Temporal distribution of the annual nymphal stock of Ixodes ricinus ticks

The human risk of contracting Lyme disease or other tick borne diseases transmitted by the tick species Ixodes ricinus is broadly linked to the tick nymph density. The study was performed in Rambouillet forest (Yvelines, France), a known focus of Lyme borreliosis, from January 1997 to December 1999. We used a nymph sampling methodology which permitted us to obtain a monthly nymph density index (from 0 to 5). Studying the seasonal nymph and larval activity patterns and estimating the larval developmental duration, we demonstrate the existence of an annual nymphal stock. Secondly, we elucidate how this stock is distributed throughout the year, month by month. Its distribution is principally dependent on two factors: the monthly mean ambient temperature and the proportion of active nymphs which find a host each month. Expected monthly nymph densities derived from a theoretical model describing the temperature-dependent stock distribution gave a good fit to the observed densities, accounting for between 76-86% of the monthly variation in observed nymph densities. Predicting the temporal distribution of nymph activity within a stable Lyme borreliosis focus enables more precise identification of risk periods.