A fungal ‘superbug’ causes global amphibian declines

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 Find out how ZSL scientists discovered the link between the frog trade and a killer fungus seriously effecting amphibians worldwide with Frog Blog

My name is Rhys and I am a postgraduate student at Imperial College London and the Zoological Society of London (ZSL). For the past two years I have been studying a chytrid fungus known as Bd, which is responsible for likely hundreds of extinctions and local extinctions (extirpations) in amphibians worldwide. Our group is interested in understanding how -until recently- an unknown disease can emerge to become a major driver of population declines in vertebrates world-wide. To answer this question, we have travelled to habitats across the globe to both sample and record the prevalence of this fungus in wild populations, focussing in particular on amphibian populations in Europe. We have taken samples from amphibians across a diverse range of habitats from the mountains of the French Pyrenees to northern Mallorca, recovering isolates of Bd that appeared to be markedly different from one another in their microscopic characteristics.

Collecting Alytes obstetricans tadpoles from a Bd+ site in Switzerland

But why do they appear different? To explain, we compared the differences between their genomes (genetic history). This showed us that Bd consists of at least three separate and divergent lineages, each likely to have been spread, and potentially to have come into contact with one another via the global trade of amphibians. The global trade of live amphibians supplies amphibians for human consumption, the pet trade, lab animal trade and zoos and in many ways is largely unregulated. One of these Bd lineages also has features within the genome that may have resulted from hybridization between two parental strains of the disease. Our data shows that this is the most common type of Bd, and also the type associated with mass-death and extinction of whole species. Lab experiments show that this lineage is hyper-virulent when compared with the other identified lineages. These findings raise an interesting possibility: that extinctions from Bd are not solely caused by introduction into naive populations, but that the largely unregulated trade in amphibians has inadvertently created this fungal super-bug. Preventing future panzootics therefore may rest in readdressing the measures used to prevent transmission of infectious diseases (biosecurity) in the amphibian trade to prevent accelerated evolution and spread of hyper-virulent diseases in the future.   Rhys Farrer

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