During the period from 2001 to 2008, 30 patients with tick-borne encephalitis (TBE) were diagnosed in Denmark. A total of 22 acquired the infection on the island of Bornholm and eight during travel to other well-known TBE-endemic areas. At present, there is no indication that TBE has sustained transmission in Denmark outside of Bornholm, but in light of the expansion of TBE in other areas of Europe and the projected climate changes, it is important to monitor the disease.
Introduction and methods
Tick-borne encephalitis (TBE) is an arthropod-borne viral encephalitis caused by a flavivirus that is transmitted to humans and animals through bites from the Ixodes ricinus tick. It often presents as a diphasic illness with a week of influenza-like illness that is followed by an asymptomatic period of a few days following which neurological signs and symptoms present in one third of the cases.
In Denmark, the disease is endemic on the island of Bornholm in the Baltic Sea (1-3). According to the Danish legislation, TBE is not a notifiable disease. However, serological diagnosis of human cases is performed by only one Danish laboratory, the Department of Virology, Statens Serum Institute. For positive cases, individual data on possible exposure and travel history are obtained. Hence, there is a good overview of the situation. The aim of this report is to review the epidemiology of TBE since 2001.
From 2001 to 2008, a total of 30 patients, 20 males and 10 females, were diagnosed with TBE. The median age was 41 years with a range from 6 to 69 years. Figure 1 shows the number of cases per year. A total of 22 patients acquired the infection on the island of Bornholm. Thirteen were residents on Bornholm while nine were visiting the island. Eight patients acquired the infection while traveling outside of Denmark; three in Sweden and one each in Austria, Estonia, Germany, Latvia and Åland in the Botnic Bay. All patients had meningitis or meningoencephalitis and, as far as we know, none of the infections were fatal.
Figure 1. Annual number of cases of TBE in Denmark acquired on the island of Bornholm (22 cases) or during travel abroad (8 cases). Statens Serum Institut, 2001-2008
It has recently been suggested that the distribution of TBE in Europe is changing. There may be several explanations including increased awareness, changes in diagnostic practices, changes in human behaviour resulting in more exposure to tick bites and the changing climate. New foci, some within Denmark, have been predicted based on these factors (4,5). Recent roe deer surveys from several Danish forest districts have indicated serological signs of TBE-virus in areas outside of Bornholm (5). However, the investigation of roe deer serum has in itself limited relevance to human medicine, partially because of uncertainty about the serological method selected. Nonetheless, the survey indicates that the situation needs to be monitored.
It has been known for more than 50 years that TBE is endemic at Bornholm, and the incidence is stable at around 4 per 100,000 (2). For unknown reasons, 2004 was an exception. Importantly, human cases of TBE have not been diagnosed in Denmark outside of Bornholm, and there is no indication that the current recommendations for TBE vaccination should be changed. Vaccination is recommended for persons who are living permanently or have a summer residence on Bornholm and who regularly leave the path in woods and scrubland. It is not usually recommended to vaccinate short-term tourists staying on Bornholm for some weeks. This applies, for example, for stays at school camps. However, in cases with a particularly great risk of infection, such as forestry work, or where the woods are the fixed location for play, sport or hobby activities, vaccination may be considered for persons staying for only a few weeks. Side effects of the TBE vaccine are relatively minor and therefore there is rarely a reason to advice against vaccination if a person requested to be vaccinated.
- Kristiansen K. TBE in Denmark-in particular on Bornholm. Int J Med Microbiol. 2002;291 S 33:62-3.
- Laursen K, Knudsen JD.Tick-borne encephalitis: a retrospective study of clinical cases in Bornholm, Denmark. Scand J Infect Dis. 2003;35:354-7.
- Statens Serum Institut: EPI-NEWS 46/2004. Available at http://www.ssi.dk/sw20875.asp
- Randolph SE. The shifting landscape of tick-borne zoonoses: tick-borne encephalitis and Lyme borreliosis in Europe. Philos Trans R Soc Lond B Biol Sci. 2001;356:1045–56.
- Skarphédinsson S, Jensen PM, Kristiansen K Survey of tick-borne infections in Denmark. Emerg Infect Dis. 2005;11:1055-61.