The effects of a fungicide and chytrid fungus on anuran larvae in aquatic mesocosms

Evidence for a metal disease refuge: The amphibian-killing fungus (Batrachochytrium dendrobatidis) is inhibited by environmentally-relevant concentrations of metals tolerated by amphibians

The amphibian-killing fungus Batrachochytrium dendrobatidis (Bd) has caused substantial declines in Bd-susceptible amphibian species worldwide. However, some populations of Bd-susceptible frogs have managed to survive at existing metal-polluted sites, giving rise to the hypothesis that frogs might persist in the presence of Bd if Bd is inhibited by metals at concentrations that frogs can tolerate. We tested this hypothesis by measuring the survival of Bd zoospores, the life stage that infects amphibians, and calculated the LC50 after exposure to environmentally-relevant elevated concentrations of copper (Cu), zinc (Zn), and their combination (Cu + Zn) in two repeated 4-day acute exposure runs. We also measured the chronic sensitivity of Bd to these metals over three generations by measuring the number of colonies and live zoospores and calculating EC50 concentrations after 42 days of exposure. We then compared acute and chronic sensitivity of Bd with amphibian sensitivities by constructing species sensitivity distributions (SSDs) using LC50 and EC50 data obtained from the literature. Acute sensitivity data showed that Bd zoospore survival decreased with increasing metal concentrations and exposure durations relative to the control, with the highest LC50 values for Cu and Zn being 2.5 μg/L and 250 μg/L, respectively. Chronic exposures to metals resulted in decreased numbers of Bd colonies and live zoospores after 42 days, with EC50 values of 0.75 μg/L and 1.19 μg/L for Cu and Zn, respectively. Bd zoospore survival was 10 and 8 times more sensitive to Cu and Zn, respectively in acute, and 2 and 5 times more sensitive to Cu and Zn in chronic exposure experiments than the most sensitive amphibian species recorded. Our findings are consistent with the hypothesis that metals in existing metal-polluted sites may have a greater impact on Bd relative to amphibians' performance, potentially enabling Bd-susceptible amphibians to persist with Bd at these sites.

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.

Current trends and mismatches on fungicide use and assessment of the ecological effects in freshwater ecosystems

Despite increasing evidence of off-site ecological impacts of pesticides and policy efforts worldwide, pesticide use is still far from being ecologically sustainable. Fungicides are among the most sold classes of pesticides and are crucial to ensure global food supply and security. This study aimed to identify potential gaps of knowledge and mismatches between research and usage data of fungicides by: (i) systematizing the current trends in global sales of fungicides, focusing on the European context in particular (where they are proportionally important); (ii) reviewing the scientific literature on the impacts of synthetic fungicides on non-target freshwater organisms. Sales data revealed important global and regional asymmetries in the relative importance of fungicides and the preferred active ingredients. The literature review on the ecological effects of fungicides disclosed a mismatch between the most studied and the most sold substances, as well as a bias towards the use of single species assays with standard test organisms. To ensure a proper evaluation, risk scenarios should focus on a regional scale, and research agendas must highlight sensitive aquatic ecorreceptors and improve the crosstalk between analytical and sales data.

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.

Efficacy of Plant-Derived Fungicides at Inhibiting Batrachochytrium salamandrivorans Growth

