Diseases of Amphibians

Pharmacokinetics, Withdrawal Time, and Physiological Effects of Single Oral Administration of Enrofloxacin in Dybowski's Frog (Rana dybowskii)

Background: As a broad-spectrum fluoroquinolone, enrofloxacin (ENR) is commonly employed to manage bacterial infections in aquatic species. Nevertheless, there have been no documented pharmacokinetic and residue studies conducted on Dybowski's frog (Rana dybowskii). Therefore, the objective of our study was to characterize the pharmacokinetics (PK) of ENR and its metabolite ciprofloxacin (CIP) in R. dybowskii, establish withdrawal times, and evaluate the physiological effects associated with ENR administration. Methods: Adult Rana dybowskii (120 individuals; 60 males and 60 females) were sex-separated and acclimated in four tanks. Prior to dosing, three males and three females were randomly selected as untreated controls (without ENR administration). Following the oral gavage of ENR (10 mg/kg), blood, liver, and kidney tissues were collected at 0.25, 0.5, 1, 1.5, 2, 4, 6, 8, 12, 24, 36, 48, and 72 h (n = 6) for pharmacokinetic analysis. Muscle and oviduct tissues were additionally sampled at 1, 3, 7, 15, and 30 days post-dose (n = 6) for ENR content determination. Serum/tissue ENR concentrations were measured via Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) and analyzed using a non-compartmental model (WinNonLin 6.1 software) to calculate PK parameters including peak time (Tmax), peak concentration (Cmax), and area under the curve (AUC0-t). In studying the physiology effects of ENR administration, biochemical enzyme activities and gene expressions in the liver and intestine were assessed post-ENR administration. Results: ENR demonstrated rapid absorption and extensive distribution in R. dybowskii. The withdrawal periods were determined to be over 33 days for females and 34 days for males in R. dybowskii. Following ENR administration, there was an increase in immune enzymes (AKP (alkaline phosphatase) and ACP (acid phosphatase)) as well as glycolytic enzymes (HK (hexokinase), PK (pyruvate kinase), PFK (phosphofructokinase)). Antioxidant enzyme levels, specifically SOD (superoxide dismutase) and CAT (catalase), peaked at 1.5 h post-ENR administration but subsequently declined by the 8 h mark. Additionally, following ENR treatment, IGF1, PI3K, and Akt exhibited up-regulation, whereas Keap1 and GYS1 showed down-regulation. Conclusions: The administration of ENR at a dosage of 10 mg/kg significantly enhances the activities of AKP and ACP, promotes glycolysis, and activates the Keap1/Nrf2 and PI3K-Akt signaling pathways in R. dybowskii. These findings establish a foundation for the rational application of ENR and the determination of withdrawal times in frog aquaculture.

Spontaneous hepatocellular and gastric tumours in African clawed frogs (Xenopus laevis)

Three seven-year-old African clawed frogs (Xenopus laevis) from an animal research facility showed weight loss, coelomic distention and an abnormal swimming gait were euthanised on welfare grounds. A complete necropsy of each animal showed a focal, firm, dark brown to green mass with multifocal haemorrhages in the left liver lobe in two animals and a transmural, firm, beige, multilobulated gastric mass in another animal. Additionally, one of the frogs with the hepatic mass had haemocoeloma. Histologically, the hepatic masses were diagnosed as hepatocellular adenomas and the gastric mass as a gastric carcinoma with trans-coelomic metastases. The three tumours were immunophenotyped using the following antibodies: cytokeratin AE1/AE3, vimentin, E-cadherin, P53 and Ki67 and, additionally, for the hepatic tumours only, synaptophysin, Prox-1, S100 and Sox-9. Masson's trichrome, Periodic acid Schiff and Gram stains was also performed in selected cases. One of the hepatocellular adenomas was weakly positively labelled with E-cadherin whereas the other showed variable positivity for Sox-9 only. Vimentin labelled the stroma and sinusoidal endothelia. Interestingly in the liver, the cytokeratin AE1/AE3 labelling was restricted to the biliary epithelium and sinusoidal endothelia. The gastric carcinoma labelled positively with cytokeratin AE1/AE3 only. This report aims to guide laboratory animal veterinarians to accurately diagnose multi-organ masses in amphibians. To the authors' knowledge, this is the first comprehensive morphological study on a case series of hepatocellular and gastric neoplasia in X. laevis.

Identification of enterobacteriaceae causing septicemia in the axolotl Ambystoma mexicanum

The Ambystoma mexicanum axolotl is a highly threatened amphibian and a valuable research model, with very little information about bacterial diseases affecting it. The aim of this study was to perform an identification of bacteria responsible for septicemia in three individuals. For all of them, necropsies were made, bacteria classification was performed by traditional and DNA-based molecular methods and tissues were histologically examined. All animals showed edema and ascites, and other tissues such as the lungs, spleen, liver, and kidney were also affected, dermatitis also occurred, in one case, the dermatitis was severe. Two bacterial isolates showed genetic identities of 99% with Aeromonas veronii, one with Citrobacter freundii, and another with 100% identity with Citrobacter portucalensis. These and other Enterobacteriaceae species of Aeromonas genus have been reported to produce septicemia in Anura amphibians and fish, pointing out that they are a health hazard for aquatic animals. Future endeavors to determine these bacteria prevalence, the search for antibiotic resistance/susceptibility, factors that can trigger the pathology, and the development of early diagnostic tools should be done to improve our understanding.

Primary regulatory T cell activator FOXP3 is present across Amphibia

The overall structure of the immune system is highly conserved across jawed vertebrates, but characterization and description of the immune system is heavily biased toward mammals. One arm of the vertebrate immune system, the adaptive immune system, mounts pathogen-specific responses that tend to be robust and effective at clearing pathogens. This system requires selection against self-recognition and modulation of the immune response. One of the mechanisms of immune modulation is the presence of regulatory T cells that suppress other effector immune cells. Regulatory T cells and their primary activator forkhead box protein P3 (FOXP3) have been well characterized in mammalian models but unexplored in most other vertebrate taxa. Amphibians are a good focal group for the characterization of FOXP3 due to their phylogenetic position on the vertebrate tree of life, and their susceptibility to emerging pathogens. In this study, we mined available transcriptomic and genomic data to confirm the presence of FOXP3 across the amphibian tree of life. We find that FOXP3 is present in all major clades of amphibians. We also test whether selection on FOXP3 shows signatures of intensification among the three main clades of amphibians, which may reflect shifts in the stringency of natural selection on this gene. Our findings provide insights into the evolutionary history of the vertebrate immune system and confirm the conservation of vertebrate immune genes within amphibians.

Morphological evidence for a new species of the Genus Cosmocercoides Wilkie, 1930 (Ascaridida: Cosmocercidae) from Iranian toads (Bufotes surdus)

BACKGROUND/OBJECTIVE

There is little data on infections of the genus Cosmocercoides Wilkie, 1930 in toad populations in Iran. Hence, the present study aims to report the first morphological description of the new genus Cosmocercoides Wilkie, 1930 (Ascaridida: Cosmocercidae) in Iranian toads (Bufotes surdus).

METHODS

During the helminthological survey from October 2023 to December 2023, 10 specimens of Bufotes surdus were collected. They were transferred to the parasitological laboratory of Ardakan University, where they were euthanized and necropsied, and parasites were isolated from the toad's large intestine. Nematodes were washed in physiological saline and then fixed and stored in 80% ethanol until studied. Moreover, for light microscopic studies, nematodes were washed in lactophenol, and the morphological and morphometric parameters of male and female nematodes, including total length, esophagus, spicule, and eggs, were evaluated.

RESULTS

The results showed that the total length of male worms was 2.83 (2.45-3.17) mm on average, and the length of the esophagus was 632.67 (631.50-634) µm. Additionally, the spicule of the male nematode was 381 (354-426) µm on average, and the eggs of female worms were 72 (59-85) × 47 (42-53) µm on average. The morphometric parameters indicate the biological differences of worms in this region compared to other countries and indicate local biodiversity.

CONCLUSION

In conclusion, the morphological and morphometric parameters evidence reported in the present study supports identifying a new species within the genus Cosmocercoides, based on specimens collected from Bufotes surdus.

