Global Epidemiology of Batrachochytrium Salamandrivorans Emergence
The outbreak of chytridiomycosis caused by Bsal in the European low countries shares many features with the emergence of Bd. While population genomic analyses have not yet been published, the rapid development of a qPCR-based molecular diagnostic has enabled the use of global surveillance to map the distribution of the pathogen which has indicated likely sources of European infection (Blooi et al.
2013). In total, over 5000 amphibians were screened by qPCR from across four continents (Martel et al. 2014). These data showed that detections of Bsal were limited to East Asia, namely, Thailand, Japan and Vietnam. The data also point to a likely Asian origin of Bsal, and assessments of host associations have strengthened this hypothesis by demonstrating that, while Palearctic caudates from Europe and North America suffer clinical disease following infection by Bsal, Asian species tolerate or clear their infections. Molecular surveys of museum collections have shown that Bsal DNA can be detected in museum specimens of Asian Cynops ensicauda that were collected over 150 years ago and molecular dating shows that Bsal separated from Bd over 30 million years ago. Taken together, these data strongly indicate an Asian evolutionary centre of origin for Bsal infections that have spilled over to infect European salamander and newt species.
As with Bd, there has been much focus on how the transcontinental vectoring of Bsal may have occurred. It is clear now that the international trade in Asian caudate species is enormous, with, for instance, over 2.3 million individuals of Cynops orientalis imported into the United States from 2001 to 2009. Further, the infection is associated with outbreaks of disease in captive European caudates, which constitute a continuous threat to vectoring the disease to wild amphibians (Cunningham et al. 2015; Sabino-Pinto et al. 2015). Therefore, the international movement of traded species of amphibians is a key mechanism contributing to the spread of Bsal and its invasion of naive disease-free ecosystems.
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