Abstract
Batrachochytrium dendrobatidis (Bd), a causal agent of the amphibian fungal skin disease chytridiomycosis, has been implicated in the decline and extinction of over 200 species worldwide since the 1970s. Despite almost two decades of research, the history of Bd and its global spread is not well understood. However, the spread of the Global Panzootic Lineage of Bd (Bd-GPL), the lineage associated with amphibian die-offs, has been linked with the American bullfrog (Rana [Aqurana] catesbeiana) and global trade. Interestingly, R. catesbeiana is native to the eastern U.S., where no Bd-related declines have been observed despite Bd’s presence since the late 1800s. In contrast Bd has been found to have emerged in California and Mexico in the 1960s and 1970s, after which epizootics (i.e., epidemics in wildlife) ensued. We hypothesize that Bd-GPL spread from the eastern U.S. with the introduction of R. catesbeiana into the western US, resulting in epizootics and declines of native host species. Using museum records, we investigated the historical relationship between R. catesbeiana and Bd invasion in the western US and found that R. catesbeiana arrived in the same year or prior to Bd in most western watersheds that had data for both species, suggesting that Bd-GPL may have originated in the eastern US and R. catesbeiana may have facilitated Bd invasion in the western US. To predict areas with greatest suitability for Bd, we created a suitability model by integrating habitat suitability and host availability. When we incorporated invasion history with high Bd suitability, we found that watersheds with non-native R. catesbeiana in the mountain ranges of the West Coast have the highest disease risk. These findings shed light on the invasion history and disease dynamics of Bd in North America. Targeted historical surveys using archived specimens in natural history collections and present-day field surveys along with more localized, community-level studies, monitoring, and surveillance are needed to further test this hypothesis and grow our understanding of the disease ecology and host-pathogen dynamics of Bd.
Highlights
Chytridiomycosis is an emerging infectious disease primarily caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd)
See Supplemental S2 Fig for a breakdown of the different watersheds types
We investigated the invasion history of R. catesbeiana and Bd in the western US and found a pattern of Bd dynamics consistent with the hypothesis that invasion of R. catesbeiana facilitated Bd spread in western North America
Summary
Chytridiomycosis is an emerging infectious disease primarily caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd). This pathogen has significantly affected global amphibian biodiversity, infecting over 500 species [1] and causing declines and extinctions in at least 200 species since the 1970s [2,3,4]. Genomic studies have led to the discovery of multiple Bd strains that range in pathogenicity [5,9,10], which provides some insight of the evolutionary history of Bd. Different Bd lineages have been identified in Brazil, Switzerland, South Africa, and South Korea, where they appear to be in an enzootic state of coexistence with native amphibian populations [5,9,11]. Genome-wide patterns suggest that mitotic recombination via asexual reproduction is the more likely mechanism to have led to the emergence of BdGPL [10]
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