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Lundsgaard NU, Cramp RL, Franklin CE. Effects of ultraviolet-B radiation on physiology, immune function and survival is dependent on temperature: implications for amphibian declines. CONSERVATION PHYSIOLOGY 2020; 8:coaa002. [PMID: 32467758 PMCID: PMC7245394 DOI: 10.1093/conphys/coaa002] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/04/2019] [Revised: 12/11/2019] [Accepted: 01/17/2020] [Indexed: 05/10/2023]
Abstract
Multiple environmental changes are thought to be contributing to the widespread decline of amphibians in montane regions, but interactions between drivers of decline are not well understood. It has been proposed previously that elevated ultraviolet-B radiation (UBVR) and low temperatures may interact in their negative effects on health, immune function and disease susceptibility in exposed amphibians. In the present study, we chronically exposed larvae of the striped-marsh frog (Limnodynastes peronii) to a factorial combination of high and low UVBR and high and low temperature to assess interactive effects on growth, survival and indices of immune function. The high UVBR treatment reduced growth and survival of larvae compared to the low UVBR treatment at both temperatures, but the effects were significantly enhanced at low temperature. High UVBR exposure also induced a chronic inflammatory response as evidenced by an increase in the leucocyte proportion of total cells and altered the ratio of neutrophils to lymphocytes in the blood, highlighting a potential mechanistic basis for increased disease susceptibility in amphibians living at high altitudes. Our findings stress the importance of investigating environmental factors in combination when assessing their effects and highlight the mechanistic basis for how key environmental drivers in montane regions affect amphibian health. Continuation of this work is necessary for the development of targeted conservation strategies that tackle the root causes of montane amphibian declines.
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Lastra González D, Baláž V, Solský M, Thumsová B, Kolenda K, Najbar A, Najbar B, Kautman M, Chajma P, Balogová M, Vojar J. Recent Findings of Potentially Lethal Salamander Fungus Batrachochytrium salamandrivorans. Emerg Infect Dis 2019; 25:1416-1418. [PMID: 31211934 PMCID: PMC6590763 DOI: 10.3201/eid2507.181001] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
The distribution of the chytrid fungus Batrachochytrium salamandrivorans continues to expand in Europe. During 2014-2018, we collected 1,135 samples from salamanders and newts in 6 countries in Europe. We identified 5 cases of B. salamandrivorans in a wild population in Spain but none in central Europe or the Balkan Peninsula.
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LETHAL AND SUBLETHAL AMPHIBIAN HOST RESPONSES TO BATRACHOCHYTRIUM DENDROBATIDIS EXPOSURE ARE DETERMINED BY THE ADDITIVE INFLUENCE OF HOST RESOURCE AVAILABILITY. J Wildl Dis 2019. [PMID: 31769713 DOI: 10.7589/2019-01-021] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Host species may differ in their responses to pathogen exposures based on host energy reserves, which could be important for long-term trends in host population growth. Batrachochytrium dendrobatidis (BD) is a pathogen associated with amphibian population declines but also occurs without causing mass mortalities. The impact of BD in populations without associated declines is not well understood, and food abundance could play a role in determining the magnitude of its effects. We exposed American toad (Anaxyrus americanus), northern leopard frog (Lithobates pipiens), and cricket frog (Acris blanchardi) metamorphs to BD under low or high food treatments. Overall, anuran species responded differently to BD exposure and the combined effect of BD exposure and food abundance was additive. American toad survival was lowered by BD exposure and low food availability. Based on these results, we developed a population model for American toads to estimate how reductions in survival could influence population growth. We found that BD could reduce population growth by 14% with high food availability and 21% with low food availability. In contrast, survival of northern leopard frogs was high across all treatments, but their growth was negatively impacted by the additive effects of BD exposure and low food availability. Cricket frog growth and survival were unaffected by BD exposure, suggesting that this species is not sensitive to the effects of this pathogen in terms of growth and survival across environments of different quality in the time period examined. Our results showed that low food availability additively increased the species-specific lethal and sublethal impacts of BD on hosts, which could have implications for long-term host population dynamics.
