Prevalence rates of Borrelia burgdorferi sensu lato in host-seeking Ixodes ricinus ticks in Europe

Borrelia burgdorferi sensu late spirochetes have been found in all examined Ixodes ricinus(L.) populations in Europe. The overall mean proportions of unfed I. ricinus infected with B. burgdorferi s.l. were 1.9% (range 0-11%), 10.8% (2-43%) and 17.4% (3-58%) for larvae (n = 5699), nymphs (n = 48 804) and adults (n = 41 666), respectively. However, the results varied according to the method used. Cultivation in BSK medium is the least sensitive technique (an average of 11% adult ticks found infected), whereas polymerase chain reaction detecting spirochetal DNA is probably the most sensitive method (29% adults found infected). Microscopic methods (dark field, phase contrast, direct or indirect fluorescence) are generally comparable to each other (17-20% adults found infected) and should be regarded as standard procedures because they also make possible a quantitative estimation of spirochetes in the vector. Some technical problems of these methods are discussed.