Tick-borne encephalitis virus natural foci emerge in western Sweden

Abstract

There has been an emergence of tick-borne encephalitis (TBE) cases in western Sweden over the past 10 years. Human cases cluster in distinct regions around major bodies of water, raising the question of TBE prevalence in ticks within these defined localities. This study was based on a collection of 7120 questing nymphs, 510 questing adults, and 132 fed female ticks from cows in 4 suspected TBE foci, based on human cases, and 2 non-endemic areas of western Gotaland. All tick pools were screened with Real-Time RT-PCR targeting the non-coding 3′-region. Prevalence in ticks in the endemic areas ranged from 0.1% to 0.42%, which is comparable with other more established TBE endemic regions in Europe. Of the 18 positive pools, viral copy numbers ranged from 500 to 3.7×10⁹ copies/pool. Sequence data from a TBE patient in western Gotaland confirmed that the western European subtype of TBEV has spread to western Sweden.

Introduction

The tick-borne encephalitis virus (TBEV) belongs to the family of Flaviviridae. The TBE complex shares 77–98% amino acid similarity in the E protein, making this protein region a prime target for sequencing in phylogeny studies (Gao et al., 1993; Mandl et al., 1993). There are three subtypes of TBE: Far Eastern (FE), Siberian (Sib), and West-European (WE) (Wallner et al., 1996; Gritsun et al., 2003b; Pogodina et al., 2004). In Europe and Siberian Russia, TBEV is a high-impact CNS pathogen with approximately 12,000 diagnoses annually (Günther and Haglund, 2005). The virus causes a variety of clinical manifestations, with neurological manifestations in up to 30% of the patients (Gustafson et al., 1993). Lethality of WE-TBEV found in Europe is <2%, but post-encephalitic syndrome is seen in over 40% of the infected patients, often severely impairing their quality of life (Günther et al., 1997). Antiviral treatment is currently lacking, although two vaccines are available that effectively prevent TBEV infection (Heinz et al., 1980; Klockmann et al., 1991).

In Scandinavia, the first reports of TBE from Sweden, Finland, Denmark, and Norway date back to 1954, 1956, 1963, and 1997, respectively (Skarpaas et al., 2006). In Sweden, routine TBEV surveillance since 1956 and neutralizing antibody assays of TBEV on cattle sera in the 1950s placed most TBE natural foci around Stockholm though recent years have seen TBE spread north and west (von Zeipel et al., 1959; Lindgren and Gustafson, 2001; Haglund, 2002). Annually, Sweden reports >100 cases each year, and that number is rising. In the western Gotaland region, situated at the south-western part of Sweden, TBE was previously a non-existent or unnoticed human disease. During the last decade, however, a growing number of cases have been diagnosed, making TBEV one of the most threatening emerging diseases in the investigated region.

TBEV in Sweden is transmitted largely by Ixodes ricinus hard ticks, which can live for 2–5 years. TBEV spreads to ticks when they feed either on viremic or non-viremic animals (Labuda et al., 1993a, Labuda et al., 1997; Randolph et al., 1999; Gritsun et al., 2003a). Vertical transstadial and sexual transmission can occur among both ticks and warm-blooded hosts (Molnarova and Mayer, 1980; Khozinsky et al., 1985). Once infected, the tick remains infected throughout its life cycle (Kožuch and Nosek, 1980; Nosek et al., 1986; Rosa et al., 2003).

The low-endemic region of western Gotaland is well-suited for studies on the epidemiology of TBEV. In addition to routine surveillance of TBE cases and seroprevalence studies, determining TBEV prevalence in free-living tick populations and quantities of TBEV RNA in ticks helps verify TBE foci, assess the transmission risk from a tick bite, and predict the epidemiology of the disease for immunoprophylactic strategies. For this purpose, samples of ticks were collected from different parts of western Gotaland, the front of the epidemic. Additionally, TBEV from western Gotaland was phylogenetically analyzed to trace the origin of the TBEV spread in that region.