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Female-biased infection and transmission of the gastrointestinal nematode Trichuris arvicolae infecting the common vole, Microtus arvalis
International Journal for Parasitology 41 (2011) 1397–1402
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Female-biased infection and transmission of the gastrointestinal nematode Trichuris arvicolae infecting the common vole, Microtus arvalis Andreas Sanchez a, Godefroy Devevey a,b, Pierre Bize a,⇑ a b
Department of Ecology & Evolution, University of Lausanne, Biophore, 1015 Lausanne, Switzerland Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA
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Article history: Received 27 July 2011 Received in revised form 30 September 2011 Accepted 30 September 2011 Available online 29 October 2011 Keywords: Parasite-biased infection Parasite-biased transmission Host gender Nematode Microtus arvalis
a b s t r a c t Previous studies addressing the importance of host gender in parasite transmission have shed light on males as the more important hosts, with the higher transmission potential of males being explained by the fact that they often harbour higher parasite loads than females. However, in some systems females are more heavily infected than males and may be responsible for driving infection under such circumstances. Using a wild population of common voles (Microtus arvalis), we showed that females were more frequently infected by the intestinal nematode Trichuris arvicolae than males (i.e. prevalence based on the presence of eggs in the faeces) and that females were shedding greater numbers of parasite eggs per gram of faeces (EPG) than males. By applying an anthelmintic treatment to either male or female voles, we demonstrated that treating females significantly reduced parasite burdens (i.e. prevalence and EPG) of both male and female hosts, while treating males only reduced parasite burden in males. These findings indicate that in this female-biased infection system females play a more important role than males in driving the dynamics of parasite transmission. Ó 2011 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction The transmission of parasites within host populations is influenced by many different sources of host heterogeneity, ranging from genetics, physiology and behaviour to spatial and temporal factors. A consequence of such heterogeneity is that a few hosts are generally responsible for the majority of the parasite transmission (and thereby parasite persistence) within host populations (Anderson and May, 1991; Woolhouse et al., 1997; Lloyd-Smith et al., 2005). This phenomenon is often referred as the Pareto rule, with 20% of the host populations contributing to at least 80% of the parasite transmission (Woolhouse et al., 1997), and it has led to the concept of high-risk ‘core groups’ (Anderson and May, 1991). Identifying which hosts are the main transmitters of parasites and why those hosts are responsible for so many transmission events are key questions in epidemiological studies. During the last decade, the contribution of host gender in parasite transmission and persistence within host populations has attracted growing attention (Perkins et al., 2003, 2008; Ferrari et al., 2004; Skørping and Jensen, 2004; Luong and Hudson, 2009). Because males are more frequently infected and suffer greater costs of infection than females (Poulin, 1996; Zuk and McKean, 1996; Schalk and Forbes, 1997; Moore and Wilson, ⇑ Corresponding author. Tel.: +41 (0)21 692 4204; fax: +41 (0)21 692 4165. E-mail address: [email protected] (P. Bize).
2002), males are often viewed as the ‘sicker sex’ in the literature (Zuk, 2009) and, in turn, as driving infection dynamics (Skørping and Jensen, 2004). In agreement with this idea, sex-biased variation in parasite transmission has been investigated in three rodent-parasite systems in the wild (Perkins et al., 2003; Ferrari et al., 2004; Luong and Hudson, 2009; see also Perkins et al., 2008), and in all three systems males were identified as driving the transmission and maintenance of parasites within host populations. Transmission of tick-borne encephalitis virus occurs when ticks feed in a ‘co-feeding aggregation’, and observation of co-feeding aggregation by sheep ticks (Ixodes ricinus) infesting yellownecked mice (Apodemus flavicollis) pointed out that sexually mature male mice of high body mass might be responsible for between 74% and 94% of the transmission events (Perkins et al., 2003). Experimental studies have clearly demonstrated the importance of male hosts in the transmission and persistence of gut parasites within yellow-necked mouse populations (Ferrari et al., 2004; see also Perkins et al., 2008) and white-footed mouse populations (Peromyscus leucopus) (Luong and Hudson, 2009). Anthelmintic treatments applied to either male or female hosts showed that the parasite burden of untreated females was significantly reduced when males were experimentally treated, while treating females had no consequences on the parasite burden of untreated males. However, male-biased infection and/or susceptibility to parasites are not universal. Although not frequently reported, in some
0020-7519/$36.00 Ó 2011 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpara.2011.09.004
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systems females are more heavily infected than males (e.g. Hillegass et al., 2008), and one intriguing question is which sex is responsible for driving the infection in those systems. We investigated the relative importance of host gender in the transmission of an intestinal nematode (Trichuris arvicolae) in a wild population of common voles (Microtus arvalis) with a female-biased infection (A. Sanchez, G. Devevey, P. Bize, unpublished data; present study). This directly transmitted parasite has previously been demonstrated to impair the reproductive success of its host, with experimentally infected females producing fewer pups of lower body mass (Deter et al., 2007), and potentially to regulate its host population dynamics (Deter et al., 2008). To explore the roles played by males and females in parasite transmission, we experimentally treated either male or female voles in alternative trapping grids using an anthelmintic, and we monitored the consequences of our treatment on the prevalence and faecal egg output simultaneously in individuals from the treated and untreated sexes. We predicted that, if transmission rate increased with the production of infectious propagules, anthelmintic treatment of female voles should reduce parasite burden not only in treated females but also in untreated males.
endoparasites in the study population, we created the three following experimental groups: on two replicate grids males alone were treated with the antiparasitic drug ivermectin (IvomecÒ, Merial, Merck Sharp & Dohme, Harlem, Netherlands); on two other grids females alone were treated with ivermectin; and on two control grids half of the males and half of the females were treated with propylene glycol which is the major solvent of IvomecÒ. Voles were treated at every trapping session by injecting subcutaneously either a dose of 0.01 mg of ivermectin in 0.01 ml of propylene glycol per gram of vole (i.e. antiparasitic treatment) or with a dose of 0.01 ml of proplyene glycol per gram of vole (i.e. control treatment). Ivermectin has a broad spectrum of activity against both ecto- and endoparasites, and it is commonly used to treat helminth infection in livestock and pets. Because juveniles below 14 g were not infected (Behnke and Wakelin, 1973; A. Sanchez, personal observations), only individuals weighting more than 14 g were treated. This experiment was conducted with the approval of the Veterinary Services of the Canton Vaud, Switzerland.
2. Materials and methods
To collect fresh faeces, voles were kept individually for 3 h in a 55 mm 40 mm 140 mm plastic box with a piece of apple as a water source before returning them to their trapping site. Faecal samples collected in each box were immediately weighed and stored in 10% formalin at 4 °C until eggs were counted in the laboratory. The number of eggs per gram of faeces (EPG) was assessed using the McMaster technique on faeces samples weighing more than 0.02 g (Mes, 2003). A total of 895 faecal samples was analysed by the same person (A. Sanchez) within 15 days of collection. Preliminary monitoring of the community of gastrointestinal parasites in the faeces showed that common voles of our study population were infected by the gastrointestinal nematode T. arvicolae (A. Sanchez, personal observations; for a description of the species, see Feliu et al., 2000), and thus we concentrated our attention on this endoparasite. Trichuris arvicolae is an obligate monoxenous nematode known to negatively affect the reproduction of its host (Deter et al., 2007). In Trichuris muris, a closely related species which parasitises murine rodents (Feliu et al., 2000), the complete life cycle takes 30–49 days (Pike, 1969). After ingestion via contaminated food, embryonated eggs hatch in the small intestine of the host, larvae develop, become adults and then establish themselves in the caeca. After reproduction, unembryonated eggs are released in the host faeces; the eggs embryonate and become infective in the soil.
