Norway

Katrine M. Paulsen^a,b, Rose Vikse^a, Arnulf Soleng^a, Kristin S. Edgar^a,
Susanne Dudman^a, Berit Sofie Wiklund^a, Åshild K. Andreassen^a

History and Current Situation

Ixodes ricinus ticks are mainly distributed along the Norwegian coastline from Østfold County in the southeast up to 66°N in Nordland County.^1–4 Ticks are in highest abundance in the southern parts of the country. The density of ticks varies between locations, even when separated by short distances. Locations with a high density of ticks are found all across the major distributional range (personal observation). The abundance of ticks declines rapidly with both increasing distance from the coast and higher altitude. According to the multi-source analysis by Jore et al. tick populations in Norway have recently undergone latitudinal and altitudinal range shifts.³ Their study identified ticks as far north as approximately 69°N. However, Jenkins et al. found few ticks attached to dogs and cats in the region north of 66°N, while Hvidsten et al. stated that further studies are needed in order to clarify if tick populations are established north of the Arctic Circle.^5,6 According to our recent study, ticks were abundant at 64.5 and 65.1°N, but few ticks were found at locations close to 66°N. At several locations from 66.3°N up to 67.5°N, no ticks were found by flagging.⁴

In Norway, tick-borne encephalitis (TBE) has been a mandatory notifiable disease to the Norwegian Surveillance System for Communicable Diseases (MSIS) since 1975. The first reported case of TBE occurred in 1997 at Tromøy in Aust-Agder County.⁷ This is a region with lots of holiday cabins and outdoor recreation for both local inhabitants and tourists, and it is known for high early spring and summer temperatures.^7,8 In addition, TBE antibodies in dogs and tick-borne encephalitis virus (TBEV) in ticks have been detected previously in the same area.^7–10

Since the first human TBE case reported in 1997, there have been a total of 127 cases reported in Norway (data per January 5, 2017). According to MSIS these represent the counties of Vest-Agder, Aust-Agder, Vestfold, Telemark, and Buskerud, all located in the southern part of the country. In addition, there are a few cases with unknown infection history. No cases have been reported from the western or northern coastal areas, nor from the area east of Oslofjorden, even though outdoor recreation activities are common in the whole country.

Recent studies in I. ricinus have detected TBEV in nymphs from the southern and eastern part of the country with a prevalence ranging from 0.14% to 1.22%.^10,11 TBEV in nymphs and adults have been detected recently in northwestern Norway and in northern Norway up to approximately 66°N, with a prevalence variation of between 0% and 3.0% in nymphs and 0% and 9.0% in adults.^4,12

In addition to tick studies, seroprevalence studies have detected TBE antibodies in specimens from cervids (deer) collected in Farsund (Vest-Agder County) and Molde (Møre og Romsdal County). In Farsund, located on the southern coast of Norway, 41% (22 of 54 animals) were found to be TBE-positive in contrast to Molde, situated mid-west, with 1.6% (1 of 64 animals).¹³ The same study detected antibodies to Louping ill virus (LIV), a closely related flavivirus, in 14.8% (8 of 54) of the analyzed cervid sera from Farsund. However, LIV has not been detected in ticks in Norway previously. Recently 6850 nymphs and 765 adult ticks from eastern, western, and northern Norway were analyzed for LIV using an in-house real-time polymerase chain reaction (PCR) assay (designed by Torstein Tengs). No positive ticks were found (unpublished data from the Norwegian Institute of Public Health).

Further, two seroprevalence studies in humans from presumed non-endemic areas have been published.^11,14 Larsen et al. detected TBE immunoglobulin G (IgG) antibodies among 0.65% of blood donors in Østfold County in southeastern Norway.¹¹ The second study in 1213 blood donors was performed in Sogn og Fjordane County, located in western Norway. TBE IgG antibodies were detected in 5 (0.4%) of the samples. However, these were all interpreted as false-positives owing to previous flavivirus vaccination causing antibody–positive status in 4 individuals and a negative result in the neutralization test for the fifth person.¹⁴

Historically, the first suggested TBEV isolate from Norway was collected in I. ricinus from Sogn og Fjordane County in June 1976 as described by Traavik and coworkers.¹⁵ Five virus strains with close serological relationship to the TBEV complex were detected in this study.¹⁶ In 1979, Traavik et al. detected a 19.6% seroprevalence from the same area. However, these results were not confirmed with a neutralization test¹⁷ and thus may be explained by cross-reactions to LIV, vaccine-related flaviviruses, or nonspecific binding in the test.

The results from both prevalence studies in ticks and seroprevalence studies in humans and wild animals indicate that TBEV might be widespread in Norway, and not limited to the southern region. Further studies on tick distribution and prevalence of TBEV in ticks, humans, domestic animals, and wild animals in Norway are currently ongoing.

Overview of TBE in Norway

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Literature