Fatal Chytridiomycosis in the Tyrrhenian Painted Frog

Batrachochytrium dendrobatidis (Bd), the causative agent of the amphibian disease chytridiomycosis, is an important factor in the global decline of amphibians. Within Europe, animals that exhibit clinical signs of the disease have only been reported in Spain despite the pathogen’s wide, but patchy, distribution on the continent. Recently, another occurrence of chytridiomycosis was reported in Euproctus platycephalus, the Sardinian brook newt, on the Mediterranean island of Sardinia, but without any evidence of fatal disease. We report further evidence of the emergence of Bd on Sardinia and the first evidence of lethal chytridiomycosis outside of Spain. Unusual mortalities of the Tyrrhenian painted frog (Discoglossus sardus) were found at three sites in the Limbara mountains of northern Sardinia. Molecular and histological screens of corpses, frogs, and tadpoles from these sites revealed infection with Bd. Infection and mortality occurred at locations that are unusual in terms of the published habitat requirements of the pathogen. Given the endemicity, the IUCN Red List status of the amphibian species on Sardinia, and the occurrence of infection and mortality caused by chytridiomycosis, there is serious reason for concern for the impact that disease emergence may have on the conservation of the amphibians of the island.     

Factors affecting host range in a generalist seed pathogen of semi-arid shrublands

Generalist pathogens can exhibit differential success on different hosts, resulting in complex host range patterns. Several factors operate to reduce realized host range relative to potential host range, particularly under field conditions. We explored factors influencing host range of the naturally occurring generalist ascomycete grass seed pathogen Pyrenophora semeniperda. We measured potential host range in laboratory experiments at high inoculum loads with 26 grass species, including the primary host Bromus tectorum, and developed models to predict susceptibility and tolerance based on host traits, including germination speed, seed hardness, seed size, and phylogenetic relations. We also examined pathogen and host density effects on infection and mortality. All species tested were at least somewhat susceptible to the pathogen at high inoculum loads, but both infection and mortality varied widely. Species more closely related to the original host (B. tectorum) were more susceptible to infection, whereas species with slower germination were less tolerant and therefore more likely to suffer mortality. Infection and mortality were sharply reduced as inoculum load was reduced. Intermediate loads had major negative impacts on dormant B. tectorum seeds but generally minimal effects on native species. In addition, field seed bank studies determined that P. semeniperda rarely exploits native grass species as hosts. This marked reduction in realized host range relative to potential host range indicates that laboratory host range studies are potentially a poor predictor of either the current or possible future realized host range for wildland plant pathogens.     

Seasonal Ecology and Behavior of an Endangered Rainforest Frog (Litoria rheocola) Threatened by Disease

One of the most devastating wildlife diseases ever recorded is chytridiomycosis, a recently emerged amphibian disease that is caused by the chytrid fungus Batrachochytrium dendrobatidis. Understanding, predicting, and managing the impacts of chytridiomycosis on any amphibian species will require detailed information on its ecology and behavior because this pathogen is transmitted by contact with water or other individuals, and pathogen growth rates are thermally sensitive. The common mistfrog (Litoria rheocola) is an endangered tropical rainforest frog that has declined due to chytridiomycosis. We tracked L. rheocola during the winter (cool/dry) and summer (warm/wet) seasons at a low- and high-elevation site. We found that seasonal differences in environmental temperatures and frog behavior should render this species most vulnerable to B. dendrobatidis during cooler months and at higher elevations, which matches observed patterns of infection prevalence in this species. During winter, frogs moved shorter distances than during summer, and they spent less time in vegetation and more time in the stream, which should increase exposure to aquatic B. dendrobatidis zoospores. At a low-elevation site (40 m ASL), estimated body temperatures were within the optimal range for B. dendrobatidis growth (15-25°C) most of the time during winter, but they reached temperatures above this threshold frequently in summer. At a higher elevation (750 m ASL), estimated body temperatures were within the range most favorable for B. dendrobatidis year-round, and did not exceed 25°C, even during summer. Our study provides the first detailed information on the ecology and behavior of L. rheocola and suggests ecological mechanisms for infection dynamics that have been observed in this endangered species.     

