Batrachochytrium salamandrivorans (Bsal)

Batrachochytrium salamandrivorans (Bsal) is a single-celled fungus (closely related to B. dendrobatidis which has had serious effects on amphibian populations around the world) that primarily infects urodelans (newts and salamanders), causing a fatal skin disease in non-resistant species. It is native to Southeast Asia where it infects native salamanders without causing significant disease. It has recently been introduced to Europe, probably with salamanders imported for the pet trade; after initial discovery in the Netherlands, it has been found in neighbouring countries as well. It has caused very serious declines in populations of native host species in the areas where it is present. In view of its virulence and the fact that it appears to have a wide host range, it is feared that it could devastate European newt and salamander populations, and that it could have a similar effect in North America if it were to be introduced there.

Batrachochytrium is the single genus in the chytrid family Batrachochytriaceae of the Order Rhizophydiales (Wijayawardene et al., 2022). It comprises only two species, Batrachochytrium salamandrivorans (Bsal) and the closely related B. dendrobatidis (Bd). Together with Ichthyochytrium vulgare, a parasite of freshwater fishes, these pathogens are the only members of the Phylum Chytridiomycota known to infect vertebrate hosts (Martel et al., 2013; Rooij et al., 2015).

Batrachochytrium salamandrivorans was described in 2013 from tissue samples of wild salamanders (Salamandra salamandra) collected in Bunderbos, southern Netherlands. The species name comes from the Greek salamandra (salamander) plus the suffix vorans meaning eating, in reference to the characteristic necrotizing skin lesions and rapid mortality observed in salamanders. The holotype is deposited at Ghent University, Belgium, where it is kept in liquid nitrogen (Martel et al., 2013).

Batrachochytrium salamandrivorans is a single-celled fungus that does not develop a mycelium. The reproductive body or ‘thallus’ consists of one to several zoosporangia (which produce asexual spores), and thin hyphal-like rhizoids for nutrient absorption. In the epidermis of amphibians, Bsal forms predominantly colonial thalli that contain several walled sporangia. Thalli located in keratinocytes measure 6.9-17.2 μm in diameter with an average of 12.2 μm. In culture, however, thalli are predominantly monocentric, i.e. containing a single zoosporangium. These zoosporangia measure 15.7-50.3 μm in diameter (average 27.9 μm) and develop one to several discharge papillae or tubes through which spores are released. Two types of spores have been identified, a motile flagellated zoospore (Martel et al., 2013), and a buoyant encysted spore (Stegen et al., 2017). The motile zoospores are 4-5.5 μm in diameter (average 4.6 μm) and have a single posterior flagellum. The encysted spores are slightly larger, have a thick wall, and lack a flagellum. Ultrastructural features include a large nucleus located outside the ribosomal mass, multiple mitochondria and numerous lipid globules (Martel et al., 2013).

Batrachochytrium salamandrivorans (Bsal) belongs, like its congener B. dendrobatidis (Bd), to the single-celled eukaryote Phylum Chytridiomycota. This novel chytrid fungus has, like Bd, two life stages: the thallus, which is the reproductive body and asexually produces spores, and the spore, which is the infectious stage. Two types of spores have been identified in Bsal: a motile flagellated zoospore and a buoyant encysted spore that can persist in both water and soil and is more resistant than the flagellated spore. Bsal develops inside the epidermal cells, where it forms predominantly colonial thalli that contain several walled sporangia. In culture, however, thalli mostly contain a single sporangium. Bsal grows well in tryptone-gelatine hydrolysate-lactose (TGhL) or in broth containing peptonized milk, tryptone and glucose (PmTG). Some strains apparently can also grow on plant material.

Sexual reproduction has not been observed in Bsal as yet. The life cycle in amphibian skin is largely similar to that of Bd, but the lesions produced by each species are different. Bd typically induces epidermal hyperplasia and hyperkeratosis, whereas Bsal causes ulcerative lesions. Bsal also has markedly different thermal preferences than Bd. Optimal growth temperatures lie between 10 and 15°C, but the fungus can still continue to grow at 4°C. Exposure to 25°C and higher is lethal and, therefore, can be used to treat infected animals. Topical treatment with fungicides (e.g. itraconazole) or a combination of fungicides (polymyxin E + voriconazole) and heat can clear Bsal infections. Most chemical disinfectants, including 1% Virkon S, 4% sodium hypochlorite, and 70% ethanol will kill the pathogen.