2.1. Study area and vole monitoring The common vole (M. arvalis) is a widely distributed Eurasian rodent that inhabits meadows and agricultural fields and that can become very abundant during population outbreaks (>1,000 voles per hectare; Briner et al., 2007). From July to November 2009, we studied a common vole population located in meadows near the University of Lausanne, Switzerland. Live trapping was performed on a set of six trapping grids, each covering an area of 0.05 hectare, located in grass meadows dominated by Bromus erectus. Grids consisted of five lines of nine Longworth traps (i.e. 45 traps) placed 2–5 m from each other and deliberately set at places showing signs of activity (i.e. trails and holes) to optimise catching success. Traps were baited with seeds, a piece of apple as a water source and hay for bedding. To maximise trap encounter and thereby capture rate, traps were pre-baited for one night before each trapping session. To minimise possible movements of individuals between the grids, each grid was separated from the neighbouring grid by at least 250 m. Traps on each grid were set one night per month for a total of 2,160 trap nights. Voles were identified using a numbered eartag. At each capture, voles were weighed to the nearest 0.1 g with a digital balance and their head width was measured to the nearest 0.1 mm with a calliper. We classified individuals as male or female based on the visual examination of their genitalia, and as juvenile, sub-adult or adult according to pelage colouration and body length. We scored the breeding status of each individual, on a scale from 0 to 3, according to whether testes in males were not (0), weakly (1), moderately (2) or well (3) developed, and whether females showed no sign of reproduction (0), had a perforated vagina (1), were lactating (2) or pregnant (3). In total, we captured 535 individual voles (222 males and 313 females), and 396 of them (150 males and 246 females, representing 67.6% and 78.6% of the initial number of males and females) were captured during at least two trapping sessions and defined as resident. Of the 396 recaptured voles, three males and five females moved to an adjacent grid; those eight individuals were discarded from the final analyses. The trapping and ear-tagging of voles was performed under the legal authorisation of the Wildlife Services of the Canton Vaud, Switzerland. 2.2. Anthelmintic treatment To investigate whether one host sex, either male or female voles, was responsible for the maintenance and transmission of
2.3. Parasite quantification
2.4. Statistical analyses To gain insight on natural variation in T. arvicolae prevalence and intensity in the common vole, we restricted our statistical analyses to the first capture of each individual trapped in the control grids. We analyzed the effect of host sex, size and trapping month on natural variation in parasite prevalence (coded 1/0 based the presence/absence of eggs in the faeces) using a logistical regression and on EPG using an analysis of covariance (ANCOVA). We performed the analyses on 96 control voles for which we had faecal parasite counts. To investigate the effect of anthelmintic treatment on T. arvicolae prevalence and intensity, we restricted our statistical analyses to the last capture of each resident individual (i.e. individuals captured, treated if required and re-captured). Most residents (80.3%) were re-captured only once. Because individuals weighing less than 14 g were not infected (A. Sanchez, personal observations), only residents weighing more than 14 g at their last capture were considered in the analyses. We analyzed the effect of host treatment, sex, size and trapping month, plus the interaction between
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treatment and sex, on parasite prevalence using a logistical regression, and on EPG using an ANCOVA. Differences between male and female voles in the transmission of T. arvicolae in the host population will be indicated by a significant treatment by sex interaction. EPG measurements were log-transformed (i.e. log (EPG + 1)) before analyses to fit a normal distribution; we verified the normality of the ANCOVA residuals using quantile–quantile plots. We performed the analyses on 47, 72 and 65 resident voles of the control, male-treated and female-treated grids, respectively. Preliminary analyses showed no significant effect of the grid identity on parasite prevalence in the faeces and EPG (all P-values >0.60), and therefore we did not control for the grid identity in the final analyses. All of the statistical analyses were performed using the statistical package JMP 7.0 (SAS Institute Inc., USA). All tests were two tailed and a P value <0.05 was considered significant.
3. Results 3.1. Natural distribution of T. arvicolae The variation in the prevalence of infection by T. arvicolae based on the presence of eggs in the host faeces was significantly explained by host sex (logistical regression: v2 = 5.27, P = 0.022), head width (v2 = 23.72, P < 0.001) and trapping month (v2 = 18.17, P = 0.001). Female voles were infected more frequently than males (Fig. 1A), the prevalence of infection increased with head width (Fig. 1B), and the prevalence of infection from July to November showed a peak of infection in September (Fig. 1C). EPG was significantly explained by host sex (ANCOVA: F1,89 = 10.38, P = 0.002) and head width (F1,89 = 25.10, P < 0.001) but not by trapping month (F4,89 = 1.64, P = 0.17). Females had greater EPGs than males (Fig. 1A), and EPGs increased with head width (Fig. 1B). The interaction between host sex and head width did not significantly contribute to the prevalence of infection (v2 = 0.96, P = 0.33) or EPG (F1,88 = 1.16, P = 0.28), indicating that the increase in shedding rates of T. arvicolae eggs with head width was similar in male and female common voles. Statistical analyses carried out separately for each sex showed no influence of male or female breeding status on prevalence and EPG (all P-values >0.32). However, analyses where individuals were grouped in reproducing females (pregnant or lactating), non-reproducing females and males revealed that pregnant and lactating female voles were more frequently infected and showed significantly greater EPGs than males, while prevalence and levels of EPG of non-reproducing females were intermediate and statistically not different from reproducing females and males (prevalence: v2 = 9.81, P = 0.007; EPG: F2,88 = 7.63, P > 0.001; Fig. 2). A survey of the pattern of the distri-
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Prior to anthelmintic treatment, neither prevalence nor EPG significantly differed among the three experimental groups (all P-values >0.05). Following the selective treatment of either male or female voles, the interaction between treatment and sex contributed significantly to the variation in T. arvicolae prevalence (v2 = 6.06, P = 0.048) and EPG (F2,173 = 4.03, P = 0.019) in the experimental host population. These interactions were explained by the fact that prevalence and EPG of female voles were significantly reduced in the female-treated grids only (prevalence in females in control, female-treated and male-treated grids = 84.4A versus 44.7B versus 61.4A; mean ± S.E. log-transformed EPG = 2.66 ± 0.37A versus 1.28 ± 0.24B versus 2.15 ± 0.25A; means with the same superscript letter were not significantly different based on pairwise comparisons; (Fig. 4), while prevalence and EPGs of male voles were significantly reduced in the male-treated and femaletreated grids compared with males in the control grids (prevalence in males in control, female-treated and male-treated grids = 80.0A versus 59.3B versus 35.7B; mean ± S.E. log-transformed EPG = 2.34 ± 0.33A versus 1.39 ± 0.26B versus 1.03 ± 0.25B; Fig. 4). Treatment alone, but not sex alone, significantly contributed to the variation in T. arvicolae prevalence (treatment: v2 = 22.37, P < 0.001; sex: v2 = 1.41, P = 0.29) and EPG intensity (treatment: F2,173 = 11.72, P < 0.001; sex: F1,173 = 2.62, P = 0.11), with voles more often being infected and showing greater EPGs in the control group compared with the treated groups (Fig. 4). The cofactor trapping month and the covariate host head width contributed significantly to the variation in parasite prevalence and EPG (all P-values <0.02), and thus they were both retained in the final statistical models together with host treatment and sex. There was no difference in trapping month among resident voles from the three treatment groups (Kruskal–Wallis test: v2 = 1.04, P = 0.60). Resident hosts had significantly greater head width in the female-treated grids (mean ± S.E. = 14.04 ± 0.98) than the male-treated grids (13.62 ± 0.97) and control girds (13.80 ± 0.83; F2,181 = 3.40, P = 0.036). There was no difference among the control, female-treated and male-treated grids in the number of voles captured at each trapping session (mean ± S.E. = 14.4 ± 2.6 versus 17.5 ± 2.4 versus 19.1 ± 2.6; F2,30 = 0.88, P = 0.43) and in the proportion of male voles
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bution of T. arvicolae showed that 87% of the total expelled eggs originated from only 20% of the population of voles, of which 83% were females (Fig. 3). The pattern of distribution of EPGs did not differ from the negative binomial distribution in males (k = 0.368, P = 1.00) and females (k = 0.136, P = 1.00). The proportion of females in the control grids was 67.7%.