Presence of Batrachochytrium dendrobatidis in feral populations of Xenopus laevis in Chile

Batrachochytrium dendrobatidis (Bd) is a causal agent of disease and population decline of amphibian populations, and the extinction of several anuran species worldwide. Diverse hypotheses have been provided for the emergence of this fungus in different continents, ranging from global climate change to the vectoring of Bd via the international trade in amphibian species. In order to address these hypotheses, it is important to assess the current distribution of Bd in the context of introduced non-native amphibian species. We sampled several populations of the African clawed frog Xenopus laevis across its distribution in Chile in order to detect the presence of B. dendrobatidis and evaluate the role of this frog as a potential vector. In three of ten sites sampled, individuals harbored B. dendrobatidis infection, with an overall prevalence of infection across the studied populations of 24% (14 positive out of 58 analyzed specimens). The rapid spread exhibited by this frog within Chile suggests that transpecific transmission of the pathogen is possible, perhaps jeopardizing native species. This finding indicates the urgent need to establish long-term monitoring population programs in order to allow early detection disease-driven changes in the sizes of native populations, allowing the prompt application of conservation practices.     

Investigating Differences across Host Species and Scales to Explain the Distribution of the Amphibian Pathogen Batrachochytrium dendrobatidis

Many pathogens infect more than one host species, and clarifying how these different hosts contribute to pathogen dynamics can facilitate the management of pathogens and can lend insight into the functioning of pathogens in ecosystems. In this study, we investigated a suite of native and non-native amphibian hosts of the pathogen Batrachochytrium dendrobatidis (Bd) across multiple scales to identify potential mechanisms that may drive infection patterns in the Colorado study system. Specifically, we aimed to determine if: 1) amphibian populations vary in Bd infection across the landscape, 2) amphibian community composition predicts infection (e.g., does the presence or abundance of any particular species influence infection in others?), 3) amphibian species vary in their ability to produce infectious zoospores in a laboratory infection, 4) heterogeneity in host ability observed in the laboratory scales to predict patterns of Bd prevalence in the landscape. We found that non-native North American bullfrogs (Lithobates catesbeianus) are widespread and have the highest prevalence of Bd infection relative to the other native species in the landscape. Additionally, infection in some native species appears to be related to the density of sympatric L. catesbeianus populations. At the smaller host scale, we found that L. catesbeianus produces more of the infective zoospore stage relative to some native species, but that this zoospore output does not scale to predict infection in sympatric wild populations of native species. Rather, landscape level infection relates most strongly to density of hosts at a wetland as well as abiotic factors. While non-native L. catesbeianus have high levels of Bd infection in the Colorado Front Range system, we also identified Bd infection in a number of native amphibian populations allopatric with L. catesbeianus, suggesting that multiple host species are important contributors to the dynamics of the Bd pathogen in this landscape.     

Widespread occurrence of Batrachochytrium dendrobatidis in Ontario,Canada, and predicted habitat suitability for the emerging Batrachochytrium salamandrivorans

Chytridiomycosis, caused by the fungi Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans, is associated with massive amphibian mortality events worldwide and with some species’ extinctions. Previous ecological niche models suggest that B. dendrobatidis is not well‐suited to northern, temperate climates, but these predictions have often relied on datasets in which northern latitudes are underrepresented. Recent northern detections of B. dendrobatidis suggest that these models may have underestimated the suitability of higher latitudes for this fungus. We used qPCR to test for B. dendrobatidis in 1,041 non‐invasive epithelial swab samples from 18 species of amphibians collected across 735,345 km² in Ontario and Akimiski Island (Nunavut), Canada. We detected the pathogen in 113 samples (10.9%) from 11 species. Only one specimen exhibited potential clinical signs of disease. We used these data to produce six Species Distribution Models of B. dendrobatidis, which classified half of the study area as potential habitat for the fungus. We also tested each sample for B. salamandrivorans, an emerging pathogen that is causing alarming declines in European salamanders, but is not yet detected in North America. We did not detect B. salamandrivorans in any of the samples, providing a baseline for future surveillance. We assessed the potential risk of future introduction by comparing salamander richness to temperature‐dependent mortality, predicted by a previous exposure study. Areas with the highest species diversity and predicted mortality risk extended 60,530 km² across southern Ontario, highlighting the potential threat B. salamandrivorans poses to northern Nearctic amphibians. Preventing initial introduction will require coordinated, transboundary regulation of trade in amphibians (including frogs that can carry and disperse B. salamandrivorans), and surveillance of the pathways of introduction (e.g., water and wildlife). Our results can inform surveillance for both pathogens and efforts to mitigate the spread of chytridiomycosis through wild populations.     

