Introduction

Introduced vertebrates have a lower parasite species diversity compared to their counterparts on their native area (Dobson and May 1986). Such a reduction in parasite diversity can result from the extinction of species very early after their host’s introduction because (1) a small number of founding hosts brought a few number of parasite species with them; (2) of a lack of obligatory intermediate hosts or too constraining abiotic conditions; (3) of limitation in the acquisition of new parasite species (Torchin et al. 2003). While the first two hypotheses need both parasitic surveys from native populations soon after the introduction of hosts, the third can be empirically tested by comparing the parasitic cortege of co-inhabiting hosts that may transfer some of their parasites to introduced hosts.

The Siberian chipmunk Tamias sibiricus was introduced into suburban French forests less than 30 years ago and has now spread across several localities (Chapuis 2005). Chipmunks share a poor species diversity in intestinal helminth (Pisanu et al. 2007) and arthropods (hard ticks: Vourc’h et al. 2007; sucking lice: Beaucournu et al. 2008; fleas: Pisanu et al. 2008). For helminths, only two dominant species of nematodes and four other rare species have been found (Pisanu et al. 2007): one of the two dominant species, Brevistriata skrjabini (Schulz and Lubimov 1932) Durette-Desset 1976, is a nematode restricted to Northeastern Asiatic Sciurids, and the other, Aonchotheca annulosa (Dujardin 1845) Bain and Wertheim 1981 is a nematode that develops in a wide spectrum of compatible host species, parasitizing numbers of Western European boreal mammals (Moravec 2000; Pisanu et al. 2007).

This study is based on the helminth survey of the two dominant native species of terrestrial rodents co-inhabiting with introduced chipmunks on three woody localities of the Ile-de-France region. This paper aims to understand why Siberian chipmunks acquired so few intestinal helminth species when introduced into French forests.

Materials and methods

Ninety-four rodents were analyzed, trapped from three locations in late May/early July. This sample includes: ten wood mice Apodemus sylvaticus and 12 bank voles Clethrionomys glareolus from the Forest of Sénart (48°39′ N, 02°29′ E; 3,200 ha) in 2004 and 2005. Thirty rodents (15 of both species) were analyzed from the urban Park of Henri Sellier (48°46′ N, 02°16′ E; 26 ha) in 2006 and 15 of both species from the Wood of Verneuil-sur-Seine (48°59′ N, 01°56′ E; 400 ha) in 2007. Additionally, 12 chipmunks were analyzed which had been trapped in the same months in 2007 on the Wood of Verneuil-sur-Seine, completing a previous helminth survey in this locality (n = 3, see Pisanu et al. 2007). Rodents were live-trapped and killed by cervical dislocation, then frozen at −20°C before analysis. The Forest of Sénart rodents were autopsied in the field and the entire digestive tract was removed and fixed in ethanol 70°C. Identification of Murid rodents was based on Le Louarn and Quéré (2003). Helminths were searched for by dissection of the complete gut walls and lumen, from esophagus to rectum, and counted using binocular lenses (×10–60). Species identification was done by morphological analysis using a microscope equipped with a camera lucida, and was based on, for the genus Aonchotheca Lopez-Neyra 1947, sensu Moravec (1982) and Eucoleus Dujardin 1845 sensu Moravec (1982) to the descriptions by Roman (1951), Bain and Wertheim (1981), Justine and de Roguin (1990), Pisanu and Bain (1999), and Moravec (2000); for the genus Trichuris Roederer 1781 by Feliu et al. (2000) ; Genus Brevistriata Durette-Desset 1971 by Durette-Desset (1970, 1976) ; Genus Heligmosomoides Hall 1916 by Durette-Desset (1971) and Biserkov et al. (1998); Genus Syphacia Seurat 1916 sensu Hugot 1988 and Aspiculuris Schulz 1924 by Roman (1951) and Quentin (1971a); Genus Mastophorus Gmelin 1790 by Quentin (1969; 1971b); Genus Strongyloides Grassi 1879 by Sato et al. (2007).