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Fig. 1. Prevalence based on the presence/absence of eggs in the faeces and mean log-transformed eggs per gram of faeces (EPG) of Trichuris arvicolae in relation to host sex (A), size (B) and trapping month (C). Prevalences are illustrated with barplots and mean EPG with dots ± 1 S.E. For illustrative purposes, host size (i.e. head width) was divided in four quartiles; however, statistical analyses were preformed with host size as a continuous covariate. Data are from 96 common voles (Microtus arvalis) captured in control girds, therefore reflecting natural levels of parasite infection.
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Fig. 2. Prevalence based on the presence/absence of eggs in the faeces and mean log-transformed eggs per gram of faeces (EPG) of Trichuris arvicolae in naturally infested common voles (Microtus arvalis) grouped in reproducing females (pregnant or lactating; n = 18), non-reproducing females (n = 47) and males (n = 31). Prevalences are illustrated with bar plots and mean EPG with dots ± 1 S.E. Bar plots and dots with the same letter were not significantly different based on a post hoc Tukey Honestly Significant Difference (HSD) test).
captured at each trapping session (0.35 ± 0.04 versus 0.35 ± 0.03 versus 0.40 ± 0.02; v2 = 2.10, P = 0.34). 4. Discussion We showed that infection of common voles (M. arvalis) by the nematode T. arvicolae was female-biased, and by applying an anthelmintic treatment to either male or female voles we demonstrated that females were responsible for endoparasite transmission and persistence under such circumstances. We discuss factors that may lead to female-biased infection and transmission rates of T. arvicolae in the common vole. Although reports of female-biased helminth infections in wild populations are few, several non-exclusive physiological or ecological factors might generate female-biased infection in the
common vole. Reviews on sex hormones, immunity and parasite resistance have recently highlighted that female reproductive status and concomitant variation in female hormonal profiles can drastically alter female immune responses and susceptibility to parasites, with the severity of a number of parasitic diseases (including helminth infection) being increased during pregnancy or lactation (Klein and Roberts, 2010). Accordingly, previous observations on patterns of infection of T. muris in wild house mice (Mus musculus) revealed higher parasite loads in females than males, a difference attributed to pregnancy and lactation making females physiologically less resistant to this nematode (Behnke and Wakelin, 1973). Along the same lines, we found that female common voles showed significantly higher average levels of EPG than male common voles, and this difference was apparent only when females were lactating or pregnant. A second factor that could heighten female-biased parasitism is behavioural differences between the sexes leading to heightened exposure of female voles to infective stages. Infection of hosts by endoparasites, here of common voles by T. arvalis, generally occurs through ingestion of food contaminated by faeces (faecal–oral transmission) (Hutchings et al., 2006). In common voles, as in yellow-necked mice (A. flavicollis) and white-footed mice (P. leucopus), the mating system varies from monogamy to promiscuity depending on density (Heise and Van Acker, 2000). Yet, conversely to female yellow-necked mice and white-footed mice who are solitary nesters, female voles have overlapping territories and share communal nests, whereas male voles spend their time visiting different female groups (Dobly and Rozenfeld, 2000). Hence, the greater sociality of female common voles compared with female yellow-necked mice and white-footed mice might expose them to higher concentrations of faeces in small home ranges and thus lead to frequent infection in female voles (see also Hillegass et al., 2008). Because lactation greatly increases food consumption (Speakman, 2008) and female common voles are the main burrowers (Dobly and Rozenfeld, 2000), females are also at greater risk (especially during lactation) of ingesting contaminated food than male voles (Cripps et al., 2011). Finally, mathematical models have highlighted that female-biased parasitism can evolve when females have higher mortality rates or are subject to greater competition for resources than males (Bacelar et al., 2011). In our study sites,
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Number of parasite eggs per gram of faeces (EPG) Fig. 3. Aggregated distribution of Trichuris arvicolae in 96 naturally infested common voles (Microtus arvalis) (n = 31 males and 65 females).
A. Sanchez et al. / International Journal for Parasitology 41 (2011) 1397–1402
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Treatment Fig. 4. Prevalence (based on the presence of eggs in the faeces) and mean (S.E.) logtransformed egg per gram of faeces (EPG) of Trichuris arvicolae by host sex and treatment in resident common voles (Microtus arvalis). In the female-treated group, females alone were treated with the antiparasitic drug ivermectin; in the maletreated group males alone were treated with ivermectin; and in the control group half of the females and half of the males were treated with the solvent alone. Prevalences are illustrated with bar plots and mean EPG with dots ± 1 S.E. The number (n) of residents in each experimental group is reported in the figure.
