Chytridiomycosis

Review

The ecology and impact of chytridiomycosis: an emerging disease of amphibians
A. Marm Kilpatrick1, Cheryl J. Briggs2 and Peter Daszak3
1 2

Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95064, USA Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106-9610, USA 3 Wildlife Trust, 460 W. 34thSt., 17th Floor, New York, NY 10001, USA

Emerging infectious diseases are increasingly recognized as key threats to wildlife. Batrachochytrium dendrobatidis (Bd), the causative agent of chytridiomycosis, has been implicated in widespread amphibian declines and is currently the largest infectious disease threat to biodiversity. Here, we review the causes of Bd emergence, its impact on amphibianpopulations and the ecology of Bd transmission. We describe studies to answer outstanding issues, including the origin of the pathogen, the effect of Bd relative to other causes of population declines, the modes of Bd dispersal, and factors influencing the intensity of its transmission. Chytridiomycosis is an archetypal emerging disease, with a broad host range and significant impacts on hostpopulations and, as such, poses a crucial challenge for wildlife managers and an urgent conservation concern. Introduction Infectious diseases are increasingly recognized as key threats to animal populations, but relatively little is understood about their ecology compared with human pathogens [1]. Several emerging diseases have caused declines in multiple families of plants and animals, includingWest Nile virus in North American birds [2], avian malaria in Hawaiian birds [3], rinderpest in African ungulates [4], sudden oak death in trees in western North America, Jarrah dieback or rootrot in trees in Australia [5], and chytridiomycosis, first described in 1998 and caused by Batrachochytrium dendrobatidis (Bd; Box 1), in amphibians in North and Central America, Europe, and Australia [6–8].For each of these diseases (many of which were introduced anthropogenically into new regions), the pathogen has a broad host range, with some species showing little pathology (Box 2), whereas others suffer mortality approaching 100%. The Global Amphibian Assessment (http://www. iucnredlist.org/amphibians) recently argued that the 6000+ species of amphibians are one of the most threatened classes ofvertebrate, with 32.5% of species threatened. In addition, 92.5% of the ‘critically endangered’ group are undergoing ‘enigmatic declines’ that might be linked to Bd [9,10]. In Latin America, Bd has been linked to possible extinctions in 30 of the 113 species of Atelopus
Corresponding author: Kilpatrick, A.M. (marm@biology.ucsc.edu)

harlequin toads [11]. Amphibian declines are currently oneof the most compelling conservation issues and Bd, first identified only a decade ago, appears to have had an important role. Here, we focus on outstanding questions in the emergence of this disease, including the origin of the pathogen, its impacts on host populations and the ecology of its transmission. Introduced or endemic pathogen? There has been much debate about whether Bd was recentlyintroduced to areas where it is causing population declines (the ‘novel pathogen hypothesis’), or whether it has been a long-term endemic pathogen, and population declines result from changes in host susceptibility, pathogen virulence, environmental changes, or a combination of these factors (the ‘endemic pathogen hypothesis’) [12–17]. This is a crucial question, because conservation actions woulddiffer substantially if it is an introduced pathogen (e.g. implementing trade restrictions to stem additional spread, or attempted eradications), or an endemic pathogen, in which case efforts to determine the environmental changes driving emergence of Bd would become paramount. Genetic analyses of Bd isolates have been used to attempt to distinguish between the novel and endemic pathogen hypotheses...