Tick-Borne Encephalitis in Norway
Published: 20.07.09 Updated: 20.07.2009 11:35:10
H. Blystad, L. Vold, K. Nygård
Department of Infectious Disease Epidemiology, Norwegian Institute of Public Health, Oslo, Norway
Citation: Blystad H, Nygård K, Vold L. Tick-Borne Encephalitis in Norway. EpiNorth.2009;10(2):75-76.
Introduction and methods
Tick-borne encephalitis (TBE) is a much feared consequence of tick bites in several European countries. In contrast to other Nordic countries like Sweden, Finland and Denmark (Bornholm Island), no human TBE cases had been diagnosed in Norway before 1998 in patients who had not travelled abroad.
The two first cases of indigenous human TBE were diagnosed in 1998 and 1999 in two patients living on a small island at the southern coast of Norway (1). These cases initiated a retrospective study that revealed a case of domestically acquired TBE who had been ill in 1997. We here report surveillance data on TBE in Norway since the first case was detected in 1998 until 2008.
We used data on cases of TBE reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). TBE and other cases of encephalitis are mandatory reportable from both laboratories and clinicians. Collected data includes information on place of infection.
In the period 1998-2008 a total of 45 cases of TBE was notified to MSIS (Figure 1) . The diagnosis was based on detection of TBEV IgM and IgG antibodies in several serum samples from each patient. Among these patients, 36 were infected by tick bites in Norway, in four different counties situated in the southern coast. Most patients were infected in East- or West Agder counties. Of the 12 municipalities in these two counties with a coastline, TBE cases have been reported in ten municipalities (Figure 2). Among the 36 domestically infected patients, 23 were males. The median age was 47 years (range from 11 months to 75 years of age).
All of the patents had meningoencephalitis and no cases were fatal. The seven patients infected abroad had been visiting Denmark, Austria, Germany, Hungary, Latvia or Slovakia.
Surveillance data indicate that since 1998 endemic foci of TBE have been established in parts of the southern coast of Norway, especially in municipalities along the coast in West- and East Agder counties. This is supported by seroprevalence studies among 317 dogs which show that IgG antibodies to TBEV were detected in 16.4 % of dogs in East-Agder county (2). This finding indicates that TBEV is present in this geographic region. These counties also have the highest incidence of Borrelia infections in Norway.
The reason for the emergence of TBE in Norway in the late 1990s is unclear. It is known that infected ticks can be carried over long distances by migrating birds and thus establishing new foci of TBE. Although climate changes have been suggested as a reason for an increase in tick-borne infection, other factors like more contacts between humans and ticks, abundance of ticks and their hosts may play a significant role in local spread of TBE and other tick-borne diseases. In addition, increased awareness of the disease and improved diagnostic practices may explain the increasing number of diagnosed cases of TBE in Norway.
There is a need for increased surveillance for the presence of TBE virus in ticks and increased vigilance among clinicians. The public also needs to be even better informed on how to avoid tick bites. In 2009 the Norwegian Institute of Public Health changed its TBE vaccination recommendations. Previously, TBE vaccination was recommended only for travellers who could expect to be exposed abroad. Since June 2009 vaccination against TBE should also be considered for people who often walk in forests and experience tick bites in municipalities in Norway where TBE is endemic.
Figure 1. Annual number of cases of reported human TBE in Norway 1998-2003 by place of infection
Figure 2. Notified indigenous TBE cases in Norway 1998-2008, by municipalities of infection
- Skarpaas T, Ljøstad U, Sundøy A. First human cases of tick-borne encephalitis, Norway. Emerg Infect Dis [serial on the Internet]. 2004 Dec [date cited]. Available from http://www.cdc.gov/ncidod/EID/vol10no12/04-0598.htm
- Csango PA, Blakstad E, Kirtz GC, Pedersen JE, Czettel B. Tick-borne encephalitis in southern Norway. Emerg Infect Dis [serial online] 2004 Mar [date cited]. Available from: http://www.cdc.gov/ncidod/EID/vol10no3/02-0734.htm