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Madison JD, Ouellette SP, Schmidt EL, Kerby JL. Serratia marcescens shapes cutaneous bacterial communities and influences survival of an amphibian host. Proc Biol Sci 2019; 286:20191833. [PMID: 31662077 DOI: 10.1098/rspb.2019.1833] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Ongoing investigations into the interactions between microbial communities and their associated hosts are changing how emerging diseases are perceived and ameliorated. Of the numerous host-microbiome-disease systems of study, the emergence of chytridiomycosis (caused by Batrachochytrium dendrobatidis, hereafter Bd) has been implicated in ongoing declines and extinction events of amphibians worldwide. Interestingly, there has been differential survival among amphibians in resisting Bd infection and subsequent disease. One factor thought to contribute to this resistance is the host-associated cutaneous microbiota. This has raised the possibility of using genetically modified probiotics to restructure the host-associated microbiota for desired anti-fungal outcomes. Here, we use a previously described strain of Serratia marcescens (Sm) for the manipulation of amphibian cutaneous microbiota. Sm was genetically altered to have a dysfunctional pathway for the production of the extracellular metabolite prodigiosin. This genetically altered strain (Δpig) and the functional prodigiosin producing strain (wild-type, WT) were compared for their microbial community and anti-Bd effects both in vitro and in vivo. In vitro, Bd growth was significantly repressed in the presence of prodigiosin. In vivo, the inoculation of both Sm strains was shown to significantly influence amphibian microbiota diversity with the Δpig-Sm treatment showing increasing alpha diversity, and the WT-Sm having no temporal effect on diversity. Differences were also seen in host mortality with Δpig-Sm treatments exhibiting significantly decreased survival probability when compared with WT-Sm in the presence of Bd. These results are an important proof-of-concept for linking the use of genetically modified probiotic bacteria to host microbial community structure and disease outcomes, which in the future may provide a way to ameliorate disease and address critical frontiers in disease and microbial ecology.
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Bates KA, Shelton JMG, Mercier VL, Hopkins KP, Harrison XA, Petrovan SO, Fisher MC. Captivity and Infection by the Fungal Pathogen Batrachochytrium salamandrivorans Perturb the Amphibian Skin Microbiome. Front Microbiol 2019; 10:1834. [PMID: 31507541 PMCID: PMC6716147 DOI: 10.3389/fmicb.2019.01834] [Citation(s) in RCA: 28] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2019] [Accepted: 07/25/2019] [Indexed: 12/13/2022] Open
Abstract
The emerging fungal pathogen, Batrachochytrium salamandrivorans (Bsal) is responsible for the catastrophic decline of European salamanders and poses a threat to amphibians globally. The amphibian skin microbiome can influence disease outcome for several host-pathogen systems, yet little is known of its role in Bsal infection. In addition, many experimental in-vivo amphibian disease studies to date have relied on specimens that have been kept in captivity for long periods without considering the influence of environment on the microbiome and how this may impact the host response to pathogen exposure. We characterized the impact of captivity and exposure to Bsal on the skin bacterial and fungal communities of two co-occurring European newt species, the smooth newt, Lissotriton vulgaris and the great-crested newt, Triturus cristatus. We show that captivity led to significant losses in bacterial and fungal diversity of amphibian skin, which may be indicative of a decline in microbe-mediated protection. We further demonstrate that in both L. vulgaris and T. cristatus, Bsal infection was associated with changes in the composition of skin bacterial communities with possible negative consequences to host health. Our findings advance current understanding of the role of host-associated microbiota in Bsal infection and highlight important considerations for ex-situ amphibian conservation programmes.
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Becker CG, Bletz MC, Greenspan SE, Rodriguez D, Lambertini C, Jenkinson TS, Guimarães PR, Assis APA, Geffers R, Jarek M, Toledo LF, Vences M, Haddad CFB. Low-load pathogen spillover predicts shifts in skin microbiome and survival of a terrestrial-breeding amphibian. Proc Biol Sci 2019; 286:20191114. [PMID: 31409249 DOI: 10.1098/rspb.2019.1114] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Wildlife disease dynamics are strongly influenced by the structure of host communities and their symbiotic microbiota. Conspicuous amphibian declines associated with the waterborne fungal pathogen Batrachochytrium dendrobatidis (Bd) have been observed in aquatic-breeding frogs globally. However, less attention has been given to cryptic terrestrial-breeding amphibians that have also been declining in tropical regions. By experimentally manipulating multiple tropical amphibian assemblages harbouring natural microbial communities, we tested whether Bd spillover from naturally infected aquatic-breeding frogs could lead to Bd amplification and mortality in our focal terrestrial-breeding host: the pumpkin toadlet Brachycephalus pitanga. We also tested whether the strength of spillover could vary depending on skin bacterial transmission within host assemblages. Terrestrial-breeding toadlets acquired lethal spillover infections from neighbouring aquatic hosts and experienced dramatic but generally non-protective shifts in skin bacterial composition primarily attributable to their Bd infections. By contrast, aquatic-breeding amphibians maintained mild Bd infections and higher survival, with shifts in bacterial microbiomes that were unrelated to Bd infections. Our results indicate that Bd spillover from even mildly infected aquatic-breeding hosts may lead to dysbiosis and mortality in terrestrial-breeding species, underscoring the need to further investigate recent population declines of terrestrial-breeding amphibians in the tropics.
