A. Jääskeläinen1, T. Korhonen2, M. Kuusi2 , O. Vapalahti1,3,4
1 Infection Biology Research Program, Department of Virology, Haartman Institute, University of Helsinki, Finland
2 National Institute for Health and Welfare, Helsinki, Finland
3 Department of Veterinary Biosciences, Faculty of Veterinary Medicine, University of Helsinki, Finland
4 Department of Virology, HUSLAB, Hospital District of Helsinki and Uusimaa, Helsinki, Finland
Citation: Jääskeläinen A, Korhonen T, Kuusi M, Vapalahti O. Tick-borne encephalitis in Finland. EpiNorth 2011; 12: 40-3.
Tick-borne encephalitis (TBE) is focally endemic within vector ticks Ixodes ricinus and Ixodes persulcatus distribution areas, especially in the northern regions including Finland. For decades, the TBE endemic foci in Finland remained the same: the Åland Islands, archipelagos of Turku, Helsinki, Kokkola and Lappeenranta region. During recent years human cases have been reported in new regions including Simo in Finnish Lapland, the northernmost TBE endemic focus in the world known. At present 20-40 human cases are registered annually in the country. Both tick species relevant for TBE virus circulation exist in Finland but their distribution areas do not appear to overlap. Interestingly, I. persulcatus is found along the western coast while I. ricinus is distributed in the southern and central parts of the country. In the Kokkola archipelago, the Siberian subtype of the TBE virus circulates, whereas in all other studied TBE endemic areas we have found the European subtype, including Simo, where it is unexpectedly carried by I. persulcatus.
Tick-borne encephalitic (TBE) is an infection of the central nervous system caused by a flavivirus tick-borne encephalitis virus (TBEV). TBEV is transmitted mainly by Ixodes ricinus and Ixodes persulcatus ticks.
Finland is situated at the northernmost region of I. ricinus and I. persulcatus distribution ranges and of the TBE-endemic area in Europe. TBE has been found on the Åland Islands since the 1940s (1, 2). In the 1960s, TBEV antibodies were found in cattle sera from the Åland Islands and Turku archipelago, Kokkola archipelago and Lappeenranta-Parikkala region in the southeast (3). These TBE-endemic areas have remained the same for decades (4, 5). In addition, the Isosaari Island in the archipelago of Helsinki was found to be a TBE-endemic in the 1990s (6) (Figure 1). Lately sporadic human cases have appeared in new areas: one case in Närpiö-Maalahti region in 2007 and three cases in 2010; one in Varkaus 2008; two in Simo in 2008, two in 2009 and one in 2010 (7); one in Kuopio region in 2009 and one in 2010; and two human cases in the archipelago of Kotka in 2010 (8) (Figure 1).
Fig. 1. Geographical distribution of TBE cases in Finland.
TBE in Finland. Red indicates old TBE-endemic foci including Åland Islands and archipelgo of Turku, archipelago of Kokkola and southeastern Finland (4); Lappeenranta region (13); Isosaari (7). Blue indicates new TBE-endemic foci or sporadic human cases, years of human cases provided. Green indicates observations of Ixodes persulcatus: Kokkola archipelago (5); Närpiö (6); Simo (10); Eastern Finland: Ilomantsi and Kuhmo (12).
TBE is a notifiable disease in Finland. The results of diagnostic antibody tests are reported to the National Infectious Diseases Register (9). In order to identify the place of acquisition, the National Institute for Health and Welfare interviewed patients diagnosed with acute TBE and/or reviewed patient records since 2009 (8). The viral zoonoses research group at the University of Helsinki collects ticks and rodents from likely sites of infection in order to determine TBEV prevalence in ticks and to study the molecular epidemiology of TBEV in the Finnish endemic foci.
Results and discussion
During recent years in Finland approximately 20-40 new cases of TBE have been reported annually (Figure 2). Traditionally, approximately two thirds of the human TBE cases in Finland have come from the Åland Islands (population about 28,000) where the incidence of TBE varied annually between 7 and 93 new cases per 100,000 inhabitants in 1990-2010. In 2006, a TBE vaccine was introduced as a part of the common vaccination program for Ålanders seven years of age and older. Since then approximately 42,000 vaccine doses have been given in Åland. In addition, many Ålanders were vaccinated before the TBE vaccine was included in the common vaccination program. Thus the vaccination coverage has been estimated to be about 65% (Tuija Leino, National Institute for Health and Welfare, personal communication). In part due to active vaccination in Åland, the geographical distribution of Finnish TBE cases has changed, and currently most of the cases are on mainland Finland (Figure 2).
Fig. 2. Number of TBE cases in Finland in 1987-2010. Data from 1987-1998: Olli Vapalahti; 1999-2009: National Institute for Health and Welfare, Infectious Diseases Register; 2010: Teija Korhonen.
The TBE endemic areas in Finland were stable for decades. In the 2000s human cases appeared in several previously unnoticed sites. We have isolated TBEV strains from ticks and rodents from Simo. Because human cases were reported in three subsequent years, we can regard Simo as an established TBE endemic focus, and the northernmost known focus in the world (10). Furthermore, four human cases were detected in 2007 and 2010 in the Närpiö region indicating the presence of another new endemic focus. Whether the other new sites of human TBE cases (Kuopio region, Varkaus and Kotka archipelago) are actually constant TBE foci remains to be confirmed. While human cases might represent the “tip of an iceberg”, it is also possible that birds have introduced individual TBEV-infected ticks to these areas that infected humans without TBEV actually circulating in the area.
Both tick species relevant for TBEV circulation are found in Finland but their distribution areas do not appear to overlap. I. ricinus is found in southern Finland, in the southeastern area close to the Russian border and in known I. persulcatus areas e.g. in Lappeenranta (6) and Varkaus (our unpublished observations). The prevalence of TBEV in I. ricinus ticks was about 1% on Kumlinge Island in the Åland Islands in 2003 and Isosaari Island, in the archipelago of Helsinki, and Lappeenranta in 2005, and 0.1% in the archipelago of Turku in 2007 (6). Interestingly, I. persulcatus seems to be distributed along the western coastline of the country in Närpiö (6), the Kokkola archipelago (5) and Simo (10). TBEV prevalence in I. persulcatus ticks was about 1% in the Kokkola archipelago in 2004 (5) and 2% in Simo in 2009 (10).
In 2010 there was a fatal TBE case in the Kokkola archipelago caused by the Siberian TBEV subtype (11). From elsewhere from Finland, there are only observations of the European TBEV subtype, even from Simo, where the European subtype is surprisingly carried by I. persulcatus (10).
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