The prevalence and incidence of clinical and asymptomatic lyme borreliosis in a population at risk

A past history of clinical Lyme borreliosis and the 6-month incidence of clinical and asymptomatic Lyme borreliosis was studied prospectively in a high-risk population.In the spring, blood samples were drawn from 950 Swiss orienteers, who also answered a questionnaire.IgG antiBorrelia burgdorferi antibodies were detected by ELISA.Positive IgG antibodies were seen in 248 (26.1%), in contrast to 3.9%-6.0% in two groups of controls (n = 101).Of the orienteers, 1.9%-3.1% had a past history of definite or probable clinical Lyme borreliosis.Six months later a second blood sample was obtained from 755 participants, 558 (73.9%) of whom were seronegative initially; 45 (8.1%) had seroconverted from negative to positive.Only 1 (2.2%) developed clinical Lyme borreliosis.Among all participants, the 6-month incidence of clinical Lyme borreliosis was 0.8% (6/755) but was much higher (8.1%) for asymptomatic seroconversion (45/558).In conclusion, positive Lyme serology was common in Swiss orienteers, but clinical disease occurred infrequently. © 1991 by The University of Chicago.