Amphibian chytridiomycosis in Japan: distribution, haplotypes and possible route of entry into Japan

Abstract

A serious disease of amphibians caused by the chytrid fungus Batrachochytrium dendrobatidis was first found in Japan in December 2006 in imported pet frogs. This was the first report of chytridiomycosis in Asia. To assess the risk of pandemic chytridiomycosis to Japanese frogs, we surveyed the distribution of the fungus among captive and wild frog populations. We established a nested PCR assay that uses two pairs of PCR primers to amplify the internal transcribed spacer (ITS) region of a ribosomal RNA cassette to detect mild fungal infections from as little as 0.001 pg (1 fg) of B. dendrobatidis DNA. We collected swab samples from 265 amphibians sold at pet shops, 294 bred at institutes and 2103 collected at field sites from northern to southwestern Japan. We detected infections in native and exotic species, both in captivity and in the field. Sequencing of PCR products revealed 26 haplotypes of the B. dendrobatidis ITS region. Phylogenetic analysis showed that three of these haplotypes were specific to the Japanese giant salamander (Andrias japonicus) and appeared to have established a commensal relationship with this native amphibian. Many other haplotypes were carried by alien amphibians. The highest genetic diversity of B. dendrobatidis was found in the American bullfrog (Rana catesbeiana). Some strains of B. dendrobatidis appeared to be endemic to Japanese native amphibians, but many alien strains are being introduced into Japan via imported amphibians. To improve chytridiomycosis risk management, we must consider the risk of B. dendrobatidis changing hosts as a result of anthropogenic disturbance of the host-specific distribution of the fungus.

Multiple emergences of genetically diverse amphibian-infecting chytrids include a globalized hypervirulent recombinant lineage

“…Research has recently suggested the existence of a Bd lineage that is associated with the Japanese giant salamander, Andrias japonicus (22). This lineage, defined by sequencing a short fragment of the ribosomal DNA, is dissimilar to the rDNA sequences of BdGPL, which nonnative North American bullfrogs have introduced to Japan.…”

Evidence for acquisition of virulence effectors in pathogenic chytrids

“…sudden appearance, strong virulence, and rapid transmission) [5]. Investigations of museum specimens collected from as far back as 1902 in Japan [6] and 1938 in Africa [7] have detected Bd in skin cross sections. On the other hand, long-term population studies since the 1900's had no records of mass mortalities until the late 1970's when the first Bd epidemics were reported [8].…”

The pandemic pathogen of amphibians,Batrachochytrium dendrobatidis(Phylum Chytridiomycota), in Italy

“…Is Bd a newly emerged pathogen recently introduced to areas by animal (including human) or other vectors where it is causing population declines (Novel Pathogen Hypothesis), or is Bd an endemic parasite and recent disease emergences are attributable to increased virulence caused by (human-mediated) ecological and environmental changes (Endemic Pathogen Hypothesis; Rachowicz et al 2005;Skerrat et al 2007)? Some authors argue that both mechanisms contribute to the existing distribution of presence, prevalence and disease (Goka et al 2009;Walker et al 2010) but the fundamental question regarding introduction remains valid, as efforts to mitigate disease may differ based on whether a pathogen is introduced or not. Evidence in support of introduction includes the presence of infection in commercially traded amphibian species both at source and in invasive populations (Mazzoni et al 2003;Weldon et al 2004;Fisher & Garner 2007).…”