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Wu NC, McKercher C, Cramp RL, Franklin CE. Mechanistic basis for loss of water balance in green tree frogs infected with a fungal pathogen. Am J Physiol Regul Integr Comp Physiol 2019; 317:R301-R311. [PMID: 31141416 DOI: 10.1152/ajpregu.00355.2018] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Chytridiomycosis, a lethal skin disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), disrupts skin function of amphibians, interfering with ionic and osmotic regulation. To regulate fungal loads, amphibians increase their rate of skin sloughing. However, sloughing also causes a temporary loss of ionic and osmotic homeostasis due to disruption of the skin, a key osmoregulatory organ. The combined effects of increased sloughing frequency and chytridiomycosis contribute to the high rates of mortality from Bd infections. However, the mechanisms responsible for the loss of cutaneous osmotic regulation remain unknown. We measured the changes in whole animal water uptake rates, in vitro transcutaneous water fluxes across the ventral skin, and the mRNA expression of epithelial water transport proteins (aquaporins, AQPs) and junctional proteins in Bd-infected and uninfected Litoria caerulea skin. We hypothesize that infected frogs would show reduction/inhibition in cutaneous water transporters responsible for regulating water balance, and sloughing would exacerbate cutaneous water fluxes. We found that infected, nonsloughing frogs had an impaired rate of water uptake and showed increased rates of in vitro water efflux across the ventral skin. In uninfected frogs, the expression of AQPs and junction genes increased significantly with sloughing, which may assist in regulating cutaneous water movements and barrier function in the newly exposed skin. In contrast, infected frogs did not show this postsloughing increase in AQP gene expression. The combination of increased sloughing frequency, impaired water uptake rates, and increased rates of water loss likely contributes to the loss of osmotic homeostasis in frogs infected with Bd.
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Greenspan SE, Lyra ML, Migliorini GH, Kersch-Becker MF, Bletz MC, Lisboa CS, Pontes MR, Ribeiro LP, Neely WJ, Rezende F, Romero GQ, Woodhams DC, Haddad CFB, Toledo LF, Becker CG. Arthropod-bacteria interactions influence assembly of aquatic host microbiome and pathogen defense. Proc Biol Sci 2019; 286:20190924. [PMID: 31238845 DOI: 10.1098/rspb.2019.0924] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The host-associated microbiome is vital to host immunity and pathogen defense. In aquatic ecosystems, organisms may interact with environmental bacteria to influence the pool of potential symbionts, but the effects of these interactions on host microbiome assembly and pathogen resistance are unresolved. We used replicated bromeliad microecosystems to test for indirect effects of arthropod-bacteria interactions on host microbiome assembly and pathogen burden, using tadpoles and the fungal amphibian pathogen Batrachochytrium dendrobatidis as a model host-pathogen system. Arthropods influenced host microbiome assembly by altering the pool of environmental bacteria, with arthropod-bacteria interactions specifically reducing host colonization by transient bacteria and promoting antimicrobial components of aquatic bacterial communities. Arthropods also reduced fungal zoospores in the environment, but fungal infection burdens in tadpoles corresponded most closely with arthropod-mediated patterns in microbiome assembly. This result indicates that the cascading effects of arthropods on the maintenance of a protective host microbiome may be more strongly linked to host health than negative effects of arthropods on pools of pathogenic zoospores. Our work reveals tight links between healthy ecosystem dynamics and the functioning of host microbiomes, suggesting that ecosystem disturbances such as loss of arthropods may have downstream effects on host-associated microbial pathogen defenses and host fitness.
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McKnight DT, Lal MM, Bower DS, Schwarzkopf L, Alford RA, Zenger KR. The return of the frogs: The importance of habitat refugia in maintaining diversity during a disease outbreak. Mol Ecol 2019; 28:2731-2745. [PMID: 31013393 DOI: 10.1111/mec.15108] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2018] [Revised: 04/07/2019] [Accepted: 04/17/2019] [Indexed: 12/01/2022]
Abstract
Recent decades have seen the emergence and spread of numerous infectious diseases, often with severe negative consequences for wildlife populations. Nevertheless, many populations survive the initial outbreaks, and even undergo recoveries. Unfortunately, the long-term effects of these outbreaks on host population genetics are poorly understood; to increase this understanding, we examined the population genetics of two species of rainforest frogs (Litoria nannotis and Litoria serrata) that have largely recovered from a chytridiomycosis outbreak at two national parks in the Wet Tropics of northern Australia. At the wetter, northern park there was little evidence of decreased genetic diversity in either species, and all of the sampled sites had high minor allele frequencies (mean MAF = 0.230-0.235), high heterozygosity (0.318-0.325), and few monomorphic markers (1.4%-4.0%); however, some recovered L. nannotis populations had low Ne values (59.3-683.8) compared to populations that did not decline during the outbreak (1,537.4-1,756.5). At the drier, southern park, both species exhibited lower diversity (mean MAF = 0.084-0.180; heterozygosity = 0.126-0.257; monomorphic markers = 3.7%-43.5%; Ne = 18.4-676.1). The diversity patterns in this park matched habitat patterns, with both species having higher diversity levels and fewer closely related individuals at sites with higher quality habitat. These patterns were more pronounced for L. nannotis, which has lower dispersal rates than L. serrata. These results suggest that refugia with high quality habitat are important for retaining genetic diversity during disease outbreaks, and that gene flow following disease outbreaks is important for re-establishing diversity in populations where it was reduced.