The emerging fungal amphibian pathogen, Batrachochytrium salamandrivorans (Bsal), is currently spreading across Europe and given its estimated invasion potential, has the capacity to decimate salamander populations worldwide. Fungicides are a promising in situ management strategy for Bsal due to their ability to treat the environment and infected individuals. However, antifungal drugs or pesticides could adversely affect the environment and non-target hosts, thus identifying safe, effective candidate fungicides for in situ treatment is needed. Here, we estimated the inhibitory fungicidal efficacy of five plant-derived fungicides (thymol, curcumin, allicin, 6-gingerol, and Pond Pimafix®) and one chemical fungicide (Virkon® Aquatic) against Bsal zoospores in vitro. We used a broth microdilution method in 48-well plates to test the efficacy of six concentrations per fungicide on Bsal zoospore viability. Following plate incubation, we performed cell viability assays and agar plate growth trials to estimate the minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of each fungicide. All six fungicides exhibited inhibitory and fungicidal effects against Bsal growth, with estimated MIC concentrations ranging from 60 to 0.156 μg/mL for the different compounds. Allicin showed the greatest efficacy (i.e., lowest MIC and MFC) against Bsal zoospores followed by curcumin, Pond Pimafix®, thymol, 6-gingerol, and Virkon® Aquatic, respectively. Our results provide evidence that plant-derived fungicides are effective at inhibiting and killing Bsal zoospores in vitro and may be useful for in situ treatment. Additional studies are needed to estimate the efficacy of these fungicides at inactivating Bsal in the environment and treating Bsal-infected amphibians.

Responsible biosecurity and risk mitigation for laboratory research on emerging pathogens of amphibians

The increasing study of emerging wildlife pathogens and a lack of policy or legislation regulating their translocation and use has heightened concerns about laboratory escape, species spillover, and subsequent epizootics among animal populations. Responsible self-regulation by research laboratories, in conjunction with institutional-level safeguards, has an important role in mitigating pathogen transmission and spillover, as well as potential interspecies pathogenesis. A model system in disease ecology that highlights these concerns and related amelioration efforts is research focused on amphibian emerging infectious diseases. Whereas laboratory escape of amphibian pathogens has not been reported and may be rare compared with introduction events from trade or human globalization, the threat that novel disease outbreaks with mass mortality effects pose to wild populations warrants thorough biosecurity measures to ensure containment and prevent spillover. Here, we present a case study of the laboratory biosecurity concerns for the emerging amphibian fungal pathogen Batrachochytrium salamandrivorans. We conclude that proactive biosecurity strategies are needed to integrate researcher and institutional oversight of aquatic wildlife pathogens generally, and we call for increased national and international policy and legislative enforcement. Furthermore, taking professional responsibility beyond current regulations is needed as development of legal guidance can be slow at national and international levels. We outline the need for annual laboratory risk assessments, comprehensive training for all laboratory personnel, and appropriate safeguards specific to pathogens under study. These strategies are critical for upholding the integrity and credibility of the scientific community and maintaining public support for research on wildlife diseases.

Effect of multiple stressors and population decline of frogs

The ongoing decline in anuran populations is linked primarily to the effects of stressor agents such as pathogens, pesticides, alterations of natural landscapes, and the introduction of exotic species. Most studies that have evaluated the effects of these stressors have focused on a single component, which is the opposite of the reality of most natural environments, where anuran populations tend to suffer the influence of multiple agents simultaneously. Studies of the effects of the interaction between these components are extremely important, given that one agent may potentialize (synergistic effect) or weaken another (antagonistic effect) or, in some cases, have a neutral effect. The present study is based on the scientometric analysis of three bibliographic databases (ISI Web of Science, Scopus, and PubMed), which identified 1376 papers that reported on the global decline of anuran populations, although only 172 of these studies focused on the interactive effects of environmental stressors. Synergistic effects were the most frequent type of interaction, followed by antagonistic effects, and a small number of studies that found no clear interaction between the stressors. Pathogens and pesticides were the classes of stressor studied most frequently, while climate-pathogen and pathogen-pesticide interactions were the combinations that featured in the largest number of studies. Overall, we would recommend a more systematic focus on the dynamics of the interactions among the stressors that impact anuran populations, in particular for the elaboration of conservation programs, given that these agents tend to have complex combined effects.

