Factors driving tick bite risk during a high-density outdoor activity

Understanding the factors driving individual exposure to tick bites is essential for predicting tick-borne disease risk. We analyzed data from a 6-day international orienteering competition held in July 2018 in the forests of Moselle, France, involving over 1200 competitors per day and 418 monitored participants. Using generalized linear mixed models, we assessed the influence of environmental and behavioral factors (racing round, race length, exposure time, departure time and repellent use) on the occurrence of tick bites in general, and specifically on the occurrence and the number of Ixodes ricinus nymph bites. Unaccounted individual-level variation was incorporated into the models through a random effect of orienteer ID. Tick bite risk was significantly influenced by racing round, departure time, race length, and repellent use, but not by exposure duration. Departure time was one of the strongest predictors, with early starters facing higher bite probabilities-which might be due to local depletion of questing ticks by earlier competitors and reduced tick activity later in the day. Race length also predicted tick bite risk, possibly reflecting differences in participant density along courses rather than distance alone. Repellent use provided limited protection. Importantly, individual identity accounted for a large portion of total variance, exceeding that of other tested factors, and recent prior tick bite history strongly predicted future bites during the last race. These findings highlight the need to further quantify inter-individual variability and to consider this variability when modeling tick bite risk.