The native distribution of B. salamandrivorans (hereafter Bsal) is thought to lie in south-east and east Asia (Martel et al., 2014). In this region, the pathogen, which diverged from its sister species Batrachochytrium dendrobatidis in the Late Cretaceous or early Paleogene, coevolved with the native urodelans, exists at low levels and does not cause apparent population declines. The oldest evidence of its presence in Asian urodelans comes from a 150-year-old museum specimen of Cynops ensicauda, a species restricted to the Amami and Okinawa islands in southern Japan (Martel et al., 2014).

To date, Bsal has been confirmed in six Asian countries: Thailand, Vietnam, China, Hong Kong, Taiwan and Japan, but it is suspected to be more widespread than currently reported. A survey in north Borneo (Malaysia and Brunei) failed to detect the pathogen (McLeod et al., 2020).

Batrachochytrium salamandrivorans has been detected in the wild in the Netherlands, Belgium, Germany and Spain. It has also been found in captive animals in Germany, the Netherlands, the UK and Spain (Sabino-Pinto et al., 2015; Fitzpatrick et al., 2018). Bsal tests have been conducted in other parts of Europe (France, Italy, Switzerland, Austria, Czechia, Poland, Latvia, Estonia, Slovakia, Croatia, Montenegro, Greece and Turkey), but so far no positives have been found in these countries (Martel et al., 2014; Saare et al., 2021; Castro-Monzon, 2022).

The pathogen has not yet been detected in the Americas. Sampling has been conducted in Canada (Govindarajulu et al., 2017), the U.S. (Bales et al., 2015; Klocke et al., 2017; Waddle et al., 2020; Hill et al., 2021), Mexico, Guatemala (Ellison et al., 2019), Costa Rica (Adams, 2020), Panama (Martel et al., 2014) and Peru (Parrot et al., 2017). However, most of these studies have tested a small fraction of species or have been limited to small geographic areas.

Batrachochytrium salamandrivorans is believed to originate from Southeast Asia. Globalization and the international pet trade in Asian salamanders have probably led to its introduction to north-western Europe (Martel et al., 2014). It could have reached wild hosts by means of wastewater from an enclosure with captive Asian salamanders or via escaped/released individuals.

The oldest known European record of Bsal is two preserved specimens of infected fire salamanders (Salamandra salamandra terrestris) collected in the Eifel region of Germany in 2004 (Lötters et al., 2020a). The pathogen, however, was first detected in the southern Netherlands in 2008-2010 and was scientifically named in 2013 from an isolate obtained from a fire salamander collected in the Bunderbos forest (Martel et al., 2013). Bsal was associated with the collapse of the salamander population at the type locality in Bunderbos 7 years after its supposed introduction (Spitzen-van der Sluijs et al., 2013; 2016). In 2013 and 2014, deaths and population declines in fire salamanders were noticed in several locations in Belgium, near the Dutch outbreak sites. Infected and dead alpine newts (Ichthyosaura alpestris) were also found in Belgium at this time (Martel et al., 2014).

In 2015, Bsal was detected in captive collections in Germany (Sabino-Pinto et al., 2015) and the UK (Cunningham et al., 2015). In the latter country, Bsal was found in quarantined amphibians, which were newly acquired from a UK amphibian breeder by a zoological collection. The infected animals either died while in quarantine or were euthanized to prevent any further spread of the disease (Cunningham et al., 2015). Additional Bsal-positive individuals tested in 2015 and 2016 were later reported from seven private amphibian collections in the Netherlands, the UK and Spain. A collection from the UK was identified as the source of the infected individuals in all other collections (including the zoological collection), providing further evidence of the risk presented by the amphibian trade in the spread of the disease (Fitzpatrick et al., 2018).

By 2016, Bsal was present in 14 field sites in the Netherlands, Belgium and Germany, with an extent of occurrence of approximately 10,000 km² (Spitzen-van der Sluijs et al., 2016). In 2019, it was reported from specimens collected between 2014 and 2018 in two regions in northern Spain, Cantabria and Asturias, more than 1000 km from the nearest known occurrence in Belgium (Lastra-González et al., 2019; 2021). Around the same time, Bsal was detected by a different team in the Montnegre i el Corredor Natural Park in Catalonia, eastern Spain (Martel et al., 2020). As of 2022, the pathogen has been reported from ca. 80 field sites in Europe, of which more than half are in Germany (Lötters et al., 2020b; Castro-Monzon et al., 2022).

