Prevalence of Borrelia burgdorferi s.l. in ticks feeding on humans in Thuringia/Germany

In 2004, a total of 618 Ixodes ricinus ticks fed on humans were collected by physicians throughout Thuringia. The prevalence rates of Borrelia burgdorferi sensu lato (s.l.) genotypes were determined by nested PCR and restriction fragment length polymorphism, targeting a 0.8-kb fragment of the ospA gene. The total prevalence of B. burgdorferi s.l. was 6.1%. B. afzelii was found in 67%, B. valaisiana in 15% and B. garinii in 15% of the positive ticks (3% could not be determined). In one tick, a double infection with B. valaisiana type II and B. garinii OspA type V was detected. Female adult ticks had the highest infection rate (11.7%), followed by nymphs (4.5%) and larvae (3.4%). The overall prevalence increased from spring (4.0%) to autumn (10.0%). Nevertheless, the risk of infection was maximal in summer, because of a much higher infestation and consequently a higher absolute number of infected ticks. The predominance of B. afzelii probably results from its resistance against human serum. The unexpectedly low total prevalence is possibly caused by immune defence mechanisms (e.g. complement) effective against less resistant B. burgdorferi s.l. strains in the tick.