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Occurrence of the amphibian chytrid fungus Batrachochytrium dendrobatidis in introduced and native species from two regions of France

In: Amphibia-Reptilia
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Martin Ouellet 1Amphibia-Nature, 469 route d’Irlande, Percé, Québec G0C 2L0, Canada

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Tony Dejean 2Parc Naturel Régional Périgord-Limousin, 24 450 La Coquille, France
3Université de Savoie, UMR CNRS 5553, Laboratoire d’Écologie Alpine, 73 376 Le-Bourget-du-Lac, France

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Patrick Galois 1Amphibia-Nature, 469 route d’Irlande, Percé, Québec G0C 2L0, Canada

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To investigate the occurrence of the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) in France, we examined 575 amphibians collected between 1875 and 2008 for the presence of the infection in the epidermis. We confirmed Bd in seven of the 15 stations investigated in the Aquitaine and Poitou-Charentes regions. Histological examination revealed the presence of the infectious agent associated with mild epidermal changes in 25 of 493 (5.1%) amphibians sampled in 2007-2008. The overall occurrence was 21.9% (21/96) in the introduced American bullfrog (Lithobates catesbeianus) and 2.3% (4/172) in the native European water frog complex (Pelophylax sp.). These prevalence data should be viewed as conservative giving the limitation of histology. Morbidity and mortality attributable to chytridiomycosis were not observed in these two species during this period. The introduced African clawed frog (Xenopus laevis) and all native museum specimens tested negative for chytrid infection. Unlike L. catesbeianus, Pelophylax sp. is broadly distributed at local and regional scales and whether or not this complex could act as a reservoir host in the dissemination of Bd remains to be determined. The presence of the chytrid fungus in amphibian populations from different locations in southwestern France calls for disease surveillance and for precautionary measures to avoid the spread of this emerging disease.

Introduction

Amphibians are globally declining due to habitat loss and commercial exploitation. Among other factors, the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) has recently raised major interest as an infectious disease threatening amphibian populations in many parts of the world (Berger et al., 1998; Carey et al., 2003; Skerratt et al., 2007; Dejean, Miaud and Ouellet, 2010). It infects the superficial layers of the amphibian epidermis. This fungus is enigmatic because it occurs in both declining and healthy amphibian populations and its recent emergence has lead to competing hypotheses regarding the endemic versus emerging nature of the pathogen (Ouellet et al., 2005; James et al., 2009; Padgett-Flohr and Hopkins, 2009). Morbidity and mortality vary among amphibian taxa and the outcome of the disease is dependent on host resistance and environmental factors. Different Bd lineages have been recently recognized and pathogen genotype may also be an important epidemiological determinant (Farrer et al., 2011). There is minimal host reaction to infection and pathological changes are somewhat inconsistent (Voyles, Rosenblum and Berger, 2011). The pathogenesis of chytridiomycosis is unclear but it may disrupt cutaneous osmoregulatory function in amphibians, leading to electrolyte imbalance and death in some species (Voyles et al., 2009). As a result of improvements in molecular diagnostics, many studies have been based solely on the magnitude of infection and few of them have actually performed detailed pathological investigations to link the emergence of the disease in amphibian populations with mortality rates large enough to cause the declines (Duffus, 2009). Yet, the negative effects of Bd on individuals do not necessarily translate into negative effects at the population level (Tobler, Borgula and Schmidt, 2012).

Among the different hypotheses trying to explain the dissemination of Bd into amphibian populations, the international trade sometimes leading to the introduction of invasive species and their pathogens has been proposed in recent years (Weldon et al., 2004; Fisher and Garner, 2007; Schloegel et al., 2009, 2010). Two prime suspects include the globally introduced American bullfrog (Lithobates catesbeianus) and the African clawed frog (Xenopus laevis). These species have been shown to be consistently carrying the fungus without any clinical signs of the disease both in their natural (Weldon et al., 2004; Ouellet et al., 2005) and feral ranges (Garner et al., 2006; Padgett-Flohr and Hopkins, 2009; Solís et al., 2010), and in the commercial trade (Schloegel et al., 2009; Bai, Garner and Li, 2010). Retrospective studies have shown that Bd has been infecting L. catesbeianus since the 1960s (Ouellet et al., 2005; Padgett-Flohr and Hopkins, 2009) and X. laevis since the 1930s (Weldon et al., 2004; Soto-Azat et al., 2010). In southwestern France, L. catesbeianus is assumed to have been introduced as ornamental garden animals in Arveyres (Gironde department) in 1968 and is now well established (Touratier, 1992; Berroneau, Détaint and Coïc, 2008). The origins of X. laevis are thought to be from a breeding centre release in western France (Deux-Sèvres department) in the early 1980s and the species is currently expanding in range (Fouquet, 2001; Fouquet and Measey, 2006).

