2-YEAR SURVEY OF THE INCIDENCE OF LYME BORRELIOSIS AND TICK-BORNE ENCEPHALITIS IN A HIGH-RISK POPULATION IN SWEDEN

A survey was made over a two-year period (September 1987 to August 1989) of a population living in an area endemic for Lyme borreliosis and tick-borne encephalitis in Sweden. For each patient a blood sample was collected and a questionnaire completed annually. All sera were tested for an antibody response to Borrelia burgdorferi in an EIA using sonicated antigen and for an antibody response to the tick-borne encephalitis virus using an EIA and a haemagglutination inhibition test. Antibodies to Borrelia burgdorferi and tick-borne encephalitis virus were detected in 89 (25.7 %) and 40 (11.6 %) respectively of 346 samples collected in August 1987. In the first year of the study 14 of 303 subjects (4.6 %) developed Lyme borreliosis and in the second year 9 of 277 subjects (3.2 %). A significant increase in the antibody titre for Borrelia burgdorferi was seen in 14 of 303 (4.6 %) subjects in the first year and 8 of 277 (2.9 %) subjects in the second year. An earlier episode of Lyme borreliosis or an elevated antibody titre did not seem to protect against reinfection. One case of tick-borne encephalitis was seen each year. Seroconversion for tick-borne encephalitis virus was found in 3 of 258 (1.2 %) subjects in the first year and 5 of 211 (2.4 %) in the second year, excluding subjects who had undergone successful immunisation or had earlier been hospitalised for tick-borne encephalitis. The study thus demonstrated a high yearly incidence of tick-borne infections in a population at risk.