**3. Results**

We sampled 36 rodents (10 bank voles and 26 wood mice) from the four locations (full dataset available in Supplementary Table S2). We identified five vector-borne microparasites using qPCR and sequencing: *Bartonella grahamii*, *Bartonella taylorii*, *Borrelia burgdorferi* s.l., *Borrelia miyamotoi*, and *Candidatus* Neoehrlichia mikurensis (Table 1). We found nine ectoparasite species: two tick species (*Ixodes ricinus* and *Ixodes trianguliceps*), four mite species (*Echinonyssus isabellinus*, *Eulaelaps stabularis*, *Haemogamasus nidi*, and *Laelaps agilis*), two flea species (*Ctenophtalmus agyrtes* and *Megabothris turbidus*), and one louse species (*Polyplax serrata*; Table 1). We also found eight gastrointestinal-parasite species: *Aonchoteca murissylvatici*, *Aspiculuris tianjinensis*, *Heligmosomoides polygyrus*, *Heterakis spumosa*, *Hymenolepis diminuta*, *Syphacia petruzewiczi*, *Syphacia stroma*, and an unidentified species of coiled nematode that was not *Trichinella* (Table 1). All serological tests were negative.


**Table 1.** Overview of infection with vector-borne microparasites and infestation with ecto- and gastrointestinal parasites in wood mice and bank voles in four Dutch forest sites.

1 Prevalence of (vector-borne) microparasites given as ratio of total number of animals with infection. 2 Average parasite load as well as prevalence (between square brackets) given as mean and range (in parentheses) of burden and ratio of total number of animals with infestation, respectively.

We found that simple trees with 3–4 nodes best described the data for all immunological and physiological indices. These trees revealed both differences among sites and differences in relation to parasite infestation or infection. Haptoglobin concentration was highest in animals with an *Ixodes trianguliceps*infestation (Mann–Witney U test, *p* = 0.035) and second highest in animals from Herperduin without *I. trianguliceps* (*p* = 0.022, Figure 1A). Haptoglobin concentration was lowest in animals with a body mass ≥16.5 gram in the other sites (*p* = 0.045, Figure 1A). Hemolysis titer was highest in bank voles (*p* = 0.00093) and intermediate in wood mice with a *Heligmosomoides polygyrus* infection (*p* = 0.050, Figure 1B). Hemagglutination titer was lowest in rodents <15.5 gram (*p* = 0.011) and highest in rodents from three sites (Buunderkamp, Herperduin and Stameren; *p* = 0.033, Figure 1C). Neutrophil to lymphocyte ratio was highest in animals infected with *Borrelia miyamotoi* (*p* = 0.0066) and lowest in animals infested with ≥3 *Laelaps agilis* (*p* = 0.12, Figure 1D). Hematocrit was lowest in rodents <13.5 gram (*p* = 0.0099) and highest in rodents between 13.5 and 17.5 gram (*p* = 0.11, Figure 1E). Body-mass corrected spleen size was highest in animals ≥17.5 gram (*p* = 0.0057) and lowest in animals from Buunderkamp, Herperduin and Maashorst (*p* = 0.036, Figure 1F).

**Figure 1.** Regression tree showing the most parsimonious split for (**A**) haptoglobin, (**B**) hemolysis, (**C**) hemagglutination, (**D**) neutrophil to lymphocyte ratio, (**E**) hematocrit, and (**F**) body-mass corrected spleen size. Boxplots show the spread of values (median and quantiles) within each group identified by the different splits in the tree. Sample size for each group is given underneath the boxplots. Body mass is given in grams.