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Fu M, Waldman B. Ancestral chytrid pathogen remains hypervirulent following its long coevolution with amphibian hosts. Proc Biol Sci 2019; 286:20190833. [PMID: 31161901 DOI: 10.1098/rspb.2019.0833] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Many amphibian species around the world, except in Asia, suffer morbidity and mortality when infected by the emerging infectious pathogen Batrachochytrium dendrobatidis (Bd). A lineage of the amphibian chytrid fungus isolated from South Korean amphibians (BdAsia-1) is evolutionarily basal to recombinant global pandemic lineages (BdGPL) associated with worldwide amphibian population declines. In Asia, the Bd pathogen and its amphibian hosts have coevolved over 100 years or more. Thus, resilience of Asian amphibian populations to infection might result from attenuated virulence of endemic Bd lineages, evolved immunity to the pathogen or both. We compared susceptibilities of an Australasian amphibian, Litoria caerulea, known to lack resistance to BdGPL, with those of three Korean species, Bufo gargarizans, Bombina orientalis and Hyla japonica, after inoculation with BdAsia-1, BdGPL or a blank solution. Subjects became infected in all experimental treatments but Korean species rapidly cleared themselves of infection, regardless of Bd lineage. They survived with no apparent secondary effects. By contrast, L. caerulea, after infection by either BdAsia-1 or BdGPL, suffered deteriorating body condition and carried progressively higher Bd loads over time. Subsequently, most subjects died. Comparing their effects on L. caerulea, BdAsia-1 induced more rapid disease progression than BdGPL. The results suggest that genomic recombination with other lineages was not necessary for the ancestral Bd lineage to evolve hypervirulence over its long period of coevolution with amphibian hosts. The pathogen's virulence may have driven strong selection for immune responses in endemic Asian amphibian host species.
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Kohli AK, Lindauer AL, Brannelly LA, Ohmer MEB, Richards-Zawacki C, Rollins-Smith L, Voyles J. Disease and the Drying Pond: Examining Possible Links among Drought, Immune Function, and Disease Development in Amphibians. Physiol Biochem Zool 2019; 92:339-348. [PMID: 30990770 DOI: 10.1086/703137] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
Drought can heavily impact aquatic ecosystems. For amphibian species that rely on water availability for larval development, drought can have direct and indirect effects on larval survival and postmetamorphic fitness. Some amphibian species can accelerate the timing of metamorphosis to escape drying habitats through developmental plasticity. However, trade-offs associated with premature metamorphosis, such as reduced body size and altered immune function in the recently metamorphosed individual, may have downstream effects on susceptibility to disease. Here, we review the physiological mechanisms driving patterns in larval amphibian development under low water conditions. Specifically, we discuss drought-induced accelerated metamorphosis and how it may alter immune function, predisposing juvenile amphibians to infectious disease. In addition, we consider how these physiological and immunological adjustments could play out in a lethal disease system, amphibian chytridiomycosis. Last, we propose avenues for future research that adopt an ecoimmunological approach to evaluate the combined threats of drought and disease for amphibian populations.
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Rebollar EA, Harris RN. Editorial: Ecology of Amphibian-Microbial Symbioses. Front Microbiol 2019; 10:766. [PMID: 31057502 PMCID: PMC6477928 DOI: 10.3389/fmicb.2019.00766] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2018] [Accepted: 03/26/2019] [Indexed: 11/13/2022] Open
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Borteiro C, Kolenc F, Verdes JM, Martínez Debat C, Ubilla M. Sensitivity of histology for the detection of the amphibian chytrid fungus Batrachochytrium dendrobatidis. J Vet Diagn Invest 2019; 31:246-249. [PMID: 30661466 DOI: 10.1177/1040638718816116] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Histology is often underappreciated for the detection of the amphibian pathogenic fungus Batrachochytrium dendrobatidis, the cause of the potentially lethal skin disease chytridiomycosis. We evaluated the sensitivity of histology to detect chytrids in 20 wild specimens of 2 frog species from Uruguay that were clinically normal, but confirmed by PCR to be infected by B. dendrobatidis. We detected maturing and sporulated sporangia in 15 of 20 (75%) frogs, which is more sensitive than previously reported for histology. The effort needed to identify chytrids in histologic skin sections of Physalaemus henselii and Pleurodema bibroni required examination of 3.2 and 8.7 mm of skin sections for each frog species, respectively.