Mapping the Global Emergence of Batrachochytrium dendrobatidis,the Amphibian Chytrid Fungus

The rapid worldwide emergence of the amphibian pathogen Batrachochytrium dendrobatidis (Bd) is having a profound negative impact on biodiversity. However, global research efforts are fragmented and an overarching synthesis of global infection data is lacking. Here, we provide results from a community tool for the compilation of worldwide Bd presence and report on the analyses of data collated over a four-year period. Using this online database, we analysed: 1) spatial and taxonomic patterns of infection, including amphibian families that appear over- and under-infected; 2) relationships between Bd occurrence and declining amphibian species, including associations among Bd occurrence, species richness, and enigmatic population declines; and 3) patterns of environmental correlates with Bd, including climate metrics for all species combined and three families (Hylidae, Bufonidae, Ranidae) separately, at both a global scale and regional (U.S.A.) scale. These associations provide new insights for downscaled hypothesis testing. The pathogen has been detected in 52 of 82 countries in which sampling was reported, and it has been detected in 516 of 1240 (42%) amphibian species. We show that detected Bd infections are related to amphibian biodiversity and locations experiencing rapid enigmatic declines, supporting the hypothesis that greater complexity of amphibian communities increases the likelihood of emergence of infection and transmission of Bd. Using a global model including all sampled species, the odds of Bd detection decreased with increasing temperature range at a site. Further consideration of temperature range, rather than maximum or minimum temperatures, may provide new insights into Bd-host ecology. Whereas caution is necessary when interpreting such a broad global dataset, the use of our pathogen database is helping to inform studies of the epidemiology of Bd, as well as enabling regional, national, and international prioritization of conservation efforts. We provide recommendations for adaptive management to enhance the database utility and relevance.     

Is Chytridiomycosis Driving Darwin’s Frogs to Extinction?

Darwin’s frogs (Rhinoderma darwinii and R. rufum) are two species of mouth brooding frogs from Chile and Argentina that have experienced marked population declines. Rhinoderma rufum has not been found in the wild since 1980. We investigated historical and current evidence of Batrachochytrium dendrobatidis (Bd) infection in Rhinoderma spp. to determine whether chytridiomycosis is implicated in the population declines of these species. Archived and live specimens of Rhinoderma spp., sympatric amphibians and amphibians at sites where Rhinoderma sp. had recently gone extinct were examined for Bd infection using quantitative real-time PCR. Six (0.9%) of 662 archived anurans tested positive for Bd (4/289 R. darwinii; 1/266 R. rufum and 1/107 other anurans), all of which had been collected between 1970 and 1978. An overall Bd-infection prevalence of 12.5% was obtained from 797 swabs taken from 369 extant individuals of R. darwinii and 428 individuals representing 18 other species of anurans found at sites with current and recent presence of the two Rhinoderma species. In extant R. darwinii, Bd-infection prevalence (1.9%) was significantly lower than that found in other anurans (7.3%). The prevalence of infection (30%) in other amphibian species was significantly higher in sites where either Rhinoderma spp. had become extinct or was experiencing severe population declines than in sites where there had been no apparent decline (3.0%; x ² = 106.407, P<0.001). This is the first report of widespread Bd presence in Chile and our results are consistent with Rhinoderma spp. declines being due to Bd infection, although additional field and laboratory investigations are required to investigate this further.     