Molecular detection of Batrachochytrium dendrobatidis (Chytridiomycota) and culturable skin bacteria associated with three critically endangered species of Atelopus (Anura: Bufonidae) in Ecuador

Chytridiomycosis is a fungal disease responsible for massive amphibian die-offs worldwide, caused by the fungus Batrachochytrium dendrobatidis (Bd). Potential symbiotic relationships between frogs and the bacteria residing on their skin-referred to as skin-bacteria-may inhibit Bd growth, aiding in resistance to this lethal disease. This research had three main objectives: (1) to detect the presence of Bd in native populations of Atelopus balios, A. bomolochos, and A. nanay in the central Andes and coastal southern regions of Ecuador; (2) to identify the culturable skin-bacteria; and (3) to analyze differences among the bacterial communities in the three Atelopus species studied. Skin swabs were collected from two populations of A. balios (107-203 m a.s.l.) and one population each of A. bomolochos and A. nanay (3,064-3,800 m a.s.l.). These swabs served two purposes: first, to detect Bd using conventional PCR; and second, to isolate culturable bacteria, which were characterized through DNA sequencing, molecular phylogeny, and community composition similarity analysis (Jaccard index). Results showed that Bd was present in all species, with positive Bd PCR amplification found in 11 of the 12 sampled amphibians. The culturable skin-bacteria were classified into 10 genera: Pseudomonas (31.4%), Stenotrophomonas (14.3%), Acinetobacter (11.4%), Serratia (11.4%), Aeromonas (5.7%), Brucella (5.7%), Klebsiella (5.7%), Microbacterium (5.7%), Rhodococcus (5.7%), and Lelliottia (2.9%). The Jaccard index revealed that bacterial genera were least similar in A. bomolochos and A. balios (J = 0.10), while the highest similarity at the genus level was between A. bomolochos and A. nanay (J = 0.33). At the clade-species level, only A. bomolochos and A. nanay show common bacteria (J = 0.13). Culturable bacterial communities of specimens diagnosed as Bd positive (n = 10) or Bd negative (n = 1) share a J value of 0.1 at genus and 0.04 at species-clade level. The prevalence of Bd and the composition of cutaneous bacteria could be influenced by Bd reservoirs, Atelopus biology, and intrinsic environmental conditions. This research contributes to understanding the relationship between endangered Andean species and Bd, and explores the potential use of native skin-bacteria as biocontrol agents against Bd.

The Influence of Environmental Conditions and Coinfection by Blood-Feeding Parasites on Red Blood Cell Physiology of an Ectothermic Host

AbstractVector-borne blood parasites cause myriad sublethal effects and can even be deadly to endotherms, but far less is known about their impacts on ectothermic hosts. Moreover, the pathologies documented in endotherms are generally linked to infection by blood parasites rather than by their vectors. Here, we measured hematocrit, hemoglobin, and relative proportions of immature red blood cells to evaluate the physiological effects of two blood-feeding parasites and coinfection on ectothermic hosts, differentiating among pathological responses, extrinsic factors, and natural variations. We investigated a population of wild eastern hellbender salamanders (Cryptobranchus alleganiensis), which harbor leeches (Placobdella appalachiensis) that transmit blood parasites (Trypanosoma spp.) to their hosts, often resulting in coinfection. We observed seasonal changes in host hematology corresponding to water temperature and demonstrated their ability to modulate hematological parameters in response to acute stress. We reveal seasonal relationships between parasite dynamics and host physiology, in which peak parasitemia occurred when hosts had seasonally high hematocrit and hemoglobin concentrations. We found that coinfected individuals expressed symptoms of anemia, including a regenerative response to depletion of their red blood cells. We also documented a more pronounced pathological response to leech vectors than to the trypanosomes they transmit. Our research underscores the complex interactions between host physiology, multiple parasites, and environmental factors and highlights the pathologies associated with the vector in coinfections. Given the contributions of climate change and disease in the rapid global decline of ectotherms such as amphibians, our study provides timely foundational insights into multiple factors that influence their red blood cell physiology.

Isolation of Naegleria lustrarea n. sp. (Excavata, Discoba, Heterolobosea) from the feces of Ambystoma annulatum (Ringed Salamander) in Northwest Arkansas

The salamander, Ambystoma annulatum, is considered a "species of special concern" in the state of Arkansas, USA, due to its limited geographic range, specialized habitat requirements and low population size. Although metazoan parasites have been documented in this salamander species, neither its native protists nor microbiome have yet been evaluated. This is likely due to the elusive nature and under-sampling of the animal. Here, we initiate the cataloguing of microbial associates with the identification of a new heterlobosean species, Naegleria lustrarea n. sp. (Excavata, Discoba, Heterolobosea), isolated from feces of an adult A. annulatum.

New Data on the Larval Stages of Leptophallus nigrovenosus (Digenea, Plagiorchiata)

(1) Background: Leptophallus nigrovenosus, an esophageal parasite that primarily affects water snakes of the genus Natrix, has a known life cycle that involves snail and amphibian hosts. However, the biological aspects, chaetotaxic patterns, and pathogeny of this parasite in its hosts have not been fully elucidated. (2) Methods: Snails (Planorbarius metidjensis) were collected in Spain and examined for cercaria emergence. The larvae were used to experimentally infect Salamandra salamandra, and metacercariae were isolated. Their chaetotaxy was studied using microscopy and scanning electron microscopy. The eye histology was also examined. (3) Results: The cercariae displayed distinctive morphological characteristics. The results of this study revealed three types of ciliated sensory papillae on the cercarial teguments, suggesting an adaptation for host detection and orientation. The metacercariae isolated from subcutaneous tissues showed oval bodies covered in spines. The chaetotaxy patterns matched those of Leptophallinae species. This is the first report of the presence of L. nigrovenosus in the snail P. metidjensis. Additionally, this study detected metacercariae in the eyes of S. salamandra, emphasizing the need for further research on trematode infections in amphibian eyes. (4) Conclusions: Members of the genus Salamandra can serve as secondary intermediate hosts for L. nigrovenosus, and the presence of metacercariae in amphibian eyes may have implications for the survival and habitat management of these amphibians. Understanding this parasite's prevalence, transmission dynamics, and impacts on host populations is crucial for conservation strategies.

Strobilurin fungicide increases the susceptibility of amphibian larvae to trematode infections

The global rise in fungal pathogens has driven the increased usage of fungicides, yet our understanding of their ecotoxicity remains largely limited to acute toxicity. While such data is critical for projecting the risk of fungicide exposure to individual species, the contamination of natural systems with fungicides also has the potential to alter species interactions within communities including host-parasite relationships. We examined the effects of the fungicide pyraclostrobin on the susceptibility of larval American bullfrogs (Rana catesbeiana) to trematode (echinostome) infections using a controlled laboratory experiment. Following a 2-wk exposure to 0, 1.0, 5.2, or 8.4 µg/L of pyraclostrobin, tadpoles were then exposed to parasites either in the 1) presence (continued/simultaneous exposure) or 2) absence (fungicide-free water) of pyraclostrobin. We found that when exposed to pyraclostrobin during parasite exposure, meta cercariae counts increased 4 to 8 times compared to control tadpoles. Additionally, parasite loads were approximately 2 times higher in tadpoles with continued fungicide exposures compared to tadpoles that were moved to fresh water following fungicide exposure. This research demonstrates that fungicides at environmentally relevant concentrations can indirectly alter host-parasite interactions, which could elevate disease risk. It also underscores the need for studies that expand beyond traditional toxicity experiments to assess the potential community and ecosystem-level implications of environmental contaminants.

Immunohistochemical Characterization of Langerhans Cells in the Skin of Three Amphibian Species

The amphibian taxon includes three orders that present different morphological characteristics: Anura, Caudata, and Apoda. Their skin has a crucial role: it acts as an immune organ constituting a physical, chemical, immunological, and microbiological barrier to pathogen insult and conducts essential physiological processes. Amphibians have developed specialized features to protect the vulnerable skin barrier, including a glandular network beneath the skin surface that can produce antimicrobial and toxic substances, thus contributing to the defense against pathogens and predators. This study aims to characterize Langerhans cells in the skin of Lithobates catesbeianus (order: Anura; Shaw, 1802), Amphiuma means (order: Caudata; Garden, 1821), and Typhlonectes natans (order: Apoda; Fischer, 1880) with the following antibodies: Langerin/CD207 (c-type lectin), Major Histocompatibility Complex (MHC)II, and Toll-like receptor (TLR)2 (expressed by different types of DCs). Our results showed Langerhans cells positive for Langerin CD/207 in the epidermis of the three species; moreover, some antigen-presenting cells (APCs) in the connective tissue expressed TLR2 and MHCII. The distribution of the Langerhans cells is very similar in the three amphibians examined, despite their different habitats. A greater knowledge of the amphibian immune system could be useful to better understand the phylogeny of vertebrates and to safeguard amphibians from population declines. Furthermore, the similarities between amphibians' and human skin concerning immunological features may be useful in both biology and translational medicine.