A Review of the Effects of the Biopesticides Bacillus thuringiensis Serotypes israelensis (Bti) and kurstaki (Btk) in Amphibians

Insecticides are important in agriculture, to reduce human disease, and to decrease the nuisance of biting insects. Despite this, many have the potential for environmental impacts and toxicity in nontarget organisms. We reviewed data on the effects of insecticides based on toxins from Bacillus thuringiensis var. israelensis (Bti) and Bacillus thuringiensis var. kurstaki (Btk) on amphibians. The few peer-reviewed publications that are available for Bti provide variable conclusions, ranging from few observable effects to evidence of acute toxicity at high concentrations. We briefly highlight the current controversies and identify key areas for future investigation.

Recovery and resiliency of skin microbial communities on the southern leopard frog (Lithobates sphenocephalus) following two biotic disturbances

BACKGROUND

Microorganisms have intimate functional relationships with invertebrate and vertebrate taxa, with the potential to drastically impact health outcomes. Perturbations that affect microbial communities residing on animals can lead to dysbiosis, a change in the functional relationship, often associated with disease. Batrachochytrium dendrobatidis (Bd), a fungal pathogen of amphibians, has been responsible for catastrophic amphibian population declines around the globe. Amphibians harbor a diverse cutaneous microbiome, including some members which are known to be antagonistic to Bd (anti-Bd). Anti-Bd microorganisms facilitate the ability of some frog populations to persist in the presence of Bd, where other populations that lack anti-Bd microorganisms have declined. Research suggests disease-antagonistic properties of the microbiome may be a function of microbial community interactions, rather than individual bacterial species. Conservation efforts have identified amphibian-associated bacteria that exhibit anti-fungal properties for use as 'probiotics' on susceptible amphibian populations. Probiotic application, usually with a single bacterial species, may benefit from a greater understanding of amphibian species-specific microbiome responses to disturbances (e.g. dysbiosis vs. recovery). We assessed microbiome responses to two microbial disturbance events over multiple time points.

RESULTS

Exposing Lithobates sphenocephalus (southern leopard frog) adults to the biopesticidal bacteria Bacillus thuringiensis, followed by exposure to the fungal pathogen Bd, did not have long term impacts on the microbiome. After initial shifts, microbial communities recovered and returned to a state that resembled pre-disturbance.

CONCLUSIONS

Our results indicate microbial communities on L. sphenocephalus are robust and resistant to permanent shifts from some disturbances. This resiliency of microbial communities may explain why L. sphenocephalus is not experiencing the population declines from Bd that impacts many other species. Conservation efforts may benefit from studies outlining amphibian species-specific microbiome responses to disturbances (e.g. dysbiosis vs. recovery). If microbial communities on a threatened amphibian species are unlikely to recover following a disturbance, additional measures may be implemented to ameliorate the impacts of physical and chemical stressors on host-associated microbial communities.

A Bacillus thuringiensis kurstaki Biopesticide Does Not Reduce Hatching Success or Tadpole Survival at Environmentally Relevant Concentrations in Southern Leopard Frogs (Lithobates sphenocephalus)

Amphibians are in global decline, and anthropogenic activities are known leading causes of their demise. Thus the interaction between agriculture and amphibian health has been examined for decades. Many facets of amphibian physiology and ecology place them at high risk among the nontarget organisms affected by agricultural byproducts. Research has shown that many chemicals and fertilizers affect amphibian growth, reproduction, and survival. The impacts differ based on the type of agricultural byproduct (e.g., chemical pesticide or nutrient-heavy fertilizer) and amphibian species, but the effects are usually negative. However, minimal research exists on how organic biopesticides interact with amphibian populations. Biopesticides utilize insecticidal bacteria as the active ingredient in lieu of synthetic chemicals. The inert ingredients present in biopesticide commercial products are considered safe to nontarget organisms. The present study tested the impacts of a commercial biopesticide on the survival of amphibian embryos and larvae. We found that expected environmental concentrations of the microbial biopesticide Monterrey B.t. did not significantly reduce survival in embryos or larvae. However, the higher doses used to assess threshold toxicity levels caused significant mortality. Our data suggest that biopesticides are not directly harmful to amphibian embryos or larvae in concentrations regularly applied for pest control. Environ Toxicol Chem 2019;39:155-161. © 2019 SETAC.

