used in dogs with pyrexia and antibiotics for secondary bacterial infections on the basis of culture. Dexamethasone in the convalescence phase has been shown to hasten recovery, but given too early can be detrimental, increasing viral proliferation.
The overall prognosis for infected dogs is very guarded and those that survive often require 6 to 12 months to make a full recovery.
The potential severity of flavivirus infection in both humans and dogs makes tick prevention vital for both dogs and owners whose lifestyle may put them at risk of exposure. This includes those who spend time in green spaces, farmland and deer-inhabited woodland.
The establishment of TBEV in the UK and associated media reports is likely to cause concern among clients who live or exercise their pets in newly discovered endemic areas. Veterinary professionals should keep the relatively low risk of exposure in perspective for clients, while encouraging preventive steps to keep risks to dogs and themselves to a minimum.
Current known and suspected endemic areas are also high incidence areas for human Lyme disease, and tick prevention will help to protect pets and owners against a range of tick-borne diseases.
Preventive measures consist of the following:
Checking for ticks after outdoor activity. Clients should be advised to check their pets after outdoor activity. Any ticks found should be carefully removed with a tick hook or other tick removal device.
Traditional techniques to loosen the tick, such as the application of petroleum jellies or burning, should be avoided, as this will increase the risk of tick stomach and salivary gland contents, and disease transmission as a result.
When checking for ticks, clients should be reminded that nymphs are very small and may easily be missed – especially in areas of long fur.
If examination is difficult or time limited, then the majority of ticks are found around the head, neck and limbs (Wright et al, 2018).
Use of tick-preventive products. The use of prophylactic compounds that rapidly kill or repel ticks are useful in reducing tick feeding and, therefore, transmission of infection. Licensed products containing an isoxazoline, permethrin, deltamethrin or flumethrin all fulfil these criteria.
It is important to consider compliance when discussing which product to use, as well as lifestyle. Whether a client prefers or is able to administer a tablet, collar or spot-on should be established, as well as whether the pet has had reactions to products in the past. Frequent swimming or bathing may make some topical products unsuitable.
It should also be remembered that no product is 100% effective; owners should, therefore, still be advised to check their pet for ticks after outdoor activity.
Vaccination. In Europe, TBE vaccines are licensed for human use, but neither have been licensed for animal use, including for dogs.
Off-licence studies in dogs have shown some efficacy, but further safety and efficacy studies are needed (Pfeffer and Dobler, 2011).
Positive dogs pose no significant risk to owners, as ticks are required for transmission. They are, however, excellent sentinels for human infection, as dog owners are likely to be at risk of infected tick exposure at the same time while walking their dog.
Infected Ixodes species ticks on imported and travelled animals may introduce the virus into new areas, with the potential for further endemic foci to establish.
Pets arriving in the UK from abroad should, therefore, be checked for ticks and if not already treated, a tick treatment applied.
LIV and TBEV are both pathogenic tick-borne flaviviruses capable of affecting humans and dogs in Europe.
The rapid spread of TBEV across Europe and establishment in the UK is, therefore, of concern, but the risk it poses to pets and owners should be kept in perspective.
The presence of both flaviviruses in the country, however, is another compelling reason for tick-preventive strategies in dogs at high risk of tick exposure.
Flavivirus infection should be considered as a differential in UK dogs presenting with neurological signs – especially if associated with pyrexia.
Veterinary professionals have a vital role in keeping the risks of exposure in perspective for pet owners, while also ensuring that adequate prevention is in place for their pets.
