Emerging infectious diseases such as chytridiomycosis and ranavirosis, caused by Batrachochytrium dendrobatidis (Bd) and ranavirus (RV), respectively, are major contributors to global amphibian declines. Despite their significance, comprehensive data on the spatial epidemiology of these pathogens in South Korea remain limited. This study aimed to assess the nationwide co-occurrence and prevalence of Bd and RV across four anuran species in five administrative regions. Infection rates were analyzed in relation to host species, sex, and life history stage. Results indicated distinct prevalence patterns driven by ecological traits. Bd was predominantly detected in mountainous and coastal habitats, whereas RV was more common in flat inland areas. Both pathogens exhibited peak occurrence in central regions, likely reflecting seasonal transmission dynamics rather than stable endemic hotspots. The observed spatial heterogeneity appears to be influenced by pathogen-specific thermal tolerance and host ecology. These findings underscore the importance of understanding host-pathogen-environment interactions for effective disease surveillance and management. Continuous monitoring and integrative ecological approaches are essential to mitigate pathogen-induced biodiversity loss and to inform amphibian conservation strategies in East Asia.

IntroductionGiven the role of microbiomes in promoting host health and homeostasis, understanding the factors shaping skin microbial communities in wild vertebrates has become increasingly important in conservation. This goal is even more pressing for amphibians, for which the skin has multiple critical functions, and pathogens currently decimating populations are linked to significant changes in skin microbiomes. However, because microbiomes are also shaped by environmental and ecological influences, as well as by host phylogeny, it is important to quantify these contributions to microbiome structure in the presence of infection.MethodsTo understand the joint influence of these diverse factors shaping microbiomes, we used 16S rRNA sequencing to characterize the skin microbial communities of six salamander species (families Plethodontidae and Salamandridae) found in Oklahoma and contrasted the effects of infection status, phylogeny, host ecology, and host environment (i.e., climate) on skin microbiomes.ResultsDifferences at the level of host family were the main factor influencing microbiome diversity; however, we did not detect a substantial phylogenetic signal. Instead, host ecology and environment were more important in driving microbiome differences among species and genera. Salamanders that tested positive for the skin fungal parasite Batrachochytrium dendrobatidis (Bd) also had slightly less diverse microbiomes than Bd-free animals, but no such differences were associated with the systemic pathogen ranavirus (RV).DiscussionTogether, these results indicate a nuanced relationship between the number and type of microbes present on salamander skin and the factors influencing them. By developing a baseline assessment of the microbiome diversity and richness present on the skin of these focal species, this work also provides a foundation for monitoring and evaluating changes in skin microbiomes as populations continue to experience stressors and diseases.

Populations of the eastern hellbender Cryptobranchus alleganiensis alleganiensis have been declining for decades, and emerging pathogens and pesticides are hypothesized to be contributing factors. However, few empirical studies have attempted to test the potential effects of these factors on hellbenders. We simultaneously exposed subadult hellbenders to environmentally relevant concentrations of either Batrachochytrium dendrobatidis (Bd) or a frog virus 3-like ranavirus (RV), a combination of the pathogens, or each pathogen following exposure to a glyphosate herbicide (Roundup). Additionally, we measured the ability of the skin mucosome to inactivate Bd and RV in growth assays. We found that mucosome significantly inactivated RV by an average of 40% but had no negative effects on Bd growth. All treatments that included RV exposure experienced reduced survival compared to controls, and the combination of RV and herbicide resulted in 100% mortality. Histopathology verified RV as the cause of mortality in all RV-exposed treatments. No animals were infected with Bd or died in the Bd-only treatment. Our results suggest that RV exposure may be a significant threat to the survival of subadult hellbenders and that Roundup exposure may potentially exacerbate this threat.

The stress-induced susceptibility hypothesis, which predicts chronic stress weakens immune defences, was proposed to explain increasing infectious disease-related mass mortality and population declines. Previous work characterized wetland salinization as a chronic stressor to larval amphibian populations. Thus, we combined field observations with experimental exposures quantifying epidemiological parameters to test the role of salinity stress in the occurrence of ranavirus-associated mass mortality events. Despite ubiquitous pathogen presence (94%), populations exposed to salt runoff had slightly more frequent ranavirus related mass mortality events, more lethal infections, and 117-times greater pathogen environmental DNA. Experimental exposure to chronic elevated salinity (0.8–1.6 g l⁻¹ Cl⁻) reduced tolerance to infection, causing greater mortality at lower doses. We found a strong negative relationship between splenocyte proliferation and corticosterone in ranavirus-infected larvae at a moderate elevation of salinity, supporting glucocorticoid-medicated immunosuppression, but not at high salinity. Salinity alone reduced proliferation further at similar corticosterone levels and infection intensities. Finally, larvae raised in elevated salinity had 10-times more intense infections and shed five-times as much virus with similar viral decay rates, suggesting increased transmission. Our findings illustrate how a small change in habitat quality leads to more lethal infections and potentially greater transmission efficiency, increasing the severity of ranavirus epidemics.