Concentration and timing of application reveal strong fungistatic effect of tebuconazole in a Daphnia-microparasitic yeast model

Given the importance of pollutant effects on host-parasite relationships and disease spread, the main goal of this study was to assess the influence of different exposure scenarios for the fungicide tebuconazole (concentration×timing of application) on a Daphnia-microparasitic yeast experimental system. Previous results had demonstrated that tebuconazole is able to suppress Metschnikowia bicuspidata infection at ecologically-relevant concentrations; here, we aimed to obtain an understanding of the mechanism underlying the anti-parasitic (fungicidal or fungistatic) action of tebuconazole. We exposed the Daphnia-yeast system to four nominal tebuconazole concentrations at four timings of application (according to the predicted stage of parasite development), replicated on two Daphnia genotypes, in a fully crossed experiment. An "all-or-nothing" effect was observed, with tebuconazole completely suppressing infection from 13.5μgl^-1 upwards, independent of the timing of tebuconazole application. A follow-up experiment confirmed that the suppression of infection occurred within a narrow range of tebuconazole concentrations (3.65-13.5μgl^-1), although a later application of the fungicide had to be compensated for by a slight increase in concentration to elicit the same anti-parasitic effect. The mechanism behind this anti-parasitic effect seems to be the inhibition of M. bicuspidata sporulation, since tebuconazole was effective in preventing ascospore production even when applied at a later time. However, this fungicide also seemed to affect the vegetative growth of the yeast, as demonstrated by the enhanced negative effect of the parasite (increasing mortality in one of the host genotypes) at a later time of application of tebuconazole, when no signs of infection were observed. Fungicide contamination can thus affect the severity and spread of disease in natural populations, as well as the inherent co-evolutionary dynamics in host-parasite systems.

A pesticide paradox: fungicides indirectly increase fungal infections

There are many examples where the use of chemicals have had profound unintended consequences, such as fertilizers reducing crop yields (paradox of enrichment) and insecticides increasing insect pests (by reducing natural biocontrol). Recently, the application of agrochemicals, such as agricultural disinfectants and fungicides, has been explored as an approach to curb the pathogenic fungus, Batrachochytrium dendrobatidis (Bd), which is associated with worldwide amphibian declines. However, the long-term, net effects of early-life exposure to these chemicals on amphibian disease risk have not been thoroughly investigated. Using a combination of laboratory experiments and analysis of data from the literature, we explored the effects of fungicide exposure on Bd infections in two frog species. Extremely low concentrations of the fungicides azoxystrobin, chlorothalonil, and mancozeb were directly toxic to Bd in culture. However, estimated environmental concentrations of the fungicides did not reduce Bd on Cuban tree frog (Osteopilus septentrionalis) tadpoles exposed simultaneously to any of these fungicides and Bd, and fungicide exposure actually increased Bd-induced mortality. Additionally, exposure to any of these fungicides as tadpoles resulted in higher Bd abundance and greater Bd-induced mortality when challenged with Bd post-metamorphosis, an average of 71 d after their last fungicide exposure. Analysis of data from the literature revealed that previous exposure to the fungicide itraconazole, which is commonly used to clear Bd infections, made the critically endangered booroolong frog (Litoria booroolongensis) more susceptible to Bd. Finally, a field survey revealed that Bd prevalence was positively associated with concentrations of fungicides in ponds. Although fungicides show promise for controlling Bd, these results suggest that, if fungicides do not completely eliminate Bd or if Bd recolonizes, exposure to fungicides has the potential to do more harm than good. To ensure that fungicide applications have the intended consequence of curbing amphibian declines, researchers must identify which fungicides do not compromise the pathogen resistance mechanisms of amphibians.

