Geographic and Temporal Variations in Population Dynamics ofIxodes ricinusand AssociatedBorreliaInfections in The Netherlands

Abstract

In a countrywide investigation of the ecological factors that contribute to Lyme borreliosis risk, a longitudinal study on population dynamics of the sheep tick Ixodes ricinus and their infections with Borrelia burgdorferi sensu lato (s.l.) was undertaken at 24 sites in The Netherlands from July 2006 to December 2007. Study sites were mature forests, dune vegetations, or new forests on land reclaimed from the sea. Ticks were sampled monthly and nymphal ticks were investigated for the presence of Borrelia spp. I. ricinus was the only tick species found. Ticks were found in all sites, but with significant spatial and temporal variations in density between sites. Peak densities were found in July and August, with lowest tick numbers collected in December and January. In some sites, questing activities of I. ricinus nymphs and adults were observed in the winter months. Mean monthly Borrelia infections in nymphs varied from 0% to 29.0% (range: 0%-60%), and several sites had significantly higher mean nymphal Borrelia infections than others. Four genospecies of Borrelia burgdorferi s.l. were found, with B. afzelii being dominant at most sites. Borrelia infection rates in nymphal ticks collected in July, September, and November 2006 were significantly higher (23.7%, p<0.01) than those in the corresponding months of 2007 (9.9%). The diversity in Borrelia genospecies between sites was significantly different (p<0.001). Habitat structure (tree cover) was an effective discriminant parameter in the determination of Borrelia infection risk, as measured by the proportion of nymphal ticks infected with B. burgdorferi s.l. Thickness of the litter layer and moss cover were positively related to nymphal and adult tick densities. The study shows that Borrelia-infected ticks are present in many forest and dune areas in The Netherlands and suggests that in such biotopes, which are used for a wide variety of recreational activities, the infection risk is high.

The wild life of tick-borne pathogens

“…This should result in a large percentage of B. afzelii-infected nymphs as B. afzelii is transmitted by small mammals 65 . Thrushes fed 10% of the infected larvae, which 40 | Chapter 2 could explain the relatively low percentages of B. garinii and B. valaisiana in fieldderived nymphs 156 .…”

“…We measured the average frequency of deer passage between March and November, the main activity season of Ixodes ricinus in the Netherlands 156 by running camera traps at 18 random points per plot. Two camera traps (HC500, RECONYX Inc., Holmen, WI, U.S.A.) were deployed in nine consecutive rounds of four weeks at sets of two random points >30 m apart, as to minimize the chance of one animal walking past both cameras in a short amount of time, resulting in 18 deployments and 504 sampling days (camera malfunction and theft reduced the sample size at some sites; Table S4.1).…”

“…We used camera trapping to estimate the relative abundance of medium-sized and large mammals during March -November, the main activity period of I. ricinus in the Netherlands 156,166 . In each plot, we placed two camera traps (HC500; RECONYX Inc., Holmen, WI, U.S.A.) on a randomly selected tree that were >30 m apart, at 40 cm above the ground, viewing north and parallel to the ground.…”

“…We measured passage rates of deer and predators using camera traps (HC500; Reconyx Inc., Holmen, WI, U.S.A.) during March -November, the period in which Ixodes ricinus is most active in the Netherlands 156 . In each plot, two camera traps were placed on two randomly selected trees, >30 m apart, and 40 cm above the ground facing north, with the view parallel to the ground.…”

“…Therefore, we used the number of animals walking past a location in the plot per day, measured as their passage rate, as a measure of host availability to ticks. We used camera traps (HC500, RECONYX Inc., Holmen, WI, U.S.A.) to determine the passage rate of mammals and birds in each forest plot as the rate of detection by camera traps during March -November, the main activity period of Ixodes ricinus in the Netherlands 156 . In each plot we used the protocol described in Chapter 3 to obtain eighteen camera positions, resulting in approximately 500 camera trap days per plot (Table S4.1).…”