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.

PILOT STUDY OF INTRACOELOMIC TERBINAFINE IMPLANTS IN GREATER SIRENS (SIREN LACERTINA)

Chytridiomycosis caused by Batrachochytrium dendrobatidis (Bd) has been documented in greater sirens (Siren lacertina) in the wild and in the pet trade. This study evaluated the use of terbinafine-impregnated implants for chytridiomycosis prophylaxis in greater sirens exposed to Bd. Implants were placed intracoelomically in both control (blank implant, n = 4) and treatment (24.5 mg of terbinafine implant, n = 4) groups. Sirens were exposed to Bd zoospores via 24-h immersion bath at 1 and 2 mon postimplant placement. Blood was collected monthly for plasma terbinafine levels, and skin swabs were collected weekly for Bd quantitative PCR. Animals with terbinafine implants had detectable concentrations of plasma terbinafine ranging from 17 to 102 ng/ml. Only one terbinafine-implanted animal had a peak concentration above the published minimum inhibitory concentration for terbinafine against Bd zoospores (63 ng/ml); however, it is unknown how plasma terbinafine concentrations relate to concentrations in the skin. There was no difference between the two treatment groups in clinical signs or Bd clearance rate, and no adverse effects from implants were observed. These findings indicate using intracoelomic drug implants for drug delivery in amphibians is safe; however, terbinafine efficacy in preventing Bd chytridiomycosis in sirens remains unclear. Further investigation of the use of intracoelomic implants and identification of effective drugs and doses in other amphibian species against Bd and other infectious diseases is warranted, as this may provide a practical method for long-term drug delivery in wildlife.

EFFICACY OF SUBCUTANEOUS IMPLANTS TO PROVIDE CONTINUOUS PLASMA TERBINAFINE IN HELLBENDERS (CRYPTOBRANCHUS ALLEGANIENSI) FOR FUTURE PROPHYLACTIC USE AGAINST CHYTRIDIOMYCOSIS

Batrachochytrium dendrobatidis (Bd) is an important fungal pathogen present in wild hellbender (Cryptobranchus alleganiensis) populations that appears to cause disease during novel exposure and acute stress. Hellbender repatriation efforts are ongoing to combat declining populations, but mortality by chytridiomycosis (disease from Bd) after release has been reported. The goal was to determine whether a safe antifungal agent could be administered and provide prolonged plasma concentrations without repeated handling. A subcutaneous implant impregnated with 24.5 mg of terbinafine was tested in three juvenile eastern hellbenders (C. a. alleganiensis) raised in human care, and plasma terbinafine concentrations were recorded from weekly to biweekly for 141 days. Plasma concentrations were variable, with peak plasma concentrations of 1,610, 112, and 66 ng/ml between 28 and 56 days postimplant. Although all hellbenders achieved plasma concentrations above the published minimum inhibitory concentration for terbinafine against Bd zoospores (63 ng/ml) at several time points, only one individual remained above this threshold for more than two consecutive time intervals. Results show the potential for these implants as a prophylaxis for chytridiomycosis in captive-to-wild hellbender releases. However, further investigation will be needed to determine the plasma concentrations required to achieve prophylaxis in vivo and implant reliability.

Strategies to Better Target Fungal Squalene Monooxygenase

Fungal pathogens present a challenge in medicine and agriculture. They also harm ecosystems and threaten biodiversity. The allylamine class of antimycotics targets the enzyme squalene monooxygenase. This enzyme occupies a key position in the sterol biosynthesis pathway in eukaryotes, catalyzing the rate-limiting reaction by introducing an oxygen atom to the squalene substrate converting it to 2,3-oxidosqualene. Currently, terbinafine—the most widely used allylamine—is mostly used for treating superficial fungal infections. The ability to better target this enzyme will have significant implications for human health in the treatment of fungal infections. The human orthologue can also be targeted for cholesterol-lowering therapeutics and in cancer therapies. This review will focus on the structural basis for improving the current therapeutics for fungal squalene monooxygenase.

Can models of percutaneous absorption based on in vitro data in frogs predict in vivo absorption?

The primary aim of in vitro testing of chemicals delivered via the percutaneous route is to predict the absorption that would ensue if exposure occurred in live animals. While there is mounting evidence that in vitro diffusion studies in mammalian skin can provide valid information regarding likely in vivo absorption, little is known whether such a correlation exists between in vitro diffusion testing and in vivo blood levels in amphibians. The current study used previously-reported in vitro absorption data for caffeine, benzoic acid, and ibuprofen across isolated skin from the cane toad (Rhinella marina) to produce a series of linear mixed-effect models of the absorption parameters flux and permeability coefficient (K_p). Models investigated the relative impacts of animal weight, physicochemical characteristics of the applied chemical (logP or molecular weight), and site of application. The top models were then used to predict the flux, K_p and serum concentrations of the same three model chemicals. Finally, the absorption of these chemicals was determined in live cane toads, and results compared to the model predictions. LogP and site of application were included in all top models. In vivo absorption rates were lower than predicted for all chemicals, however, the models provided reasonable predictions of serum concentration, with factors of difference (FOD) ranging from 2.5–10.5. Ibuprofen, the chemical with the highest relative lipophilicity, had the poorest predictive performance, consistently having the highest FOD for all predictions. This report presents the first models of percutaneous absorption in an amphibian. These models provide a basic method to establish the approximate in vivo absorption of hydrophilic and moderately-lipophilic chemicals through frog skin, and could therefore be used to predict absorption when formulating such chemicals for treatment of disease in frogs, or for risk-assessments regarding chemical pollutants in frog habitats.

Percutaneous absorption between frog species: Variability in skin may influence delivery of therapeutics

Frogs have permeable skin, so transdermal delivery provides a practical alternative to traditional dosing routes. However, little is known about how frog skin permeability differs interspecifically, and there are different reported clinical outcomes following topical application of the same chemical in different frog species. This study collated in vitro absorption kinetic data previously reported for two frog species: the green tree frog (Litoria caerulea) and the cane toad (Rhinella marina), and used linear mixed-effects modelling to produce a model of absorption. Histology of skin samples from each species was performed to observe morphological differences that may affect absorption. Absorption kinetics differed significantly between species, with the logP of the applied chemical a better predictor of permeability than molecular weight. Application site also influenced permeability, with dorsal permeability consistently higher in cane toads. Ventral permeability was more consistent between species. Skin thickness differed between species and skin regions, and this may explain the differences in absorption kinetics. Guidelines for selecting chemicals and dosing site when treating frogs are presented. The permeability differences identified may explain the poor reproducibility reported in the treatment of disease across frog species, and reinforces the importance of considering interspecies differences when designing therapeutic treatments for frogs.

Regional variation in percutaneous absorption in the tree frog Litoria caerulea

Frog skin structure and physiology differs between skin regions, however little is known about how these differences affect transdermal absorption of chemicals. Further, no information is available regarding how the relative lipophilicity of a chemical influences its transdermal pharmacokinetics in frog skin. This study investigated the in vitro percutaneous absorption of three model chemicals - benzoic acid, caffeine, and ibuprofen - through dorsal and ventral skin of the tree frog Litoria caerulea. Flux was significantly higher through the ventral skin for all chemicals. Relative lipophilicity affected flux differently in different skin regions. These differences are likely due to significantly thicker dorsal skin increasing absorption path length, and also possibly owing to lipoid secretions on the dorsum providing an additional diffusional barrier. This knowledge can advise risk mitigation of xenobiotics in agricultural and industrial settings, and also guide selection of chemicals and doses when considering transdermal drug therapy in captive frogs.