The risk of introducing Bsal is very high due to the high numbers of amphibians traded to and within North America and Europe each year. In the U.S. alone, for example, an average of 3.73 million amphibians were imported annually between 1999 and 2021 (Connelly et al., 2023). Import regulations imposed by the USFWS in 2016 have effectively reduced the import of targeted salamander species and subsequent risk of introduction. However, other potential Bsal carriers (e.g. Alytes, Anaxyrus, Bombina, Hyla, Osteopilus, Rana and Scaphiopus) that were not included in the USFWS listing are still imported in large quantities, representing a continued threat to the urodele fauna of the U.S. and North America (Grear et al., 2021; Connelly et al., 2023).

North America holds the highest urodelan diversity globally, with nearly 50% of all species occurring on the continent. In particular, the Appalachian Mountains, the Pacific Northwest, and the highlands of Central and Southern Mexico are notable for their high number of endemic and relictual salamander species (Gray et al., 2015; Basanta et al., 2019). These regions have been identified as the most important high-risk areas of Bsal introduction (Yap et al., 2015; Richgels et al., 2016; Basanta et al., 2019). In addition to their greater salamander species richness and environmental suitability, these regions are in close proximity to some of the most active ports of entry for live amphibians, namely Los Angeles (45.6% of individuals came through this port between 1999 and 2021), New York (20.9%) and San Francisco (15%). Given that a number of North American species have been shown to be susceptible to Bsal in laboratory tests, the introduction of this pathogen could be catastrophic for the native amphibian fauna (Carter et al., 2019; Gray et al., 2023).

Spores are the primary means of dispersal and can be transmitted through both water and soil. Encysted spores have been shown to remain infective in an aquatic environment for at least a month. These spores can also persist in soil and are more environmentally resistant than flagellated spores. In an experiment by Stegen et al. (2017), infected salamanders were found to be able to contaminate forest soil. Bsal transmission was observed when healthy individuals came into contact with this spore-contaminated soil (Stegen et al., 2017).

Batrachochytrium salamandrivorans transmission includes direct contact with an infected animal, but other non-amphibian vectors might also play an indirect role in the pathogen’s spread. The encysted spores have been shown to adhere to the scales of the feet of waterfowl, potentially promoting their spread over long distances (Stegen et al., 2017).

Batrachochytrium salamandrivorans is thought to have been accidentally introduced to Europe through the amphibian pet trade. Like Bd, it is predicted to continue to spread via this pathway given its high prevalence in amphibian collections, the occurrence of asymptomatic carriers, and the large volume of animals traded globally each year (Fitzpatrick et al., 2018; Sabino-Pinto et al., 2018; Brunner et al., 2023; Connelly et al., 2023).

Batrachochytrium salamandrivorans primarily affects urodel amphibians (newts and salamanders), but can also attack some anurans. It was previously thought that anurans could not be infected (Martel et al., 2014; Fitzpatrick et al., 2018) or were only asymptomatic carriers of the pathogen (Nguyen et al., 2017; Stegen et al., 2017). However, recent studies indicate that certain frog species can also develop chytridiomycosis and experience mortality when exposed to high zoospore doses (Towe et al., 2021; Gray et al., 2023). Caecilians apparently do not become infected, although few species have been investigated (Martel et al., 2014; Flach et al., 2020).

To date, 90 species of amphibians belonging to 13 different families have been reported as hosts of Bsal (Castro-Monzon et al., 2022; Chen et al., 2023; Connelly et al., 2023; Gray et al., 2023). About 75 percent belong to the families Salamandridae and Plethodontidae.

Diagnostic methods to detect the presence of Bsal include histopathology, cell culture and quantitative real-time PCR (qPCR). Histological and culture methods require invasive sampling of the host and specialist expertise for accurate identification. qPCR, on the other hand, can be performed on non-invasively collected skin swabs and can easily be implemented by diagnostic laboratories without the need for specialist expertise (Thomas et al., 2018). This assay can detect and quantify the pathogen load, even in individuals with no signs of disease. A duplex qPCR developed by Blooi et al. (2013) allows the detection of Bd and Bsal simultaneously.

