Infectiousness for humans of Ixodes ricinus containing Borrelia burgdorferi.

We studied the rate of transmission of Borrelia burgdorferi from ticks (Ixodes ricinus) to man under field conditions in a case control study. At a holiday camp in southern Germany 384 ticks were removed from 272 persons. Information on symptoms possibly related to Borrelia infection were obtained by a questionnaire to be sent back six weeks after the tick bite. Ticks were examined by immunofluorescence microscopy (IFT) for Borrelia and 49 (12.8%) were found positive. Blood was obtained from 41 persons bitten by Borrelia positive and 41 age and sex matched persons bitten by Borrelia negative ticks. Sera from age and sex matched patients of local hospitals and clinics served as additional controls. Antibody titers were obtained by indirect IFT about 13 weeks after tick bite. Titers 1 : greater than 32 suggested recent infection and 1 : 32 immunity. In the exposed group there were about half as many persons with titers 1 : less than 32 (n = 14) than in the control group (n = 27) suggesting that either part of the infected ticks was in a non-infectious state or the hosts were immune. In the exposed group there were 46.4% (n = 19) and in the control group, bitten by Borrelia negative ticks, 14.7% (n = 6) persons with titers 1 : greater than 32, but 5/6 of these persons in the control group recalled additional tick bites in 1984. Only one child (in the exposed group) developed an Erythema chronicum migrans, and no other Borrelia related manifestations were reported. The manifestation rate of the Borrelia-related disease was 4%.