Local environmental factors characterizing Ixodes ricinus nymph abundance in grazed permanent pastures for cattle.

Although Ixodes ricinus ticks are mainly associated with woodland, they are also present in open habitat such as pastures. The distribution of nymphal I. ricinus was monitored by drag sampling the vegetation in May-June 2003 on 61 grazed permanent pastures for cattle located in central France. After selecting explanatory variables from among a set of 155, tick abundance was modelled on the perimeter of the pasture using a negative binomial model that took into account data overdispersion. An abundant tree layer at the perimeter of the pasture associated with a high humidity before sampling greatly enhanced the average number of captured I. ricinus nymphs. The presence of apple or cherry trees around the pasture perimeter, the presence of trees or bushes at the pasture edge, woodland around the pasture and a high number of I. ricinus nymphs in the nearest woodland to the pasture were also favourable to nymph abundance in the pasture. The study highlighted that woodland vegetation associated with humidity and the presence of attractive foraging areas for tick hosts around the pasture played a key role in the abundance of I. ricinus. Finally, the results raised the question of whether and how transfer of ticks between woodland and grazed pastures occurs.