The Last South American Redoubt? Tested Surinamese Anurans Still Chytrid Free

Batrachochytrium dendrobatidis, a chytrid fungus infecting amphibians’ cutaneous layer, is responsible for the greatest contemporary loss of amphibian biodiversity. In South America, Suriname is one of the only three countries where B. dendrobatidis infections of anurans (frogs and toads) have not been documented. To further examine this apparent gap in pathogen occurrence, frogs were sampled for B. dendrobatidis spores at eight disparate geographic locations in Suriname, including locations with high and low levels of anthropogenic activities, and near Suriname’s border with Brazil and French Guiana, countries where B. dendrobatidis infections have been documented. None of the 347 frogs sampled, representing 37 species from eight families, tested positive for B. dendrobatidis. Our results provide the baseline data for future comparative testing and one of the last opportunities for a country in South America to proactively plan mitigation measures to protect amphibians from B. dendrobatidis’ presumed eventual incursion into Suriname.

     

Spatial Assessment of Amphibian Chytrid Fungus (Batrachochytrium dendrobatidis) in South Africa Confirms Endemic and Widespread Infection

Chytridiomycosis has been identified as a major cause of global amphibian declines. Despite widespread evidence of Batrachochytrium dendrobatidis infection in South African frogs, sampling for this disease has not focused on threatened species, or whether this pathogen poses a disease risk to these species. This study assessed the occurrence of Bd-infection in South African Red List species. In addition, all known records of infection from South Africa were used to model the ecological niche of Bd to provide a better understanding of spatial patterns and associated disease risk. Presence and prevalence of Bd was determined through quantitative real-time PCR of 360 skin swab samples from 17 threatened species from 38 sites across the country. Average prevalence was 14.8% for threatened species, with pathogen load varying considerably between species. MaxEnt was used to model the predicted distribution of Bd based on 683 positive records for South Africa. The resultant probability threshold map indicated that Bd is largely restricted to the wet eastern and coastal regions of South Africa. A lack of observed adverse impacts on wild threatened populations supports the endemic pathogen hypothesis for southern Africa. However, all threatened species occur within the limits of the predicted distribution for Bd, exposing them to potential Bd-associated risk factors. Predicting pathogen distribution patterns and potential impact is increasingly important for prioritising research and guiding management decisions.     

The Lethal Fungus Batrachochytrium dendrobatidis Is Present in Lowland Tropical Forests of Far Eastern Panamá

The fungal disease chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd), is one of the main causes of amphibian population declines and extinctions all over the world. In the Neotropics, this fungal disease has caused catastrophic declines in the highlands as it has spread throughout Central America down to Panamá. In this study, we determined the prevalence and intensity of Bd infection in three species of frogs in one highland and four lowland tropical forests, including two lowland regions in eastern Panamá in which the pathogen had not been detected previously. Bd was present in all the sites sampled with a prevalence ranging from 15–34%, similar to other Neotropical lowland sites. The intensity of Bd infection on individual frogs was low, ranging from average values of 0.11–24 zoospore equivalents per site. Our work indicates that Bd is present in anuran communities in lowland Panamá, including the Darién province, and that the intensity of the infection may vary among species from different habitats and with different life histories. The population-level consequences of Bd infection in amphibian communities from the lowlands remain to be determined. Detailed studies of amphibian species from the lowlands will be essential to determine the reason why these species are persisting despite the presence of the pathogen.     

Endemic infection of the amphibian chytrid fungus in a frog community post-decline

The chytrid fungus Batrachochytrium dendrobatidis has been implicated in the decline and extinction of numerous frog species worldwide. In Queensland, Australia, it has been proposed as the cause of the decline or apparent extinction of at least 14 high-elevation rainforest frog species. One of these, Taudactylus eungellensis, disappeared from rainforest streams in Eungella National Park in 1985–1986, but a few remnant populations were subsequently discovered. Here, we report the analysis of B. dendrobatidis infections in toe tips of T. eungellensis and sympatric species collected in a mark-recapture study between 1994 and 1998. This longitudinal study of the fungus in individually marked frogs sheds new light on the effect of this threatening infectious process in field, as distinct from laboratory, conditions. We found a seasonal peak of infection in the cooler months, with no evidence of interannual variation. The overall prevalence of infection was 18% in T. eungellensis and 28% in Litoria wilcoxii/jungguy, a sympatric frog that appeared not to decline in 1985–1986. No infection was found in any of the other sympatric species. Most importantly, we found no consistent evidence of lower survival in T. eungellensis that were infected at the time of first capture, compared with uninfected individuals. These results refute the hypothesis that remnant populations of T. eungellensis recovered after a B. dendrobatidis epidemic because the pathogen had disappeared. They show that populations of T. eungellensis now persist with stable, endemic infections of B. dendrobatidis.     

