Temporal Variation of Ixodes ricinus Intensity on the Rodent Host Apodemus flavicollis in Relation to Local Climate and Host Dynamics

Abstract

The risk to humans of contracting tick-borne zoonotic diseases depends on the risk of a bite from an infected tick, which can be broken down into its component parts as the number of host-seeking ticks in the environment, in particular nymphs, and the prevalence of tick-borne pathogens they are carrying. In turn, the prevalence of tick-borne pathogens is dependent upon tick biting intensity on hosts that support transmission between ticks; namely rodents. These ticks once fed moult into the next life stage and search for the next blood meal, thus posing a zoonotic risk. Here, we analyse tick biting intensity on rodents in a known tick-borne encephalitis (TBE) focus in Trentino (northern Italy). We examine patterns of tick demography and the influence of host densities and climate on ticks' generation time, development rates, tick density and intensity. During the period 2000–2004, a population of the yellow-necked mouse, Apodemus flavicollis, the most important TBE transmission host, was intensively monitored. Ticks feeding on individual rodents were counted, distinguishing between the larval and nymph life-stages. Local temperature and relative humidity was calculated using both data-loggers in the field site and regional weather stations. We investigated which factors had a predictive value both on feeding tick intensity and on the overall density of larvae or nymphs feeding on rodents in a year. We observed a negative effect of rodent density on tick intensity, while temperature influenced positively both larvae and nymph intensity. Overall larval density was higher in the years and trapping grids where rodent density was higher, while for nymphs no such effect was observed. The best explanatory variable for nymph density was the larval density in the previous year, confirming the discrete nature of tick demography. This provides important information in terms of monitoring the risk to humans of acquiring pathogen-infected ticks.