Emerging infectious diseases are reducing biodiversity on a global scale. Recently, the emergence of the chytrid fungus Batrachochytrium salamandrivorans resulted in rapid declines in populations of European fire salamanders. Here, we screened more than 5000 amphibians from across four continents and combined experimental assessment of pathogenicity with phylogenetic methods to estimate the threat that this infection poses to amphibian diversity. Results show that B. salamandrivorans is restricted to, but highly pathogenic for, salamanders and newts (Urodela). The pathogen likely originated and remained in coexistence with a clade of salamander hosts for millions of years in Asia. As a result of globalization and lack of biosecurity, it has recently been introduced into naïve European amphibian populations, where it is currently causing biodiversity loss.

Amphibian declines and extinctions are emblematic for the current sixth mass extinction event. Infectious drivers of these declines include the recently emerged fungal pathogens Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans (Chytridiomycota). The skin disease caused by these fungi is named chytridiomycosis and affects the vital function of amphibian skin. Not all amphibians respond equally to infection and host responses might range from resistant, over tolerant to susceptible. The clinical outcome of infection is highly dependent on the amphibian host, the fungal virulence and environmental determinants. B. dendrobatidis infects the skin of a large range of anurans, urodeles and caecilians, whereas to date the host range of B. salamandrivorans seems limited to urodeles. So far, the epidemic of B. dendrobatidis is mainly limited to Australian, neotropical, South European and West American amphibians, while for B. salamandrivorans it is limited to European salamanders. Other striking differences between both fungi include gross pathology and thermal preferences. With this review we aim to provide the reader with a state-of-the art of host-pathogen interactions for both fungi, in which new data pertaining to the interaction of B. dendrobatidis and B. salamandrivorans with the host’s skin are integrated. Furthermore, we pinpoint areas in which more detailed studies are necessary or which have not received the attention they merit.Electronic supplementary materialThe online version of this article (doi:10.1186/s13567-015-0266-0) contains supplementary material, which is available to authorized users.

Batrachochytrium dendrobatidis ( Bd ) is the causative agent of chytridiomycosis, a fungal skin disease in amphibians and driver of worldwide amphibian declines. We focussed on the early stages of infection by Bd in 3 amphibian species with a differential susceptibility to chytridiomycosis. Skin explants of Alytes muletensis , Litoria caerulea and Xenopus leavis were exposed to Bd in an Ussing chamber for 3 to 5 days. Early interactions of Bd with amphibian skin were observed using light microscopy and transmission electron microscopy. To validate the observations in vitro , comparison was made with skin from experimentally infected frogs. Additional in vitro experiments were performed to elucidate the process of intracellular colonization in L. caerulea . Early interactions of Bd with amphibian skin are: attachment of zoospores to host skin, zoospore germination, germ tube development, penetration into skin cells, invasive growth in the host skin, resulting in the loss of host cell cytoplasm. Inoculation of A. muletensis and L. caerulea skin was followed within 24 h by endobiotic development, with sporangia located intracellularly in the skin. Evidence is provided of how intracellular colonization is established and how colonization by Bd proceeds to deeper skin layers. Older thalli develop rhizoid-like structures that spread to deeper skin layers, form a swelling inside the host cell to finally give rise to a new thallus. In X. laevis , interaction of Bd with skin was limited to an epibiotic state, with sporangia developing upon the skin. Only the superficial epidermis was affected. Epidermal cells seemed to be used as a nutrient source without development of intracellular thalli. The in vitro data agreed with the results obtained after experimental infection of the studied frog species. These data suggest that the colonization strategy of B. dendrobatidis is host dependent, with the extent of colonization most likely determined by inherent characteristics of the host epidermis.

Infections with Batrachochytrium dendrobatidis ( B. dendrobatidis ), the causal agent of chytridiomycosis, have been shown to play an important role in the decline of amphibians worldwide. Spread of the fungus is poorly understood. Bird movement might possibly contribute to the spread of B. dendrobatidis in the environment. Therefore, 397 wild geese in Belgium were screened for presence of B. dendrobatidis on their toes using real-time quantitative PCR (qPCR). In addition, chemotaxis towards, adhesion, survival after desiccation and proliferation of B. dendrobatidis on keratinous toe scales from waterfowl were examined in vitro . qPCR revealed that 76 geese (15%) were positive for B. dendrobatidis . Results of the in vitro tests showed that B. dendrobatidis is attracted to the keratinous toes of aquatic birds on which they can adhere and even proliferate. However, desiccation is poorly tolerated. This suggests waterfowl are potential environmental reservoirs for B. dendrobatidis .

Batrachochytrium dendrobatidis is one of the most pathogenic microorganisms affecting amphibians in both captivity and in nature. The establishment of B. dendrobatidis free, stable, amphibian captive breeding colonies is one of the emergency measures that is being taken to save threatened amphibian species from extinction. For this purpose, in vitro antifungal susceptibility testing and the development of efficient and safe treatment protocols are required. In this study, we evaluated the use of amphotericin B and voriconazole to treat chytridiomycosis in amphibians. The concentration at which the growth of five tested B. dendrobatidis strains was inhibited was 0.8 μg/ml for amphotericin B and 0.0125 μg/ml for voriconazole. To completely eliminate a mixture of sporangia and zoospores of strain IA042 required 48 h of exposure to 8 μg/ml of amphotericin B or 10 days to 1.25 μg/ml of voriconazole. Zoospores were killed within 0.5 h by 0.8 μg/ml of amphotericin B, but even after 24 h exposure to 1.25 μg/ml of voriconazole they remained viable. Amphotericin B was acutely toxic for Alytes muletensis tadpoles at 8 μg/ml, whereas toxic side effects were not noticed during a seven-day exposure to voriconazole at concentrations as high as 12.5 μg/ml. The voriconazole concentrations remained stable in water during this exposure period. On the basis of this data, experimentally inoculated postmetamorphic Alytes cisternasii were sprayed once daily for 7 days with a 1.25 μg/ml solution of voriconazole in water which eliminated the B. dendrobatidis infection from all treated animals. Finally, treatment of a naturally infected colony of poison dart frogs (Dendrobatidae) using this protocol, combined with environmental disinfection, cleared the infection from the colony.