Infection with Batrachochytrium dendrobatidis is common in tropical lowland habitats: Implications for amphibian conservation

Numerous species of amphibians declined in Central America during the 1980s and 1990s. These declines mostly affected highland stream amphibians and have been primarily linked to chytridiomycosis, a deadly disease caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd). Since then, the majority of field studies on Bd in the Tropics have been conducted in midland and highland environments (>800 m) mainly because the environmental conditions of mountain ranges match the range of ideal abiotic conditions for Bd in the laboratory. This unbalanced sampling has led researchers to largely overlook host–pathogen dynamics in lowlands, where other amphibian species declined during the same period. We conducted a survey testing for Bd in 47 species (n = 348) in four lowland sites in Costa Rica to identify local host–pathogen dynamics and to describe the abiotic environment of these sites. We detected Bd in three sampling sites and 70% of the surveyed species. We found evidence that lowland study sites exhibit enzootic dynamics with low infection intensity and moderate to high prevalence (55% overall prevalence). Additionally, we found evidence that every study site represents an independent climatic zone, where local climatic differences may explain variations in Bd disease dynamics. We recommend more detection surveys across lowlands and other sites that have been historically considered unsuitable for Bd occurrence. These data can be used to identify sites for potential disease outbreaks and amphibian rediscoveries.     

Development and Infectious Disease in Hosts with Complex Life Cycles

Metamorphosis is often characterized by profound changes in morphology and physiology that can affect the dynamics of species interactions. For example, the interaction between a pathogen and its host may differ depending on the life stage of the host or pathogen. One pathogen that infects hosts with complex life cycles is the emerging fungal pathogen of amphibians, Batrachochytrium dendrobatidis (Bd). We sought to determine how conditions at the larval stage can affect variation in development and patterns of Bd infection across amphibian life stages. We used outdoor experimental mesocosms to simulate natural pond habitats and manipulated the presence of Bd, the larval density, and the number of host species in larvae of two co-occurring amphibian species (Rana cascadae and Pseudacris regilla). We found that infection differed between species throughout development; P. regilla consistently had higher infection severity compared to R. cascadae. Additionally, while up to 100% of larvae were infected, only 18.2% of R. cascadae and 81.5% of P. regilla were infected after metamorphosis. This indicates that amphibians have the ability to recover from Bd infection as they undergo metamorphosis. Higher larval densities in P. regilla led to a shorter larval period, and individuals with a shorter larval period had lower infection severity. This led to a trend where P. regilla larvae reared at high densities tended to have lower infection prevalence after metamorphosis. We also found that exposure to Bd increased larval mortality and prolonged the larval period in P. regilla, indicating that P. regilla are susceptible to the negative effects of Bd as larvae. This study demonstrates that host density, species composition, and pathogen exposure may all interact to influence development and infection in hosts with complex life cycles.     

The Amphibian Chytrid Batrachochytrium dendrobatidis Occurs on Freshwater Shrimp in Rain Forest Streams in Northern Queensland, Australia

Chytridiomycosis is a disease of amphibians caused by the chytrid fungus Batrachochytrium dendrobatidis. It can be highly virulent and is unusual in that it appears to drive many host species to local extinction during outbreaks. One mechanism that could facilitate this is the ability to grow saprophytically or on alternative hosts. This is common in other chytrids but has not been demonstrated for B. dendrobatidis in the field. B. dendrobatidis can grow on arthropod exoskeletons in the laboratory, and freshwater shrimp can be the most abundant animals in tropical rain forest streams. We therefore used diagnostic quantitative polymerase chain reaction to determine the infection status of freshwater shrimp from areas in which they are sympatric with frog species that have suffered declines in association with outbreaks of chytridiomycosis. We detected B. dendrobatidis on three individual shrimp belonging to two genera and collected from two widely separated streams. Two of the individuals had high levels of infection. This indicates that the presence of alternative hosts is likely to contribute to the extreme virulence of chytridiomycosis outbreaks in some systems. The presence of alternative hosts may allow B. dendrobatidis to remain in the environment after local extinctions of amphibian hosts, preventing the recovery of amphibian populations.     