Diverging effects of host density and richness across biological scales drive diversity-disease outcomes

Understanding how biodiversity affects pathogen transmission remains an unresolved question due to the challenges in testing potential mechanisms in natural systems and how these mechanisms vary across biological scales. By quantifying transmission of an entire guild of parasites (larval trematodes) within 902 amphibian host communities, we show that the community-level drivers of infection depend critically on biological scale. At the individual host scale, increases in host richness led to fewer parasites per host for all parasite taxa, with no effect of host or predator densities. At the host community scale, however, the inhibitory effects of richness were counteracted by associated increases in total host density, leading to no overall change in parasite densities. Mechanistically, we find that while average host competence declined with increasing host richness, total community competence remained stable due to additive assembly patterns. These results help reconcile disease-diversity debates by empirically disentangling the roles of alternative ecological drivers of parasite transmission and how such effects depend on biological scale.

The effects of preformed vitamin A and provitamin A carotenoid supplementation on tadpoles of the poison frog Phyllobates vittatus

Understanding the nutritional requirements of captive animals is necessary for proper animal husbandry, however, the specific dietary requirements for many amphibian species commonly kept in captivity are unknown. Like most vertebrates, frogs cannot synthesize carotenoids and must therefore obtain these essential nutrients through diet. It is unclear if amphibians can cleave provitamin A carotenoids to form vitamin A metabolically within the body, so common practice is to supplement their captive diets with both preformed vitamin A and provitamin A carotenoids. We carried out a feeding experiment in tadpoles of Phyllobates vittatus, a commonly kept poison frog species, to test the effects of supplementing a fish flake diet with a provitamin A carotenoid (2.5 mg/g β-carotene) and vitamin A (0.033-0.066 µg/mL retinyl acetate), both individually and in combination. Contrary to our expectations, supplementation had either no effect or adverse effects on tadpole growth and survivorship. Tadpoles reared under supplemented diets with vitamin A showed higher mortality rates, coupled with symptoms of hypervitaminosis A. Survivors had a smaller body size and mass at metamorphosis. β-carotene supplementation alone had no detectable effect. The vitamin A and β-carotene levels in our supplemented diet have been shown to be harmless or benefit tadpoles of other species, yet our results indicate that adding these amounts to what is found in a generalist fish flake mix can have detrimental effects on P. vittatus tadpoles. More broadly, this study highlights the importance of creating husbandry guidelines based on the specific physiological needs of the species (or species groups) being kept in captivity, rather than general ones for all amphibians, as is often done.

Anti-Trypanosomal Bufadienolides from the Oocytes of the Toad Rhinella alata (Anura, Bufonidae)

Amphibians are widely known as a prolific source of bioactive metabolites. In this work, we isolated and characterized compounds with antiparasitic activity from the oocytes of the toad Rhinella alata collected in Panama. Bio-guided isolation and structural elucidation were carried out using chromatographic and spectroscopic techniques, respectively. The organic extract was subjected to solid phase extraction followed by HPLC purification of the fraction with in vitro activity against Trypanosoma cruzi trypomastigotes. Seven steroids (1-7) of the bufadienolide family were isolated, and their structures were determined using NMR and MS analyses; of these 19-formyl-dyscinobufotalin, (3) is reported as a new natural product. Compounds 1 and 3-7 resulted in a good anti-trypanosomal activity profile. Among these, 16β-hydroxyl-hellebrigenin (1) and bufalin (7) showed significant selectivity values of >5 and 2.69, respectively, while the positive control benznidazole showed a selectivity of 18.81. Furthermore, molecular docking analysis showed compounds 1, 3 and 7 interact through H-bonds with the amino acid residues GLN-19, ASP-158, HIS-159 and TRP-177 from cruzipain at the catalytic site. Given the lack of therapeutic options to treat American trypanosomiasis, this work can serve as the basis for further studies that aim for the development of bufadienolides or their derivatives as drugs against Chagas disease.

Mr. Toad's Wild Fungi: Fungal Isolate Diversity on Colorado Boreal Toads and their Capacity for Pathogen Inhibition

The amphibian skin pathogen Batrachochytrium dendrobatidis (Bd) has caused an ongoing biodiversity crisis, including in the locally endangered Colorado boreal toad (Anaxyrus boreas boreas). Although researchers have investigated the bacteria living on amphibian skin and how they interact with Bd, there is less information about fungal community members. This study describes (1) the diversity of culturable fungi from boreal toad skin, (2) which subset of these isolates is Bd-inhibitory, and (3) how Bd affects these isolates' growth and morphology. Most isolates were from the orders Capnodiales, Helotiales, and Pleosporales. Of 16 isolates tested for Bd-inhibition, two from the genus Neobulgaria and three from Pseudeurotium inhibited Bd. Fungal growth in co-culture with Bd varied with weak statistical support for Neobulgaria sp. (isolate BTF_36) and cf Psychrophila (isolate BTF_60) (p-values = 0.076 and 0.092, respectively). Fungal morphology remained unchanged in co-culture with Bd, however, these results could be attributed to low replication per isolate. Nonetheless, two fungal isolates' growth may have been affected by Bd, implying that fungal growth changes in Bd co-culture could be a variable worth measuring in the future (with higher replication). These findings add to the sparse but growing literature on amphibian-associated fungi and suggest further study may uncover the relevance of fungi to amphibian health and Bd infection.

Hemoparasites Do Not Affect Life-History Traits and Cellular Immune Response in Treefrog Hosts Boana cordobae

We provide the first evidence for hemoparasites in the endemic Cordoba treefrog Boana cordobae. We collected 37 adult frogs at 1200 m a.s.l. in the Comechingones Mountains in the Córdoba province (Argentina). Each individual was sexed, then snout-vent length and body mass were recorded, a toe was collected for skeletochronological age determination, and a slide with a blood smear was prepared for hemoparasite screening, before releasing the frogs in situ. A total of 81% (n = 30) of the frogs were infected by hemogregarines and trypanosomes with a high intensity of infections. Dactylosoma was found for the first time in Argentina. Hemoparasites had no significant effect on the leukocyte profile, which we assessed from the May-Grünwald-Giemsa-stained blood smears. The neutrophils/lymphocytes ratio, indicative of stress, was insignificantly higher (0.06) in parasitized frogs than in parasite-free individuals (0.04). Infected frogs were larger than the controls, but this effect vanished when correcting size data for age. Young frogs (first-breeders) dominated the age distribution of parasite-free individuals, suggesting that infection of frogs takes usually place after sexual maturation. Vectors transmitting hemoparasites to B. cordobae remain to be identified. We demonstrate that moderate to high intensities of hemoparasites do not significantly affect the cellular immune response of B. cordobae, or any of the life-history traits studied, nor did they show any external sign of disease.

Human Disturbance Increases Health Risks to Golden Snub-Nosed Monkeys and the Transfer Risk of Pathogenic Antibiotic-Resistant Bacteria from Golden Snub-Nosed Monkeys to Humans

From the perspective of interactions in the human-animal-ecosystem, the study and control of pathogenic bacteria that can cause disease in animals and humans is the core content of "One Health". In order to test the effect of human disturbance (HD) on the health risk of pathogenic antibiotic-resistant bacteria (PARBs) to wild animals and transfer risk of the PARBs from wild animals to humans, golden snub-nosed monkeys (Rhinopithecus roxellana) were used as sentinel animals. Metagenomic analysis was used to analyze the characteristics of PARBs in the gut microbiota of golden snub-nosed monkeys. Then, the total contribution of antibiotic resistance genes (ARGs) and virulence factors (VFs) of the PARBs were used to assess the health risk of PARBs to golden snub-nosed monkeys, and the antimicrobial drug resistance and bacterial infectious disease of PARBs were determined to assess the transfer risk of PARBs from golden snub-nosed monkeys to humans. There were 18 and 5 kinds of PARBs in the gut microbiota of golden snub-nosed monkeys under HD (HD group) and wild habitat environments (W group), respectively. The total health risks of PARBs to the W group and the HD group were -28.5 × 10-3 and 125.8 × 10-3, respectively. There were 12 and 16 kinds of KEGG pathways of human diseases in the PARBs of the W group and the HD group, respectively, and the gene numbers of KEGG pathways in the HD group were higher than those in the W group. HD increased the pathogenicity of PARBs to golden snub-nosed monkeys, and the PARBs in golden snub-nosed monkeys exhibited resistance to lincosamide, aminoglycoside, and streptogramin antibiotics. If these PARBs transfer from golden snub-nosed monkeys to humans, then humans may acquire symptoms of pathogens including Tubercle bacillus, Staphylococcus, Streptococcus, Yersinia, Pertussis, and Vibrio cholera.

