Tick-borne infections in Sogn og Fjordane, western Norway. Seroprevalence, risk factors and subjective health complaints in blood donors
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Background: The tick Ixodes ricinus is involved in the transmission of a large variety of pathogens of medical and veterinary importance in Norway. The most prevalent human tickborne disease in the country is Lyme borreliosis, caused by the bacterium Borrelia burgdorferi sensu lato (s.l.). Granulocytic anaplasmosis, caused by the bacterium Anaplasma phagocytophilum, is prevalent in livestock, but only a few human cases have been published. The viral infection tick-borne encephalitis (TBE), caused by the TBE-virus (TBEV), is endemic in the southernmost parts of the country.
Aims: The aims of the present thesis were to assess the frequency and risk factors of tick bites, and the seroprevalence of antibodies to B. burgdorferi s.l., A. phagocytophilum and TBEV in Sogn og Fjordane county on the western coast of Norway. In addition, we wanted to assess any association between tick bites or seropositivity for B. burgdorferi s.l. and common subjective health complaints. Finally, we wanted to compare different laboratory methods for detection of antibodies to B. burgdorferi s.l.
Methods: During the first half of 2010, serum samples and questionnaires were collected from 1,213 blood donors at the four blood banks in the county. The questionnaire included questions about demographics, various life style factors, data on tick bites, and a set of questions designed to measure common and prevalent health complaints in the general population. Antibodies to B. burgdorferi s.l. were tested in Enzygnost Lyme link VlsE/IgG, Enzygnost Borreliosis IgM and Immunetics C6 Lyme ELISA kit. Sera showing positive or grey-zone reactivities in any of these tests were further tested in Euroimmun Borrelia-EUROLine-RN-AT IgG and Borrelia-EUROLine-RN-AT IgM. A random subgroup of 301 sera was examined for IgG-antibodies to A. phagocytophilum by an indirect immunofluorescence assay (IFA). All 1,213 sera were analysed for IgG-antibodies to TBEV in Serion ELISA classic TBE IgG.
Results: Among the participants, 65.7% had experienced tick bites during their lifetime, and 30% had experienced tick bites during the last 12 months. Donors from the easternmost blood bank in Lærdal reported the lowest occurrence of ticks in their living area as well as the lowest number of tick bites. In the younger age-groups, males reported more bites than females. This was reversed in subjects older than 50 years of age, with females reporting more tick bites than males. Tick bites were more common among participants with the highest educational level, increased outdoor activity and among hunters and owners of domestic animals. Using the laboratory’s routine tests for detecting antibodies to B. burgdorferi s.l., Enzygnost IgG and IgM, 9.6% were positive for IgG and 8.2% for IgM. There was a positive association of IgG-seropositivity with age, and more males than females were positive for IgG (13.0% and 5.5%, respectively). IgG prevalence was higher in persons spending more time outdoors. There was a delayed age-related rise in seroprevalence in women compared to men. Subjects from the blood bank in Lærdal had the lowest prevalence of IgG. We found a substantial agreement between Enzygnost IgG and Immunetics C6 ELISA, most discrepancies were found in weakly reactive sera. IgM only was seen in 55 subjects (4.5%), of which more than half had a positive immunoblot for IgM. This pattern was seen more often in women and younger age-groups. Among the 301 blood donors tested for IgG-antibodies to A. phagocytophilum, 49 (16.2%) were positive with a titer ≥80 (range 80-1280). Among the 1,213 sera tested, six (0.5%) gave positive or grey-zone results in the ELISA test for TBEV IgG. Five of these were from persons having received vaccines that might give positive reactions in the TBE ELISA, and the last was further examined by neutralising antibodies to TBEV, with negative result. We found no association between the number of tick bites or antibodies to B. burgdorferi s.l. and subjective health complaints, reduced general function or reduced physical fitness. The number of tick bites was positively associated with good physical fitness.
Conclusions: The results provide insight into the epidemiology of tick bites and tick-borne diseases in western Norway, and confirm the endemicity of Lyme borreliosis in the region. There were no indications that TBE is established as a human disease in the area, but there were serological indications that human granulocytic anaplasmosis should be considered in patients with compatible symptoms after a tick bite. The results also give insight into strengths and weaknesses of serological methods in diagnosing Lyme borreliosis, and may help to establish prudent test algorithms for this disease. There were no indications of adverse chronic health effects of tick bites or B. burgdorferi s.l. infection in this overall healthy population.