Prevalence of tick-borne pathogens in questing Ixodes ricinus ticks in urban and suburban areas of Switzerland

Abstract

Background: Throughout Europe, Ixodes ricinus transmits numerous pathogens. Its widespread distribution is not limited to rural but also includes urbanized areas. To date, comprehensive data on pathogen carrier rates of I. ricinus ticks in urban areas of Switzerland is lacking.

Results: Ixodes ricinus ticks sampled at 18 (sub-) urban collection sites throughout Switzerland showed carrier rates of 0% for tick-borne encephalitis virus, 18.0% for Borrelia burgdorferi (sensu lato), 2.5% for Borrelia miyamotoi, 13.5% for Rickettsia spp., 1.4% for Anaplasma phagocytophilum, 6.2% for "Candidatus Neoehrlichia mikurensis", and 0.8% for Babesia venatorum (Babesia sp., EU1). Site-specific prevalence at collection sites with n > 45 ticks (n = 9) significantly differed for B. burgdorferi (s.l.), Rickettsia spp., and "Ca. N. mikurensis", but were not related to the habitat type. Three hundred fifty eight out of 1078 I. ricinus ticks (33.2%) tested positive for at least one pathogen. Thereof, about 20% (71/358) were carrying two or three different potentially disease-causing agents. Using next generation sequencing, we could detect true pathogens, tick symbionts and organisms of environmental or human origin in ten selected samples.

Conclusions: Our data document the presence of pathogens in the (sub-) urban I. ricinus tick population in Switzerland, with carrier rates as high as those in rural regions. Carriage of multiple pathogens was repeatedly observed, demonstrating the risk of acquiring multiple infections as a consequence of a tick bite.

Keywords: "Candidatus Midichloria mitochondrii"; "Candidatus Neoehrlichia mikurensis"; Anaplasma; Babesia; Borrelia; Ixodes ricinus; NGS; Rickettsia; Tick-borne encephalitis virus; Urban.

Fig. 1

Urban areas in Switzerland analyzed for the presence of pathogens in questing I. ricinus ticks. Tick collection was successful at 18 collection sites: (a) Basel (2 sites), (b) Bern (1 site), (c) Geneva (2 sites), (d) Lausanne (1 site), (e) Lugano (1 site), (f) Luzern (1 site), (g) Neuchâtel (2 sites), (h) Sion (1 site), (i) St. Gallen (1 site), (j) Winterthur (1 site), and (k) Zürich (5 sites)

Fig. 2

Number of ticks positive for different tick-borne pathogens. The overall height of the bars represents the percentage of infected ticks tested positive for the respective pathogen. The proportions at which the pathogens were detected alone or in combination with one or two others are shown in light gray, dark gray, and black, respectively. Abbreviations: B.g., B. garinii; B.a., B. afzelii; B.b.(s.s.), B. burgdorferi (sensu stricto); B.va., B. valaisiana; B.m., B. miyamotoi; R.h., R. helvetica; R.m., R. monacensis; A.p., A. phagocytophilum; B.ve., B. venatorum (Babesia sp., EU1); N.m., "Candidatus N. mikurensis"; B.b.(s.l.), two (or more) different B. burgdorferi (sensu lato) species. R. helvetica was significantly more often detected alone than in association with another pathogen (GLM with developmental stage as a dependent variable; Chi-square test with Bonferroni correction, P = 0.023)

Fig. 3

Correlation-plot showing the pathogen combinations observed in urban I. ricinus ticks in Switzerland. The more frequent a pathogen combination, the bigger the respective circle in the plot. In addition, the absolute counts of ticks with the particular pathogen combination are given in numbers. Abbreviations: B.g., B. garinii; B.a., B. afzelii; B.b.(s.s.), B. burgdorferi (sensu stricto); B.va., B. valaisiana; B.m., B. miyamotoi; R.h., R. helvetica; R.m., R. monacensis; A.p., A. phagocytophilum; B.ve., B. venatorum (Babesia sp., EU1); N.m., "Candidatus N. mikurensis"; B.b.(s.l.), two (or more) different B. burgdorferi (sensu lato) species, not distinguishable. Other combinations of three different pathogens are not shown in this plot; these were 1× B. afzelii + R. helvetica + A. phagocytophilum and 1× B. afzelii + B. venatorum (Babesia sp., EU1) + "Ca. N. mikurensis"

Fig. 4

NGS results for 10 I. ricinus tick samples. Two samples positive in one or more pathogen screening PCR (a) and 8 samples negative in all screening PCRs (b) were analyzed. Whole genome amplified samples were sequenced on an Ion S5™, Kraken was used for taxonomic profiling of trimmed reads, and species with low read support were filtered out. Species pathogenic for humans, i.e. R. helvetica (R.h.), A. phagocytophilum (A.p.), B. afzelii (B.a.), as well as the tick endosymbiont "Candidatus Midichloria mitochondrii" (C.M.m.), are represented individually. The remaining species are grouped in Pseudomonas spp. (P.spp.), other residents of soil and water, commensals, human microbiota, and plant associated bacteria. The bars indicate the percentages of reads assigned to the respective species or groups in a logarithmic scale