Chytridiomycosis is an amphibian skin disease which sometimes occurs following infection by Batrachochytrium dendrobatidis (Bd) or Batrachochytrium salamandrivorans (Bsal). Bd and Bsal are fungal pathogens and members of the phylum Chytridiomycota. Chytridiomycosis has been implicated in the decline or extinction of over 200 species of amphibian worldwide (1).
The defining characteristic of the chytrid fungi is their reproduction via the production of motile zoospores. These then settle and develop into large sporangia, which are the reproductive stage, and go on to produce more zoospores (Figure 1) (2).
The chytrids are a paraphyletic group that diverge early on in the evolution of fungi, occupying a basal position in the fungal tree of life. They can be found all over the world and in almost every environment from tropical to arctic conditions. Only two chytrids are known to be capable of parasitizing vertebrates, however – Bd and Bsal.
Bd was described in 1999 in Australia (3) and appears to have undergone a rapid, global expansion in distribution which continues to this day (Figure 2) (4). It is now found on every continent except Antarctica, where no amphibian hosts exist. In Australia, Bd’s estimated mean rate of advance was about 100km per year and amphibian declines began within months of the pathogen’s arrival in a naïve population (5). The impact of Chytridiomycosis on amphibians has been so great that it has been described as “the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted and its propensity to drive them to extinction (6).
The motile zoospores of Bd seek out the keratin rich skin of adult amphibians, and the keratinised mouthparts of tadpoles as Bd survives by digesting keratin. Once infection is complete they encyst and develop into sporangia (Figure 3).
Infection with Bd causes the cells to undergo hyperkeratosis which results in a thickening in the skin of adult amphibians. It is thought that this hyperkeratosis disrupts osmoregulation across the skin, resulting in fatal electrolyte depletion (5, 7, 8, 9). Tadpoles are able to tolerate infection as only their mouthparts are keratinised, but the fungus migrates to the skin following metamorphosis. This means that there is often a high mortality rate among juvenile amphibians (Figure 4).
Bsal was described from European fire salamanders (Salamandra salamandra) in the Netherlands in 2013 (Figure 5) (2).
This chytrid causes an erosive skin disease and can induce extremely rapid mortality in susceptible amphibians. it is closely related to Bd, but appears to be restricted to the urodeles (salamanders and newts) although it is highly pathogenic in those animals. Bd has a tendency to be found infecting and causing mortality in frogs and toads. Bsal also has a lower thermal growth preference compared to Bd. The research group that described Bsal found that infected fire salamanders died within seven days of exposure to the pathogen, displaying skin erosions and deep ulcerations all over the body. In the Netherlands, the discovery of Bsal followed a catastrophic 96% decline in fire salamander populations between 1997 and 2012 (2, 10).
Bsal’s ‘unholy trinity’ of pathogen characteristics – high virulence, no host immunity and an environmentally resistant transmission stage – make it extremely dangerous (11, 12). We are only just beginning to understand the huge impacts that this newly emerging pathogen is likely to have as it rapidly continues its range expansion.
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