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Ellison S, Knapp RA, Sparagon W, Swei A, Vredenburg VT. Reduced skin bacterial diversity correlates with increased pathogen infection intensity in an endangered amphibian host. Mol Ecol 2018; 28:127-140. [PMID: 30506592 DOI: 10.1111/mec.14964] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2017] [Revised: 09/28/2018] [Accepted: 10/30/2018] [Indexed: 12/13/2022]
Abstract
The fungal pathogen Batrachochytrium dendrobatidis (Bd) infects the skin of amphibians and has caused severe declines and extinctions of amphibians globally. In this study, we investigate the interaction between Bd and the bacterial skin microbiome of the endangered Sierra Nevada yellow-legged frog, Rana sierrae, using both culture-dependent and culture-independent methods. Samples were collected from two populations of R. sierrae that likely underwent Bd epizootics in the past, but that continue to persist with Bd in an enzootic disease state, and we address the hypothesis that such "persistent" populations are aided by mutualistic skin microbes. Our 16S rRNA metabarcoding data reveal that the skin microbiome of highly infected juvenile frogs is characterized by significantly reduced species richness and evenness, and by strikingly lower variation between individuals, compared to juveniles and adults with lower infection levels. Over 90% of DNA sequences from the skin microbiome of highly infected frogs were derived from bacteria in a single order, Burkholderiales, compared to just 54% in frogs with lower infection levels. In a culture-dependent Bd inhibition assay, the bacterial metabolites we evaluated all inhibited the growth of Bd. Together, these results illustrate the disruptive effects of Bd infection on host skin microbial community structure and dynamics, and suggest possible avenues for the development of anti-Bd probiotic treatments.
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Jenkinson TS, Rodriguez D, Clemons RA, Michelotti LA, Zamudio KR, Toledo LF, Longcore JE, James TY. Globally invasive genotypes of the amphibian chytrid outcompete an enzootic lineage in coinfections. Proc Biol Sci 2018; 285:20181894. [PMID: 30963903 PMCID: PMC6304064 DOI: 10.1098/rspb.2018.1894] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2018] [Accepted: 11/19/2018] [Indexed: 01/22/2023] Open
Abstract
Competition between genotypes is likely to be a key driver of pathogen evolution, particularly following a geographical invasion by distant strains. Theory predicts that competition between disease strains will result in the most virulent strain persisting. Despite its evolutionary implications, the role of strain competition in shaping populations remains untested for most pathogens. We experimentally investigated the in vivo competitive differences between two divergent lineages of the amphibian-killing chytrid fungus ( Batrachochytrium dendrobatidis, Bd). These Bd lineages are hypothesized to have diverged in allopatry but been recently brought back into secondary contact by human introduction. Prior studies indicate that a panzootically-distributed, global lineage of Bd was recently introduced into southern Brazil, and is competitively excluding enzootic lineages in the southern Atlantic Forest. To test for differences in competitive ability between invasive and enzootic Brazilian Bd isolates, we coinfected a model host frog system which we developed for this study ( Hymenochirus curtipes). We tracked isolate-specific zoospore production over the course of the coinfection experiment with chip-based digital PCR (dPCR). The globally invasive panzootic lineage had a competitive advantage in spore production especially during the first one to four weeks of infection, and on frogs that eventually succumbed to Bd infection. Our study provides new evidence that competitive pressure resulting from the human movement of pathogen strains can rapidly alter the genetics, community dynamics and spatial epidemiology of pathogens in the wild.
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Grogan LF, Robert J, Berger L, Skerratt LF, Scheele BC, Castley JG, Newell DA, McCallum HI. Review of the Amphibian Immune Response to Chytridiomycosis, and Future Directions. Front Immunol 2018; 9:2536. [PMID: 30473694 PMCID: PMC6237969 DOI: 10.3389/fimmu.2018.02536] [Citation(s) in RCA: 88] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2018] [Accepted: 10/15/2018] [Indexed: 12/27/2022] Open
Abstract
The fungal skin disease, chytridiomycosis (caused by Batrachochytrium dendrobatidis and B. salamandrivorans), has caused amphibian declines and extinctions globally since its emergence. Characterizing the host immune response to chytridiomycosis has been a focus of study with the aim of disease mitigation. However, many aspects of the innate and adaptive arms of this response are still poorly understood, likely due to the wide range of species' responses to infection. In this paper we provide an overview of expected immunological responses (with inference based on amphibian and mammalian immunology), together with a synthesis of current knowledge about these responses for the amphibian-chytridiomycosis system. We structure our review around four key immune stages: (1) the naïve immunocompetent state, (2) immune defenses that are always present (constitutive defenses), (3) mechanisms for recognition of a pathogen threat and innate immune defenses, and (4) adaptive immune responses. We also evaluate the current hot topics of immunosuppression and immunopathology in chytridiomycosis, and discuss their respective roles in pathogenesis. Our synthesis reveals that susceptibility to chytridiomycosis is likely to be multifactorial. Susceptible amphibians appear to have ineffective constitutive and innate defenses, and a late-stage response characterized by immunopathology and Bd-induced suppression of lymphocyte responses. Overall, we identify substantial gaps in current knowledge, particularly concerning the entire innate immune response (mechanisms of initial pathogen detection and possible immunoevasion by Bd, degree of activation and efficacy of the innate immune response, the unexpected absence of innate leukocyte infiltration, and the cause and role of late-stage immunopathology in pathogenesis). There are also gaps concerning most of the adaptive immune system (the relative importance of B and T cell responses for pathogen clearance, the capacity and extent of immunological memory, and specific mechanisms of pathogen-induced immunosuppression). Improving our capacity for amphibian immunological research will require selection of an appropriate Bd-susceptible model species, the development of taxon-specific affinity reagents and cell lines for functional assays, and the application of a suite of conventional and emerging immunological methods. Despite current knowledge gaps, immunological research remains a promising avenue for amphibian conservation management.
