Longitudinal study of Lyme borreliosis in a high risk population in Switzerland

Orienteers from all parts of Switzerland (n = 416) were included in a longi tudinal study for lyme borreliosis. In spring 1986, the seroprevalence was 28.1 %. At the beginning of the study, 84.3 % of orienteers reported a hist ory of tick bite, and 3.8 % reported a past history of lyme borreliosis. Du ring the first (spring 1986-autumn 1986), second (autumn 1986-spring 1987) and third (spring 1987-autumn 1987) period, rates of seroconversion were 0. 6 % 2.7 % and 2.1 % respectively. During the first and second period, clini cal incidence were 1.0 % and 0.25 % respectively. No active lyme borreliosi s was detected during the third period. Among orienteers who seroconverted during the study (n = 16), only two developed clinical symptoms. Hence, Bor relia burgdorferi infection is often asymptomatic.