Mitigating amphibian chytridiomycoses in nature

Amphibians across the planet face the threat of population decline and extirpation caused by the disease chytridiomycosis. Despite consensus that the fungal pathogens responsible for the disease are conservation issues, strategies to mitigate their impacts in the natural world are, at best, nascent. Reducing risk associated with the movement of amphibians, non-amphibian vectors and other sources of infection remains the first line of defence and a primary objective when mitigating the threat of disease in wildlife. Amphibian-associated chytridiomycete fungi and chytridiomycosis are already widespread, though, and we therefore focus on discussing options for mitigating the threats once disease emergence has occurred in wild amphibian populations. All strategies have shortcomings that need to be overcome before implementation, including stronger efforts towards understanding and addressing ethical and legal considerations. Even if these issues can be dealt with, all currently available approaches, or those under discussion, are unlikely to yield the desired conservation outcome of disease mitigation. The decision process for establishing mitigation strategies requires integrated thinking that assesses disease mitigation options critically and embeds them within more comprehensive strategies for the conservation of amphibian populations, communities and ecosystems.This article is part of the themed issue 'Tackling emerging fungal threats to animal health, food security and ecosystem resilience'.

Temperature modulates the interaction between fungicide pollution and disease: evidence from a Daphnia-microparasitic yeast model

Temperature is expected to modulate the responses of organisms to stress. Here, we aimed to assess the influence of temperature on the interaction between parasitism and fungicide contamination. Specifically, using the cladoceran Daphnia as a model system, we explored the isolated and interactive effects of parasite challenge (yeast Metschnikowia bicuspidata) and exposure to fungicides (copper sulphate and tebuconazole) at two temperatures (17 and 20 °C), in a fully factorial design. Confirming a previous study, M. bicuspidata infection and copper exposure caused independent effects on Daphnia life history, whereas infection was permanently suppressed with tebuconazole exposure. Here, we show that higher temperature generally increased the virulence of the parasite, with the hosts developing signs of infection earlier, reproducing less and dying at an earlier age. These effects were consistent across copper concentrations, whereas the joint effects of temperature (which enhanced the difference between non-infected and infected hosts) and the anti-parasitic action of tebuconazole resulted in a more pronounced parasite × tebuconazole interaction at the higher temperature. Thus, besides independently influencing parasite and contaminant effects, the temperature can act as a modulator of interactions between pollution and disease.

Interplay between fungicides and parasites: Tebuconazole, but not copper, suppresses infection in a Daphnia-Metschnikowia experimental model

Natural populations are commonly exposed to complex stress scenarios, including anthropogenic contamination and their biological enemies (e.g., parasites). The study of the pollutant-parasite interplay is especially important, given the need for adequate regulations to promote improved ecosystem protection. In this study, a host-parasite model system (Daphnia spp. and the microparasitic yeast Metschnikowia bicuspidata) was used to explore the reciprocal effects of contamination by common agrochemical fungicides (copper sulphate and tebuconazole) and parasite challenge. We conducted 21-day life history experiments with two host clones exposed to copper (0.00, 25.0, 28.8 and 33.1 μg L^-1) or tebuconazole (0.00, 154, 192 and 240 μg L^-1), in the absence or presence of the parasite. For each contaminant, the experimental design consisted of 2 Daphnia clones × 4 contaminant concentrations × 2 parasite treatments × 20 replicates = 320 experimental units. Copper and tebuconazole decreased Daphnia survival or reproduction, respectively, whilst the parasite strongly reduced host survival. Most importantly, while copper and parasite effects were mostly independent, tebuconazole suppressed infection. In a follow-up experiment, we tested the effect of a lower range of tebuconazole concentrations (0.00, 6.25, 12.5, 25.0, 50.0 and 100 μg L^-1) crossed with increasing parasite challenge (2 Daphnia clones × 6 contaminant concentrations × 2 parasite levels × 20 replicates = 480 experimental units). Suppression of infection was confirmed at environmentally relevant concentrations (> 6.25 μg L^-1), irrespective of the numbers of parasite challenge. The ecological consequences of such a suppression of infection include interferences in host population dynamics and diversity, as well as community structure and energy flow across the food web, which could upscale to ecosystem level given the important role of parasites.