An overview of all available diagnostic methods for Bsal can be found in Thomas et al. (2018) and OIE (2023). Culture techniques for Bsal growth, culturing protocols, and a method for isolating the pathogen from tissue are described by Robinson et al. (2020).

Batrachochytrium salamandrivorans differs from Bd in the presence of numerous colonial thalli throughout all epidermal cell layers. In contrast, most thalli are monocentric in Bd (Berger et al., 2005; Rooij et al., 2015). The lesions produced by each species are also different. Bd typically induces epidermal hyperplasia (cell proliferation) and hyperkeratosis (thickening of the skin), whereas Bsal causes characteristic ulcerative lesions (Martel et al., 2013). In culture, the encysted zoospores produce germ tubes or tubular extensions from which new sporangia arise. Germ tubes are produced by Bd in the host epidermis, but have not been observed in culture (Rooij et al., 2015). Photos comparing the morphology of each species, both in culture and in the amphibian skin, can be found in Rooij et al. (2015).

Another important difference between both species is their dissimilar thermal preferences. The thermal preference of Bd is 17-25°C, while the optimum range for Bsal is 10-15°C (Martel et al., 2013; Rooij et al., 2015).

Batrachochytrium species inhabit aquatic or semi-aquatic environments. Animals testing positive for Bsal have been reported from ponds, streams and moist sites in forests (Spitzen-van der Sluijs et al., 2016; Laking et al., 2017; Yuan et al., 2018).

The Batrachochytrium salamandrivorans genome, which was sequenced independently by Kelly et al. (2021) and Wacker et al. (2023), is much larger and more complex than that of Bd and other chytrid fungi. Kelly et al. (2021) sequenced nine isolates collected from six outbreak sites across Europe. Substantial differences in genome size (range 34.9-41.6 Mbp), repetitive sequence content (7.1-16.2 Mbp) and gene count (10,353-17,091) were found among the isolates, indicating extensive intraspecific variation. Several gene families associated with pathogenicity were found to have undergone significant expansions and divergence within Bsal, including the Crinkler proteins (effector proteins with diverse roles in infection), M36 metalloproteases (involved in the breakdown of amphibian skin) and genes involved in chitin metabolism. Such vast variation and isolate-specific gene acquisitions and expansions suggest diverse metabolic capacities and great adaptive potential, which helps explain Bsal’s devastating effect on amphibian populations. From a biogeographic standpoint, the occurrence of these highly divergent lineages suggests that Bsal may have been present in Europe for much longer than previously thought and that the current epidemic is the result of multiple introductions to the continent (Kelly et al., 2021).

Wacker et al. (2023) reported a total genome length of 73.3 Mbp, with 40.9% (30 Mbp) consisting of repetitive sequences. This makes the Bsal genome the second-largest in the Phylum Chytridiomycota after Cladochytrium polystomum (81.2 Mbp), and the most repeat-rich of any chytrid sequenced to date. Pathogenicity genes (e.g. M36 metalloproteases) and genes upregulated during infection are enriched in transposable elements and other repetitive sequences, suggesting these regions are a cradle for the evolution of pathogenicity in this species (Wacker et al., 2023).

Batrachochytrium salamandrivorans reproduces only asexually via mitotically produced spores. Upon release from zoosporangia, the flagellated zoospores actively swim to find a suitable host (or reinfect the same individual), while encysted non-motile spores float at the water’s surface and passively attach to a passing host (Stegen et al., 2017). The motile zoospores first encyst on the skin by developing a cell wall and absorbing the flagellum. The encysted spores then enter the skin cells, where they produce one to several rhizoids and develop into the thallus. Thalli (immature sporangia) increase in volume, and when they reach a specific size, they produce a large number of spores, which are released through discharge tubes (Martel et al., 2013; Robinson et al., 2020). Severe to lethal signs of chytridiomycosis can appear in susceptive individuals as early as 8-10 days after exposure to zoospores, although this time frame varies greatly among host species (Martel et al., 2014; Gray et al., 2023).

The lifecycle in culture, from spore to zoosporangium, is completed within 5 days at 15°C. As with many members of Chytridiomycota, sexual reproduction has not been observed in this species (Martel et al., 2013; Rooij et al., 2015).