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Bletz MC, Kelly M, Sabino-Pinto J, Bales E, Van Praet S, Bert W, Boyen F, Vences M, Steinfartz S, Pasmans F, Martel A. Disruption of skin microbiota contributes to salamander disease. Proc Biol Sci 2018; 285:rspb.2018.0758. [PMID: 30135150 DOI: 10.1098/rspb.2018.0758] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2018] [Accepted: 07/27/2018] [Indexed: 12/15/2022] Open
Abstract
Escalating occurrences of emerging infectious diseases underscore the importance of understanding microbiome-pathogen interactions. The amphibian cutaneous microbiome is widely studied for its potential to mitigate disease-mediated amphibian declines. Other microbial interactions in this system, however, have been largely neglected in the context of disease outbreaks. European fire salamanders have suffered dramatic population crashes as a result of the newly emerged Batrachochytrium salamandrivorans (Bsal). In this paper, we investigate microbial interactions on multiple fronts within this system. We show that wild, healthy fire salamanders maintain complex skin microbiotas containing Bsal-inhibitory members, but these community are present at a remarkably low abundance. Through experimentation, we show that increasing bacterial densities of Bsal-inhibiting bacteria via daily addition slowed disease progression in fire salamanders. Additionally, we find that experimental-Bsal infection elicited subtle changes in the skin microbiome, with selected opportunistic bacteria increasing in relative abundance resulting in septicemic events that coincide with extensive destruction of the epidermis. These results suggest that fire salamander skin, in natural settings, maintains bacterial communities at numbers too low to confer sufficient protection against Bsal, and, in fact, the native skin microbiota can constitute a source of opportunistic bacterial pathogens that contribute to pathogenesis. By shedding light on the complex interaction between the microbiome and a lethal pathogen, these data put the interplay between skin microbiomes and a wildlife disease into a new perspective.
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Bell SC, Garland S, Alford RA. Increased Numbers of Culturable Inhibitory Bacterial Taxa May Mitigate the Effects of Batrachochytrium dendrobatidis in Australian Wet Tropics Frogs. Front Microbiol 2018; 9:1604. [PMID: 30072970 PMCID: PMC6058028 DOI: 10.3389/fmicb.2018.01604] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2017] [Accepted: 06/27/2018] [Indexed: 11/20/2022] Open
Abstract
Symbiotic bacterial communities resident on amphibian skin can benefit their hosts. For example, antibiotic production by community members can control the pathogen Batrachochytrium dendrobatidis (Bd) and it is possible for these community members to be used as probiotics to reduce infection levels. In the early 1990s, the emergence of Bd caused declines and disappearances of frogs in the Australian Wet Tropics; the severity of its effects varied among species and sites. Some species have since recolonized despite enzootic Bd within their populations. This variation in history among species and sites provided an opportunity to investigate the role of anti-fungal cutaneous bacteria in protecting frogs against Bd infection. We collected cutaneous swab samples from three species of frogs at two upland and two lowland sites in the Wet Tropics, and used in vitro challenge assays to identify culturable Bd-inhibitory bacterial isolates for further analysis. We sequenced DNA from cultured inhibitory isolates to identify taxa, resulting in the classification of 16 Bd-inhibitory OTUs, and determined whether inhibitory taxa were associated with frog species, site, or intensity of infection. We present preliminary results showing that the upper limit of Bd infection intensity was negatively correlated with number of inhibitory OTUs present per frog indicating that increased numbers of Bd-inhibiting taxa may play a role in reducing the intensity of Bd infections, facilitating frog coexistence with enzootic Bd. One upland site had a significantly lower prevalence of Bd infection, a significantly higher proportion of frogs with one or more culturable Bd-inhibitory OTUs, a greater number of inhibitory bacterial genera present per frog, and statistically significant clustering of individual frogs with similar Bd-inhibitory signatures when compared to all other sites. This suggests that Bd-inhibitory taxa are likely to be particularly important to frogs at this site and may have played a role in their ability to recolonize following population declines. Our findings suggest that the use of multi-taxon Bd-inhibitory probiotics to support at-risk amphibian populations may be more effective than single-taxon alternatives.