Prevalence and mean number of worms per infected host, i.e., mean intensity of infection, along with their 95% confidence intervals, were calculated using a bootstrapped method provided by the Quantitative Parasitology 3.0 software (Reiczigel and Rózsa 2005). Prevalence was compared between sites using the Fisher’s exact test and mean intensity was compared using the Mood’s median test.

Results

A. annulosa was found in the small intestine of all the hosts species analyzed (Table 1). In the Forest of Sénart sample, it was found in one wood mouse (ten worms) and in eight chipmunks with a mean intensity of two worms (range in counts one to seven worms), but not in voles. There was neither difference in prevalence (P Fisher = 0.21) between species, nor intensities (P Mood = 0.44). All host species harbored A. annulosa in the Park Henri Sellier sample: 13 mice with a mean intensity of 15 worms (range 1–51), seven voles with a mean intensity of 11 worms (2–21), and four chipmunks with a mean of 117 worms (3–431), and no differences in prevalence (P Fisher = 0.07) and mean intensity (P Mood = 0.73) between hosts. In the Wood of Verneuil-sur-Seine sample, all hosts also harbored A. annulosa: five chipmunks with a mean intensity of four worms (between 1 and 7), two mice (2; 39), and two voles (6; 11), without differences in prevalence (P Fisher = 0.45) or mean intensity (P Mood = 1.00). A. annulosa was significantly more frequent in wood mice on the Park Henri Sellier than on the other sites analyzed (P Fisher < 0.01) but not in mean intensity (P Mood = 1.00). There were no differences in chipmunks between sites (P Fisher = 0.26; P Mood = 0.08), as in bank voles (P Fisher = 0.11; P Mood = 1.00).

Table 1 Mean intensity (mI) of infection, followed by the 95% confidence interval in parentheses, and number of hosts infected (n) by helminth in rodents on the Forest of Sénart, the Park of Henri Sellier, and the Wood of Verneuil-sur-Seine
Full size table

Mastophorus muris was found in the stomach of wood mice and bank voles. Only one vole had one worm in the Sénart sample (Table 1). In the Henri Sellier sample, four mice had M. muris with a mean intensity of two worms (one to two) and six voles with a mean of six worms (1–16), without differences between species (P Fisher = 0.70; P Mood = 0.20). In the Verneuil-sur-Seine sample, six mice had M. muris with a mean intensity of three worms (one to five), and 14 voles with a mean of seven worms (1–34). Voles were more frequently infected than mice (P Fisher < 0.01), but there were no differences in mean intensity of infection by M. muris (P Mood = 1.00). There were no differences in mice between sites (P Fisher = 0.70; P Mood = 0.20); voles were more frequently infected in Verneuil-sur-Seine than in Sellier (P Fisher < 0.01), and no differences were found in mean intensity between sites (P Mood = 1.00). M. muris was not found in chipmunks.

Aonchotheca murissylvatici was found in the small intestine of two mice from Sénart (one and two worms). This worm was found encysted in the stomach of eight voles in Sénart with a mean of 35 worms (5–100), and 11 in Henri Sellier with a mean of 159 worms (40–500), but not in Verneuil-sur-Seine. There were no differences in infection in voles between sites (P Fisher = 1.00; P Mood = 0.06). A. murissylvatici was not found in chipmunks.