In the laboratory, Bsal can grow in different culture media, but the growth rate is highest in tryptone-gelatin hydrolysate-lactose (TGhL) and tryptone media. On half-strength TGhL plates with 1% agar incubated at 15°C, sporulation begins after 3 days of growth with robust zoospore release after 4 days (Robinson et al., 2020).

Some strains apparently have the potential for sustained growth on plant material. Kelly et al. (2021) reported an isolate with saprotrophic capacity that grew better on lima bean (Phaseolus lunatus) culture medium, hay and beech (Fagus sylvatica) litter than in the standard TGhL medium. This suggests that some strains might be able to withstand prolonged periods without an amphibian host, particularly after causing the population collapse (Kelly et al., 2021).

Batrachochytrium salamandrivorans is an amphibian-specific pathogen primarily associated with urodeles, although it can also infect some anurans. Non-Asian species in the Plethodontidae (lungless salamanders) and Salamandridae (newts) appear to be particularly susceptible (Martel et al., 2014; Gray et al., 2023).

The growth and survival of Bsal is strongly dependent on temperature, the optimum range being 10-15°C. The pathogen can still grow and infect the host at 4°C, but the progression of infection is slower (Stegen et al., 2017). Temperatures of 25°C and higher are usually lethal (Martel et al., 2013; Blooi et al., 2015a), although infected individuals have been detected at sites with water temperature up to 26.4°C, suggesting different thermal tolerances across lineages (Laking et al., 2017).

In the laboratory, Bsal can be stored in culture for weeks to months at 4°C, but it will die rapidly if left at room temperature for an extended period (Robinson et al., 2020). Like Bd, the fungus is likely highly susceptible to desiccation, although the encysted spore might have some tolerance (Stegen et al., 2017).

Batrachochytrium salamandrivorans spores are predated upon by zooplankton. In a laboratory experiment using pond water containing copepods, ciliates, rotifers, ostracods, heliozoans and water fleas, Stegen et al. (2017) found that almost half of motile zoospores were ingested by these organisms after 4 h incubation. The encysted spores, in contrast, were avoided by these micro-predators.

Salamanders are vital components of ecosystems, contributing to various ecological processes that maintain the health and integrity of terrestrial and aquatic environments (Davic and Welsh, 2004). Their decline or extirpation could have negative implications for the timber industry, fishing and tourism, which contribute billions of dollars to the economy.

The implementation of biosecurity measures has direct economic consequences on the pet trade industry. Nearly 2.5 million live salamanders were imported into the U.S. from 2004 to 2014, primarily for the pet trade (annual average = 228,000 individuals). The economic losses, including direct and indirect effects from loss in revenue to pet stores as a result of the trade restrictions imposed by the U.S. Fish and Wildlife Service in 2016, were estimated to be 10 million U.S. dollars/year (USFWS, 2016).

Severe declines in the population of fire salamanders (Salamandra salamandra), smooth newts (Lissotriton vulgaris) and Alpine newts (Ichthyosaura alpestris) have already been observed in places where Bsal has been introduced in Europe. Fire salamander populations in parts of the extreme south of the Netherlands (the only part of the country where the species is found) have been reduced by as much as 99.99% (Spitzen-van der Sluijs et al., 2013; 2016).

Given the virulence, apparent wide host range, and the effects that B. dendrobatidis has had worldwide, Bsal has the potential to devastate urodelan biodiversity in Europe, North Africa, western Asia and the Americas (Martel et al., 2014; Gray et al., 2015; Yap et al., 2015; Spitzen-van der Sluijs et al., 2016). In Europe, 30 species (75% of urodelan diversity) are considered to be at high risk of extinction at the population level due to Bsal (Gilbert et al., 2020). In North America, which is home to the world’s richest and most diverse salamander fauna, predicted losses could exceed 140 species (Gray et al., 2023).