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de Jong MS, van Dyk R, Weldon C. Antifungal efficacy of F10SC veterinary disinfectant against Batrachochytrium dendrobatidis. Med Mycol 2018; 56:60-68. [PMID: 28371902 DOI: 10.1093/mmy/myx023] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2016] [Accepted: 02/28/2017] [Indexed: 11/13/2022] Open
Abstract
The Infectious disease chytridiomycosis, which is caused by the fungal pathogen Batrachochytrium dendrobatidis, has been identified as one of the most important drivers of amphibian declines and extinction. In vitro B. dendrobatidis is susceptible to a range of disinfectants, but not all have been tested on animals and some that have been proven effective have harmful side effects on the surrounding environment or the animals being treated. We tested the efficacy of F10SC veterinary disinfectant to treat B. dendrobatidis in experimentally infected tadpole and juvenile Sclerophrys gutturalis and tadpoles of Sclerophrys poweri and Amietia hymenopus. The minimum inhibitory concentration for F10SC on in vitro B. dendrobatidis ranged between 1:7000 for 5-min contact time and 1:10000 for 10-min contact time. Based on the survival data of test animals the no observed effect concentration for 15-min contact time was estimated to be 1:2000 dilution for juveniles, and 1:10000 for tadpoles. In S. gutturalis juveniles an 86% infection clearance rate was achieved after five 15-min doses of 1:3000 dilution. A 100% clearance was achieved in A. hymenopus tadpoles after seven 15-min doses of 1:10000 dilution, and after nine doses of the same treatment in S. poweri tadpoles. F10SC has the benefit of being a concentrated compound that provides a treatment protocol which is nontoxic to tadpoles and post-metamorphic individuals, has a short half-life and is effective against B. dendrobatidis during short contact times, but further testing on different species of amphibians is advised.
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Christie MR, Searle CL. Evolutionary rescue in a host-pathogen system results in coexistence not clearance. Evol Appl 2018; 11:681-693. [PMID: 29875810 PMCID: PMC5979755 DOI: 10.1111/eva.12568] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2017] [Accepted: 10/17/2017] [Indexed: 01/14/2023] Open
Abstract
The evolutionary rescue of host populations may prevent extinction from novel pathogens. However, the conditions that facilitate rapid evolution of hosts, in particular the population variation in host susceptibility, and the effects of host evolution in response to pathogens on population outcomes remain largely unknown. We constructed an individual-based model to determine the relationships between genetic variation in host susceptibility and population persistence in an amphibian-fungal pathogen (Batrachochytrium dendrobatidis) system. We found that host populations can rapidly evolve reduced susceptibility to a novel pathogen and that this rapid evolution led to a 71-fold increase in the likelihood of host-pathogen coexistence. However, the increased rates of coexistence came at a cost to host populations; fewer populations cleared infection, population sizes were depressed, and neutral genetic diversity was lost. Larger adult host population sizes and greater adaptive genetic variation prior to the onset of pathogen introduction led to substantially reduced rates of extinction, suggesting that populations with these characteristics should be prioritized for conservation when species are threatened by novel infectious diseases.
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Bosch J, Fernández-Beaskoetxea S, Garner TWJ, Carrascal LM. Long-term monitoring of an amphibian community after a climate change- and infectious disease-driven species extirpation. GLOBAL CHANGE BIOLOGY 2018; 24:2622-2632. [PMID: 29446515 DOI: 10.1111/gcb.14092] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/06/2017] [Revised: 01/11/2018] [Accepted: 01/26/2018] [Indexed: 05/28/2023]
Abstract
Infectious disease and climate change are considered major threats to biodiversity and act as drivers behind the global amphibian decline. This is, to a large extent, based on short-term studies that are designed to detect the immediate and strongest biodiversity responses to a threatening process. What few long-term studies are available, although typically focused on single species, report outcomes that often diverge significantly from the short-term species responses. Here, we report the results of an 18-year survey of an amphibian community exposed to both climate warming and the emergence of lethal chytridiomycosis. Our study shows that the impacts of infectious disease are ongoing but restricted to two out of nine species that form the community, despite the fact all species can become infected with the fungus. Climate warming appears to be affecting four out of the nine species, but the response of three of these is an increase in abundance. Our study supports a decreasing role of infectious disease on the community, and an increasing and currently positive effect of climate warming. We caution that if the warming trends continue, the net positive effect will turn negative as amphibian breeding habitat becomes unavailable as water bodies dry, a pattern that already may be underway.
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Spitzen-van der Sluijs A, Canessa S, Martel A, Pasmans F. Fragile coexistence of a global chytrid pathogen with amphibian populations is mediated by environment and demography. Proc Biol Sci 2018; 284:rspb.2017.1444. [PMID: 28978729 DOI: 10.1098/rspb.2017.1444] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2017] [Accepted: 08/30/2017] [Indexed: 01/05/2023] Open
Abstract
Unravelling the multiple interacting drivers of host-pathogen coexistence is crucial in understanding how an apparently stable state of endemism may shift towards an epidemic and lead to biodiversity loss. Here, we investigate the apparent coexistence of the global amphibian pathogen Batrachochytrium dendrobatidis (Bd) with Bombina variegata populations in The Netherlands over a 7-year period. We used a multi-season mark-recapture dataset and assessed potential drivers of coexistence (individual condition, environmental mediation and demographic compensation) at the individual and population levels. We show that even in a situation with a clear cost incurred by endemic Bd, population sizes remain largely stable. Current environmental conditions and an over-dispersed pathogen load probably stabilize disease dynamics, but as higher temperatures increase infection probability, changing environmental conditions, for example a climate-change-driven rise in temperature, could unbalance the current fragile host-pathogen equilibrium. Understanding the proximate mechanisms of such environmental mediation and of site-specific differences in infection dynamics can provide vital information for mitigation actions.