All the other helminth species were found in a single host species. Heligmosomoides polygyrus was found only in the small intestine of wood mice: five in Sénart with a mean of 13 worms (1–32), 14 in Sellier with a mean of 7 worms (1–28), and all mice trapped in Verneuil-sur-Seine with a mean of 32 worms (1–119). Mice in Sénart were significantly less frequently infected by H. polygyrus than in the two other locations (P Fisher < 0.01), without any significant difference in mean intensity of infection between sites (P Mood = 0.13). Syphacia (Syphacia) stroma was found in two mice in Sénart (1 and 123 worms), in 14 mice in Henri Sellier with a mean of 24 worms (1–73), and 14 in Verneuil-sur-Seine with a mean of 85 worms (1–119). Mice in Sénart were significantly less frequently infected by S. stroma than in the two other localities (P Fisher < 0.01), without any significant difference in mean intensity of infection between sites (P Mood = 0.35). Trichuris muris was found in three mice in Sénart (one, one and two worms) and eight mice in Henri Sellier with a mean of two worms (one to four), but not in Verneuil-sur-Seine. No difference between sites was found in T. muris infection in mice (P Fisher = 0.41; P Mood = 1.00). Syphacia (Syphacia) frederici was found in a single mouse in Henri Sellier (three worms), and in four mice in Sénart (1, 20, 74 and 108 worms). Eucoleus sp. (? bacillatus) was found in three mice in Sellier (two, three, and five worms) and in two mice in Sénart, each harboring a single worm encysted in the stomach wall. Aspiculuris tetraptera was found in only three mice (one, one, and four worms) in Sénart.

Heligmosomoides glareoli was found in the small intestine of one bank vole (two worms) in Verneuil-sur-Seine, in eight voles on Sellier with a mean of five worms (two to ten), and in ten voles in Sénart with a mean of four worms (1–11). Voles in Verneuil-sur-Seine were significantly less frequently infected by H. glareoli than in the two other locations (P Fisher < 0.01), without any significant difference in mean intensity of infection between sites (P Mood = 0.50). Only one specimen belonging to the genus Trichuris was found in the cecum of a vole in Sénart.

B. skrjabini (Schulz and Lubimov 1932) Durette-Desset 1976 was found in the small intestine of three chipmunks in Sellier (20, 30, and 38 worms), 15 in Verneuil-sur-Seine with a mean of 175 worms (1–1,819), and 20 in Sénart with a mean of 44 worms (1–225). There was no difference in prevalence in B. skrjabini in chipmunks according to site (P Fisher = 0.08), and chipmunks in Verneuil-sur-Seine were more intensely infected by B. skrjabini than in the other sites (P Mood = 0.05). Strongyloides callosciureus (Sato et al. 2007) was found only in eight chipmunks in Verneuil-sur-Seine, with a mean of seven worms (four to ten). In the Sénart sample, four chipmunks hosted a Trichuroid female in their cecum, and one had an Oxyurid male also in the cecum. One chipmunk had an Ascaroid male in the duodenum at Verneuil-sur-Seine, and three chipmunks had immature stages of a Trichostrongyloid (one, two and four worms) in the small intestine.

Unidentified cestodes Cyclophyllidea were found in the small intestine of a mouse in Verneuil-sur-Seine (one worm), in Sénart (two mice each infected by a single worm), and in Henri Sellier, where ten mice harbored a mean of four worms (three to six). One vole had one specimen of a Cestode. An unidentified Trematode was found in the small intestine of two mice (one and six worms) from Sénart.

Discussion

Introduced chipmunks acquired only one of the species shared by sympatric native Murid species that dominate in the rodent community: A. annulosa. This nematode was the third most often identified in the rodent’s helminth community, with a total of 849 worms counted. It infects a very wide number of host species in Western Europe (Pisanu et al. 2007), mainly rodents and even insectivores or primates (Moravec 2000). This result highlights that such a generalist helminth species (i.e., with a wide spectrum of compatible host species; Combes 1995) is more prone to invade new host species, providing its taxonomic position is valuable (for the genus Aonchotheca, see Moravec 1982; Pisanu and Bain 1999). Its life-cycle is unknown.

Eleven other helminth species were restricted to the native Murid species. Among these, at least three are heteroxenous species: M. muris, and the Platyhelminths. Six are monoxenous species, H. polygyrus, S. stroma, T. muris, S. frederici, and A. tetraptera. The life cycles of the Capillariids nematodes Eucoleus sp. (? bacillatus) and A. murissylvatici are unknown.