Extinctions or large reductions in salamander populations could have significant ecological effects. Salamanders throughout their global range are often abundant vertebrates in a wide variety of habitats and connect aquatic and terrestrial food webs and contribute to ecosystem stability. They regulate food webs directly as mid-level predators and indirectly by controlling grazers and detritivores; in some cases they effectively contribute to the carbon cycle (Davic and Welsh, 2004; Best and Welsh, 2014). In North American forests, Plethodontid salamanders often outnumber any vertebrate species in biomass -- for example Ensatina enscholtzii predates on many invertebrates and effectively promotes leaf litter retention and fixation or slow release of carbon (Best and Welsh, 2014). Many species burrow and contribute to soil quality and dynamics as well as providing tertiary consumers with energy and nutrients (Davic and Welsh, 2004). Salamanders, as all amphibians, are also indicators for ecosystem health. If Bsal were to be introduced in such communities and devastate them such ecosystem services might well be lost.

In addition to their appeal as pets, salamanders have many medical applications ranging from bioactive compounds in the skin secretions to applications gained from research on their regenerative abilities. For example, the red-legged salamander (Plethodon shermani) and the marbled salamander (Ambystoma opacum) produce adhesive secretions with potential applications as bioadhesives, such as tissue glue for skin healing (Byern et al., 2017). Several newt genera (Cynops, Notophthalmus, Paramesotriton, Taricha and Triturus) contain tetradotoxin and its analogs (Yotsu-Yamashita et al., 2017), which are potent neurotoxins with many therapeutic uses (Bucciarelli et al., 2021). The loss of these and other caudate species will result in the loss of unique compounds that could positively impact human and animal health.

Clinical signs of chytridiomycosis due to Bsal include superficial erosions or ulcers, skin shedding, lethargy, anorexia and death. The skin ulcerations are usually visible as circular spots with black margins and may occur at any site on the body, including the head, limbs and tail (Martel et al., 2013; Rooij et al., 2015). These signs, however, are not unique to Bsal or may be absent in asymptomatic individuals (Sabino-Pinto et al., 2018). In the case of diseased animals, the microscopic observation of intracellular structures consistent with colonial thalli in a tissue sample taken from a skin lesion may indicate Bsal infection. However, a definitive diagnosis can only be made by PCR (OIE, 2023).

Prevention strategies such as wildlife trade regulations, stricter quarantine rules and import bans are among the most important and effective measures for limiting pathogen spread and are therefore considered a top priority to prevent Bsal invasion (Garner et al., 2016; Grant et al., 2016; EFSA AHAW Panel (EFSA Panel on Animal Health and Animal Welfare) et al., 2018; Thomas et al., 2019).

Amphibian trade restrictions were instituted by the U.S. and Canada in 2016 and 2017, respectively, as a precautionary action in response to the Bsal threat. In the U.S., the Fish and Wildlife Service imposed strict regulations to limit the importation and interstate transportation of 201 species of salamanders considered potential Bsal carriers (USFWS, 2016). The Canadian government, for its part, established a ban on the importation of all salamander species unless accompanied by a permit issued (for scientific or research purposes) by the Environmental and Climate Change Canada (ECCC) (Government of Canada, 2018). These bans, however, are insufficient to prevent Bsal translocation, given that several anuran species known to carry the pathogen are still traded in large quantities (Connelly et al., 2023).

In Europe, the threat posed by Bsal was first recognized by the Standing Committee of the Bern Convention, which recommended imposing immediate restrictions on salamander and newt trade to prevent further introduction and spread (Standing Committee to the Convention on the Conservation of European Wildlife and Natural Habitats, 2015). In 2018, the European Commission issued decision 2018/320, which establishes animal health protection measures for the import and trade of salamanders within the EU. These include rejecting animals with signs of infection, quarantining salamanders, testing salamanders to certify they are free from Bsal, and implementing appropriate sanitary and biosecurity measures (Commission Implementing Decision (EU), 2018).

Sanitary measures to minimize Bsal transmission include disinfecting equipment and material that might have been contaminated or exposed to the fungus (EFSA AHAW Panel (EFSA Panel on Animal Health and Animal Welfare) et al., 2018; Thomas et al., 2019). Most commonly used chemical disinfectants can kill Bsal within a relatively short time. The recommended options are 1% Virkon S, 4% sodium hypochlorite and 70% ethanol, with a minimal contact time of 5 min for Virkon S and 1 min for NaOCl and EtOH (Rooij et al., 2017). Heat treatments are also effective at killing Bsal, but its use as a disinfectant requires more study (Thomas et al., 2019).