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Drawert B, Griesemer M, Petzold LR, Briggs CJ. Using stochastic epidemiological models to evaluate conservation strategies for endangered amphibians. J R Soc Interface 2018; 14:rsif.2017.0480. [PMID: 28855388 PMCID: PMC5582134 DOI: 10.1098/rsif.2017.0480] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2016] [Accepted: 08/07/2017] [Indexed: 01/02/2023] Open
Abstract
Recent outbreaks of chytridiomycosis, the disease of amphibians caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), have contributed to population declines of numerous amphibian species worldwide. The devastating impacts of this disease have led researchers to attempt drastic conservation measures to prevent further extinctions and loss of biodiversity. The conservation measures can be labour-intensive or expensive, and in many cases have been unsuccessful. We developed a mathematical model of Bd outbreaks that includes the effects of demographic stochasticity and within-host fungal load dynamics. We investigated the impacts of one-time treatment conservation strategies during the disease outbreak that occurs following the initial arrival of Bd into a previously uninfected frog population. We found that for all versions of the model, for a large fraction of parameter space, none of the one-time treatment strategies are effective at preventing disease-induced extinction of the amphibian population. Of the strategies considered, treating frogs with antifungal agents to reduce their fungal load had the greatest likelihood of a beneficial outcome and the lowest risk of decreasing the persistence of the frog population, suggesting that this disease mitigation strategy should be prioritized over disinfecting the environment or reducing host density.
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McCoy KA, Peralta AL. Pesticides Could Alter Amphibian Skin Microbiomes and the Effects of Batrachochytrium dendrobatidis. Front Microbiol 2018; 9:748. [PMID: 29731742 PMCID: PMC5919957 DOI: 10.3389/fmicb.2018.00748] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2017] [Accepted: 04/03/2018] [Indexed: 11/13/2022] Open
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Jani AJ, Briggs CJ. Host and Aquatic Environment Shape the Amphibian Skin Microbiome but Effects on Downstream Resistance to the Pathogen Batrachochytrium dendrobatidis Are Variable. Front Microbiol 2018; 9:487. [PMID: 29619014 PMCID: PMC5871691 DOI: 10.3389/fmicb.2018.00487] [Citation(s) in RCA: 50] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2017] [Accepted: 03/01/2018] [Indexed: 01/01/2023] Open
Abstract
Symbiotic microbial communities play key roles in the health and development of their multicellular hosts. Understanding why microbial communities vary among different host species or individuals is an important step toward understanding the diversity and function of the microbiome. The amphibian skin microbiome may affect resistance to the fungal pathogen Batrachochytrium dendrobatidis (Bd). Still, the factors that determine the diversity and composition of the amphibian skin microbiome, and therefore may ultimately contribute to disease resistance, are not well understood. We conducted a two-phase experiment to first test how host and environment shape the amphibian skin microbiome, and then test if the microbiome affects or is affected by Bd infection. Most lab experiments testing assembly of the amphibian skin microbiome so far have compared sterile to non-sterile environments or heavily augmented to non-augmented frogs. A goal of this study was to evaluate, in an experimental setting, realistic potential drivers of microbiome assembly that would be relevant to patterns observed in nature. We tested effects of frog genetic background (2 source populations) and 6 natural lake water sources in shaping the microbiome of the frog Rana sierrae. Water in which frogs were housed affected the microbiome in a manner that partially mimicked patterns observed in natural populations. In particular, frogs housed in water from disease-resistant populations had greater bacterial richness than frogs housed in water from populations that died out due to Bd. However, in the experiment this difference in microbiomes did not lead to differences in host mortality or rates of pathogen load increase. Frog source population also affected the microbiome and, although none of the frogs in this study showed true resistance to infection, host source population had a small effect on the rate of pathogen load increase. This difference in infection trajectories could be due to the observed differences in the microbiome, but could also be due to other traits that differ between frogs from the two populations. In addition to examining effects of the microbiome on Bd, we tested the effect of Bd infection severity on the microbiome. Specifically, we studied a time series of the microbiome over the course of infection to test if the effects of Bd on the microbiome are dependent on Bd infection severity. Although limited to a small subset of frogs, time series analysis suggested that relative abundances of several bacterial phylotypes changed as Bd loads increased through time, indicating that Bd-induced disturbance of the R. sierrae microbiome is not a binary effect but instead is dependent on infection severity. We conclude that both host and aquatic environment help shape the R. sierrae skin microbiome, with links to small changes in disease resistance in some cases, but in this study the effect of Bd on the microbiome was greater than the effect of the microbiome on Bd. Assessment of the microbiome differences between more distantly related populations than those studied here is needed to fully understand the role of the microbiome in resistance to Bd.
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