sex ratios were strongly skewed toward females during the reproductive season (for similar findings, see Bryja et al., 2005; Briner et al., 2007), probably due to females living in groups and males living solitarily (Dobly and Rozenfeld, 2000). Thus, female-biased parasitism in the common vole might also be driven by greater competition for resources in females than males. The combination of two individual characteristics can lead a host to be a super-spreading host: first, those individuals should produce more infectious propagules than others, and second their behaviour should favour contact with and transmission of the parasites to susceptible hosts (Anderson and May, 1991; Woolhouse et al., 1997; Lloyd-Smith et al., 2005; Ferrari et al., 2007; Perkins et al., 2008). By applying an anthelmintic treatment to either male or female voles, we showed that female treatment significantly reduced parasite burden (i.e. prevalence and EPG) of both male and female hosts, while male treatment only reduced parasite burdens in males. It indicates that females played a more important role than males in driving the dynamics of transmission of T. arvicolae. Our findings of female-biased transmission are in contrast with two previous experiments similarly addressing the importance of gender in transmission of the parasitic worms Heligmosomoides polygyrus in yellow-necked mice and of Pterygodermatites peromysci in white-footed mice which have shed the light on males as super-spreading hosts (Ferrari et al., 2004; Luong and Hudson, 2009). At least two mechanisms might have worked in concert to produce super-spreading females in this vole–nematode system. First and conversely to previous studies, female voles were shedding more parasite eggs in their faeces than males, thus potentially leading to greater environmental contamination by female faeces. Secondly, the communal breeding system of female voles greatly increases the number of contacts between infected and noninfected females, and thereby parasite transmission rates. The biology of common voles is also characterised by high female philopatry and high male natal and breeding dispersal (Bryja et al., 2005), which leads to female-biased sex-ratios in resident vole populations (i.e. 62% of resident voles were females in our study). Hence, the combination of high female densities and high female shedding rates is likely to account for the influential role of female common voles in driving the dynamics of parasite
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transmission, at least at small spatial scales. Note finally that we also found a bias in host size, with resident hosts being significantly larger in female-treated grids than in male-treated and control girds. However, since parasite prevalence and shedding rates increased with host size, this bias in host size might have weakened (rather than increased) the probability of detecting a reduction of parasite burden in large resident hosts within femaletreated grids. The comparison of previous results in the absence of sex-biased infection (Luong and Hudson, 2009) or in the presence of malebiased infection (Ferrari et al., 2004) and current results in the presence of female-biased infection highlights the importance of host infectivity, independently of host gender per se, in parasite transmission. Host infectivity is shaped by parasite shedding rates and contact rates, and experimental studies addressing the importance of host gender in parasite transmission have focused primarily on parasite shedding rates. However, shedding rates and contact rates are often intermingled, as for instance female voles showing greater EPGs and living in social groups, and thus research addressing the role of gender in parasite transmission would greatly benefit by accounting for contact rates in the experimental design. To this end, experiments are required where male and female social networks are investigated and anthelmintic treatment is specifically applied to male and female individuals with high or low connectedness within the social networks (Godfrey et al., 2009; Perkins et al., 2009). In the common vole, such an approach at a large spatial scale and identifying males that are linked solely to one group of females (‘territorial’ males) versus more than one group of females (‘floaters’) might provide additional insights on the dynamics of parasite transmission with, hypothetically, female voles responsible for parasite transmission at low spatial scales (i.e. within-group of breeding females) and male voles at larger spatial scales (i.e. among groups of breeding females). Furthermore, studies to date have focused on endoparasites with faecal–oral transmission (Ferrari et al., 2004; this study) or trophic transmission (Luong and Hudson, 2009). The production of large numbers of parasite eggs in the faeces of a few hosts will result in transmission patterns that are shaped foremost by the most heavily infected hosts. Conversely to endoparasites, ectoparasites are often transmitted by contact between infested and susceptible hosts (Harbison et al., 2008), and work is also needed to clarify the relative importance of the production of infectious propagules versus the number of contacts in the transmission of ectoparasites. Acknowledgements We are grateful to the many students, especially Christophe Seppey, Vincent Sonnay and Susana Pinto, for their help with the trapping of voles, Patrick Boujon for his help with the faecal analyses and Philippe Christe for support and advice. This research was funded by the Swiss National Science Foundation (Grants No. 31003A_124988 to PB and 3100A0_104118 and PBLAP3-127724/ 1 to GD). References Anderson, R.M., May, R.M., 1991. Infectious Diseases of Humans: Dynamics and Control. Oxford University Press, Oxford, UK. Bacelar, F.S., White, A., Boots, M., 2011. Life history and mating systems select for male biased parasitism mediated through natural selection and ecological feedbacks. J. Theoret. Biol. 269, 131–137. Behnke, J.M., Wakelin, D., 1973. The survival of Trichuris muris in wild populations of its natural hosts. Parasitology 67, 157–164. Briner, T., Favre, N., Nentwig, W., Airoldi, J.-P., 2007. Population dynamics of Microtus arvalis in a weed strip. Mammal. Biol. 72, 106–115. Bryja, J., Nesvadbová, J., Heroldová, M., Jánová, E., Losík, J., Trebatická, L., Tkadlec, E., 2005. Common vole (Microtus arvalis) population sex ratio: biases and process variation. Can. J. Zool. 83, 1391–1399.
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Cripps, J.K., Wilson, M.E., Elgar, M.A., Coulson, G., 2011. Experimental manipulation of fertility reveals potential lactation costs in a free-ranging marsupial. Biol. Lett.. doi:10.1098/rsbl.2011.0526. Deter, J., Charbonnel, N., Cosson, J.-F., Morand, S., 2008. Regulation of vole populations by the nematode Trichuris arvicolae: insights from modelling. Eur. J. Wildl. Res. 54, 60–70. Deter, J., Cosson, J.F., Chaval, Y., Charbonnel, N., Morand, S., 2007. The intestinal nematode Trichuris arvicolae affects the fecundity of its host, the common vole Microtus arvalis. Parasitol. Res. 101, 1161–1164. Dobly, A., Rozenfeld, F.M., 2000. Burrowing by common voles (Microtus arvalis) in various social environments. Behaviour 137, 1443–1462. Feliu, C., Spakulova, M., Casanova, J.C., Renaud, F., Morand, S., Hugot, J.P., Santalla, F., Durand, P., 2000. Genetic and morphological heterogeneity in small rodent whipworms in southwestern Europe: characterization of Trichuris muris and description of Trichuris arvicolae n. sp (Nematoda: Trichuridae). J. Parasitol. 86, 442–449. Ferrari, N., Cattadori, I.M., Nespereira, J., Rizzoli, A., Hudson, P.J., 2004. The role of host sex in parasite dynamics: field experiments on the yellow-necked mouse Apodemus flavicollis. Ecol. Lett. 7, 88–94. Ferrari, N., Rosa, R., Pugliese, A., Hudson, P.J., 2007. The role of sex in parasite dynamics: model simulations on transmission of Heligmosomoides polygyrus in populations of yellow-necked mice, Apodemus flavicollis. Int. J. Parasitol. 37, 341–349. Godfrey, S., Bull, C., James, R., Murray, K., 2009. Network structure and parasite transmission in a group living lizard, the gidgee skink, Egernia stokesii. Behav. Ecol. Sociobiol. 63, 1045–1056. Harbison, C.W., Bush, S.E., Malenke, J.R., Clayton, D.H., 2008. Comparative transmission dynamics of competing parasite species. Ecology 89, 3186–3194. Heise, S.R., Van Acker, A., 2000. The effect of social environment on the immune response of female common voles in matriarchal laboratory groups. Physiol. Behav. 71, 289–296. Hillegass, M.A., Waterman, J.M., Roth, J.D., 2008. The influence of sex and sociality on parasite loads in an African ground squirrel. Behav. Ecol. 19, 1006–1011. Hutchings, M.R., Judge, J., Gordon, I.J., Athanasiadou, S., Kyriazakis, I., 2006. Use of trade-off theory to advance understanding of herbivore–parasite interactions. Mam. Rev. 36, 1–16. Klein, L.K., Roberts, C.W., 2010. Sex Hormones and Immunity to Infection. Springer Verlag, Berlin, Heidelberg.
