Volume 27, Number 7—July 2021
Confirmed Cases of Ophidiomycosis in Museum Specimens from as Early as 1945, United States
Ophidiomycosis represents a conservation threat to wild snake populations. The disease was reported in North America early in the 21st century, but the history of ophidiomycosis has not been investigated. We examined museum specimens and confirmed cases of ophidiomycosis >50 years before the disease’s reported emergence.
Emerging fungal pathogens of wildlife are recognized as major threats to global biodiversity, causing population declines and extinction events in a variety of host species (1). Ophidiomyces ophidiicola, the causative agent of ophidiomycosis, is one such pathogen recognized as a conservation threat to wild snakes (2). The disease first gained attention in 2008 when fatal infections emerged in eastern massasauga rattlesnakes (Sistrurus catenatus) in Illinois, USA (3), and has since been documented throughout North America and Europe (2,4). The earliest retrospective detection of O. ophidiicola in snakes was from 2000 (5). We report the earliest known confirmed cases of ophidiomycosis in free-living snakes in the United States, dating back to 1945.
We investigated the historical occurrence of ophidiomycosis in snakes in the United States by examining specimens preserved in formalin or ethanol at the University of Wisconsin Zoological Museum (UWZM; Madison, WI, USA) and Morehead State University Museum Collection (Morehead, KY, USA). We visually examined 524 specimens representing 30 snake species from 19 states in the eastern United States collected during 1900–2012 (Appendix 1). To reduce risk for cross-contamination, we first examined snakes for clinical signs of ophidiomycosis within the glass jars in which they were stored. When specimens were removed from the jars for sampling, new gloves were worn to handle each snake. We observed clinical signs consistent with ophidiomycosis (Figure) in 47 (9.0%) snakes (6). These specimens represented 12 species from 7 states with collection dates ranging from 1929 to 1983 (Appendix 1).
Clinical signs of ophidiomycosis are not pathognomonic, and a confirmed diagnosis requires compatible histopathologic lesions and the detection of O. ophidiicola (6). Because these confirmatory steps involve destructive sampling of museum material, we selected a subset of snakes (n = 12) for these analyses. We targeted specimens with large (>0.5 cm2) or multiple skin lesions from distant geographic areas and collected >25 years before the reported 2008 emergence of ophidiomycosis (3) (Table). From selected snakes, we excised and formalin-fixed portions of lesioned skin, routinely processed them for light microscopy, and stained with periodic acid-Schiff and Grocott methenamine silver methods. We also collected small pieces of lesioned skin (≈4 mm2) for PCR-based detection of O. ophidiicola. We extracted DNA from dehydrated tissue by using the Gentra Puregene Tissue Kit (QIAGEN, https://www.qiagen.com); we used 10 µL of the kit-provided proteinase K per sample. Negative controls consisted of blank extractions. For PCR, we used existing primers that specifically target the internal transcribed spacer region (ITS) of O. ophidiicola (6) and a newly designed PCR assay that targets mitochondrial NADH dehydrogenase subunit 1 (nad1) (Appendix 2). We targeted these 2 loci, which exist at high copy numbers in the genome, because amplifiable DNA was expected to be at low abundance in the preserved specimens. We cloned and sequenced PCR amplicons of the appropriate size to confirm the presence of O. ophidiicola. We conducted tissue collection, DNA extraction, and PCR under strict protocols (e.g., unidirectional workflow and regular decontamination of work surfaces and equipment) to prevent contamination of samples.
Of the 12 snakes subjected to histopathological analyses, 7 (58.3%) had microscopic lesions with intralesional fungi consistent with ophidiomycosis (6) (Table; Figure). We detected DNA from O. ophidiicola in 3 (50%) of the 6 specimens from UWZM that had been stored in 70% ethanol (Table). We did not detect DNA of O. ophidiicola in snakes from the Morehead State University Museum Collection (n = 6), likely because these specimens were stored long-term in formalin, which is known to affect the recovery of amplifiable nucleic acid. These results highlight the importance of targeting specimens stored in ethanol rather than formalin for molecular-based detection of pathogens in archival material.
We amplified the ITS target from 2 of the 3 specimens and nad1 target from all 3 specimens; these sequences were 100% identical to existing O. ophidiicola sequences in GenBank. The 3 additional specimens from UWZM were strongly suspected to represent cases of ophidiomycosis on the basis of the presence of arthroconidia in histologic sections of lesioned skin (6); however, fungal DNA from these specimens may not have been suitable for PCR amplification. Negative controls performed as expected. The 3 PCR-positive specimens met the diagnostic criteria for confirmed cases of ophidiomycosis (6); they were collected in Florida in 1945, Wisconsin in 1958, and Tennessee in 1973 (Table). These cases predate the earliest previously known detection of O. ophidiicola in free-living snakes in North America by as much as 55 years (5).
Museum specimens can provide crucial insights into the history of emerging infectious diseases. Preserved animal specimens have been used to trace the origin and spread of other fungal pathogens, such as Pseudogymnoascus destructans (white-nose syndrome in bats) and Batrachochytrium spp. (chytridiomycosis in amphibians) (8–10). By using a similar approach, we demonstrate that ophidiomycosis was circulating in the eastern United States for decades before its recognition as an emerging disease. Future work focusing on how such factors as climate change, environmental disturbance, and underlying health of snake populations influence ophidiomycosis dynamics might reveal the mechanism by which ophidiomycosis is emerging (2).
Dr. Lorch is a diagnostic microbiologist and research scientist at the US Geological Survey National Wildlife Health Center, Madison, Wisconsin. His research focuses on emerging infectious diseases of wildlife and the development of molecular tools for use in wildlife disease diagnostics.
We thank the University of Wisconsin Zoological Museum and Morehead State University Museum Collection for allowing us to examine and destructively sample specimens.
This work was funded by the US Geological Survey and McIntire-Stennis Cooperative Forestry Research Program (#1014910).
Data for this study are available at https://doi.org/10.5066/P9FLC1XK.
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Original Publication Date: June 09, 2021