TBE report 2024
Annual Report on TBE (Tick-borne Encephalitis) detected in Denmark in 2024
Tick-borne encephalitis (TBE), also known as Central European encephalitis, is a viral infection transmitted to humans via tick bites. The disease can cause flu-like symptoms and, in some cases, can develop into encephalitis, particularly in adults/elderly individuals. In Denmark, the number of registered TBE cases has slightly decreased from 28 in 2023 to 22 in 2024, primarily due to fewer cases of infection from Sweden. However, there has been a slight increase in cases with suspected infection in Denmark and a spread to new geographic areas in Denmark.
Since November 2023, TBE has been subject to laboratory notification to Statens Serum Institut (SSI), which reports annually on the occurrence of TBE detected in Denmark. This annual report covers 2024.
Transmission and disease occurrence
TBE (Tick-borne encephalitis, Central European encephalitis) is a viral infection transmitted to humans through tick bites and, in rare cases, by consuming unpasteurized milk from infected animals.
The virus is found among deer, rodents, and cats and can be transmitted to forest ticks when they bite and suck the animals' blood. Subsequently, the tick can transmit the virus to humans through a new bite.
The vast majority (75-95%) of those infected with the TBE virus show no symptoms. The remainder develop symptoms, either monophasically in the form of flu-like symptoms such as fever, malaise, and headache, or biphasically, as approximately 10% of cases develop encephalitis, where, after a symptom-free period of about a week, more severe symptoms such as balance problems, paralysis, seizures, and altered consciousness occur. Additionally, sequelae such as memory and concentration difficulties may arise.
TBE virus can be introduced into new areas either through the migration of tick-carrying or infected mammalian hosts or birds. Forest ticks are found throughout Danish nature, but areas at risk of TBE infection are typically highly localized. TBE has been known to be found in Bornholm since the 1950s, with approximately 1-2 cases annually. Until 2017, with the exception of two cases in Tokkekøb Hegn in 2008 and 2009, respectively, TBE cases were only detected with presumed infection sites in Bornholm, EPI-NEWS 40 /2017.
In 2017, a TBE case was again identified in a patient with presumed infection outside of Bornholm. The patient was believed to have been infected at Feddet, located in Faxe Municipality (retrospectively assessed, as the patient was initially believed to have been infected abroad). Since then, several new potential infection areas have emerged, especially in the North Zealand forests, as well as sporadic cases in other parts of Denmark, including South and West Zealand, Falster, Funen, and Jutland, EPI-NEWS 12-14/2019 and 34-35/2020. Notably, the area around Tisvilde Hegn has been classified as a known risk area following repeated detections of the TBE virus in ticks collected from the area, but there are also localized areas with infection risk elsewhere. See here for previous reports on the disease occurrence of TBE.
TBE surveillance and infection areas in 2024
The total number of TBE cases detected in Denmark has decreased from 28 in 2023 to 22 in 2024. Of the 22 cases, two were detected in the primary phase without symptoms of encephalitis; these are included in the report as information about the infection site contributes to a better understanding of how widespread the TBE virus is, both within and outside Denmark.
The decrease in total TBE cases over the past year is primarily due to a reduction in cases with presumed infection in Sweden, where the number of TBE cases has otherwise doubled over the past ten years.
The number of diagnosed TBE cases with presumed infection in Denmark has been increasing since 2018. Despite the small numbers, the trend remains upward moving, with 17 cases in 2024 with presumed infection in Denmark, compared to 13 detected cases in 2023.
Of the 17 Danish TBE cases in 2024, two had presumed infection in Bornholm, fourteen in Zealand – primarily in North Zealand – and one case in Jutland in the Randers area.
As in previous years, 2024 saw a geographical spread to new municipalities classified as potential infection areas. In 2024, for example, two cases were seen in Egedal Municipality, one in Odsherred Municipality, and one in Randers Municipality.
The case in Randers Municipality is presumed to be related to movement in the area around Fussing Lake. Previously, there had only been one other known infection case in Jutland in 2023, where the infection most likely occurred in the natural area of Harrild Hede in Ikast-Brande Municipality. In 2018, the first case of presumed infection in Jutland was reported, but the exact infection site was unknown.
The infection in Egedal Municipality is presumed to have occurred in the forest areas of Ganløse Egede and/or Ganløse Ore. There had not previously been any presumed infections in Egedal Municipality, but the forest areas in North Zealand have been considered potential infection areas since 2008 and 2009.
The case in Odsherred Municipality is presumed to be related to movement in Veddinge Bakker, which, along with the infection cases in Jutland, contributes to a westward geographical spread of infection areas in Denmark. Figure 2 shows the number of TBE cases by municipality cumulatively from 2013-2024, and Figure 3 shows the number of TBE cases as isolated cases detected in 2024.