Lloyd-Smith, J.O., Schreiber, S.J., Kopp, P.E., Getz, W.M., 2005. Superspreading and the effect of individual variation on disease emergence. Nature 438, 355–359. Luong, L.T., Hudson, D.A.G.J., 2009. Male hosts are responsible for the transmission of a trophically transmitted parasite, Pterygodermatites peromysci, to the intermediate host in the absence of sex-biased infection. Int. J. Parasitol. 39, 1263–1268. Mes, T.H.M., 2003. Technical variability and required sample size of helminth egg isolation procedures. Vet. Parasitol. 115, 311–320. Moore, S.L., Wilson, K., 2002. Parasites as a viability cost of sexual selection in natural populations of mammals. Science 297, 2015–2017. Perkins, S.E., Cagnacci, F., Stradiotto, A., Arnoldi, D., Hudson, P.J., 2009. Comparison of social networks derived from ecological data: implications for inferring infectious disease dynamics. J. Anim. Ecol. 78, 1015–1022. Perkins, S.E., Cattadori, I.M., Tagliapietra, V., Rizzoli, A.P., Hudson, P.J., 2003. Empirical evidence for key hosts in persistence of a tick-borne disease. Int. J. Parasitol. 33, 909–917. Perkins, S.E., Ferrari, M.F., Hudson, P.J., 2008. The effects of social structure and sexbiased transmission on macroparasite infection. Parasitology 135, 1561–1569. Pike, E.H., 1969. Egg output of Trichuris muris (Schrank, 1788). J. Parasitol. 55, 1046– 1049. Poulin, R., 1996. Sexual inequalities in helminth infections: a cost of being a male? Am. Nat. 147, 287–295. Schalk, G., Forbes, M.R., 1997. Male biases in parasitism of mammals: effects of study type, host age, and parasite taxon. Oikos 78, 67–74. Skørping, A., Jensen, K.H., 2004. Disease dynamics: all caused by males? Trends Ecol. Evol. 19, 219–220. Speakman, J.R., 2008. The physiological costs of reproduction in small mammals. Phil. Trans. R. Soc. B 363, 375–398. Woolhouse, M.E.J., Dye, C., Etard, J.-F., Smith, T., Charlwood, J.D., Garnett, G.P., Hagan, P., Hii, J.L.K., Ndhlovu, P.D., Quinnell, R.J., Watts, C.H., Chandiwana, S.K., Anderson, R.M., 1997. Heterogeneities in the transmission of infectious agents: implications for the design of control programs. Proc. Natl. Acad. Sci. USA 94, 338–342. Zuk, M., 2009. The sicker sex. PLoS Pathog. 5, e1000267. Zuk, M., McKean, K.A., 1996. Sex differences in parasite infections: patterns and processes. Int. J. Parasitol. 26, 1009–1023.
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International Journal for Parasitology journal homepage: www.elsevier.com/locate/ijpara
Female-biased infection and transmission of the gastrointestinal nematode Trichuris arvicolae infecting the common vole, Microtus arvalis Andreas Sanchez a, Godefroy Devevey a,b, Pierre Bize a,⇑ a b
Department of Ecology & Evolution, University of Lausanne, Biophore, 1015 Lausanne, Switzerland Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA
a r t i c l e
i n f o
Article history: Received 27 July 2011 Received in revised form 30 September 2011 Accepted 30 September 2011 Available online 29 October 2011 Keywords: Parasite-biased infection Parasite-biased transmission Host gender Nematode Microtus arvalis
a b s t r a c t Previous studies addressing the importance of host gender in parasite transmission have shed light on males as the more important hosts, with the higher transmission potential of males being explained by the fact that they often harbour higher parasite loads than females. However, in some systems females are more heavily infected than males and may be responsible for driving infection under such circumstances. Using a wild population of common voles (Microtus arvalis), we showed that females were more frequently infected by the intestinal nematode Trichuris arvicolae than males (i.e. prevalence based on the presence of eggs in the faeces) and that females were shedding greater numbers of parasite eggs per gram of faeces (EPG) than males. By applying an anthelmintic treatment to either male or female voles, we demonstrated that treating females significantly reduced parasite burdens (i.e. prevalence and EPG) of both male and female hosts, while treating males only reduced parasite burden in males. These findings indicate that in this female-biased infection system females play a more important role than males in driving the dynamics of parasite transmission. Ó 2011 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction The transmission of parasites within host populations is influenced by many different sources of host heterogeneity, ranging from genetics, physiology and behaviour to spatial and temporal factors. A consequence of such heterogeneity is that a few hosts are generally responsible for the majority of the parasite transmission (and thereby parasite persistence) within host populations (Anderson and May, 1991; Woolhouse et al., 1997; Lloyd-Smith et al., 2005). This phenomenon is often referred as the Pareto rule, with 20% of the host populations contributing to at least 80% of the parasite transmission (Woolhouse et al., 1997), and it has led to the concept of high-risk ‘core groups’ (Anderson and May, 1991). Identifying which hosts are the main transmitters of parasites and why those hosts are responsible for so many transmission events are key questions in epidemiological studies. During the last decade, the contribution of host gender in parasite transmission and persistence within host populations has attracted growing attention (Perkins et al., 2003, 2008; Ferrari et al., 2004; Skørping and Jensen, 2004; Luong and Hudson, 2009). Because males are more frequently infected and suffer greater costs of infection than females (Poulin, 1996; Zuk and McKean, 1996; Schalk and Forbes, 1997; Moore and Wilson, ⇑ Corresponding author. Tel.: +41 (0)21 692 4204; fax: +41 (0)21 692 4165. E-mail address: [email protected] (P. Bize).
2002), males are often viewed as the ‘sicker sex’ in the literature (Zuk, 2009) and, in turn, as driving infection dynamics (Skørping and Jensen, 2004). In agreement with this idea, sex-biased variation in parasite transmission has been investigated in three rodent-parasite systems in the wild (Perkins et al., 2003; Ferrari et al., 2004; Luong and Hudson, 2009; see also Perkins et al., 2008), and in all three systems males were identified as driving the transmission and maintenance of parasites within host populations. Transmission of tick-borne encephalitis virus occurs when ticks feed in a ‘co-feeding aggregation’, and observation of co-feeding aggregation by sheep ticks (Ixodes ricinus) infesting yellownecked mice (Apodemus flavicollis) pointed out that sexually mature male mice of high body mass might be responsible for between 74% and 94% of the transmission events (Perkins et al., 2003). Experimental studies have clearly demonstrated the importance of male hosts in the transmission and persistence of gut parasites within yellow-necked mouse populations (Ferrari et al., 2004; see also Perkins et al., 2008) and white-footed mouse populations (Peromyscus leucopus) (Luong and Hudson, 2009). Anthelmintic treatments applied to either male or female hosts showed that the parasite burden of untreated females was significantly reduced when males were experimentally treated, while treating females had no consequences on the parasite burden of untreated males. However, male-biased infection and/or susceptibility to parasites are not universal. Although not frequently reported, in some
0020-7519/$36.00 Ó 2011 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpara.2011.09.004
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systems females are more heavily infected than males (e.g. Hillegass et al., 2008), and one intriguing question is which sex is responsible for driving the infection in those systems. We investigated the relative importance of host gender in the transmission of an intestinal nematode (Trichuris arvicolae) in a wild population of common voles (Microtus arvalis) with a female-biased infection (A. Sanchez, G. Devevey, P. Bize, unpublished data; present study). This directly transmitted parasite has previously been demonstrated to impair the reproductive success of its host, with experimentally infected females producing fewer pups of lower body mass (Deter et al., 2007), and potentially to regulate its host population dynamics (Deter et al., 2008). To explore the roles played by males and females in parasite transmission, we experimentally treated either male or female voles in alternative trapping grids using an anthelmintic, and we monitored the consequences of our treatment on the prevalence and faecal egg output simultaneously in individuals from the treated and untreated sexes. We predicted that, if transmission rate increased with the production of infectious propagules, anthelmintic treatment of female voles should reduce parasite burden not only in treated females but also in untreated males.