Reports of the presence of Bd are few in France. Its presence, in particular, has recently been detected in the common midwife toad (Alytes obstetricans) and the fire salamander (Salamandra salamandra) from the Parc National des Pyrénées, Pyrénées-Atlantiques department (Dejean, Miaud and Ouellet, 2010; Walker et al., 2010; Farrer et al., 2011). In this study, we searched for the presence of Bd in amphibian populations from two regions of France across different introduced and native species. We processed by histological examination in order to evaluate the severity of epidermal changes in cases of positive infection. In a first attempt to explore the time and location of Bd emergence, we conducted a retrospective survey in two museum collections. Our objective was to assess the presence of the chytrid fungus in native amphibians before the first introductions of L. catesbeianus and X. laevis in southwestern and western France, respectively.

Materials and methods

We caught 493 postmetamorphic amphibians in April through August 2007 and February through July 2008 from 15 different stations located in the French departments of Deux-Sèvres (Poitou-Charentes) and Dordogne (Aquitaine) (Appendix 1, fig. 1, table 1). Among these specimens, 44 were in the juvenile and 449 in the subadult-adult life stages. They belonged to 6 anuran and 3 salamander species and included two introduced species, L. catesbeianus and X. laevis. Retrospectively, we also examined 79 amphibians (8 juveniles, 71 adults) of the Muséum National d’Histoire Naturelle (Paris) opportunistically collected between 1875 and 1967 from different localities in France (table 2). An additional 3 anurans (1 juvenile, 2 adults) captured in 1890 were analysed from the Muséum des Sciences Naturelles d’Angers. We restricted our museum sampling to amphibians collected before 1968. We clipped the fourth toe from the right hindlimb at the articulation proximal to the fourth phalange in wild specimens while it was the third toe from the left hindlimb at the articulation proximal to the third phalange in museum specimens. Tissue samples were stored in absolute alcohol. All wild animals were considered healthy at the time of sampling (no gross anomalies observed upon physical examination) and were subsequently released. We ensured that no contamination occurred between animals by changing gloves and sterilizing our surgical instruments. We also cleaned and disinfected all field equipment using a 1% Virkon solution between each sampling location (Dejean, Miaud and Ouellet, 2007).

Figure 1.
Figure 1.

Map of France showing 15 stations investigated for the occurrence of Batrachochytrium dendrobatidis (Bd) in amphibian populations during 2007-2008.

Citation: Amphibia-Reptilia 33, 3-4 (2012) ; 10.1163/15685381-00002845

Table 1.

Occurrence of Batrachochytrium dendrobatidis in amphibian populations from two regions of France during 2007-2008. Pelophylax sp. represents a complex of water frogs that was not differentiated in the field.

Table 1.

Toes were later fixed in 10% neutral buffered formalin before processing for histopathology. Paraffin-embedded toes were sectioned longitudinally at 4 μm, and stained with hematoxylin-(eosine-phloxine)-saffron for light microscopic examination. Each slide was examined blind with no knowledge of species or date of collection. A second observer confirmed all positive cases and rejected occasional suspicious sections due to histologic artifacts. We diagnosed infection by Bd when zoosporangia (empty or with zoospores) were observed in the stratum corneum and/or stratum granulosum of the epidermis (Ouellet et al., 2005).

We performed Pearson chi-square tests to compare the proportions of infected individuals in the species examined. Data from the different stations and years were pooled for each species in order to increase the sample size. Significance level was set at α=0.05.

Results

In total, 575 samples were tested for the presence of Bd (tables 1 and 2). We detected the infection in 25 of 493 (5.1%) amphibians from two species collected in the Dordogne department during the period 2007-2008, including the introduced L. catesbeianus and the native European water frog complex (Pelophylax sp.). We confirmed Bd in seven of the 15 stations investigated. Except for station 2, L. catesbeianus was present at the time of sampling in the other six positive stations. However, stations 1 through 14 were located in close proximity to a L. catesbeianus-invaded area. We did not detect a significant difference in the occurrence of Bd among the 7 native species (χ2=3.204, df = 6, P=0.783). When adding the introduced species in the analysis, we detected a significant difference in the occurrence of Bd among species examined (χ2=71.025, df = 8, P<0.001). Detailed infection prevalence was 21.9% (21/96) in L. catesbeianus, 2.3% (4/172) in Pelophylax sp., and 0% (0/225) in the seven other species. Among the 25 infected specimens, 8 were in the juvenile and 17 in the subadult-adult life stages. All museum specimens collected between 1875 and 1967 tested negative (0/82). These data will be deposited at http://www.bd-maps.eu/.