Host specificity of microsporidia pathogenic to the gypsy moth, Lymantria dispar (L.): Field studies in Slovakia

Several species of microsporidia are important chronic pathogens of Lymantria dispar in Europe but have never been recovered from North American gypsy moth populations. The major issue for their introduction into North American L. dispar populations is concern about their safety to native non-target insects. In this study, we evaluated the susceptibility of sympatric non-target Lepidoptera to two species of microsporidia, Nosema lymantriae and Vairimorpha disparis, isolated from European populations of L. dispar and applied in field plots in Slovakia. Application of ultra low volume sprays of the microsporidia maximized coverage of infective spores in a complex natural environment and, thus, exposure of non-target species to the pathogens. Of 653 non-target larvae collected from plots treated with V. disparis in 2002, 18 individual larvae representing nine species in four families were infected. These plots were monitored for two subsequent seasons and V. disparis was not recovered from non-target species. Of 2571 non-target larvae collected in N. lymantriae-treated sites, one larva was found to be infected. Both species of microsporidia, particularly N. lymantriae, appear to have a very narrow host range in the field, even when an inundative technique is used for their introduction. V. disparis infections in L. dispar exceeded 40% of recovered larvae in the treated study sites; infection rates were lower in sites sprayed with N. lymantriae. Several naturally-occurring pathogens were recorded from the non-target species. The most common pathogen, isolated from 21 species in eight families, was a microsporidium in the genus Cystosporogenes.     

The Bacterially Produced Metabolite Violacein Is Associated with Survival of Amphibians Infected with a Lethal Fungus

The disease chytridiomycosis, which is caused by the chytrid fungus Batrachochytrium dendrobatidis, is associated with recent declines in amphibian populations. Susceptibility to this disease varies among amphibian populations and species, and resistance appears to be attributable in part to the presence of antifungal microbial species associated with the skin of amphibians. The betaproteobacterium Janthinobacterium lividum has been isolated from the skins of several amphibian species and produces the antifungal metabolite violacein, which inhibits B. dendrobatidis. In this study, we added J. lividum to red-backed salamanders (Plethodon cinereus) to obtain an increased range of violacein concentrations on the skin. Adding J. lividum to the skin of the salamander increased the concentration of violacein on the skin, which was strongly associated with survival after experimental exposure to B. dendrobatidis. As expected from previous work, some individuals that did not receive J. lividum and were exposed to B. dendrobatidis survived. These individuals had concentrations of bacterially produced violacein on their skins that were predicted to kill B. dendrobatidis. Our study suggests that a threshold violacein concentration of about 18 μM on a salamander''s skin prevents mortality and morbidity caused by B. dendrobatidis. In addition, we show that over one-half of individuals in nature support antifungal bacteria that produce violacein, which suggests that there is a mutualism between violacein-producing bacteria and P. cinereus and that adding J. lividum is effective for protecting individuals that lack violacein-producing skin bacteria.The amphibian fungal pathogen Batrachochytrium dendrobatidis causes a lethal skin disease that has caused substantial declines in amphibian populations (18). However, some species, such as the bullfrog (Rana catesbeiana) and the tiger salamander (Ambystoma tigrinum), are relatively asymptomatic when they are infected with this pathogen (4, 5). Variation in survival among species has been attributed to differences in innate immune factors, such as antimicrobial peptides (20) and skin-associated microbial species (8-11), as well as behavior (16). The presence of antifungal microbes is of particular interest because it suggests that these organisms are mutualistic associates of amphibian species. In addition, augmentation of the cutaneous microbial community by adding species of bacteria that inhibit B. dendrobatidis has the potential to provide resistance to chytridiomycosis (9).We have identified a number of bacteria associated with the skin of amphibians that inhibit B. dendrobatidis in vitro via secretion of antifungal metabolites (2, 3, 10, 11). The bacterial species used in this study, Janthinobacterium lividum, produces the anti-B. dendrobatidis metabolites violacein and indole-3-carboxaldehyde (MIC, 1.82 μM and 69 μM, respectively) (3). We have shown that violacein inhibits B. dendrobatidis in laboratory assays (3) and is strongly correlated with survival in vivo of the frog species Rana muscosa (9). Violacein was also present on three of seven wild-collected red-backed salamanders (Plethodon cinereus) at concentrations that inhibit B. dendrobatidis in vitro (3), suggesting that this salamander species has a mutualistic community of violacein-producing bacteria on its skin. In this study, we added J. lividum to salamander skins to generate a wide range of violacein concentrations in order to determine what concentration is needed to prevent mortality caused by chytridiomycosis in vivo.     

