Determination of species diversity
A total of 5691 ectoparasite specimens belonging to 27 species were collected from 775 small mammals. Species richness was the lowest in the urban location with 12 ectoparasite species, compared to 19 species in the forest and 20 in the recreational area (Additional file 1: Table S1). The ectoparasites within the class of Insecta belonged to the order Siphonaptera (fleas) comprising four families (Leptopsyllidae, Ceratophylliodea, Ctenophthalmidae and Hystrichopsyllidae) with 11 species. Within the class of Arachnida, subclass Acari, three species of ticks (order Ixodida; family Ixodidae), one species of the order Sarcoptiformes and nine species within four families (Euryparasitidae, Laelapidae, Macrochelidae and Hamogamasidae) of the order Mesostigmata were detected.
In total, 689 of 775 (88.9%; 95% CI 86.50, 90.93) small mammals were infested with ectoparasites, with an average of 7.34 (standard deviation [SD] 11.58) specimens per host. Hosts were infested with up to six species of ectoparasites simultaneously. An infestation with one or two ectoparasite species per host was most frequently observed, with 253 (32.6%) and 242 (31.2%) hosts infested with one or two species, respectively. Apodemus flavicollis and A. agrarius were more frequently infested with two ectoparasite species than with one, with 64 A. flavicollis (25.9%) infested with one ectoparasite species and 75 (30.4%) infested with two species. None of the four trapped A. agrarius were infested with only one species of ectoparasite, and three (75%) were infested with two species (Table 1).
Investigation of the effect of host species, trapping location and season on the infestation of small mammals with ectoparasites
An overview of the prevalence of small mammals being affected with the three different ectoparasite groups and their mean intensity across the three trapping locations is provided in Table 2.
The prevalence of ticks was higher than that of mites or fleas. More than 80% of the hosts in urban and forest areas had ticks, while fleas were present in around 60% of the hosts and mites only in 20% of the hosts in the recreational area.
Mean intensity and prevalence of ticks
Ticks were the most frequently observed group of ectoparasites, with an overall prevalence of 68.77% (95% CI 65.43, 71.94). Three species of ticks were found during the study period. Ixodes ricinus was the most commonly observed species of ticks, with a prevalence of 68.52% (95% CI 65.16, 71.69) and was found in all three trapping locations and on all host species. Ixodes trianguliceps was exclusively found in the forest location, while D. reticulatus was only observed in the recreational area. Prevalence was low for both these species, ranging between 3.32% (95% CI 1.69, 6.41) and 3.01% (95% CI 1.83, 4.91), with no significant difference in infestation of A. flavicollis or M. glareolus (I. trianguliceps: Fisher test P = 0.634; D. reticulatus: Fisher test P = 0.289). The mean intensity of the infestation was 1.0 for all occurrences of I. trianguliceps and D. reticulatus, except for M. glareolus in the recreational area, which was infested with a mean intensity of 9.2 (SE 2.7) with D. reticulatus.
Factors significantly influencing the prevalence of ticks in the forest location and the recreational area were host species, trapping location and season, and factors influencing the mean intensity were host species and season (spring) (Fig. 1). Apodemus flavicollis was more likely to be infested than M. glareolus (LR-OR 1.99, P < 0.001; TNB-IRR 1.91, P < 0.001). The season also significantly influenced the prevalence and the mean intensity of ticks on small mammals with spring and summer as the seasons with the highest odds to find ticks compared to winter (LR-OR: 1.97, P = 0.009 spring; 2.22, P < 0.001 summer). The mean intensity of ticks was highest during spring (TNB-IRR 3.00, P = 0.001) compared to winter, but was not statistically significantly higher during summer (TNB-IRR 1.36, P = 0.179). The trapping location did not significantly influence the mean intensity of ticks (TNB-IRR 1.00, P = 0.986), but it significantly influenced the prevalence of ticks, with small mammals in the forest being much more frequently infested than in the recreational area (LR-OR 3.30, P < 0.001).
Mean intensity and prevalence of mites
Mites had a prevalence of 26.32% (95% CI 23.34, 29.53), with the most prevalent family being Laelapidae (21.55%, 95% CI 18.80, 24.58). The most frequently observed species of mite was L. agilis, with a prevalence between 72.45% (95% CI 62.88, 80.32) and 42.28% (95% CI 34.64, 50.31) for A. flavicollis in the forest location and the recreational area. Myodes glareolus was only infested in the forest location, with a prevalence of 5.04% (95% CI 2.46, 10.03). Laelaps agilis also affected Apodemus sylvaticus in the urban location, but was not found on A. agrarius, Sorex spp. or Microtus arvalis. Eulaelaps stabularis was the second most frequently observed mite species, with a low prevalence in both the forest location (1.66%; 95% CI 0.65, 4.19) and recreational area (3.6%; 95% CI 2.21, 5.76) but a higher prevalence in the urban park (19.44%; 95% CI 9.75; 35.03). Haemogamasus nidi was also observed frequently and had the highest prevalence in the urban location (5.5%; 95% CI 1.54, 18.14). Other mite species were observed less frequently. Oribatid mites and Androlaelaps farenholzi were only found in the recreational area while Haemogamasus hirsutosimilis and Euryparasitus emerginatus were only found in the forest. Like ticks, mites were found on small mammals in all three trapping locations, but the prevalence significantly differed between locations (Chi2-test: 33.423, P < 0.001), with the lowest prevalence found in the recreational area and the highest in the urban area (Table 2). Only a few individuals belonging to the species A. agrarius and M. arvalis (n = 6 for each) were trapped in the recreational areas and these were not infested with mites at all (Additional file 1: Table S3). From the Sorex spp. trapped in the forest, one specimen was infested with mites (Eur. emerginatus).
