The impacts of water quality on the amphibian chytrid fungal pathogen: A systematic review

The pathogenic fungus Batrachochytrium dendrobatidis has caused declines of amphibians worldwide. Yet our understanding of how water quality influences fungal pathogenicity is limited. Here, we reviewed experimental studies on the effect of water quality on this pathogen to determine which parameters impacted disease dynamics consistently. The strongest evidence for protective effects is salinity which shows strong antifungal properties in hosts at natural levels. Although many fungicides had detrimental effects on the fungal pathogen in vitro, their impact on the host is variable and they can worsen infection outcomes. However, one fungicide, epoxiconazole, reduced disease effects experimentally and likely in the field. While heavy metals are frequently studied, there is weak evidence that they influence infection outcomes. Nitrogen and phosphorous do not appear to impact pathogen growth or infection in the amphibian host. The effects of other chemicals, like pesticides and disinfectants on infection were mostly unclear with mixed results or lacking an in vivo component. Our study shows that water chemistry does impact disease dynamics, but the effects of specific parameters require more investigation. Improving our understanding of how water chemistry influences disease dynamics will help predict the impact of chytridiomycosis, especially in amphibian populations affected by land use changes.

Glutathione is required for growth and cadmium tolerance in the amphibian chytrid fungus, Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd) is a lethal amphibian pathogen, partly due to its ability to evade the immune system of susceptible frog species. In many pathogenic fungi, the antioxidant glutathione is a virulence factor that helps neutralise oxidative stressors generated from host immune cells, as well as other environmental stressors such as heavy metals. The role of glutathione in stress tolerance in Bd has not been investigated. Here, we examine the changes in the glutathione pool after stress exposure and quantify the effect of glutathione depletion on cell growth and stress tolerance. Depletion of glutathione repressed growth and release of zoospores, suggesting that glutathione is essential for life cycle completion in Bd. Supplementation with <2 mM exogenous glutathione accelerated zoospore development, but concentrations >2 mM were strongly inhibitory to Bd cells. While hydrogen peroxide exposure lowered the total cellular glutathione levels by 42 %, glutathione depletion did not increase the sensitivity to hydrogen peroxide. Exposure to cadmium increased total cellular glutathione levels by 93 %. Glutathione-depleted cells were more sensitive to cadmium, and this effect was attenuated by glutathione supplementation, suggesting that glutathione plays an important role in cadmium tolerance. The effects of heat and salt were exacerbated by the addition of exogenous glutathione. The impact of glutathione levels on Bd stress sensitivity may help explain differences in host susceptibility to chytridiomycosis and may provide opportunities for synergistic therapeutics.

Mechanisms underlying host persistence following amphibian disease emergence determine appropriate management strategies

Emerging infectious diseases have caused many species declines, changes in communities and even extinctions. There are also many species that persist following devastating declines due to disease. The broad mechanisms that enable host persistence following declines include evolution of resistance or tolerance, changes in immunity and behaviour, compensatory recruitment, pathogen attenuation, environmental refugia, density-dependent transmission and changes in community composition. Here we examine the case of chytridiomycosis, the most important wildlife disease of the past century. We review the full breadth of mechanisms allowing host persistence, and synthesise research on host, pathogen, environmental and community factors driving persistence following chytridiomycosis-related declines and overview the current evidence and the information required to support each mechanism. We found that for most species the mechanisms facilitating persistence have not been identified. We illustrate how the mechanisms that drive long-term host population dynamics determine the most effective conservation management strategies. Therefore, understanding mechanisms of host persistence is important because many species continue to be threatened by disease, some of which will require intervention. The conceptual framework we describe is broadly applicable to other novel disease systems.

Identifying sex-linked markers in Litoria aurea: a novel approach to understanding sex chromosome evolution in an amphibian

Few taxa exhibit the variability of sex-determining modes as amphibians. However, due to the presence of homomorphic sex chromosomes in many species, this phenomenon has been difficult to study. The Australian frog, Litoria aurea, has been relatively well studied over the past 20 years due to widespread declines largely attributable to chytrid fungus. However, it has been subject to few molecular studies and its mode of sex determination remained unknown. We applied DArTseq™ to develop sex-linked single nucleotide polymorphisms (SNPs) and restriction fragment presence/absence (PA) markers in 44 phenotypically sexed L. aurea individuals from the Molonglo River in NSW, Australia. We conclusively identified a male heterogametic (XX-XY) sex determination mode in this species, identifying 11 perfectly sex-linked SNP and six strongly sex-linked PA markers. We identified a further 47 moderately sex-linked SNP loci, likely serving as evidence indicative of XY recombination. Furthermore, within these 47 loci, a group of nine males were found to have a feminised Y chromosome that significantly differed to all other males. We postulate ancestral sex-reversal as a means for the evolution of this now pseudoautosomal region on the Y chromosome. Our findings present new evidence for the ‘fountain of youth’ hypothesis for the retention of homomorphic sex chromosomes in amphibians and describe a novel approach for the study of sex chromosome evolution in amphibia.