Histological changes of the epidermis associated with Bd infection were mild and included focal hyperkeratosis and minimal stratum corneum sloughing. Most zoosporangia were empty spherical and ovoidal structures localized to areas of infection. Typical zoosporangia containing zoospores and/or empty zoosporangia were observed in the stratum corneum (fig. 2). These observations were in accord with the lack of clinical signs of disease observed in all amphibians examined under field conditions.

Table 2.

Occurrence of Batrachochytrium dendrobatidis in museum amphibian specimens collected between 1875 and 1967 from different localities in France. Pelophylax sp. was originally identified as Pelophylax kl. esculentus and the exact localities were sometimes unknown in older specimens. MNHN = Muséum National d’Histoire Naturelle (Paris), MSNA = Muséum des Sciences Naturelles d’Angers.

Table 2.

Discussion

Bd infection was observed with a prevalence of 21.9% in apparently healthy populations of the introduced L. catesbeianus in southwestern France. A lower prevalence of 2.3% was also diagnosed in the native Pelophylax sp. complex. This low prevalence in Pelophylax sp. was not statistically different from the apparent absence of Bd in the other native species. However, caution should be used when interpreting these results due to relatively small sample sizes where neither absence can be reliably inferred nor prevalence reliably estimated. In regard to the limitation of histological examination (Ouellet et al., 2005), our results probably underestimate the overall prevalence of the infection in the species examined. With species harbouring a low level of infection, molecular techniques (qPCR or real-time quantitative polymerase chain reaction) are usually considered as a more sensitive test for the detection of Bd zoospores, particularly at early stages of infection (Boyle et al., 2004). Accounting for the lower sensitivity of histology, we analysed the whole toe instead of a smaller patch of epidermal tissue. We also achieved our objective to evaluate the severity of epidermal changes associated to infection. Indeed, histology has the ability to detect epidermal changes associated with chytridiomycosis, not only the presence of zoospores (Smith, 2007). The absence of Bd in museum specimens collected before 1968 is of interest although further samples should be tested to better support this absence. Hence, we may have missed the pathogen considering the apparently low prevalence in the indigenous amphibians. Anurans of the Pelophylax sp. complex and A. obstetricans (high susceptibility to Bd) should be given priority, especially those collected in the early 1960s.

Figure 2.
Figure 2.

Histological section of the toe skin of an introduced American bullfrog (Lithobates catesbeianus) infected by Batrachochytrium dendrobatidis from Bridarias (station 1) in southwestern France. This asymptomatic specimen constituted the most infected individual in our study with localized areas of sloughed stratum corneum containing zoosporangia both empty and with zoospores. This figure is published in colour in the online version.

Citation: Amphibia-Reptilia 33, 3-4 (2012) ; 10.1163/15685381-00002845

In Europe, Bd has previously been identified in introduced L. catesbeianus from France (Garner et al., 2006), Italy (Garner et al., 2006; Adams et al., 2008), and the United Kingdom (Garner et al., 2006). Garner et al. (2006) detected the presence of Bd in 61.5% (16/26) of French L. catesbeianus specimens from three different Bd-positive locations in the Gironde and Loir-et-Cher departments. Our L. catesbeianus data expand the range of the amphibian chytrid fungus in five other locations in the Dordogne department. Thus, in all three French departments where populations of L. catesbeianus are currently observed (ACEMAV, 2003), individuals have been found to harbour Bd infection. In both its natural range and in introduced regions, L. catesbeianus is considered as a reservoir host of the chytrid fungus and relatively asymptomatic when naturally or experimentally-infected. For example, in native populations, a prevalence of 60.2% (80/133) was observed in L. catesbeianus from the Mont Saint-Hilaire Biosphere Reserve of Québec during the period 1960-2001 (Ouellet et al., 2005). The rate was 21.8% (39/179) for the same species in the northeastern United States during 2000-2002 (Longcore et al., 2007).

