<i>Ixodes ricinus</i> Density, and Distribution and Prevalence of <i>Borrelia burgdorferi</i> Sensu Lato Infection Along an Altitudinal Gradient

In this study, we measured the phenology of <i>Ixodes ricinus</i> ticks and their infection with <i>Borrelia burgdorferi</i> sensu lato (sl) simultaneously along an altitudinal gradient to assess the impact of climate on the phenology of ticks and on their infection with <i>B. burgdorferi</i> sl. From 1999 to 2001, free-living <i>I. ricinus</i> ticks were collected monthly by flagging vegetation at three different altitudes (620, 740, and 900 m above sea level) on the slope of a mountain in Chaumont (Neuchâtel, Switzerland). <i>I. ricinus</i> ticks were examined for the presence of <i>B. burgdorferi</i> sl by using direct fluorescent antibody assay and isolation of spirochetes. Borrelia species were characterized by polymerase chain reaction followed by restriction fragment-length polymorphism. Tick density and tick phenology varied with altitude. Although the peak tick density decreased and the onset of ticks was delayed with altitude, the phenology was much more stable among years at the highest altitudes than at the lowest. The prevalence of <i>B. burgdorferi</i> infection in nymphs and adults decreased with altitude. The prevalence of infection differed significantly among years, and it was significantly higher in adults (30%) than in nymphs (21%). <i>B. burgdorferi</i> infection in adults was positively related with adult density, but this was not observed for nymphs. Five <i>B. burgdorferi</i> sl genospecies were successfully isolated: <i>B. garinii, B. burgdorferi</i> sensu stricto, <i>B. afzelii, B. valaisiana</i>, and <i>B. lusitaniae</i>. Mixed infections were obtained from five of 140 infected ticks. The greatest diversity in Borrelia species was observed at the lowest altitude where all five <i>Borrelia</i> species were present, whereas at the two highest altitudes, <i>B. lusitaniae</i> was not observed.