Metabolites of Xenorhabdus bacteria are potent candidates for mitigating amphibian chytridiomycosis

Chytridiomycosis, caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd), has caused extreme losses in amphibian biodiversity. Finding bacteria that produce metabolites with antifungal properties may turn out to be invaluable in the fight against this devastating disease. The entomopathogenic bacteria, Xenorhabdus szentirmaii and X. budapestensis produce secondary metabolites that are effective against a wide range of fungal plant pathogens. To assess whether they may also be effective against Bd, we extracted cell-free culture media (CFCM) from liquid cultures of X. szentirmaii and X. budapestensis and tested their ability to inhibit Bd growth in vitro. As a second step, using juvenile common toads (Bufo bufo) experimentally infected with Bd we also tested the in vivo antifungal efficacy of X. szentirmaii CFCM diluted to 2 and 10% (v/v), while also assessing possible malign side effects on amphibians. Results of the in vitro experiment documented highly effective growth inhibition by CFCMs of both Xenorhabdus species. The in vivo experiment showed that treatment with CFCM of X. szentirmaii applied at a dilution of 10% resulted in infection intensities reduced by ca. 73% compared to controls and to juvenile toads treated with CFCM applied at a dilution of 2%. At the same time, we detected no negative side effects of treatment with CFCM on toad survival and development. Our results clearly support the idea that metabolites of X. szentirmaii, and perhaps of several other Xenorhabdus species as well, may prove highly useful for the treatment of Bd infected amphibians.

Pathogenic selection promotes adaptive immune variations against serious bottlenecks in early invasions of bullfrogs

Adaptive genetic variations are key for understanding evolutionary processes influencing invasions. However, we have limited knowledge on how adaptive genetic diversity in invasive species responds to new pathogenic environments. Here, we compared variations in immune major histocompatibility complex (MHC) class-II β gene and neutral loci in relation to pathogenic chytrid fungus (Batrachochytrium dendrobatidis, Bd) infection across invasive and native populations of American bullfrog between China and United States (US). Chinese invasive populations show a 60% reduction in neutral cytb variations relative to US native populations, and there were similar MHC variation and functional diversity between them. One MHC allele private to China was under recent positive selection and associated with decreased Bd infection, partly explaining the lower Bd prevalence for Chinese populations than for native US populations. These results suggest that pathogen-mediated selection favors adaptive MHC variations and functional diversity maintenance against serious bottlenecks during the early invasions (within 15 generations) of bullfrogs.

Introduction to the special issue Amphibian immunity: stress, disease and ecoimmunology

Amphibian populations have been declining worldwide, with global climate changes and infectious diseases being among the primary causes of this scenario. Infectious diseases are among the primary drivers of amphibian declines, including ranavirosis and chytridiomycosis, which have gained more attention lately. While some amphibian populations are led to extinction, others are disease-resistant. Although the host's immune system plays a major role in disease resistance, little is known about the immune mechanisms underlying amphibian disease resistance and host-pathogen interactions. As ectotherms, amphibians are directly subjected to changes in temperature and rainfall, which modulate stress-related physiology, including immunity and pathogen physiology associated with diseases. In this sense, the contexts of stress, disease and ecoimmunology are essential for a better understanding of amphibian immunity. This issue brings details about the ontogeny of the amphibian immune system, including crucial aspects of innate and adaptive immunity and how ontogeny can influence amphibian disease resistance. In addition, the papers in the issue demonstrate an integrated view of the amphibian immune system associated with the influence of stress on immune-endocrine interactions. The collective body of research presented herein can provide valuable insights into the mechanisms underlying disease outcomes in natural populations, particularly in the context of changing environmental conditions. These findings may ultimately enhance our ability to forecast effective conservation strategies for amphibian populations. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Conserved Evolution of MHC Supertypes among Japanese Frogs Suggests Selection for Bd Resistance

The chytrid fungus Batrachochytrium dendrobatidis (Bd) is a major threat to amphibians, yet there are no reports of major disease impacts in East Asian frogs. Genetic variation of the major histocompatibility complex (MHC) has been associated with resistance to Bd in frogs from East Asia and worldwide. Using transcriptomic data collated from 11 Japanese frog species (one individual per species), we isolated MHC class I and IIb sequences and validated using molecular cloning. We then compared MHC from Japanese frogs and other species worldwide, with varying Bd susceptibility. Supertyping analysis, which groups MHC alleles based on physicochemical properties of peptide binding sites, identified that all examined East Asian frogs contained at least one MHC-IIb allele belonging to supertype ST-1. This indicates that, despite the large divergence times between some Japanese frogs (up to 145 million years), particular functional properties in the peptide binding sites of MHC-II are conserved among East Asian frogs. Furthermore, preliminary analysis using NetMHCIIpan-4.0, which predicts potential Bd-peptide binding ability, suggests that MHC-IIb ST-1 and ST-2 have higher overall peptide binding ability than other supertypes, irrespective of whether the peptides are derived from Bd, other fungi, or bacteria. Our findings suggest that MHC-IIb among East Asian frogs may have co-evolved under the same selective pressure. Given that Bd originated in this region, it may be a major driver of MHC evolution in East Asian frogs.

Genomic insights into local adaptation in the Asiatic toad Bufo gargarizans, and its genomic offset to climate warming

Genomic signatures of local adaptation have been identified in many species but remain sparsely studied in amphibians. Here, we explored genome-wide divergence within the Asiatic toad, Bufo gargarizans, to study local adaptation and genomic offset (i.e., the mismatch between current and future genotype-environment relationships) under climate warming scenarios. We obtained high-quality SNP data for 94 Asiatic toads from 21 populations in China to study spatial patterns of genomic variation, local adaptation, and genomic offset to warming in this wide-ranging species. Population structure and genetic diversity analysis based on high-quality SNPs revealed three clusters of B. gargarizans in the western, central-eastern, and northeastern portions of the species' range in China. Populations generally dispersed along two migration routes, one from the west to the central-east and one from the central-east to the northeast. Both genetic diversity and pairwise F ST were climatically correlated, and pairwise F ST was also correlated with geographic distance. Spatial genomic patterns in B. gargarizans were determined by the local environment and geographic distance. Global warming will increase the extirpation risk of B. gargarizans.

Proliferative parathyroid lesions in captive-bred American bullfrogs (Lithobates catesbeianus) with metabolic bone disease

Parathyroid gland lesions in anurans are infrequently reported and most often occur secondary to experimental interventions. Husbandry-related parathyroid changes have not been documented in this order of Amphibia. Three American bullfrogs (Lithobates catesbeianus) living in a captive colony were euthanized due to clinical concern for metabolic bone disease secondary to lack of consistent dietary supplementation with vitamin D₃. Necropsy revealed cystic dilation and variable proliferation of unidentified structures within the cranial coelom corresponding to the anatomical location of anuran parathyroid glands. Histologically, the structures consisted of sheets and whorls of elongated cells. Immunohistochemistry for pan-cytokeratin revealed strong cytoplasmic staining and Grimelius staining identified neuroendocrine granules in the elongated cells of these structures, supportive of a parathyroid origin.