Indiscriminate feeding by an alien population of the spotted-thighed frog (Litoria cyclorhyncha) in southern Australia and potential impacts on native biodiversity

Litoria cyclorhyncha (Hylidae) is native to southern Western Australia, but a naturalised population has established on the Eyre Peninsula, South Australia. We investigated the diet of this exotic population to assess potential impacts on biodiversity and ecosystems. Seventy-six frogs were collected from three different habitats and their diet items assigned to parataxonomic units (PU) within orders. Stomach contents were diverse, containing 467 prey items from 19 orders and 135 PU, with extrapolation suggesting a diet of ~200 PU. Shannon diversity estimates of prey items consumed produced different rankings for the three habitats at the PU and order level. Therefore, estimates at the order level may not be representative of the actual diversity of prey items. L. cyclorhyncha consumed mainly arthropods and low numbers of conspecific young frogs, geckos and a juvenile house mouse. This generalist, indiscriminate predatory diet is similar to that of other hylids and implies that the species poses a risk to native biodiversity and ecosystem processes by predation and competition. Therefore, further spread of this species needs to be prevented. Our findings can inform effective policies and management actions to mitigate future impacts of L. cyclorhyncha.

Assessing host response to disease treatment: how chytrid-susceptible frogs react to increased water salinity

Context The severity and prevalence of the amphibian fungal pathogen, Batrachochytrium dendrobatidis (Bd) is correlated with several environmental variables, including salinity, temperature, and moisture content, which influence the pathogen’s growth and survival. Habitats that contain these environmental variables at levels outside of those optimal for Bd growth and survival may facilitate the survival of susceptible host species. Therefore, manipulation of environmental salinity is a potential management strategy to help conserve Bd-susceptible species. However, host behaviour also influences disease dynamics, and the success of habitat manipulation programs depends on how hosts use this altered habitat. Aims To assess if the Bd-susceptible green and golden bell frog, Litoria aurea, will select waterbodies with a salinity increased to S=3; if this selection is affected by infection; and if a frog’s time in a waterbody of this salinity affects infection load or blood physiology. Methods We conducted a filmed choice experiment and a 3-year field study where infected and uninfected frogs could choose between fresh or saline waterbodies. Key results In both the laboratory experiment and field study, Bd-infected L. aurea spent a significantly greater amount of time in or closer to a waterbody than uninfected frogs. Experimentally infected frogs tended to prefer the saline water over fresh, but their choice of water usage did not differ statistically from uninfected frogs. In the field, frogs began to avoid ponds when salinities rose above S=5. Conclusions Because both wild and captive, and infected and uninfected L. aurea readily selected waterbodies with a salinity of S=3, this salinity could potentially be used as a passive method for reducing the severity of Bd when managing this species. However, further testing is needed to understand the efficacy of this treatment, and care must be taken to prevent salinities rising above S=5, because this level seems to produce an avoidance response and therefore may not be suitable in every location. Implications Manipulation of aquatic habitats may be a worthwhile focus for Bd management in habitats where water level fluctuations are minimal.

Evidence of a salt refuge: chytrid infection loads are suppressed in hosts exposed to salt

With the incidence of emerging infectious diseases on the rise, it is becoming increasingly important to identify refuge areas that protect hosts from pathogens and therefore prevent population declines. For the chytrid fungus Batrachochytrium dendrobatidis, temperature and humidity refuge areas for amphibian hosts exist but are difficult to manipulate. Other environmental features that may affect the outcome of infection include water quality, drying regimes, abundance of alternate hosts and isolation from other hosts. We identified relationships between water bodies with these features and infection levels in the free-living hosts inhabiting them. Where significant relationships were identified, we used a series of controlled experiments to test for causation. Infection loads were negatively correlated with the salt concentration of the aquatic habitat and the degree of water level fluctuation and positively correlated with fish abundance. However, only the relationship with salt was confirmed experimentally. Free-living hosts inhabiting water bodies with mean salinities of up to 3.5 ppt had lower infection loads than those exposed to less salt. The experiment confirmed that exposure to sodium chloride concentrations >2 ppt significantly reduced host infection loads compared to no exposure (0 ppt). These results suggest that the exposure of amphibians to salt concentrations found naturally in lentic habitats may be responsible for the persistence of some susceptible species in the presence of B. dendrobatidis. By manipulating the salinity of water bodies, it may be possible to create refuges for declining amphibians, thus allowing them to be reintroduced to their former ranges.

Potential influence of plant chemicals on infectivity of Batrachochytrium dendrobatidis

We explored whether extracts of trees frequently found associated with amphibian habitats in Australia and Arizona, USA, may be inhibitory to the fungal pathogen Batrachochytrium dendrobatidis (Bd), which has been associated with global amphibian declines. We used salamanders Ambystoma tigrinum as the model system. Salamanders acquired significantly lower loads of Bd when exposed on leaves and extracts from the river red gum Eucalyptus camaldulensis, and loads were also low in some animals exposed on extracts of 2 oak species, Quercus emoryi and Q. turbinella. Some previously infected salamanders had their pathogen loads reduced, and some were fully cured, by placing them in leaf extracts, although some animals also self cured when housed in water alone. A significant number of animals cured of Bd infections 6 mo earlier were found to be resistant to reinfection. These results suggest that plants associated with amphibian habitats should be taken into consideration when explaining the prevalence of Bd in these habitats and that some amphibians may acquire resistance to the fungus if previously cured.