endoparasites in the study population, we created the three following experimental groups: on two replicate grids males alone were treated with the antiparasitic drug ivermectin (IvomecÒ, Merial, Merck Sharp & Dohme, Harlem, Netherlands); on two other grids females alone were treated with ivermectin; and on two control grids half of the males and half of the females were treated with propylene glycol which is the major solvent of IvomecÒ. Voles were treated at every trapping session by injecting subcutaneously either a dose of 0.01 mg of ivermectin in 0.01 ml of propylene glycol per gram of vole (i.e. antiparasitic treatment) or with a dose of 0.01 ml of proplyene glycol per gram of vole (i.e. control treatment). Ivermectin has a broad spectrum of activity against both ecto- and endoparasites, and it is commonly used to treat helminth infection in livestock and pets. Because juveniles below 14 g were not infected (Behnke and Wakelin, 1973; A. Sanchez, personal observations), only individuals weighting more than 14 g were treated. This experiment was conducted with the approval of the Veterinary Services of the Canton Vaud, Switzerland.
2. Materials and methods
To collect fresh faeces, voles were kept individually for 3 h in a 55 mm 40 mm 140 mm plastic box with a piece of apple as a water source before returning them to their trapping site. Faecal samples collected in each box were immediately weighed and stored in 10% formalin at 4 °C until eggs were counted in the laboratory. The number of eggs per gram of faeces (EPG) was assessed using the McMaster technique on faeces samples weighing more than 0.02 g (Mes, 2003). A total of 895 faecal samples was analysed by the same person (A. Sanchez) within 15 days of collection. Preliminary monitoring of the community of gastrointestinal parasites in the faeces showed that common voles of our study population were infected by the gastrointestinal nematode T. arvicolae (A. Sanchez, personal observations; for a description of the species, see Feliu et al., 2000), and thus we concentrated our attention on this endoparasite. Trichuris arvicolae is an obligate monoxenous nematode known to negatively affect the reproduction of its host (Deter et al., 2007). In Trichuris muris, a closely related species which parasitises murine rodents (Feliu et al., 2000), the complete life cycle takes 30–49 days (Pike, 1969). After ingestion via contaminated food, embryonated eggs hatch in the small intestine of the host, larvae develop, become adults and then establish themselves in the caeca. After reproduction, unembryonated eggs are released in the host faeces; the eggs embryonate and become infective in the soil.
2.1. Study area and vole monitoring The common vole (M. arvalis) is a widely distributed Eurasian rodent that inhabits meadows and agricultural fields and that can become very abundant during population outbreaks (>1,000 voles per hectare; Briner et al., 2007). From July to November 2009, we studied a common vole population located in meadows near the University of Lausanne, Switzerland. Live trapping was performed on a set of six trapping grids, each covering an area of 0.05 hectare, located in grass meadows dominated by Bromus erectus. Grids consisted of five lines of nine Longworth traps (i.e. 45 traps) placed 2–5 m from each other and deliberately set at places showing signs of activity (i.e. trails and holes) to optimise catching success. Traps were baited with seeds, a piece of apple as a water source and hay for bedding. To maximise trap encounter and thereby capture rate, traps were pre-baited for one night before each trapping session. To minimise possible movements of individuals between the grids, each grid was separated from the neighbouring grid by at least 250 m. Traps on each grid were set one night per month for a total of 2,160 trap nights. Voles were identified using a numbered eartag. At each capture, voles were weighed to the nearest 0.1 g with a digital balance and their head width was measured to the nearest 0.1 mm with a calliper. We classified individuals as male or female based on the visual examination of their genitalia, and as juvenile, sub-adult or adult according to pelage colouration and body length. We scored the breeding status of each individual, on a scale from 0 to 3, according to whether testes in males were not (0), weakly (1), moderately (2) or well (3) developed, and whether females showed no sign of reproduction (0), had a perforated vagina (1), were lactating (2) or pregnant (3). In total, we captured 535 individual voles (222 males and 313 females), and 396 of them (150 males and 246 females, representing 67.6% and 78.6% of the initial number of males and females) were captured during at least two trapping sessions and defined as resident. Of the 396 recaptured voles, three males and five females moved to an adjacent grid; those eight individuals were discarded from the final analyses. The trapping and ear-tagging of voles was performed under the legal authorisation of the Wildlife Services of the Canton Vaud, Switzerland. 2.2. Anthelmintic treatment To investigate whether one host sex, either male or female voles, was responsible for the maintenance and transmission of
2.3. Parasite quantification
2.4. Statistical analyses To gain insight on natural variation in T. arvicolae prevalence and intensity in the common vole, we restricted our statistical analyses to the first capture of each individual trapped in the control grids. We analyzed the effect of host sex, size and trapping month on natural variation in parasite prevalence (coded 1/0 based the presence/absence of eggs in the faeces) using a logistical regression and on EPG using an analysis of covariance (ANCOVA). We performed the analyses on 96 control voles for which we had faecal parasite counts. To investigate the effect of anthelmintic treatment on T. arvicolae prevalence and intensity, we restricted our statistical analyses to the last capture of each resident individual (i.e. individuals captured, treated if required and re-captured). Most residents (80.3%) were re-captured only once. Because individuals weighing less than 14 g were not infected (A. Sanchez, personal observations), only residents weighing more than 14 g at their last capture were considered in the analyses. We analyzed the effect of host treatment, sex, size and trapping month, plus the interaction between
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treatment and sex, on parasite prevalence using a logistical regression, and on EPG using an ANCOVA. Differences between male and female voles in the transmission of T. arvicolae in the host population will be indicated by a significant treatment by sex interaction. EPG measurements were log-transformed (i.e. log (EPG + 1)) before analyses to fit a normal distribution; we verified the normality of the ANCOVA residuals using quantile–quantile plots. We performed the analyses on 47, 72 and 65 resident voles of the control, male-treated and female-treated grids, respectively. Preliminary analyses showed no significant effect of the grid identity on parasite prevalence in the faeces and EPG (all P-values >0.60), and therefore we did not control for the grid identity in the final analyses. All of the statistical analyses were performed using the statistical package JMP 7.0 (SAS Institute Inc., USA). All tests were two tailed and a P value <0.05 was considered significant.
