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.

The wild life of tick-borne pathogens

“…These differences have been attributed both to differences in day range between host individuals 215 and differences in resistance against or tolerance for parasites between host species 13,23 . Our results regarding extrinsic factors (deer and predator abundance, and rodent density) are also consistent with previous studies, which Determinants of Tick Burden on Rodents | 89 found an increase in larval burden on rodents with increasing deer density 32 and a decreasing larval burden with increasing rodent density 95,227 . However, we are the first to report a decrease in tick burden with increasing relative abundance of predators.…”

Prevalence and genetic variability of tick-borne encephalitis virus in host-seeking Ixodes ricinus in northern Italy

“…The pattern of TBEV infection prevalence observed in ticks could be explained by the specific pattern of tick infestation on competent hosts within the TBE foci investigated. In this part of Italy, the ratio of nymphs to larvae feeding on rodents (Apodemus flavicollis) is approximately 1 : 20 (Rosà et al, 2007), which is comparable to other endemic TBE sites in Europe (Randolph et al, 1999). Estimates of the TBEV transmission potential obtained from the ratio of nymphs to larvae recorded on the most infected transmission cohort (adult male) probably explain the sustained endemic TBE cycles in this area (Perkins et al, 2003).…”

Range expansion of Ixodes ricinus to higher altitude, and co-infestation of small rodents with Dermacentor marginatus in the Northern Apennines, Italy

“…The time pattern of infestation of small rodents by immature ticks might be the combined result of the effects of factors associated with both the parasites, and the hosts (Rosà et al, 2007;Paulauskas et al, 2009;Paziewska et al, 2010;Pérez et al, 2012;Estrada-Peña et al, 2013). As a consequence, conclusions on seasonal activity of ticks, based upon rodent infestation, must be drawn with caution.…”