Interdisciplinary approach to solve unusual mortalities in the European common frog (Rana temporaria) in two high-mountain ponds affected by climate change

The global decline in amphibian populations is a major environmental issue. Chytridiomycosis, Ranaviruses and the red-leg syndrome have been identified in unusual mortality events. However, these infections do not account for all causes of declining amphibian populations. Moreover, several cases of amphibian mortality are difficult to solve without resorting to an interdisciplinary approach. Two cases of unusual mortality in Rana temporaria occurred at two high-mountain ponds (northwest Italy) in April and May 2021. Water and frog samples were analysed to understand the possible causes responsible for the unusual mortalities. Results of the main physicochemical (pH, conductivity, dissolved oxygen, chemical and biochemical oxygen demand) and nutrient (ammonia/ammonium, nitrite, nitrate, total phosphorus) parameters revealed a good condition of the water quality, with the absence of the main cyanotoxins (microcystins/nodularins). However, unseasonably high spring water temperatures were recorded in both ponds (12.73 °C and 14.21 °C for Frog Pond and Selleries Pond, respectively). Frogs (n = 50; snout-vent length: 7.0-9.8 cm; body mass: 85-123 g) collected from Frog Pond mainly presented bumps on the ventral cavity and dermal ulceration associated with the isolation of Carnobacterium maltaromaticum. On the other hand, frogs (n = 5; snout-vent length: 8.0-9.1 cm; body mass: 87-92 g) from Selleries Pond presented petechiae and dermal ulcerations on the rear limbs associated with the isolation of Aeromonas salmonicida and A. sobria. In both mortality events, the interdisciplinary approach revealed an association between frog mortalities and the isolation of bacteria. Isolated bacteria are considered opportunistic pathogens, and the high values of the water temperature has certainly led a stress on the frogs, favouring the spread of bacteria and the death of the frogs. Further studies are needed to assess the pathophysiological effects of the opportunistic bacteria here isolated, clarifying the interactions between emerging pathogens and climate change.

Vitamin A, Vitamin E, and Carotenoid Concentrations in Livers of Marine Toads (Rhinella marina)

Global amphibian population decline is a major concern. Therefore, conservation centers have become increasingly needed for population sustainability breeding programs. This need makes proper nutrition programs while in human care imperative. The specific nutritional focus of this current research was to analyze vitamin A, vitamin E, and carotenoid (apocarotenoid, β-carotene, β-carotene ester, β-cryptoxanthin, lutein, lutein ester, zeaxanthin, and zeaxanthin ester) concentrations in the liver of 66 free-range marine toads (Rhinella marina) over a two-month period after entering human management. Toads were fed supplemented crickets randomly assigned to one of two diets: Diet 1 consisted of brown house crickets (Acheta domestica) gut loaded with Mazuri® Cricket Diet 5M38 and small amounts of sweet potato and carrots; Diet 2 consisted of an identical diet with the gut loaded crickets additionally dusted with Repashy® Superfoods Vitamin A Plus. Ten toads were euthanized prior to human management on Day 0 to assess baseline liver nutrients. Seven toads consuming each of the two diets (14 toads total) were euthanized on Days 9, 15, 32, and 62. Regardless of diet treatment, there were decreases (p < 0.05) in all the analyzed nutrient concentrations over the 62-day human management period. The results from this study indicate that higher dietary vitamin A, vitamin E, and carotenoid content may be needed and/or the cricket gut loading and dusting techniques used to feed the amphibians food source insects may be inadequate.

Hematologic variables of free-living Leptodactylus luctator with and without hemoparasites and thrombidiform mites in southern Brazil

It has been suggested that anuran amphibian parasites can cause clinical signs in situations of environmental imbalance. In the family Leptodactylidae, information about hematology is scarce, although these are well-known tools for the diagnosis and prognosis in clinical practice and potential bioindicators of environmental stress. The objective of this study is to describe Leptodactylus luctator hematology, to report the occurrence of hemoparasites and thrombidiform mites, and to compare the hematological variables under the presence and absence of these organisms. Ectoparasites and heparinized blood samples from 40 free-living specimens of L. luctator were collected for analysis. Hematologic variables and total plasma protein were compared between groups with and without hemoparasites and intradermal mites. As results, structures compatible with hemogregarines, Lankesterella sp., five morphotypes of Trypanosoma spp., microfilaria, Aegyptianella sp., an unidentified intraleukocytic hemoparasite, and frog erythrocytic virus (FEV) inclusion bodies were identified in the blood samples, besides Hannemania spp. intradermal mites. The hemoparasite occurrence was higher than previously reported in other anuran families and locations. Also, L. luctator has smaller red blood cells (RBCs) and white blood cells (WBCs), and a hyposegmentation of the neutrophil nucleus, when compared to many other amphibians. White blood cell, neutrophil, and monocyte counts were higher in animals parasitized by mites. There was no correlation between the number of parasitized RBCs and hematologic variables. This study provides anuran hematologic information, in addition to indicating a host reaction to infestation by Hannemania spp. mites, besides constituting the first record of the distribution of hemoparasites and intradermal mites in L. luctator of the study region.

Neglected skin-associated microbial communities: a unique immune defense strategy of Bufo raddei under environmental heavy metal pollution

Amphibians defend against pathogens using skin microbial communities, in addition to innate and adaptive immunity. Despite skin microbial communities play a key role in the immune function of amphibians, few studies have focused on the changes in its composition and function. In the present study, we identified the variation in adaptive immunity, as well as the corresponding changes in skin microbiome of Bufo raddei living in a heavy metal polluted area. The adaptive immunity of B. raddei in heavy metal polluted area was significantly lower than that in relatively unpolluted area. Further, different skin bacterial communities were found in the two areas. In the heavy metal polluted area, Actinobacteria and Microbacterium were the dominant bacteria in the skin microbiome of B. raddei, which showed broad-spectrum antibacterial activity. Besides, the antibiotic synthesis was also increased in metabolic pathways. The present study suggested that the adaptive immunity of B. raddei was weakened under long-term heavy metal stress. However, the toads increased the abundance of bacteriostatic bacteria by regulating the composition of skin microbiome, which released a large number of bacteriostatic metabolites and enhanced the host resistance to external pathogens in turn.

Establishing a New Research Axolotl Colony

The field of regenerative biology has taken a keen interest in the Mexican axolotl (Ambystoma mexicanum) over the past few decades, as this salamander successfully regenerates amputated limbs and injured body parts. Recent progress in research tool development has also made possible axolotl genetic manipulation and single-cell analysis, which will help understand the molecular mechanisms of complex tissue regeneration. To support the growing popularity of this model, we describe how to set up a new axolotl housing facility at a research laboratory. We also review husbandry practices for raising axolotls and using them in biological research, with a focus on diet, water quality, breeding, and anesthesia.

Embryo mortality in a captive-bred, Critically Endangered amphibian

The Critically Endangered southern corroboree frog Pseudophryne corroboree is dependent upon captive assurance colonies for its continued survival. Although the captive breeding programme for this species has largely been successful, embryonic mortality remains high (40-90% per year). This study aimed to investigate the causes of mortality in P. corroboree embryos in the captive collection at Melbourne Zoo. During the 2021 breeding season, we investigated 108 abnormal embryos to determine the impact of infections and anatomical deformities on survival and used culture and molecular methods to identify microbes. Overall, 100% of abnormal embryos had fungal infections, and of these, 41.6% also had anatomical deformities. The mortality rate in abnormal embryos was 89.8%; however, we detected no difference in survival in any of the 3 observed fungal growth patterns or between deformed and non-deformed embryos. Sanger sequencing of the ITS region identified fungal isolates belonging to the genus Ilyonectria, the first record in a vertebrate host, and another as a Plectosphaerella sp., which is the first record of infection in an embryo. Dominant bacteria identified were of the genera Herbaspirillum and Flavobacterium; however, their role in the mortality is unknown. Fungal infection and deformities have a significant impact on embryo survival in captive-bred P. corroboree. In a species which relies on captive breeding, identifying and reducing the impacts of embryonic mortality can inform conservation efforts and improve reintroduction outcomes.

A Variety of Fungal Species on the Green Frogs' Skin (Pelophylax esculentus complex) in South Banat

In the last several decades, amphibian populations have been declining worldwide. Many factors have been linked to global amphibian decline, including habitat destruction, pollution, introduced species, global environmental changes, and emerging infectious diseases. Recent studies of amphibian skin infections were mainly focused on the presence of chytridiomycosis, neglecting other members of the frogs' skin communities. The diversity pattern of fungal dwellers on the skin of green frogs (Pelophylax esculentus complex) was investigated. A total of 100 adults were sampled from three localities in South Banat (northern Serbia) over three consecutive years and detected fungal dwellers were identified using light microscopy and ITS and BenA gene sequencing. Structures belonging to fungi and fungus-like organisms including a variety of spores and different mycelia types were documented in the biofilm formed on amphibian skin, and are classified into 10 groups. In total, 42 fungal isolates were identified to species, section, or genus level. The difference in mycobiota composition between sampling points (localities and green frog taxa) was documented. The highest number of fungal structures and isolates was recorded on the hybrid taxon P. esculentus and locality Stevanove ravnice. Parental species showed a markedly lower diversity than the hybrid taxon and were more similar in diversity patterns and were placed in the same homogenous group. The locality Stevanove ravnice exhibited more pronounced differences in diversity pattern than the other two localities and was placed in a distinct and separate homogenous group. Among the fungal isolates, the highest isolation frequency was documented for Alternaria alternata, Aspergillus sp. sect. Nigri, Epicoccum nigrum, Fusarium proliferatum, and Trichoderma atroviride. Among the documented species, dematiaceous fungi, causative agents of chromomycosis in amphibians, were also recorded in this research with high isolation frequency. Also, some rare fungal species such as Quambalaria cyanescens and Pseudoteniolina globosa are documented for the first time in this research as microbial inhabitants of amphibian skin.