Chiggers (Acariformes: Trombiculoidea) do not increase rates of infection by Batrachochytrium dendrobatidis fungus in the endemic Dwarf Mexican Treefrog Tlalocohyla smithii (Anura: Hylidae)

The commercial production of lumpfish Cyclopterus lumpus L. is expanding with the increased demand for their use as cleaner fish, to control sea-lice numbers, at marine Atlantic salmon Salmo salar L. aquaculture sites throughout Northern Europe. A new ranavirus has been isolated from lumpfish at multiple locations in the North Atlantic area. First isolated in 2014 in the Faroe Islands, the virus has subsequently been found in lumpfish from Iceland in 2015 and from Scotland and Ireland in 2016. The Icelandic lumpfish ranavirus has been characterized by immunofluorescent antibody test, optimal growth conditions and transmission electron microscopy. Partial sequences of the major capsid protein gene from 12 isolates showed 99.79-100% nt identity between the lumpfish ranaviruses. Complete genome sequencing from three of the isolates and phylogenetic analysis based on the concatenated 26 iridovirus core genes suggest these lumpfish ranavirus isolates form a distinct clade with ranaviruses from cod Gadus morhua L. and turbot Scophthalmus maximus L. isolated in Denmark in 1979 and 1999, respectively. These data suggest that these viruses should be grouped together as a new ranavirus species, European North Atlantic Ranavirus, which encompasses ranaviruses isolated from marine fishes in European North Atlantic waters.

Global amphibian decline continues to be a great concern despite our increased understanding of the causes behind the observed patterns of the decline, such as habitat modification and infectious diseases. Although there is a large body of literature on the topic of amphibian infectious diseases, pathogen prevalence and distribution among entire communities of species in many regions remain poorly understood. In addition to these geographic gaps in our understanding, past work has focused largely on individual pathogens, either Batrachochytrium dendrobatidis (Bd) or ranavirus (RV), rather than dual infection rates among host species. We sampled for prevalence and infection load of both pathogens in 514 amphibians across 16 total sites in northeastern Oklahoma. Amphibians were caught by hand, net, or seine; they were swabbed to screen for Bd; and liver tissue samples were collected to screen for RV. Overall results of quantitative PCR assays showed that 7% of screened individuals were infected with RV only, 37% were infected with Bd only, and 9% were infected with both pathogens simultaneously. We also documented disease presence in several rare amphibian species that are currently being monitored as species of concern due to their small population sizes in Oklahoma. This study synthesizes a growing body of research regarding infectious diseases among amphibian communities in the central United States.

Amphibian declines have been occurring world wide over the past several decades. Infectious disease and environmental changes have been implicated as the causative agents in these declines. Fish stocking from hatcheries provides a unique opportunity for organisms, including infectious diseases, to travel over long distances. Ranavirus was previously found in two ponds at Harrison Lake National Fish Hatchery in Charles City County, Virginia. Therefore, the primary objective of this study was to determine if ranavirus is present in tadpoles from four warm water fish hatcheries in Virginia. A secondary objective of this study was to determine if there are relationships between environmental variables and the proportion of tadpoles that test positive for ranavirus. Ranavirus was detected via PCR in three of the four warm water fish hatcheries, Harrison Lake National Fish Hatchery, Front Royal Fish Hatchery, and King and Queen Fish Hatchery, but was not detected at Vic Thomas Fish Hatchery. Temperature and the length of time a pond is filled with water were significant predictors of the proportion of tadpoles that tested positive for ranavirus, as determined by logistic regression analysis. Results from this study indicate that ranavirus is present in Virginia warm water fish hatcheries and can be found over multiple years. Precautions should be taken to ensure that ranavirus is not spread when fish are transferred from one hatchery to another or to the wild.

A ranavirus (RV) was isolated from Chinese giant salamanders (Andrias davidianus) in China in 2010 and provisionally designated Andrias davidianus ranavirus (ADRV). The complete genome sequence is 106,719 nucleotides long. Genomic sequence and phylogenetic analyses showed that ADRV has a high degree of conservation with other RVs.