High-throughput sequencing technologies now allow for rapid cost-effective surveys of multiple pathogens in many host species including rodents, but it is currently unclear if the organ chosen for screening influences the number and identity of bacteria detected. We used 16S rRNA amplicon sequencing to identify bacterial pathogens in the heart, liver, lungs, kidneys and spleen of 13 water voles (Arvicola terrestris) collected in Franche-Comté, France. We asked if bacterial pathogen assemblages within organs are similar and if all five organs are necessary to detect all of the bacteria present in an individual animal. We identified 24 bacteria representing 17 genera; average bacterial richness for each organ ranged from 1·5 ± 0·4 (mean ± standard error) to 2·5 ± 0·4 bacteria/organ and did not differ significantly between organs. The average bacterial richness when organ assemblages were pooled within animals was 4·7 ± 0·6 bacteria/animal; Operational Taxonomic Unit accumulation analysis indicates that all five organs are required to obtain this. Organ type influences bacterial assemblage composition in a systematic way (PERMANOVA, 999 permutations, pseudo-F4,51 = 1·37, P = 0·001). Our results demonstrate that the number of organs sampled influences the ability to detect bacterial pathogens, which can inform sampling decisions in public health and wildlife ecology.

Rodents are shared intermediate or paratenic hosts for Echinococcus multilocularis, Toxocara spp. and Toxoplasma gondii, and may serve as valuable indicators for assessing the occurrence and the level of environmental contamination and infection pressure with free-living stages of these zoonotic parasites. We investigated 658 non-commensal rodents for parasite infections in the canton of Geneva, Switzerland. The prevalence of infection with E. multilocularis was highest in Arvicola terrestris captured in the north-western area (16·5%, CI: 10·1%–24·8%), possibly reflecting a higher red fox density due to the low incidence of sarcoptic mange in this part of the canton. The exposure rate to Toxocara spp. was highest in the urban area (13·2%, CI: 7·9%–20·3%), and may account for higher densities of domestic carnivore and red fox definitive hosts within the city. Exposure to T. gondii was widespread (5·0%, CI: 3·2–7·4%), indicating a ubiquitous distribution of infected cat definitive hosts. Interestingly, a widespread distribution of Taenia taeniaeformis, a parasite mainly transmitted by cats, was similarly evidenced in A. terrestris. Distinct spatial patterns for the different zoonotic parasites likely reflected differences in distribution, abundance, and habitat use of the respective definitive hosts. These results highlight the potential value of rodents as shared indicators for these pathogens.

Infection by the cestode Taenia taeniaeformis was investigated within numerous cyclic populations of the fossorial water vole Arvicola terrestris sampled during 4 years in Franche-Comté (France). The relative influence of different rodent demographic parameters on the presence of this cestode was assessed by considering (1) the demographic phase of the cycle; (2) density at the local geographical scale (<0·1 km2); (3) mean density at a larger scale (>10 km2). The local scale corresponded to the rodent population (intermediate host), while the large scale corresponded to the definitive host population (wild and feral cats). General linear models based on analyses of 1804 voles revealed the importance of local density but also of year, rodent age, season and interactions between year and season and between age and season. Prevalence was significantly higher in low vole densities than during local outbreaks. By contrast, the large geographical scale density and the demographic phase had less influence on infection by the cestode. The potential impacts of the cestode on the fitness of the host were assessed and infection had no effect on the host body mass, litter size or sexual activity of voles.

There is a considerable amount of literature on the diet and effect of feral American mink Mustela vison on the distribution and abundance of water voles Arvicola terrestris in Britain. Few recent studies, however, have attempted to examine the occurrence of water voles in the diets of native predators. Fox Vulpes vulpes scat and grey heron Ardea cinerea pellets were collected during winter 2003–2004 at a location known to contain a large number of water voles. These scats and pellets were subsequently analysed to determine the frequency of water vole occurrence in the diet of these opportunistic predators. Water vole remains were found in 30% of fox scats analysed and accounted for 13% of the total weight of scats. A total of 18% of heron pellets contained the fur of water voles in addition to the fur of four other rodent species. The potential role of native predators in the loss and fragmentation of site-specific populations of water voles in Britain is further discussed.

The fox tapeworm Echinococcus multilocularis is typically perpetuated in a cycle with red foxes as definitive hosts and various rodent species as intermediate hosts. In this study, foxes were baited with a highly efficient drug against cestodes (praziquantel) in 5 blocks of 1 km2. Voles, Arvicola terrestris, the most abundant intermediate host species, were trapped in the 5 baited blocks and in 5 non-baited control blocks. Baiting the foxes reduced the prevalence of E. multilocularis in fox faecal samples in the baited blocks, but voles trapped in the two blocks did not differ in their infection rates. However, voles from the baited blocks had significantly smaller spleen masses and were more likely to be infested with mites than those from the control blocks, possibly reflecting different immunological activities. Our study suggests that the environmental contamination with E. multilocularis eggs, and perhaps those of other tapeworms, influences the immune system of the intermediate host species A. terrestris in the wild.

The distribution of fossorial water voles Arvicola terrestris on 15 small islands in the Sound of Jura, Argyll, Scotland, was established and populations were characterized in terms of size, structure and genetic composition (12 microsatellites). The results are compared with similar data from four metapopulations of riparian water voles in mainland Scotland. Water voles occurred on six islands, one of which was occupied in 1994, but not on two islands occupied during the 1960s. Probability of occupancy was significantly associated with proximity to the nearest occupied island. Unlike voles on the mainland, which occupy fragmented patches of exclusively riparian habitat, voles on islands were fossorial and occupied large areas of continuous habitat. Spring population sizes were c. 10 times higher on the islands than colonies of mainland metapopulations. Individuals from three islands were genotyped and these populations had 50% lower levels of microsatellite polymorphism than the mainland metapopulations. Rates of inferred contemporary dispersal among islands separated by 1 km or more of sea were very low, according to large differences among islands in genetic composition, proportions of coat colour types and incidence of skin infection. None the less, the long-term persistence of the island populations is likely to depend on such dispersal. Comparisons between island populations and mainland metapopulations indicate that in both fragmented systems the level of dispersal is the primary factor influencing genetic structure. The low genetic variability in the island populations may reflect either past historical events (founder effect and genetic drift) or present ecology (lack of immigration in closed and temporally fluctuating populations).

Over a period of 26 months from January 1996 to February 1998, 388 foxes from the city of Zürich, Switzerland, were examined for intestinal infections with Echinococcus multilocularis and other helminths. The prevalence of E. multilocularis in foxes sampled during winter increased significantly from 47% in the urban to 67% in the adjacent recreational area, whereas prevalence rates of other helminths were similar in both areas. Seasonal differences in the prevalence of E. multilocularis were only found in urban subadult male foxes which were significantly less frequently infected in summer than in winter. The distribution of the Echinococcus biomass, as expressed by worm numbers per fox was overdispersed in 133 infected foxes randomly sampled in winter. Ten of these foxes (8%) were infected with more than 10000 specimens and carried 72% of the total biomass of E. multilocularis (398653 worms). Prevalences did not differ significantly in these foxes in regard to age and sex but worm burdens were significantly higher in subadult foxes as compared with adult foxes. In voles (Arvicola terrestris) trapped in a city park of Zürich, E. multilocularis metacestodes were identified by morphological examination and by PCR. The prevalence was 20% among 60 rodents in 1997 and 9% among 75 rodents in 1998. Protoscoleces occurred in 2 of the cases from 1997. The possible risk for human infection is discussed with respect to the established urban E. multilocularis cycle.