A century of Batrachochytrium dendrobatidis in Illinois amphibians (1888-1989)

Abstract

The fungal pathogen, Batrachochytrium dendrobatidis (Bd), causes the disease chytridiomycosis in amphibians and is responsible for the worst epizootics in vertebrate history. In some regions of the world (e.g., the Neotropics and Western United States), Bd has caused recent reductions in amphibian population abundance and species richness, while in other regions the impacts are less clear. Although Bd is present in the Midwestern United States, its history and impact in the region is not known. We used a qPCR assay to determine historic Bd prevalence in Illinois, testing 1028 specimens representing 10 anuran species, collected 1888-1989. We used two complementary sets of samples to first assess historic prevalence with the primary set, and used a secondary set to confirm Bd presence and examine older samples with a more sensitive technique. Prevalence varied among species; in the primary dataset of 1008 samples extracted with PrepMan Ultra collected 1892-1989, Bd was found in four species (11.1%, CI: 9.3-13.2%). Rana (. Lithobates) sphenocephala, the southern leopard frog, had the highest prevalence (38.3%, CI 32.7-44.2%); prevalence among other infected species was <7%. Overall prevalence was <10% in most decades but >40% in the 1940s. In the secondary set of 50 samples extracted with Qiagen Blood and Tissue Kits (30 re-swabbed of the original 1008 and 20 additional older specimens), 17 of the 20 additional samples were Bd+ (85.0%, CI 64.0-94.8%) including the oldest Bd+ specimen, which was collected in 1888. We confirmed Bd presence by sequencing 42 Bd+ samples and found ≥99% homology with Bd sequences in Genbank. By 1900, Bd was geographically widespread throughout Illinois-40. years earlier than the current oldest estimate in the U.S.-meaning that Illinois amphibians have been coexisting with Bd for at least 126. years. This long period of coexistence from our results raises new questions about the history of Bd in North America, possible coevolution between host and pathogen, and the potential role of Bd in historic population declines.