Early exposure to UV radiation causes telomere shortening and poorer condition later in life

Determining the contribution of elevated ultraviolet-B radiation (UVBR; 280–315 nm) to amphibian population declines is being hindered by a lack of knowledge about how different acute UVBR exposure regimes during early life-history stages might affect post-metamorphic stages via long-term carryover effects. We acutely exposed tadpoles of the Australian green tree frog (Litoria caerulea) to a combination of different UVBR irradiances and doses in a multi-factorial laboratory experiment, and then reared them to metamorphosis in the absence of UVBR to assess carryover effects in subsequent juvenile frogs. Dose and irradiance of acute UVBR exposure influenced carryover effects into metamorphosis in somewhat opposing manners. Higher doses of UVBR exposure in larvae yielded improved rates of metamorphosis. However, exposure at a high irradiance resulted in frogs metamorphosing smaller in size and in poorer condition than frogs exposed to low and medium irradiance UVBR as larvae. We also demonstrate some of the first empirical evidence of UVBR-induced telomere shortening in vivo, which is one possible mechanism for life-history trade-offs impacting condition post-metamorphosis. These findings contribute to our understanding of how acute UVBR exposure regimes in early life affect later life-history stages, which has implications for how this stressor may shape population dynamics.

Summary: Ultraviolet radiation exposure in amphibian larvae generates detrimental carryover effects on body condition and relative telomere length post-metamorphosis, a mechanism that may influence amphibian population dynamics.

Cultivable Skin Mycobiota of Healthy and Diseased Blind Cave Salamander (Proteus anguinus)

Proteus anguinus is a neotenic cave salamander, endemic to the Dinaric Karst and a symbol of world natural heritage. It is classified as “vulnerable” by the International Union for Conservation of Nature (IUCN) and is one of the EU priority species in need of strict protection. Due to inaccessibility of their natural underground habitat, scientific studies of the olm have been conducted mainly in captivity, where the amphibians are particularly susceptible to opportunistic microbial infections. In this report, we focused on the diversity of cultivable commensal fungi isolated from the skin of asymptomatic and symptomatic animals obtained from nature (20 specimens) and captivity (22 specimens), as well as from underground water of two karstic caves by direct water filtration and by exposure of keratin-based microbial baits and subsequent isolation from them. In total 244 fungal isolates were recovered from the animals and additional 153 isolates were obtained from water samples. Together, these isolates represented 87 genera and 166 species. Symptomatic animals were colonized by a variety of fungal species, most of them represented by a single isolate, including genera known for their involvement in chromomycosis, phaeohyphomycosis and zygomycosis in amphibians: Acremonium, Aspergillus, Cladosporium, Exophiala, Fusarium, Mucor, Ochroconis, Phialophora and Penicillium. One symptomatic specimen sampled from nature was infected by the oomycete Saprolegnia parasitica, the known causative agent of saprolegniosis. This is the first comprehensive report on cultivable skin mycobiome of this unique amphibian in nature and in captivity, with an emphasis on potentially pathogenic fungi and oomycetes.

Detection and characterization of Hyaloklossia kasumiensis (Apicomplexa: Sarcocystidae) from Nagoya Daruma pond frog, Pelophylax porosus brevipodus

The Nagoya Daruma pond frog (Pelophylax porosus brevipodus) is an endangered anuran species endemic to the semi-aquatic environment of Japan. On June 11, 2021, four of the six Nagoya Daruma pond frogs captured from the wild and kept in an aquarium died in rapid succession. Pathological and/or parasitological examinations of two of the four carcasses revealed protozoan infection in their kidneys. The pathogen identified as Hyaloklossia kasumiensis (Apicomplexa: Sarcocystidae: Hyaloklossiinae) based on their morphological and molecular characterization. This is a new host record of H. kasumiensis. Mitochondrial cox1 sequence analysis suggest that the present species detected from Nagoya Daruma pond frog belongs to the same lineage as those found in the Tokyo Daruma pond frog (P. p. porosus) from Ibaraki Prefecture, Japan. This particular lineage may be prevalent among Pelophylax porosus ssp. distributed in Japan.

FELASA-AALAS Recommendations for Monitoring and Reporting of Laboratory Fish Diseases and Health Status, with an Emphasis on Zebrafish (Danio Rerio)

The exchange of fish for research may expose an aquatic laboratory to pathogen contamination as incoming fish can introduce bacteria, fungi, parasites, and viruses capable of affecting both experimental results and fish and personnel health and welfare. To develop risk mitigation strategies, FELASA and AALAS established a joint working group to recommend good practices for health monitoring of laboratory fish. The recommendations address all fish species used for research, with a particular focus on zebrafish (Danio rerio). First, the background of the working group and key definitions are provided. Next, fish diseases of high impact are described. Third, recommendations are made for health monitoring of laboratory fishes. The recommendations emphasize the importance of daily observation of the fish and strategies to determine fish colony health status. Finally, report templates are proposed for historical screening data and aquatic facility description to facilitate biohazard risk assessment when exchanging fish.

Saprolegniosis in Amphibians: An Integrated Overview of a Fluffy Killer Disease

Amphibians constitute the class of vertebrates with the highest proportion of threatened species, with infectious diseases being considered among the greatest causes for their worldwide decline. Aquatic oomycetes, known as “water molds,” are fungus-like microorganisms that are ubiquitous in freshwater ecosystems and are capable of causing disease in a broad range of amphibian hosts. Various species of Achlya sp., Leptolegnia sp., Aphanomyces sp., and mainly, Saprolegnia sp., are responsible for mass die-offs in the early developmental stages of a wide range of amphibian populations through a disease known as saprolegniosis, aka, molding or a “Saprolegnia-like infection.” In this context, the main objective of the present review was to bring together updated information about saprolegniosis in amphibians to integrate existing knowledge, identify current knowledge gaps, and suggest future directions within the saprolegniosis–amphibian research field. Based on the available literature and data, an integrated and critical interpretation of the results is discussed. Furthermore, the occurrence of saprolegniosis in natural and laboratory contexts and the factors that influence both pathogen incidence and host susceptibility are also addressed. The focus of this work was the species Saprolegnia sp., due to its ecological importance on amphibian population dynamics and due to the fact that this is the most reported genera to be associated with saprolegniosis in amphibians. In addition, integrated emerging therapies, and their potential application to treat saprolegniosis in amphibians, were evaluated, and future actions are suggested.

The Amphibian Heart

Currently, there are more than 8200 amphibian species described, including the orders Anura (frogs and toads), Caudata (salamanders and newts) and Gymnophiona (caecilians). Amphibians have 3 heart chambers: 2 atria and 1 ventricle. Their heart anatomy, histology, and physiology are reviewed. The basic morphology of the heart is similar in all amphibians with some differences due to their lifestyle. Blood flow, blood mixing, and blood oxygenation show variation due to interindividual and interspecific differences. Finally, different diagnostic methods to investigate the amphibian heart are described and reported amphibian heart diseases are summarized, including genetic, congenital, infectious, and neoplastic heart diseases.

A New Disease Caused by an Unidentified Etiological Agent Affects European Salamanders

New pathologies are causing dramatic declines and extinctions of multiple amphibian species. In 2013, in one fire salamander population of Northern Italy, we found individuals with undescribed cysts at the throat level, a malady whose existence has not previously been reported in amphibians. With the aim of describing this novel disease, we performed repeated field surveys to assess the frequency of affected salamanders from 2014 to 2020, and integrated morphological, histological, and molecular analyses to identify the pathogen. The novel disease affected up to 22% of salamanders of the study population and started spreading to nearby populations. Cysts are formed by mucus surrounding protist-like cells about 30 µm long, characterized by numerous cilia/undulipodia. Morphological and genetic analyses did not yield a clear match with described organisms. The existence of this pathogen calls for the implementation of biosecurity protocols and more studies on the dynamics of transmission and the impact on wild populations.