M. muris is commonly transferred between wood mice and bank voles. This nematode is acquired from the consumption of insect prey which is an obligate intermediate host (Anderson 1992). M. muris was not found in chipmunks, although these terrestrial Sciurid are omnivorous (Kawamichi 1980), and prey on several insect and snail species in the Forest of Sénart (J.-L. Chapuis, unpublished). This result confirms that heteroxenous helminth can accidentally develop at least in related host species at the family level, i.e. Murid rodents for M. muris.

Unidentified trematodes were only found in wood mice in Sénart. This result highlights the importance of local peculiarities in helminth community composition of rodents, such as in wood mice (Abu-Madi et al. 2000; Eira et al. 2006) or bank voles (Behnke et al. 2001). These platyhelminth species were not found in chipmunks, indicating that heteroxeny is not a key condition of helminth biology for successful host transfer.

All other helminth species are monoxenous nematodes, which can be seen as specific of their host at the subfamily or genus level: H. polygyrus, S. stroma, T. muris, S. frederici, and A. tetraptera in Murinae (i.e. Apodemus spp., Mus spp.; Anderson 1992), and H. glareoli in Arvicolinae (Clethrionomys spp., Microtus spp.; Biserkov et al. 1998). The life cycle of Eucoleus sp. (? bacillatus) and A. murissylvatici is unknown. However, Eucoleus spp. has been reported as being specific to the genus level within the Murinae (Roman 1951; Bain and Wertheim 1981; Moravec 2000). A. murissylvatici only very occasionally infects hosts in Murinae, as in Sénart. It is more frequently and intensely found in C. glareolus (Moravec 2000). According to the observation in this study, bank voles can be considered as primary host for A. murissylvatici. Thus, reduced spectrum of compatible host species available to fully develop must explains why these helminths were not found in chipmunks.

Chipmunks imported two nematode species in Ile-de-France: B. skrjabini and S. callosciureus. B. skrjabini is most probably a direct life cycle nematode, and it is restricted to Sciurid hosts in Eastern Asia (Pisanu et al. 2007). S. callosciureus was missed in the previous study. This nematode was recently described from feral Pallas’s squirrels Callosciurus erythraeus introduced as zoo or park animals from Taiwan, and captive Plantain squirrels Callosciurus notatus originating from Malaysia and sold in Japan in 2004–2005 (Sato et al. 2007). Again, affinities of this nematode species with the Sciurids could explain why they were not found in the Murid rodents co-inhabiting with chipmunks in Ile-de-France.

A total of 13 nematode specimens belonging to at least five taxa were also found in nine chipmunks that could not be fully identified at the species level. These represent accidental infections that may have been acquired from other hosts species inhabiting the localities investigated here. In the case of seven immature stages of Trichostrongyloidea, a male was observed which did not corresponded to the species found in native rodents. The four females Trichuris sp. found in the cecum of four chipmunks in Sénart may be accidental infection by T. muris, which is mainly found in A. sylvaticus on this locality. Only molecular identification could help in identifying these specimens, as morphology alone remains insufficient to discriminate between Trichuris sp. from Murid rodents (Feliu et al. 2000).

At least 14 helminth species were found in the native small rodents living sympatrically in the three woody localities investigated in Ile-de-France, of which only one was transferred to introduced chipmunks: A. annulosa, a generalist nematode species. The lack of inter-transmission by the other species found in these rodents is mainly explained by the ability for helminth to develop only in phylogenetically related host species (Klimpel et al. 2007). These results are in accordance with other studies on the parasite diversity of these introduced Sciurid (Pisanu et al. 2008; Beaucournu et al. 2008) and more largely with introduced rodents (Asakawa 2005). If native host species are threatened by parasites imported with introduced hosts (Prenter et al. 2004; Smith and Carpenter 2006), then careful attention should be paid to closely related native host species, like European red squirrels Sciurus vulgaris L., that live sympatrically with introduced chipmunks in French forests (Pisanu et al. 2007, 2008).