Tick-borne encephalitis is an inflammation of the brain or meninges that is caused by the tick-borne encephalitis virus (TBE virus). Infected ticks transmit the TBE virus to other animals, and sometimes to human beings.

What is tick-borne encephalitis?

Tick-borne encephalitis is an inflammation of the brain or meninges that is caused by the tick-borne encephalitis virus (TBE virus). Until recently, the virus only occurred abroad, but in the spring of 2016, there were signs that in the Netherlands deer had been infected by the virus and the virus was found in ticks in the Netherlands. There is one known case of a person having been infected by the virus in the Netherlands. The virus is transmitted by tick bites. 

Symptoms

There are several types of TBE virus. In general, all  types cause infections with a similar disease progression. The TBE virus which occurs in Far-Eastern Russia often causes more serious disease. The risk of an infection after a tick bite is very small, because only very few ticks are infected with the TBE virus. People who have been infected after a tick bite usually do not present any clinical manifestations. Otherwise the disease often comes in 2 phases. The incubation period of 7-14 days is followed by a phase  in which a patient suffers from fever, fatigue, general malaise and headache. This usually lasts 2-7 days and is followed by an asymptomatic period of approximately 1 week, In the second phase of the disease the patient has symptoms such as severe headache and encephalitis, meningitis or meningoencephalitis. In this stage the patient needs to be admitted to a hospital. Some patients will develop unresolved neurological aspects. Approximately 1-2% do not survive. There is no specific medication to treat tick-borne encephalitis. 

How common is the tick-borne encephalitis virus?

Since 2016 we know that ticks can be infected with TBE virus in some regions of the Netherlands. The risk of an infection after a tick bite is minimal in the Netherlands because only very few ticks are infected with the TBE virus. In areas where the TBE virus was found, about 1 in 1500 ticks were infected with this virus. This is significantly lower than the borrelia bacterium, which is found in about one in five ticks. This Borrelia bacterium causes about 27,000 times Lyme disease in the Netherlands each year. The risk of contracting a TBE infection after a tick bite is therefore much smaller than the risk of contracting Lyme disease. In recent years, 1-2 patients a year have become ill due to the TBE virus in the Netherlands.

The TBE virus is more common in other European countries, including Austria, Switzerland, Germany, southern Scandinavian countries, northeastern France and southern England. TBE virus also occurs in Russia and in certain areas of Central Asia.

Where in the Netherlands does the tick-borne encephalitis virus occur?

The risk of an infection after a tick bite is minimal in the Netherlands, because only very few ticks are infected with the TBE virus. RIVMNational Institute for Public Health and the Environment keeps track of the number of patients that become seriously ill due to  contracting TBE in the Netherlands. We are also investigating the spread of TBE virus in wild animals and ticks, in collaboration with DWHC, WUR and Artemis.

The spread of the TBE virus in the Netherlands can be seen in the map below. Between 2016 and 2019, seven patients have been reported to have contracted tick-borne encephalitis in the Netherlands. The most likely places of infection are the Sallandse Heuvelrug, the Utrechtse Heuvelrug, Twente and the Achterhoek.  At the border with Germany in the Achterhoek and east of Nijmegen, the TBE virus was found in small rodents. Research for antibodies showed that roe deer in North Brabant, Limburg, Overijssel and the Achterhoek had been bitten by ticks with the TBE virus. Further research is ongoing into the spread and ecology of the TBE virus in the Netherlands.

This map shows the spread of the TBE virus in the Netherlands (source: DWHC, WUR and Artemis, June 2020).

Infection and prevention

The TBE virus is transmitted by infected ticks to humans . There is a vaccine that gives 95% protection. People who stay for a long period in areas where tick-borne encephalitis occurs, can have themselves vaccinated; this is recommended for example in parts of Central and Eastern Europe. For information about countries where vaccination is recommend,  we refer to the website of the National Coordination of Advice for Travellers (Landelijke Coördinatie Reizigersadvisering). For the time being, there is no reason to vaccinate people in the Netherlands.

Removing  ticks as soon as possible reduces the infection risks of viruses they may carry, such as the virus that causes Lyme disease. TBE virus, however, is transmitted shortly after the bite. So, quickly removing the tick will not always prevent infection. Tick bites can be prevented by wearing protective clothing and by  using  repellents that contain DEET on exposed skin. Repellants do not protect 100%, so tick bite checks remain a necessity after being outdoor/in nature. TBE virus is rarely found in farm animals, such as sheep, goats and cows. The virus is than excreted in milk. As a result people could be infected by drinking unpasteurized infected milk or eating cheese made from infected milk.