Development and comparison of loop-mediated isothermal amplification with quantitative PCR for the specific detection of Saprolegnia spp

Saprolegniasis is an important disease in freshwater aquaculture, and is associated with oomycete pathogens in the genus Saprolegnia. Early detection of significant levels of Saprolegnia spp. pathogens would allow informed decisions for treatment which could significantly reduce losses. This study is the first to report the development of loop-mediated isothermal amplification (LAMP) for the detection of Saprolegnia spp. and compares it with quantitative PCR (qPCR). The developed protocols targeted the internal transcribed spacer (ITS) region of ribosomal DNA and the cytochrome C oxidase subunit 1 (CoxI) gene and was shown to be specific only to Saprolegnia genus. This LAMP method can detect as low as 10 fg of S. salmonis DNA while the qPCR method has a detection limit of 2 pg of S. salmonis DNA, indicating the superior sensitivity of LAMP compared to qPCR. When applied to detect the pathogen in water samples, both methods could detect the pathogen when only one zoospore of Saprolegnia was present. We propose LAMP as a quick (about 20–60 minutes) and sensitive molecular diagnostic tool for the detection of Saprolegnia spp. suitable for on-site applications.

Investigating the Effect of Disturbance on Prey Consumption in Captive Congo Caecilians Herpele squalostoma

Maintaining Gymnophiona in captivity provides opportunities to study the behaviour and life-history of this poorly known Order, and to investigate and provide species-appropriate welfare guidelines, which are currently lacking. This study focuses on the terrestrial caecilian Herpele squalostoma to investigate its sensitivity to disturbances associated with routine husbandry needed for monitoring and maintaining adequate wellbeing in captivity. Fossorial caecilians gradually pollute their environment in captivity with waste products, and substrate must be replaced at intervals; doing so disturbs the animals directly and via destruction of burrow networks. As inappetence is frequently associated with stress in amphibians, the percentage consumption of offered food types, river shrimp (Palaemon varians) and brown crickets (Gryllus assimilis), was measured as an indicator of putative stress following three routine substrate changes up to 297 days post-substrate change. Mean daily variation in substrate temperatures were also recorded in order to account for environmental influences on food consumption, along with nitrogenous waste in tank substrate prior to a substrate change and fresh top soil in order to understand the trade-off between dealing with waste accumulation and disturbing animals. We found a significant negative effect of substrate disturbance on food intake, but no significant effect of prey type. Variations in daily soil temperatures did not have a significant effect on food intake, but mean substrate temperature did. Additionally, substrate nitrogenous waste testing indicated little difference between fresh and tank substrate. In conclusion, this study provides a basis from which to develop further welfare assessment for this and other rarely kept and rarely observed terrestrial caecilian species.

Changes in the gut microbiota diversity of brown frogs (Rana dybowskii) after an antibiotic bath

Background

Captive amphibians frequently receive antibiotic baths to control bacterial diseases. The potential collateral effect of these antibiotics on the microbiota of frogs is largely unknown. To date, studies have mainly relied on oral administration to examine the effects of antibiotics on the gut microbiota; in contrast, little is known regarding the effects of bath-applied antibiotics on the gut microbiota. The gut microbiota compositions of the gentamicin, recovery, and control groups were compared by Illumina high-throughput sequencing, and the functional profiles were analysed using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt). Furthermore, the relationship between the structure and predicted functional composition of the gut microbiota was determined.

Results

The alpha diversity indices were significantly reduced by the gentamicin bath, illustrating that this treatment significantly changed the composition of the gut microbiota. After 7 days, the gut microbiota of the recovery group was not significantly different from that of the gentamicin group. Forty-four indicator taxa were selected at the genus level, comprising 42 indicators representing the control group and 2 indicators representing the gentamicin and recovery groups. Potential pathogenic bacteria of the genera Aeromonas, Citrobacter, and Chryseobacterium were significantly depleted after the gentamicin bath. There was no significant positive association between the community composition and functional composition of the gut microbiota in the gentamicin or control frogs, indicating that the functional redundancy of the gut bacterial community was high.

Conclusions

Gentamicin significantly changed the structure of the gut microbiota of R. dybowskii, and the gut microbiota exhibited weak resilience. However, the gentamicin bath did not change the functional composition of the gut microbiota of R. dybowskii, and there was no significant correlation between the structural composition and the functional composition of the gut microbiota.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-03044-z.

Identifying fungal-host associations in an amphibian host system

Host-associated microbes can interact with macro-organisms in a number of ways that affect host health. Few studies of host-associated microbiomes, however, focus on fungi. In addition, it is difficult to discern whether a fungal organism found in or on an ectotherm host is associating with it in a durable, symbiotic interaction versus a transient one, and to what extent the habitat and host share microbes. We seek to identify these host-microbe interactions on an amphibian, the Colorado boreal toad (Anaxyrus boreas boreas). We sequenced the ITS1 region of the fungal community on the skin of wild toads (n = 124) from four sites in the Colorado Rocky Mountains, across its physiologically dynamic developmental life stages. We also sampled the common habitats used by boreal toads: water from their natal wetland and aquatic pond sediment. We then examined diversity patterns within different life stages, between host and habitat, and identified fungal taxa that could be putatively host-associated with toads by using an indicator species analysis on toad versus environmental samples. Host and habitat were strikingly similar, with the exception of toad eggs. Post-hatching toad life stages were distinct in their various fungal diversity measures. We identified eight fungal taxa that were significantly associated with eggs, but no other fungal taxa were associated with other toad life stages compared with their environmental habitat. This suggests that although pre- and post-metamorphic toad life stages differ from each other, the habitat and host fungal communities are so similar that identifying obligate host symbionts is difficult with the techniques used here. This approach does, however, leverage sequence data from host and habitat samples to predict which microbial taxa are host-associated versus transient microbes, thereby condensing a large set of sequence data into a smaller list of potential targets for further consideration.

Gaps in global wildlife trade monitoring leave amphibians vulnerable

As the biodiversity crisis continues, we must redouble efforts to understand and curb pressures pushing species closer to extinction. One major driver is the unsustainable trade of wildlife. Trade in internationally regulated species gains the most research attention, but this only accounts for a minority of traded species and we risk failing to appreciate the scale and impacts of unregulated legal trade. Despite being legal, trade puts pressure on wild species via direct collection, introduced pathogens, and invasive species. Smaller species-rich vertebrates, such as reptiles, fish, and amphibians, may be particularly vulnerable to trading because of gaps in regulations, small distributions, and demand of novel species. Here, we combine data from five sources: online web searches in six languages, Convention on International Trade in Endangered Species (CITES) trade database, Law Enforcement Management Information System (LEMIS) trade inventory, IUCN assessments, and a recent literature review, to characterise the global trade in amphibians, and also map use by purpose including meat, pets, medicinal, and for research. We show that 1215 species are being traded (17% of amphibian species), almost three times previous recorded numbers, 345 are threatened, and 100 Data Deficient or unassessed. Traded species origin hotspots include South America, China, and Central Africa; sources indicate 42% of amphibians are taken from the wild. Newly described species can be rapidly traded (mean time lag of 6.5 years), including threatened and unassessed species. The scale and limited regulation of the amphibian trade, paired with the triptych of connected pressures (collection, pathogens, invasive species), warrants a re-examination of the wildlife trade status quo, application of the precautionary principle in regard to wildlife trade, and a renewed push to achieve global biodiversity goals.

In the last few decades, exotic pets have become much more common. In the UK in 2008, reptiles and amphibians were more popular than dogs, with over eight million in captivity. But while almost all pet cats and dogs are born and bred in captivity, exotic pets are often taken from the wild, putting species and their habitats at risk.

An international trade agreement called the Convention on International Trade in Endangered Species (CITES) strives to prevent unsustainable animal trade. But to get CITES protection, species depend on data showing that wildlife trade threatens their survival. In addition, their range countries need to first propose them to be listed. For most wild animal species, there are no data on population size or population decline. In the case of amphibians, CITES regulates the trade of just 2.5% of species. This leaves the rest with no protection from overarching international trade regulations. To protect these animals, researchers need to find out which species are in trade, where they are coming from, and how many are already threatened.

To address this, Hughes, Marshall and Strine combined data from five sources, including official CITES trade records, recent research and an online search for amphibian sales in six languages. The data showed evidence of trade in at least 1,215 amphibian species, representing 17% of all amphibians. The figure is three times higher than previous estimates. Of the species in trade, more than one in five is vulnerable to extinction, endangered, or critically endangered. For a further 100 of the traded species, data on population were unavailable. Moreover, analysis of the origins of traded individuals showed that around 42% came from the wild. Tropical parts of the world had the highest number of species in trade, but the data showed exchanges happening across the globe.

