Tick-borne encephalitis virus (TBEV) is a flavivirus closely related to, but distinct from, louping ill virus (LIV) in sheep. Although TBEV is less virulent than LIV in sheep (Gritsun et al., 2003), it can cause a neurologic disease known as tick-borne encephalitis (TBE) in humans and, less commonly, in dogs. Its detection in the UK has therefore been a concern, but how concerned should veterinary professionals, pet owners and the wider public be? To assess the risk that TBE poses in the UK and the need to put preventative measures in place, a number of factors need to be considered.
Prevalence and spread of TBEV in Europe
There are three subtypes of TBEV currently recognised (European, Siberian and Far Eastern), although Baikalian and Himalayan subtypes have also recently 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 (Figure 1) 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 can serve as reproductive and transport hosts for infected ticks. These hosts allow maintenance of tick populations and contribute to geographical spread of TBEV. The abundance of Ixodes ricinus ticks, deer and rodent hosts in Central and Western Europe has allowed the virus to become endemic in Scandinavia and many Eastern, Western and Southern European countries. The spread of TBEV in Western Europe was confirmed recently when the Netherlands reported its first human case in 2016 (Jahfari et al., 2017). This increase in European distribution makes the introduction of the virus to UK tick populations more likely via the movement of pets and animals from these countries to the UK. Migratory birds also play a significant role in carrying infected ticks over large distances and the chances of birds being exposed increases as more countries are affected.
Current distribution in the UK
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 percent of samples were ELISA-positive for TBEV with foci in the New and Thetford forests. The Thetford Forest area had the highest proportion (47.7 percent) of seropositive samples. Engorged ticks collected from culled deer within seropositive regions were tested for viral RNA, with 5 out of 2,041 ticks testing positive by LIV/TBEV real-time reverse transcription polymerase chain reaction (RT-qPCR). 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 focus in the UK, and possibly more. This is of concern to pet owners and the wider public regularly using these areas for work or recreation. The large numbers of deer in the UK means that infected ticks are likely to be moved from one area of deer habitat to another, making the risk of spread from these endemic foci likely.
Risk of exposure and severity of disease in humans and dogs
In endemic areas, the prevalence of TBEV in questing ticks rarely exceeds 1 percent, 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 and the risk of people or pets being infected from short visits to endemic areas should be kept in perspective. People and pets living in, frequently visiting or working in endemic areas, however, would be at significantly greater risk of exposure over time.
Clinical disease in humans occurs in an estimated one third of 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 percent of cases are fatal. TBE should therefore be considered a serious zoonosis with significant risk for people frequently visiting or spending prolonged periods of time in endemic areas without adequate tick protection.
In contrast, the incidence of TBE in dogs appears to be relatively low when compared to humans. In Switzerland there is a higher seroprevalence in dogs than humans and yet the incidence of clinical cases in dogs in Switzerland, Austria, Germany and Sweden 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 is often fatal. Although the risk of disease developing in dogs is lower than in humans, its severity means that dogs should therefore have adequate tick prevention if visiting or residing in endemic areas.
Diagnostic tools and prognosis
Diagnosis of TBE is difficult in both humans and dogs. A definitive diagnosis relies on isolation of the causative virus from blood or cerebrospinal fluid but carries a poor sensitivity. Negative results, therefore, do not rule out infection. IgG serology can be used with a fourfold increase in titres indicative of acute infection.
There is no specific treatment which can target the virus itself and treatment therefore is supportive and symptomatic. Pain relief and intravenous fluid therapy may be required alongside anticonvulsives, sedatives and muscle relaxants (Pfeffer and Dobler, 2011). Non-steroidal anti-inflammatory drugs (NSAIDs) can be used in dogs with pyrexia, and antibiotics used 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 dogs with TBE is poor and those that survive often require 6 to 12 months to make a full recovery.
Prevention of disease
The potential severity of TBE makes tick prevention vital for dogs and owners whose lifestyle may put them at risk of exposure in endemic areas. This includes those who spend time in rural areas, farmland, bracken and deer-inhabited woodland. Dogs travelling to endemic countries should also be protected. 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 endemic areas. Veterinary professionals should keep the relatively low risk of exposure in perspective for clients, while encouraging preventative 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 both diseases.
Daily monitoring for ticks
Clients should be advised to check their pets every 24 hours or after outdoor activity in endemic areas. 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 will increase the risk of tick stomach and salivary gland contents being released, and disease transmission as a result. They should therefore be avoided. When checking for ticks, clients should be reminded that nymphs are very small (Figure 2) 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 preventative products
The use of prophylactic compounds that rapidly kill or repel ticks are useful in reducing tick feeding and therefore transmission of infection. Products containing an isoxazoline, permethrin, deltamethrin and 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 percent effective and owners should therefore still be advised to check their pet for ticks at least every 24 hours and after outdoor activity.
Positive dogs pose no significant risk to owners as ticks are required for transmission. They are, however, sentinels for human infection as dog owners are likely to be at risk of infected tick exposure themselves from walking in the same areas as their dog.
Infected Ixodes ricinus 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.
In Europe, there are currently two vaccines licensed for human use but neither have been licensed for animal use including dogs. Off-licence studies in dogs have shown some efficacy but further safety and efficacy studies are needed before they could be routinely used in high-risk areas (Pfeffer and Dobler, 2011).
Tick-borne encephalitis virus is a pathogenic tick-borne virus affecting humans and dogs in Europe, and a potentially fatal zoonosis. Its rapid spread 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 arrival of this pathogen in the country, however, is another compelling reason for tick preventative strategies in dogs at high risk of exposure. TBE should also now be considered 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. Vigilance in imported and travelled pets is also an important component of preventing or slowing future spread.