Tick bites in healthy adults from western Norway: Occurrence, risk factors, and outcomes
Introduction
Ixodes ricinus, the predominating tick species in Norway, is present in coastal areas with its highest abundance along the southernmost coast. In earlier studies, its northern distribution limit was estimated to approximately 66° N (Tambs-Lyche, 1943, Mehl, 1983). Recent data indicate that the latitudinal and altitudinal borders for I. ricinus in Norway are expanding and that the tick now is present as far north as 69° N (Jore et al., 2011).
Human tick-borne diseases in Norway are dominated by Lyme borreliosis, with a distribution corresponding to that of I. ricinus, with the highest incidence along the southern coastline. Cases of systemic disease and chronic manifestations of Lyme borreliosis are notifiable to the Norwegian Surveillance System for Communicable Diseases (MSIS), i.e., erythema migrans alone is not reported. In the period of 2001–2010, the mean reported annual national incidence was 5.3 cases per 100,000 inhabitants, with 14.1 in Sogn and Fjordane, compared to 26.1 in the southernmost county of Vest-Agder (http://www.msis.no/). Borrelia burgdorferi sensu lato has been detected in host-seeking I. ricinus ticks in 22.1–31.3% of nymphs and adults in the southernmost part of Norway (Kjelland et al., 2010). In Sogn and Fjordane county, B. burgdorferi s.l. has been demonstrated in 12.0% and 3.5% of ticks at 2 different locations (Olav Rosef, personal communication). Tick-borne encephalitis has recently been detected along the southern coastline of Norway (Skarpaas et al., 2004, Andreassen et al., 2012). The incidence of human granulocytic anaplasmosis is not known, but 2 cases have been described so far (Kristiansen et al., 2001), and serological evidence for human infection has been demonstrated (Bakken et al., 1996).
Sogn and Fjordane county, located at the western coast of Norway, encompasses coastal, fjord, and mountainous areas at 61–62° N and 5–8° E; it stretches from the coast nearly 200 km eastwards (Fig. 1). The climate is temperate, with a high yearly rain fall in the western and middle areas, but with a more inland-like climate in the eastern part, with less precipitation and lower winter temperatures. Ticks are prevalent in the coastal areas and along the fjords and neighbouring valleys, up to about 400 m above sea level, but less so in the eastern parts (Jore et al., 2011). This distribution is reflected in the skewed incidence within the county of notified cases of disseminated Lyme borreliosis in humans (http://www.msis.no/).
The literature on occurrence and risk factors for tick bites in the general population is sparse. Two surveys from the Netherlands demonstrated marked geographical differences in the occurrence of tick bites, and the incidence seemed to double from 1994 to 2001 (de Mik et al., 1997, den Boon et al., 2004). A recent Belgian investigation likewise demonstrated local geographical differences (Vanthomme et al., 2012). In a study from the island of Åland in Finland, 85% of the general population more than 8 years old reported having been bitten by ticks (Wahlberg, 1990), and in Sweden, Stjernberg and Berglund (2002) found a 4% risk of being tick-bitten per 10 h spent outdoors. Another Swedish study found an increased risk of contracting tick bites in women more than 40 years old (Bennet et al., 2007). In Connecticut and Wisconsin, 4.1% of blood donors reported having been bitten by ticks during 6 months (Leiby et al., 2002).
In the present study, we wanted to assess the frequency of tick bites in a healthy adult population (blood donors) from Sogn and Fjordane with regard to demographics and other risk factors. We also wanted to assess the frequency of symptoms following tick bites, visits to a medical doctor and antibiotic treatment, as well as to estimate whether tick occurrence leads to avoidance of outdoor activities in certain areas.
Section snippets
Materials and methods
The Helse Førde Hospital Trust has 4 blood banks in Sogn and Fjordane county (Fig. 1). One is situated at the western coast (Florø), 2 by fjords somewhat further east (Førde and Nordfjordeid), and one is located in the easternmost part of the county (Lærdal). During the period January 13th to June 15th 2010, blood donors at the 4 blood banks were asked to participate in the Tick-borne Infection Study in Sogn and Fjordane. A total of 1213 blood donors participated, a response rate of 76%. Owing
Results
The self-reported numbers of total and recent tick bites are presented in Table 2. The table also includes the respondents’ subjective estimation of the occurrence of ticks in his/her living area. Among participants, 65.7% had experienced tick bites during their life time, whereas 30% had experienced tick bites during the latest 12 months.
Discussion
The main findings of this study are that about two thirds of healthy blood donors in Sogn and Fjordane had experienced tick bites, and about 30% during the preceding 12 months. There were significant differences according to geography, age, gender, education, outdoor time, animal contact, and smoking. Among bitten subjects, 22.7% had experienced a rash surrounding the bite.
Tick bite is a sine qua non for acquisition of tick-borne diseases. Whether or not an infectious agent is actually
Acknowledgements
This study was supported by grants from the Antibiotic Centre for Primary Care, University of Oslo, and from Helse Førde Hospital Trust. The employees of the blood banks kindly assisted in recruiting respondents and handling questionnaires.
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