Distribution and Pathogenicity of Batrachochytrium dendrobatidis in Boreal Toads from the Grand Teton Area of Western Wyoming

The pathogen Batrachochytrium dendrobatidis (Bd), which causes the skin disease chytridiomycosis, has been linked to amphibian population declines and extinctions worldwide. Bd has been implicated in recent declines of boreal toads, Bufo boreas boreas, in Colorado but populations of boreal toads in western Wyoming have high prevalence of Bd without suffering catastrophic mortality. In a field and laboratory study, we investigated the prevalence of Bd in boreal toads from the Grand Teton ecosystem (GRTE) in Wyoming and tested the pathogenicity of Bd to these toads in several environments. The pathogen was present in breeding adults at all 10 sites sampled, with a mean prevalence of 67%. In an experiment with juvenile toadlets housed individually in wet environments, 10⁶ zoospores of Bd isolated from GRTE caused lethal disease in all Wyoming and Colorado animals within 35 days. Survival time was longer in toadlets from Wyoming than Colorado and in toadlets spending more time in dry sites. In a second trial involving Colorado toadlets exposed to 35% fewer Bd zoospores, infection peaked and subsided over 68 days with no lethal chytridiomycosis in any treatment. However, compared with drier aquaria with dry refuges, Bd infection intensity was 41% higher in more humid aquaria and 81% higher without dry refuges available. Our findings suggest that although widely infected in nature, Wyoming toads may escape chytridiomycosis due to a slight advantage in innate resistance or because their native habitat hinders Bd growth or provides more opportunities to reduce pathogen loads behaviorally than in Colorado.     

Correlates of virulence in a frog-killing fungal pathogen: evidence from a California amphibian decline

The fungal pathogen Batrachochytrium dendrobatidis (Bd) has caused declines and extinctions in amphibians worldwide, and there is increasing evidence that some strains of this pathogen are more virulent than others. While a number of putative virulence factors have been identified, few studies link these factors to specific epizootic events. We documented a dramatic decline in juvenile frogs in a Bd-infected population of Cascades frogs (Rana cascadae) in the mountains of northern California and used a laboratory experiment to show that Bd isolated in the midst of this decline induced higher mortality than Bd isolated from a more stable population of the same species of frog. This highly virulent Bd isolate was more toxic to immune cells and attained higher density in liquid culture than comparable isolates. Genomic analyses revealed that this isolate is nested within the global panzootic lineage and exhibited unusual genomic patterns, including increased copy numbers of many chromosomal segments. This study integrates data from multiple sources to suggest specific phenotypic and genomic characteristics of the pathogen that may be linked to disease-related declines.     

Experimental Evidence for American Bullfrog (Lithobates catesbeianus) Susceptibility to Chytrid Fungus (Batrachochytrium dendrobatidis)

The emerging fungal pathogen, Batrachochytrium dendrobatidis (Bd), has been associated with global amphibian population declines and extinctions. American bullfrogs (Lithobates catesbeianus) are widely reported to be a tolerant host and a carrier of Bd that spreads the pathogen to less tolerant hosts. Here, we examined whether bullfrogs raised from eggs to metamorphosis in outdoor mesocosms were susceptible to Bd. We experimentally exposed metamorphic juveniles to Bd in the laboratory and compared mortality rates of pathogen-exposed animals to controls (non-exposed) in two separate experiments; one using a Bd strain isolated from a Western toad and another using a strain isolated from an American bullfrog. We wanted to examine whether metamorphic bullfrogs were susceptible to either of these strains. We show that bullfrogs were susceptible to one strain of Bd and not the other. In both experiments, infection load detected in the skin decreased over time, suggesting that metamorphic bullfrogs from some populations may be inefficient long-term carriers of Bd.