3. Results 3.1. Natural distribution of T. arvicolae The variation in the prevalence of infection by T. arvicolae based on the presence of eggs in the host faeces was significantly explained by host sex (logistical regression: v2 = 5.27, P = 0.022), head width (v2 = 23.72, P < 0.001) and trapping month (v2 = 18.17, P = 0.001). Female voles were infected more frequently than males (Fig. 1A), the prevalence of infection increased with head width (Fig. 1B), and the prevalence of infection from July to November showed a peak of infection in September (Fig. 1C). EPG was significantly explained by host sex (ANCOVA: F1,89 = 10.38, P = 0.002) and head width (F1,89 = 25.10, P < 0.001) but not by trapping month (F4,89 = 1.64, P = 0.17). Females had greater EPGs than males (Fig. 1A), and EPGs increased with head width (Fig. 1B). The interaction between host sex and head width did not significantly contribute to the prevalence of infection (v2 = 0.96, P = 0.33) or EPG (F1,88 = 1.16, P = 0.28), indicating that the increase in shedding rates of T. arvicolae eggs with head width was similar in male and female common voles. Statistical analyses carried out separately for each sex showed no influence of male or female breeding status on prevalence and EPG (all P-values >0.32). However, analyses where individuals were grouped in reproducing females (pregnant or lactating), non-reproducing females and males revealed that pregnant and lactating female voles were more frequently infected and showed significantly greater EPGs than males, while prevalence and levels of EPG of non-reproducing females were intermediate and statistically not different from reproducing females and males (prevalence: v2 = 9.81, P = 0.007; EPG: F2,88 = 7.63, P > 0.001; Fig. 2). A survey of the pattern of the distri-
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Prior to anthelmintic treatment, neither prevalence nor EPG significantly differed among the three experimental groups (all P-values >0.05). Following the selective treatment of either male or female voles, the interaction between treatment and sex contributed significantly to the variation in T. arvicolae prevalence (v2 = 6.06, P = 0.048) and EPG (F2,173 = 4.03, P = 0.019) in the experimental host population. These interactions were explained by the fact that prevalence and EPG of female voles were significantly reduced in the female-treated grids only (prevalence in females in control, female-treated and male-treated grids = 84.4A versus 44.7B versus 61.4A; mean ± S.E. log-transformed EPG = 2.66 ± 0.37A versus 1.28 ± 0.24B versus 2.15 ± 0.25A; means with the same superscript letter were not significantly different based on pairwise comparisons; (Fig. 4), while prevalence and EPGs of male voles were significantly reduced in the male-treated and femaletreated grids compared with males in the control grids (prevalence in males in control, female-treated and male-treated grids = 80.0A versus 59.3B versus 35.7B; mean ± S.E. log-transformed EPG = 2.34 ± 0.33A versus 1.39 ± 0.26B versus 1.03 ± 0.25B; Fig. 4). Treatment alone, but not sex alone, significantly contributed to the variation in T. arvicolae prevalence (treatment: v2 = 22.37, P < 0.001; sex: v2 = 1.41, P = 0.29) and EPG intensity (treatment: F2,173 = 11.72, P < 0.001; sex: F1,173 = 2.62, P = 0.11), with voles more often being infected and showing greater EPGs in the control group compared with the treated groups (Fig. 4). The cofactor trapping month and the covariate host head width contributed significantly to the variation in parasite prevalence and EPG (all P-values <0.02), and thus they were both retained in the final statistical models together with host treatment and sex. There was no difference in trapping month among resident voles from the three treatment groups (Kruskal–Wallis test: v2 = 1.04, P = 0.60). Resident hosts had significantly greater head width in the female-treated grids (mean ± S.E. = 14.04 ± 0.98) than the male-treated grids (13.62 ± 0.97) and control girds (13.80 ± 0.83; F2,181 = 3.40, P = 0.036). There was no difference among the control, female-treated and male-treated grids in the number of voles captured at each trapping session (mean ± S.E. = 14.4 ± 2.6 versus 17.5 ± 2.4 versus 19.1 ± 2.6; F2,30 = 0.88, P = 0.43) and in the proportion of male voles
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bution of T. arvicolae showed that 87% of the total expelled eggs originated from only 20% of the population of voles, of which 83% were females (Fig. 3). The pattern of distribution of EPGs did not differ from the negative binomial distribution in males (k = 0.368, P = 1.00) and females (k = 0.136, P = 1.00). The proportion of females in the control grids was 67.7%.
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Fig. 1. Prevalence based on the presence/absence of eggs in the faeces and mean log-transformed eggs per gram of faeces (EPG) of Trichuris arvicolae in relation to host sex (A), size (B) and trapping month (C). Prevalences are illustrated with barplots and mean EPG with dots ± 1 S.E. For illustrative purposes, host size (i.e. head width) was divided in four quartiles; however, statistical analyses were preformed with host size as a continuous covariate. Data are from 96 common voles (Microtus arvalis) captured in control girds, therefore reflecting natural levels of parasite infection.
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Fig. 2. Prevalence based on the presence/absence of eggs in the faeces and mean log-transformed eggs per gram of faeces (EPG) of Trichuris arvicolae in naturally infested common voles (Microtus arvalis) grouped in reproducing females (pregnant or lactating; n = 18), non-reproducing females (n = 47) and males (n = 31). Prevalences are illustrated with bar plots and mean EPG with dots ± 1 S.E. Bar plots and dots with the same letter were not significantly different based on a post hoc Tukey Honestly Significant Difference (HSD) test).
