Tick-borne encephalitis virus (TBEV) is a tick-borne virus that occurs in much of Europe and Asia. Infection with this virus can lead to long-term meningitis and neurological disorders. In 2015 infected ticks were found for the first time on the Sallandse Heuvelrug. New research shows that different variants of the virus are now present in different areas of the Netherlands, but the chance of infection is very small.
The vast majority of people never or rarely get sick after being bitten by a TBEV-infected tick. Of the people who get sick, 1-2% die. Within the European Union, the number of cases fluctuates between 2,500 and 3,500 per year, mainly in central and eastern Europe, the Baltic states and Scandinavia.
Locally, however, there is great variation in the number of new cases of the disease. Ecological, socio-economic and cultural aspects play a role in this, such as changing wildlife density and climate change, but also mushroom and berry picking, which are very popular in Eastern Europe. Therefore, a typical feature of TBEV is that its distribution is highly fragmented, despite the wide distribution of the main vector and hosts.
In 2015, infected ticks were found for the first time in the Netherlands on the Sallandse Heuvelrug. These ticks were collected after a blood test of deer showed that some animals in this area had antibodies to TBEV. Follow-up research on roe deer killed in 2017 showed the virus may have spread more widely since then.
Between 2018 and 2020, Wageningen University & Research, in collaboration with RIVM and Artemis One Health, conducted large-scale research into the presence of TBEV in wild ticks and mice. In total, more than 46,000 ticks and 320 wild mice from 48 locations across the Netherlands were tested. Ticks were tested in groups. Of 3708 tick pools tested, 7 tested positive for TBEV RNA at 3 sites. This means that only 0.02% of the ticks tested were infected with the virus.
By way of comparison: around 15% of Dutch ticks are infected with the bacterium Borrelia burgdorferi sl, which causes Lyme disease. The positive sign RNA search also showed that each region had its own viral variant. This suggests multiple independent introductions and confirms the idea that TBEV may be spread over long distances, presumably through migratory birds.
While further spread of the virus is not inconceivable, the chance of being bitten by an infected tick and subsequently falling ill is very small. Currently, 2 to 5 patients develop tick-borne encephalitis every year in the Netherlands. An effective vaccine is available for people with high exposure to ticks.
Further information can be found in the publication ‘Continued circulation of tick-borne encephalitis virus variants and detection of new transmission outbreaks, The Netherlandsin the scientific journal Emerging Infectious Diseases.
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