Invasive pathogens threaten species recovery programs

Captive breeding and re-introduction is integral to the recovery of many threatened species [1], but such practices carry an associated risk of introducing exotic and potentially unknown pathogens into naïve settings. Amphibians are facing a mass extinction crisis and an emerging pathogen, Batrachochytrium dendrobatidis, described only in 1998, is now recognised as a principal driver of these declines [2]. Debate rages about the role of invasion [2,3] versus climate change [4] in determining the distribution of B. dendrobatidis and chytridiomycosis. The severity of the threat from anthropogenic spread is recognized by the recent decision to list chytridiomycosis as a notifiable disease by the OIE (World Organisation for Animal Health) [5]. Case-studies documenting anthropogenic spread are rare, however. Here, we report that native island populations of the IUCN red-listed Mallorcan Midwife Toad Alytes muletensis are infected by B. dendrobatidis and suffering from chytridiomycosis. We trace the source of this infection by screening archived mortalities from a captivebreeding facility that had been used for re-introduction of the species to its native habitat. Our study provides the first strong evidence that the anthropogenic movement of amphibians is spreading B. dendrobatidis; it also provides a salutatory lesson of the need to ensure that breeding-programs are not hotbeds for crossspecific disease transmission, and that species are free of infectious agents prior to reintroduction. In 1997, mass mortalities of Alytes obstetricans in central Spain [6] led to the first report of chytridiomycosis in a wild European amphibian population. In Europe, the pathogen is now known to be widespread ([7] and our unpublished data). The finding of a dead A. muletensis juvenile in 2004, and the subsequent confirmation of a B. dendrobatidispositive status prompted us to screen archived captive populations of A. muletensis that had been used for reintroductions; to examine the pattern of occurrence of infection in native A. muletensis populations; and to determine the multilocus genotypes of isolates of B. dendrobatidis from Mallorca relative to other localities worldwide. Our screening of archived mortalities of captive A. muletensis using whole-genome amplification, quantitative PCR [8] and histology found clear evidence of B. dendrobatidis in 1991, 1992, 1994 and 1995 (Figure S1 and Table S1 in the Supplemental data available online). Xenopus gilli, an endangered frog endemic to the Western Cape, South Africa, was brought into the same breeding facilities in 1991 and housed in the same room as the A. muletensis colony. Of the two specimens of X. gilli mortalities examined, one was positive for B. dendrobatidis. Following the import of X. gilli, 23 captive A. muletensis died. Of the five A. muletensis mortalities archived in 1991, three were positive for B. dendrobatidis. Surveillance of wild populations in Mallorca found that B. dendrobatidis was present in four of the 21 populations surveyed. The distribution of B. dendrobatidis among the larval populations was heterogeneous and highly clustered (Potthoff-Whittinghill test statistic, p < 0.001; Figure 1; Table S2 in the Supplemental data). In two of these infected populations, Cocó de sa Bova and Torrent des Ferrerets, a prevalence of, or almost, 100% was recorded. The population at Cocó de sa Bova received animals from the aforementioned breeding facility in 1991. Multilocus genotypes from five Mallorcan isolates of B. dendrobatidis were all identical to each other, and different to those known from both mainland Spain and the UK (Figure 2). This finding is consistent with there having been a single introduction of an exotic strain of B. dendrobatidis into Mallorca. Taking into account the heterogeneous distribution of infection on Mallorca, the presence of B. dendrobatidis in both the captive A. muletensis and X. gilli, and the lack of similarity between the identical genotypes of B. dendrobatidis from Mallorca compared to those from mainland Spain and elsewhere, we believe that a recent introduction of B. dendrobatidis to