Unsustainable wildlife trade can have devastating consequences for wild animals. It has already driven at least 21 reptile species to extinction, and data of amphibian species are unknown. To prevent further species going extinct, legal wildlife trade should follow the precautionary principle when it comes to wildlife trade. Rather than allowing people to trade a species until CITES regulates it, a blanket ban should come into force for species that have not been assessed or are threatened. Trade would be able to resume for a species only when assessments show that it would not cause major population decline, or secure, captive breeding facilities can be guaranteed.

Ultraviolet-B irradiance and cumulative dose combine to determine performance and survival

Despite decades of research, the role of elevated solar ultraviolet-B radiation (UVBR; 280-315 nm) in shaping amphibian populations remains ambiguous. These difficulties stem partly from a poor understanding of which parameters of UVBR exposure - dose, irradiance, and time interval - determine UVBR exposure health risk, and the potentially erroneous assumption that effects are proportional to the dose of exposure, irrespective of the administered regime (Bunsen-Roscoe Law of Reciprocity; BRL). We tested if the BRL holds with respect to UVBR-induced physiological effects in amphibians by acutely exposing tadpoles of the Australian green tree frog (Litoria caerulea) to a combination of different UVBR irradiances and doses in a fully factorial experiment. The BRL was invalid across all metrics assessed, with UVBR irradiance influencing the effects of a given dose on growth, coloration and burst swimming performance of larvae. We demonstrated some of the first empirical evidence for beneficial physiological effects of UVBR exposure in a larval amphibian, with improvements in growth, burst swimming performance and survival at the highest UVBR doses, contrary to hypotheses. Our findings demonstrate the species-specific nature of amphibian responses to UVBR, and the importance of UVBR irradiance in influencing the long-term physiological effects of a given dose of radiation. This work enhances our understanding of which parameters of complex UVBR exposures determine amphibian health risk.

Bufadienolides from the Skin Secretions of the Neotropical Toad Rhinella alata (Anura: Bufonidae): Antiprotozoal Activity against Trypanosoma cruzi

Toads in the family Bufonidae contain bufadienolides in their venom, which are characterized by their chemical diversity and high pharmacological potential. American trypanosomiasis is a neglected disease that affects an estimated 8 million people in tropical and subtropical countries. In this research, we investigated the chemical composition and antitrypanosomal activity of toad venom from Rhinella alata collected in Panama. Structural determination using mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy led to the identification of 10 bufadienolides. Compounds identified include the following: 16β-hydroxy-desacetyl-bufotalin-3-adipoyl-arginine ester (1), bufotalin (2), 16β-hydroxy-desacetyl-bufotalin-3-pimeloyl-arginine ester (3), bufotalin-3-pimeloyl-arginine ester (4), 16β-hydroxy-desacetyl-bufotalin-3-suberoyl-arginine ester (5), bufotalin-3-suberoyl-arginine ester (6), cinobufagin-3-adipoyl-arginine ester (7), cinobufagin-3-pimeloyl-arginine ester (8), cinobufagin-3-suberoyl-arginine ester (9), and cinobufagin (10). Among these, three new natural products, 1, 3, and 5, are described, and compounds 1-10 are reported for the first time in R. alata. The antitrypanosomal activity assessed in this study revealed that the presence of an arginyl-diacid attached to C-3, and a hydroxyl group at C-14 in the structure of bufadienolides that is important for their biological activity. Bufadienolides showed cytotoxic activity against epithelial kidney Vero cells; however, bufagins (2 and 10) displayed low mammalian cytotoxicity. Compounds 2 and 10 showed activity against the cancer cell lines MCF-7, NCI-H460, and SF-268.

MYCOBACTERIA IN SKIN LESIONS AND THE HABITAT OF THE ENDANGERED HOUSTON TOAD (ANAXYRUS HOUSTONENSIS)

Head-starting of the federally endangered Houston toad (Anaxyrus houstonensis), that is, the release of egg strands, tadpoles, and metamorphic juveniles produced in captivity into the original breeding ponds, requires assessment of potential threats for the transmission of pathogens from captive to free-ranging toads. We used Illumina-based 16S rRNA V3 amplicon sequencing to investigate the community structure of bacteria from skin lesions of captive Houston toad and habitat (pond) samples. Proteobacteria, alone or together with Actinobacteria and, in some samples, Cyanobacteria represented virtually all reads in tissue lesion samples, whereas pond samples were much more diverse, with Acidobacteria, Actinobacteria, Bacteriodetes, Chloroflexi, Cyanobacteria, Firmicutes, Planctomycetes, Proteobacteria, and Verrucomicrobia present with little variation between samples. If present in lesions, Actinobacteria were largely represented by Mycobacteriaceae, and here mainly by one sequence identical to sequences of members of the Mycobacterium chelonae-abscessus complex. In pond samples, mycobacteria represented only a small portion of the actinobacteria, although at higher diversity with six distinct reads. Sequences for reads obtained from pond samples were identical to those representing the M. chelonae-abscessus complex, a group with Mycobacterium marinum, Mycobacterium kansasii, Mycobacterium avium, a group with Mycobacterium vaccae, Mycobacterium fortuitum, Mycobacterium poriferae, and a group with Mycobacterium elephantis and Mycobacterium celeriflavum, whereas sequences of high similarity were detected for reads related to those of Mycobacterium holsaticum, Mycobacterium pallens, and Mycobacterium obuense, and Mycobacterium goodii. Our results indicated that lesions observed on the Houston toad in captivity are not the result of mycobacteria in every case, and that the presence of mycobacteria in the captive colony does not represent a novel pathogen threat to the wild populations because such bacteria are also seen in the natural pond habitats for the Houston toad.

First record of Trypanosoma sp. (Kinetoplastea: Trypanosomatidae) parasiting Rhinella major in the Brazilian Amazon

Rhinella major is one of the component species of the Rhinella granulosa group and is distributed in the neotropical region of the South American continent, being found in anthropized environments. The occurrence of trypanosomes in anurans involves a yet unknown diversity of species and hosts. In the present study, we aimed to describe the first record of Trypanosoma sp. at R. major as a new host. Of the species captured, four were positive for trypomastigote forms of Trypanosoma sp. The morphometry of the trypomastigote forms revealed parasitemia by only one morphotype. There is a need for reports and studies of parasite-host relationships in anurans and a lack of records regarding hemoparasite diversity linked to the Rhinella granulosa group.

Host-multiparasite interactions in amphibians: a review

Parasites, including viruses, bacteria, fungi, protists, helminths, and arthropods, are ubiquitous in the animal kingdom. Consequently, hosts are frequently infected with more than one parasite species simultaneously. The assessment of such co-infections is of fundamental importance for disease ecology, but relevant studies involving non-domesticated animals have remained scarce. Many amphibians are in decline, and they generally have a highly diverse parasitic fauna. Here we review the literature reporting on field surveys, veterinary case studies, and laboratory experiments on co-infections in amphibians, and we summarize what is known about within-host interactions among parasites, which environmental and intrinsic factors influence the outcomes of these interactions, and what effects co-infections have on hosts. The available literature is piecemeal, and patterns are highly diverse, so that identifying general trends that would fit most host–multiparasite systems in amphibians is difficult. Several examples of additive, antagonistic, neutral, and synergistic effects among different parasites are known, but whether members of some higher taxa usually outcompete and override the effects of others remains unclear. The arrival order of different parasites and the time lag between exposures appear in many cases to fundamentally shape competition and disease progression. The first parasite to arrive can gain a marked reproductive advantage or induce cross-reaction immunity, but by disrupting the skin and associated defences (i.e., skin secretions, skin microbiome) and by immunosuppression, it can also pave the way for subsequent infections. Although there are exceptions, detrimental effects to the host are generally aggravated with increasing numbers of co-infecting parasite species. Finally, because amphibians are ectothermic animals, temperature appears to be the most critical environmental factor that affects co-infections, partly via its influence on amphibian immune function, partly due to its direct effect on the survival and growth of parasites. Besides their importance for our understanding of ecological patterns and processes, detailed knowledge about co-infections is also crucial for the design and implementation of effective wildlife disease management, so that studies concentrating on the identified gaps in our understanding represent rewarding research avenues.