Factors influencing the prevalence of mites in the group level regression models were host species and season, with a higher prevalence for the host species being A. flavicollis and a lower prevalence for the season being spring (Fig. 2). The mean intensity was affected by host species, trapping location and season (summer). Apodemus flavicollis was more often infested than M. glareolus (LR-OR 14.99, P < 0.001) and with a much higher mean intensity (TNB-IRR 5.07, P < 0.001) (Fig. 2).
Mean intensity and prevalence of fleas
Fleas had an overall prevalence of 59.35% (95% CI 55.86, 62.76), with Ctenophthalmidae being the most prevalent family (51.87%; 95% CI 48.35, 55.37). The most prevalent species of fleas was C. agyrtes. Apodemus flavicollis in the forest location was slightly more often infested (66.33%; 95% CI 56.51, 74.91) than M. glareolus in the forest or the recreational area, which ranged between 48.20% (95% CI 40.06; 56.44) and 50.60% (95% CI 45.26; 55.92), respectively. The flea species Me. turbidus had a prevalence of 15.87% (95% CI 12.34; 20.17) in the recreational area and of 5.04% (95% CI 2.46; 10.03) in the forest on M. glareolus, whereas its prevalence on A. flavicollis was 5.37% (95% CI 2.75, 10.24) and 2.04% (95% CI 0.56, 7.14) for the forest location and recreational area, respectively.
In the regression analysis no hurdle model could be fitted; therefore the abundance was modeled in a negative binomial model. The abundance of fleas was not significantly influenced by either host species or the location of trapping, but by season (NB-IRR spring 1.06, P = 0.751; NB-IRR summer 1.94, P < 0.001) (Fig. 3).
Assessment of the parameters influencing the abundance of the most frequently observed ectoparasite species
Ixodes ricinus, Laelaps agilis, Ctenophthalmus agyrtes and Megabothris turbidus were each observed with an overall prevalence of > 5% and thus analyzed separately. Due to the low sample size of Ctenophthalmus congener congener and C. bisoctodentatus, these flea species were grouped and analyzed together.
Ixodes ricinus was more prevalent in the forest (LR-OR 3.19, P < 0.001) and during the spring and summer (LR-OR spring 2.99, P < 0.001; LR-OR summer 1.73, P = 0.024); however, A. flavicollis was not more likely to be infested by I. ricinus (Table 3) even though the host species had a significant influence on the mean intensity of I. ricinus (TNB-IRR 2.45, P < 0.001). Being infested with other ectoparasite species also had a strong influence on infestation with I. ricinus ticks, but it did not have an influence on the mean intensity of I. ricinus. Each additional infestation with an ectoparasite species was associated with an increase in the odds (4.76; P < 0.001) of being infested with I. ricinus. Being a male host or in a higher age category also significantly influenced the mean intensity of I. ricinus ticks (TNB-IRR-male 1.84, P < 0.001; TNB-IRR young adult 1.79, P = 0.034, TNB-IRR adult 1.76, P = 0.049).
Laelaps agilis mites were almost exclusively found on A. flavicollis (LR-OR 121.95, P < 0.001) with only five M. glareolus in the forest being infested with this species (Additional file 1: Table S3). Even though small mammals trapped in the forest did not have a higher prevalence of L. agilis compared to those in the recreational area, they did have a higher mean intensity (LR-OR 1.57, P = 0.173; TNB-IRR 1.92, P = 0.031) (Table 4). The same was true for male hosts (TNB-IRR 1.76, P = 0.029). Polyparasitism also had a statistically significant positive influence on the prevalence of L. agilis mites (LR-OR 3.07, P < 0.001). Hosts trapped in the spring had slightly lower odds to be infested with L. agilis than those trapped in the winter (LR-OR 0.23, P = 0.005). This association was not statistically significant for hosts trapped in the summer (LR-OR 0.79, P = 0.524).
The prevalence of C. agyrtes was higher on M. glareolus and on hosts trapped during the summer (LR-OR A. flavicollis 0.35, P < 0.001; LR-OR summer 1.82, P = 0.015) and also for small mammals infested with multiple ectoparasite species, with each additional infestation associated with an increase in the odds of 6.74 (P < 0.001) (Table 5).
The prevalence of Me. turbidus was higher for M. glareolus (LR-OR A. flavicollis 0.13, P < 0.001) in the recreational area (LR-OR-forest 0.24, P < 0.001) and 3.62-fold as high for each additional infestation with an ectoparasite species (LR-OR polyparasitism 3.62, P < 0.001) (Table 6). The same factors statistically significantly affected the prevalence of C. bisoctodentatus and C. congener congener (LR-OR A. flavicollis 0.32, P = 0.008; LR-OR-forest 0.07, P < 0.001; LR-OR-polyparasitism 2.84, P < 0.001) (Table 7).