The tick-borne encephalitis virus (TBEV) is a tick-borne virus that occurs in large parts of Europe and Asia. Infection with this virus can lead to meningitis and long-term neurological complaints. 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 several areas in the Netherlands, but the chance of infection is very small.
The vast majority of people do not or hardly become ill after being bitten by a TBEV-infected tick. Of the people who do get sick, 1 to 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 the variation in density of wildlife and climate change, but also the picking of mushrooms and collecting berries, which are very popular in Eastern Europe. Thus, a typical feature of TBEV is that its distribution is highly fragmented, despite the widespread 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 from this area had antibodies to TBEV. Follow-up research on roe deer shot in 2017 showed that 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 ticks and wild mice. In total, more than 46,000 ticks and 320 wild mice from 48 locations throughout the Netherlands were tested. The ticks have been tested in groups. Of the 3708 tick pools tested, 7 tested positive for TBEV RNA at 3 sites. This means that only 0.02% of the tested ticks were infected with the virus.
By way of comparison: about 15% of Dutch ticks are infected with the bacterium Borrelia burgdorferi sl, which causes Lyme disease. Research into the RNA of the positive sign also showed that each region had its own virus variant. This suggests multiple independent introductions and confirms the idea that TBEV can be spread over long distances, presumably via migratory birds.
Although further spread of the virus is not inconceivable, the chance of being bitten by an infected tick and subsequently becoming ill is very small. Currently, 2 to 5 patients in the Netherlands develop tick-borne encephalitis every year. An effective vaccine is available for people with high exposure to ticks.
More information can be found in the publication ‘Continued circulation of tick-borne Encephalitis virus variants and detection of novel transmission foci, the Netherlands’ in the scientific journal ‘Emerging infectious diseases’.
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