captured at each trapping session (0.35 ± 0.04 versus 0.35 ± 0.03 versus 0.40 ± 0.02; v2 = 2.10, P = 0.34). 4. Discussion We showed that infection of common voles (M. arvalis) by the nematode T. arvicolae was female-biased, and by applying an anthelmintic treatment to either male or female voles we demonstrated that females were responsible for endoparasite transmission and persistence under such circumstances. We discuss factors that may lead to female-biased infection and transmission rates of T. arvicolae in the common vole. Although reports of female-biased helminth infections in wild populations are few, several non-exclusive physiological or ecological factors might generate female-biased infection in the
common vole. Reviews on sex hormones, immunity and parasite resistance have recently highlighted that female reproductive status and concomitant variation in female hormonal profiles can drastically alter female immune responses and susceptibility to parasites, with the severity of a number of parasitic diseases (including helminth infection) being increased during pregnancy or lactation (Klein and Roberts, 2010). Accordingly, previous observations on patterns of infection of T. muris in wild house mice (Mus musculus) revealed higher parasite loads in females than males, a difference attributed to pregnancy and lactation making females physiologically less resistant to this nematode (Behnke and Wakelin, 1973). Along the same lines, we found that female common voles showed significantly higher average levels of EPG than male common voles, and this difference was apparent only when females were lactating or pregnant. A second factor that could heighten female-biased parasitism is behavioural differences between the sexes leading to heightened exposure of female voles to infective stages. Infection of hosts by endoparasites, here of common voles by T. arvalis, generally occurs through ingestion of food contaminated by faeces (faecal–oral transmission) (Hutchings et al., 2006). In common voles, as in yellow-necked mice (A. flavicollis) and white-footed mice (P. leucopus), the mating system varies from monogamy to promiscuity depending on density (Heise and Van Acker, 2000). Yet, conversely to female yellow-necked mice and white-footed mice who are solitary nesters, female voles have overlapping territories and share communal nests, whereas male voles spend their time visiting different female groups (Dobly and Rozenfeld, 2000). Hence, the greater sociality of female common voles compared with female yellow-necked mice and white-footed mice might expose them to higher concentrations of faeces in small home ranges and thus lead to frequent infection in female voles (see also Hillegass et al., 2008). Because lactation greatly increases food consumption (Speakman, 2008) and female common voles are the main burrowers (Dobly and Rozenfeld, 2000), females are also at greater risk (especially during lactation) of ingesting contaminated food than male voles (Cripps et al., 2011). Finally, mathematical models have highlighted that female-biased parasitism can evolve when females have higher mortality rates or are subject to greater competition for resources than males (Bacelar et al., 2011). In our study sites,
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Treatment Fig. 4. Prevalence (based on the presence of eggs in the faeces) and mean (S.E.) logtransformed egg per gram of faeces (EPG) of Trichuris arvicolae by host sex and treatment in resident common voles (Microtus arvalis). In the female-treated group, females alone were treated with the antiparasitic drug ivermectin; in the maletreated group males alone were treated with ivermectin; and in the control group half of the females and half of the males were treated with the solvent alone. Prevalences are illustrated with bar plots and mean EPG with dots ± 1 S.E. The number (n) of residents in each experimental group is reported in the figure.
sex ratios were strongly skewed toward females during the reproductive season (for similar findings, see Bryja et al., 2005; Briner et al., 2007), probably due to females living in groups and males living solitarily (Dobly and Rozenfeld, 2000). Thus, female-biased parasitism in the common vole might also be driven by greater competition for resources in females than males. The combination of two individual characteristics can lead a host to be a super-spreading host: first, those individuals should produce more infectious propagules than others, and second their behaviour should favour contact with and transmission of the parasites to susceptible hosts (Anderson and May, 1991; Woolhouse et al., 1997; Lloyd-Smith et al., 2005; Ferrari et al., 2007; Perkins et al., 2008). By applying an anthelmintic treatment to either male or female voles, we showed that female treatment significantly reduced parasite burden (i.e. prevalence and EPG) of both male and female hosts, while male treatment only reduced parasite burdens in males. It indicates that females played a more important role than males in driving the dynamics of transmission of T. arvicolae. Our findings of female-biased transmission are in contrast with two previous experiments similarly addressing the importance of gender in transmission of the parasitic worms Heligmosomoides polygyrus in yellow-necked mice and of Pterygodermatites peromysci in white-footed mice which have shed the light on males as super-spreading hosts (Ferrari et al., 2004; Luong and Hudson, 2009). At least two mechanisms might have worked in concert to produce super-spreading females in this vole–nematode system. First and conversely to previous studies, female voles were shedding more parasite eggs in their faeces than males, thus potentially leading to greater environmental contamination by female faeces. Secondly, the communal breeding system of female voles greatly increases the number of contacts between infected and noninfected females, and thereby parasite transmission rates. The biology of common voles is also characterised by high female philopatry and high male natal and breeding dispersal (Bryja et al., 2005), which leads to female-biased sex-ratios in resident vole populations (i.e. 62% of resident voles were females in our study). Hence, the combination of high female densities and high female shedding rates is likely to account for the influential role of female common voles in driving the dynamics of parasite
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transmission, at least at small spatial scales. Note finally that we also found a bias in host size, with resident hosts being significantly larger in female-treated grids than in male-treated and control girds. However, since parasite prevalence and shedding rates increased with host size, this bias in host size might have weakened (rather than increased) the probability of detecting a reduction of parasite burden in large resident hosts within femaletreated grids. The comparison of previous results in the absence of sex-biased infection (Luong and Hudson, 2009) or in the presence of malebiased infection (Ferrari et al., 2004) and current results in the presence of female-biased infection highlights the importance of host infectivity, independently of host gender per se, in parasite transmission. Host infectivity is shaped by parasite shedding rates and contact rates, and experimental studies addressing the importance of host gender in parasite transmission have focused primarily on parasite shedding rates. However, shedding rates and contact rates are often intermingled, as for instance female voles showing greater EPGs and living in social groups, and thus research addressing the role of gender in parasite transmission would greatly benefit by accounting for contact rates in the experimental design. To this end, experiments are required where male and female social networks are investigated and anthelmintic treatment is specifically applied to male and female individuals with high or low connectedness within the social networks (Godfrey et al., 2009; Perkins et al., 2009). In the common vole, such an approach at a large spatial scale and identifying males that are linked solely to one group of females (‘territorial’ males) versus more than one group of females (‘floaters’) might provide additional insights on the dynamics of parasite transmission with, hypothetically, female voles responsible for parasite transmission at low spatial scales (i.e. within-group of breeding females) and male voles at larger spatial scales (i.e. among groups of breeding females). Furthermore, studies to date have focused on endoparasites with faecal–oral transmission (Ferrari et al., 2004; this study) or trophic transmission (Luong and Hudson, 2009). The production of large numbers of parasite eggs in the faeces of a few hosts will result in transmission patterns that are shaped foremost by the most heavily infected hosts. Conversely to endoparasites, ectoparasites are often transmitted by contact between infested and susceptible hosts (Harbison et al., 2008), and work is also needed to clarify the relative importance of the production of infectious propagules versus the number of contacts in the transmission of ectoparasites. Acknowledgements We are grateful to the many students, especially Christophe Seppey, Vincent Sonnay and Susana Pinto, for their help with the trapping of voles, Patrick Boujon for his help with the faecal analyses and Philippe Christe for support and advice. This research was funded by the Swiss National Science Foundation (Grants No. 31003A_124988 to PB and 3100A0_104118 and PBLAP3-127724/ 1 to GD). References Anderson, R.M., May, R.M., 1991. Infectious Diseases of Humans: Dynamics and Control. Oxford University Press, Oxford, UK. Bacelar, F.S., White, A., Boots, M., 2011. Life history and mating systems select for male biased parasitism mediated through natural selection and ecological feedbacks. J. Theoret. Biol. 269, 131–137. Behnke, J.M., Wakelin, D., 1973. The survival of Trichuris muris in wild populations of its natural hosts. Parasitology 67, 157–164. Briner, T., Favre, N., Nentwig, W., Airoldi, J.-P., 2007. Population dynamics of Microtus arvalis in a weed strip. Mammal. Biol. 72, 106–115. Bryja, J., Nesvadbová, J., Heroldová, M., Jánová, E., Losík, J., Trebatická, L., Tkadlec, E., 2005. Common vole (Microtus arvalis) population sex ratio: biases and process variation. Can. J. Zool. 83, 1391–1399.
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