Influence of some climatic factors on Ixodes ricinus ticks studied along altitudinal gradients in two geographic regions in Switzerland

In the context of climate change, the seasonal activity of questing Ixodes ricinus and their infection with <i>Borrelia burgdorferi</i> sensu lato (s.l.) were examined in relation to some climatic data along altitudinal gradients in Switzerland. The first study took place in an Alpine area (Valais) from 750 to 1020 m above sea level. The other gradient was located on a mountain in the foothills of the Jura chain (Neuchâtel) from 620 to 1070 m above sea level. In the Alpine area, the highest questing tick density was observed at the highest altitude. At the lowest altitudes (750 and 880 m), very high saturation deficits, >10 mmHg, were present during most of the tick activity season and they seem to have impaired the thriving of tick populations. The second study in Neuchâtel (2003–2005) was a follow-up of a previous study (1999–2001) in which it was observed that tick density decreased with increasing altitude. During the follow-up study, substantial differences in questing tick density and phenology of ticks were observed: At high elevations, questing tick densities were 2.25 and 3.5 times higher for nymphs and adults, respectively, than during 1999–2001. As observed during 1999–2001, questing tick density decreased with increasing altitude in this site in 2003–2005. Tick questing density remained higher at the lowest altitude. Increased temperatures during summer months, more favorable for ticks, reaching values similar to those registered in the first study at the lowest elevations are probably responsible for the higher tick questing density at high altitudes. <i>B. burgdorferi</i> s.l. infection prevalence in ticks decreased with increasing altitudes along both altitudinal gradients. Long-lasting high saturation deficit values may limit the development of tick populations as too high a moisture stress has a negative effect on tick survival. This factor may have a permanent impact, as it is probably the case at the lowest altitudes in the Alpine area or a more transient effect like in the Neuchâtel gradient.