Friday, 31 May 2013

Amphibian chytrid disease likely originated in Africa


Amphibian chytridiomycosis is an infectious disease caused by the fungus Batrachochytrium dendrobatidis (Bd), and is widely believed to be the cause of catastrophic worldwide declines in amphibian populations. In Gabon, the fungus has been identified in two National Parks (Monts de Cristal and Ivindo; Bell et al., 2011), but not in other areas studied (Lope, Lastourville and Lebamba; Daversa et al., 2011; Gamba and Libreville; Gratwicke et al., 2011), and with no evidence of associated disease or mortality, so its epidemiology and impacts on native amphibians is not yet fully understood.
Two new studies out this month shed light on the origin of the disease and its global spread. The first, published in PNAS , is a phylogenetic study that suggests the recent pandemic strain is much older than previously research had suggested and evolved thousands of years ago, predating the recent global epidemic (Rosenblum et al., 2013). The researchers suggest the disease is most likely endemic in some parts of its range, such as Africa or Asia, but is a novel pathogen in other areas, and blame recent globalization through trade as the most likely cause of its spread.
Other research supports this. Xenopus laevis, the south African clawed frog, has long been suspected of spreading the disease: the species has been traded globally since the 1900’s, and the earliest known case of the fungus was found in native populations in South Africa. By examining museum specimens from as far back as 1871, a new study published in PLOS One is the first to find evidence of Bd fungus in recent, introduced feral populations of Xenopus in North America, as well as in older, native populations from Africa (Vredenburg et al., 2013). This study supports the hypothesis that Bd was present as a stable, endemic infection in Xenopus populations in Africa prior to their worldwide distribution likely via international live-amphibian trade.
Further research is required to understand if Bd is an introduced or endemic infection in Gabon, and how prevalent and pathogenic it is among native amphibian species.
Rosenblum EB, James TY, Zamudio KR, Poorten TJ, Ilut D, Rodriguez D, Eastman JM, Richards-Hrdlicka K, Joneson S, Jenkinson TS, Longcore JE, Parra Olea G, Toledo LF, Arellano ML, Medina EM, Restrepo S, Flechas SV, Berger L, Briggs CJ, Stajich JE, 2013. Complex history of the amphibian-killing chytrid fungus revealed with genome resequencing data. Proceedings of the National Academy of Sciences.10.1073/pnas.1300130110

Vredenburg V, Stephen A. Felt S, Morgan E, McNally S, Wilson S, Green S, 2013. Prevalence of Batrachochytrium dendrobatidis in Xenopus Collected in Africa (1871–2000) and in California (2001–2010). PloS one 8:e63791. 

Gratwicke, B., Alonso, A., Elie, T., Kolowski, J., Lock, J., Rotzel, N., Sevin, J. & Fleischer., R.C. (2011) Batrachochytrium dendrobatidis not detected on amphibians from two lowland sites in Gabon, Africa. Herpetological Review, 42, 69–71.
Bell RC, Gata Garcia A V, Stuart BL, Zamudio KR (2011) High Prevalence of the Amphibian Chytrid Pathogen in Gabon. EcoHealth 8: 116–120.
Daversa D, Bosch J, Jeffery K (2011) First survey of the disease-causing fungus, Batrachochytrium dendrobatidis, in amphibian populations of tropical areas of Gabon, Africa. Herpetological Review 42: 67–69.

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