Collection of ticks from possible risk areas
By dragging a large, 1x1 meter white flag over the forest floor (“flagging”), ticks can be collected and sent to SSI for examination for the TBE virus (TBEV). It is generally very difficult to find microfoci with TBEV, as these are typically the size of a handball court or smaller.
In 2009, SSI/DTU found a TBEV microfocus by flagging in Tokkekøb Hegn, which could be detected for eight years until 2016. Another TBEV microfocus has been identified in ticks at Rubinsøen in western Bornholm. Yet another focus was found in 2019 near a nature playground in Tisvilde Hegn, and in June 2024, TBEV was again detected in collected ticks from this area, with a prevalence of 0.44%.
Previously, it had not been possible to identify a specific infection area in Hareskoven from which it was relevant to collect ticks for TBEV examination. However, in 2024, two human TBE cases were detected with a possible infection site in the part of Hareskoven surrounding Søndersø, known as Jonstrup Vang. It was possible to obtain a very precise account of one of the patients' movements in Jonstrup Vang prior to symptom onset, which led to subsequent flagging in the area.
Two areas in Jonstrup Vang (designated JV1 and JV2) were flagged, and the collected ticks were pooled and tested for TBEV in October 2024. Despite a high tick density in area JV1, none tested positive for TBEV. In contrast, area JV2, which had lower vegetation and a lower tick density, tested positive for TBEV with a prevalence of 0.86%. A follow-up flagging to confirm the original finding showed that the area remained positive for TBEV in November 2024, with a prevalence of 0.3%.
Genomic characterization from whole-genome sequencing of the findings from Jonstrup Vang indicates a new introduction of TBEV, likely from Central Europe. Sequence analyses suggest that the virus is closely related to strains previously found in southern Germany and Austria. This indicates that the virus did not spread from an already known focus in Denmark. In contrast, analysis of new virus findings from the known focus in Tisvilde in 2024 shows a high degree of similarity with previous sequences from the same area, supporting the idea that the original introduction in that region continues to circulate.
Most infections occur in summer and autumn
The tick season in Denmark typically begins in April-May when people start frequenting forests again and usually lasts until October-November. However, ticks can also be active during the winter months, though the risk of infection is very low.
As shown in Figure 4, most individuals developed symptoms in June and July 2024. The number of cases then declined throughout late summer before rising slightly again in autumn 2024. This late increase may be explained by a warmer autumn, which allows ticks to remain active while more people spend time in forested areas.
TBE is most common among older individuals
Figure 5 illustrates the age and gender distribution of the 22 individuals infected in 2024. In all age groups—except 0-4 years and 45-64 years—more men were affected. TBE can affect individuals of all ages, but as shown in Figure 5, the disease occurs most frequently among older individuals. In 2024, 15 of the 22 infected individuals were over 45 years old, while two cases were also recorded in individuals under 15 years old.
Reports indicate that the disease generally progresses more mildly in children than in adults. Children often exhibit nonspecific and mild symptoms that can be difficult to detect, and they are less likely to experience lasting neurological complications after infection. Of the two cases in children under 15 years old, one was described as mild, with no symptoms affecting the nervous system.
The biphasic course is most common
Of the 22 confirmed cases of TBEV in Denmark in 2024, two showed no symptoms related to the central nervous system—one was infected abroad, and one was infected in Denmark. Therefore, these cases do not meet the case definition for Central European encephalitis. However, they have been included in the report as they contribute to knowledge about the spread of TBEV and enable the identification of new potential transmission areas.
In 16 cases, the infection followed the classical biphasic disease course as previously described. However, four cases presented directly with symptoms affecting the central nervous system, without prior flu-like symptoms or a subsequent symptom-free period.
Prevention of TBE
Protection against the TBE virus can be achieved by avoiding tick bites, removing ticks promptly, and/or getting vaccinated if one frequently spends time in forests and dense vegetation outside of trails between May and October or if one encounters ticks while walking on trails in known TBE risk areas. However, it is important to note that vaccination does not provide 100% protection.
Of the 22 individuals infected in 2024, three had previously been vaccinated against TBE with either the Encepur or Ticovac vaccine.
In 2023, the SSI conducted a risk assessment of TBE. Overall, the likelihood of an individual contracting TBEV in Denmark and developing TBE (encephalitis) was assessed as very low—both at that time and over the following five years. This assessment held true despite expectations of an increase in TBE cases and a wider geographical spread of the virus. Currently, only Bornholm and the municipalities that make up the North Zealand risk area have a disease incidence of >1/100,000 per year.
Due to the relatively low number of cases, the SSI estimates that approximately 50,000 individuals would need to be vaccinated to prevent a single case of TBE among those exhibiting risk behavior in known risk areas—defined as an area with at least two TBE cases within a three-year period. However, this estimate is sensitive to the actual at-risk population, which is challenging to determine with certainty.
This annual report is also mentioned in EPI-NEWS 10-11/2025.