For professional and private breeders, it is of the utmost importance to dispose of wastewater, substrate, and other materials according to hygiene protocols as described by Cunningham (2018) and the EFSA AHAW Panel (EFSA Panel on Animal Health and Animal Welfare) et al. (2018), as water, soil, or the escape or release of infected animals risk spreading the disease to wild populations. People visiting Bsal infected areas should also adhere to hygiene protocols such as disinfecting hands, footwear, field equipment and even vehicles (EFSA AHAW Panel (EFSA Panel on Animal Health and Animal Welfare) et al., 2018; Thomas et al., 2019).

The environmental application of fungicides might be a viable strategy for controlling Bsal in certain wild amphibian populations. However, this type of treatment is often ineffective in the long term (unless fungicides are applied on a regular basis), and has many potential negative effects that must be carefully considered, such as the emergence of anti-fungal resistance (Garner et al., 2016).

In captivity, heat treatments or a combination of heat with anti-fungals have been shown to effectively eliminate Bsal infections (Blooi et al., 2015a, b). Exposure to the critical thermal maximum (25°C) for 10 days completely clears Bsal infection and resolves the lesions in affected animals. However, this temperature also approaches the upper thermal limit of some urodeles, rendering this method of limited use for these species (Blooi et al., 2015a, b). In these cases, the infected individuals can be treated with a combination of polymyxin E and voriconazole for 10 days at an ambient temperature of 20°C (Blooi et al., 2015b). Itraconazole, an anti-fungal often used to clear Bd infections, is also effective, although more experimental evidence is needed (Plewnia et al., 2023).

Improving amphibian resistance to chytridiomycosis has been explored as a management option against Bd, but has not yet been attempted against Bsal. Both (topical) vaccination and the application of probiotics to augment the skin microbiome have been proposed as potential prophylactic strategies for Bsal (Grant et al., 2016). However, to date, these approaches have shown limited success in the laboratory and might not be viable under field conditions. Compared to bacteria and viruses, the development of a fungal vaccine is very difficult and expensive, and any vaccine would need to be useful in a range of amphibian species (Garner et al., 2016; Thomas et al., 2019). In addition, laboratory experiments have shown that infected salamanders do not mount any protective immune response after repetitive exposure to Bsal, which essentially excludes vaccination as a feasible mitigation strategy (Stegen et al., 2017).

Batrachochytrium salamandrivorans surveillance systems, both passive and active, have been in place in at least 12 European countries since 2013 (EFSA AHAW Panel (EFSA Panel on Animal Health and Animal Welfare) et al., 2018). In North America, ongoing surveillance studies are being conducted by different agencies in all three countries (Gray et al., 2015; Bsal Task Force, 2022). The Bsal Surveillance and Monitoring Group, one of the eight working groups within the North American Bsal Task Force, is tasked with coordinating the sampling efforts across the region (Bsal Task Force, 2022).

Passive surveillance consists of opportunistic detection of sick and dead individuals and testing these for Bsal infection. It relies on the public for reports of diseased or dead animals and submission of samples. Active surveillance includes the regular screening of wild and captive populations, as well as animals in the pet trade. This type of surveillance provides the most accurate picture of the pathogen occurrence but requires substantial resources and capacity, which may be out of reach for many countries (EFSA AHAW Panel (EFSA Panel on Animal Health and Animal Welfare) et al., 2018; Brunner, 2020). For example, screening animals in the pet trade alone is already a daunting task given the massive volume of trade. One approach that can substantially reduce sample sizes and facilitate pathogen surveillance is the analysis of DNA from environmental samples (eDNA), e.g. taken from water bodies or housing containers (Brunner, 2020; Brunner et al., 2023). The eDNA method has proven useful for Bsal screening in both natural (Lastra-González et al., 2021) and captive settings (Brunner et al., 2023).

Much needs to be learned about the host range and host-pathogen interaction, including the mechanisms conferring disease tolerance in Bsal’s native range.

In north-western Europe, in some locations where Bsal is present population declines appear absent even in a very susceptible species as the fire salamander (Salamandra salamandra) (Spitzen-van der Sluijs et al., 2016); it is not yet known whether this is due to recent introduction which has not given declines time to happen or whether there are other reasons such as environmental factors.

How Bsal survives outside its host is not yet known in detail. Why it primarily affects urodelans, how the pathogen establishes itself in the skin and induces death, possible immune responses (innate and acquired) and host susceptibility are all fields that require more investigation in the future (Rooij et al., 2015).