The presence of Bd infection in native European species has been reported in asymptomatic pool frogs (Pelophylax lessonae) from Italy (Simoncelli et al., 2005), edible frogs (Pelophylax kl. esculentus) from Italy (Adams et al., 2008; Federici et al., 2008), Denmark (Scalera, Adams and Galvan, 2008) and Luxembourg (Wood, Griffiths and Schley, 2009), and Perez’s frogs (Pelophylax perezi) from Spain (Walker et al., 2010). It has been also observed in the hybridogenetic complex Pelophylax lessonae-esculentus from Switzerland with a mean proportion of individuals carrying Bd per positive site of 27% (Tobler, Borgula and Schmidt, 2012). In Québec, Bd has been detected in North American green frog specimens (Lithobates clamitans) collected as early as 1961 with an overall prevalence of 26.7% (135/506) during 1960-2001 (Ouellet et al., 2005). The prevalence was 25.5% (52/204) in L. clamitans from the northeastern United States in 2000-2002 (Longcore et al., 2007). In light of these findings, it will be interesting to determine in future studies whether anurans of the European water frog complex could act as a reservoir to more sensitive native species. At local and regional scales, Pelophylax sp. is broadly distributed (Pagano et al., 2001; ACEMAV, 2003) and, like L. catesbeianus, might play a role in the spread of Bd. The apparent absence of Bd in introduced X. laevis from station 15 also supports this hypothesis. The diversity of indigenous water frog assemblages may be already considered under threat due to the multiple introductions of the marsh frog (Pelophylax ridibundus) in some parts of France (Neveu, 1997; Schmeller et al., 2007).

Chytridiomycosis and ranaviral disease (an emerging viral disease in amphibians caused by ranaviruses, family Iridoviridae) (e.g., Teacher, Cunningham and Garner, 2010) are now listed as globally notifiable diseases by the World Organisation for Animal Health (OIE) with the aim to assure the sanitary safety of international trade in live amphibians and their products (Schloegel et al., 2010). Considering the many uncertainties surrounding the real significance of Bd infection into native and introduced amphibian populations, we recommend being proactive in France. Disease surveillance is thus warranted and any suspected cases of disease-induced mortality in the field should be followed by detailed pathological investigations. The presence of disease must be confirmed, not only the presence of an infection with fast molecular tests (Smith, 2007; Duffus, 2009). In Europe, rapid dissemination of Bd data can be made through the RACE (Risk Assessment of Chytridiomycosis to European Amphibian Biodiversity) project at http://www.bd-maps.eu/. Amphibian disease research is a rapidly evolving discipline and many diseases have yet to be discovered. Programs to eradicate or at least prevent the progression of introduced non-native amphibian species (e.g., Berroneau, Détaint and Coïc, 2008) should also be implemented in a concerted way. Finally, a more systematic application of precautionary measures (e.g., Dejean, Miaud and Ouellet, 2007), to avoid the dissemination of these emerging diseases through human activities in France and elsewhere in Europe, is also highly desirable.

Acknowledgements

We thank the Direction Régionale de l’Environnement (Aquitaine) for the collecting permits, A.-M. Ohler for providing access to the amphibian collection of the Muséum National d’Histoire Naturelle (Paris), and M. Beucher for the Muséum des Sciences Naturelles d’Angers. We also thank P. Grillet, S. Guibert (Parc Naturel Régional Périgord-Limousin), M. Millerioux (VetAgro Sup), and J.-M. Thirion for their help in the field, and J. Cardin (Faculté de Médecine Vétérinaire, Université de Montréal) for her technical assistance. We are grateful to C. Miaud (Laboratoire d’Écologie Alpine, Université de Savoie) for reviewing an earlier draft of this manuscript, anonymous referees for their comments, and B. Schmidt (Associate Editor, Amphibia-Reptilia) for his suggestions on the final version. This article was written as part of a research partnership on amphibian disease between the Parc Naturel Régional Périgord-Limousin, the Université de Savoie, and Amphibia-Nature (Québec). Funding was provided by the European Union (FEDER Limousin), the Conseil Régional du Limousin, the Association Nationale de la Recherche et de la Technologie, the Conseil Régional d’Aquitaine, the Agence de l’Eau Adour-Garonne, the Conseil Général de la Dordogne, the Conseil Général de la Haute-Vienne, and the Direction Régionale de l’Environnement (Aquitaine).

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Footnotes

Associated Editor: Benedikt Schmidt

Appendix 1.

Summary information of the 15 stations investigated for the occurrence of Batrachochytrium dendrobatidis in amphibian populations from the French departments of Deux-Sèvres (Poitou-Charentes) and Dordogne (Aquitaine) during 2007-2008.

Appendix 1.

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