Leveraging preserved specimens of Nerodia to infer the spatiotemporal dynamics of Ophidiomyces ophidiicola via quantitative polymerase chain reaction

Abstract

Ophidiomyces ophidiicola (Oo) is a fungal pathogen and the causative agent of ophidiomycosis that has affected multiple snake taxa across the United States, Europe, and Asia. Ophidiomycosis has often been referred to as an emerging infectious disease (EID); however, its status as an EID has recently come under debate. Oo infections have been confirmed in wild snake populations in Texas; however, it is unknown if the pathogen is novel (i.e., invasive) or endemic to the state. To address this knowledge gap, we conducted surveys for Oo among preserved Nerodia deposited at three university museums in Texas. First, we visually assessed snakes for signs of infection (SOI), and if SOI were present, we sampled the affected area. We then used quantitative polymerase chain reaction to diagnose the presence of Oo DNA on areas with SOI and used these data to evaluate spatiotemporal patterns of Oo prevalence. We also tested for significant spatial clusters of Oo infenction using a Bernoulli probability model as implemented in the program SatScan. We found that the proportion of snakes exhibiting SOI was constant over time while the prevalence of Oo DNA among those SOI increased across space and time. Within these data, we detected an incidence pattern consistent with an introduction and then spread. We detected six spatial clusters of Oo infection, although only one was significant. Our results support the hypothesis that Oo is an emerging, novel pathogen to Texas snakes. These data narrow the knowledge gap regarding the history of Oo infections in Texas and establish a historical record of confirmed Oo detections in several counties across the state. Thus, our results will guide future research to those areas with evidence of past Oo infections but lacking confirmation in contemporary hosts.

Keywords: EID; Texas; museum survey; novel pathogen; qPCR; retrospective; water snakes.

FIGURE 1

Distribution of the proportion of Ophidiomyces ophidiicola detections (Oo +) among preserved Nerodia with potential signs of Oo infection (SOI) collected in Texas from 1905 through 2019. The size of the closed circle is scaled to the proportion of O. ophidiicola detections for each county. The county of record (shaded gray) for preserved snakes when SOI or O. ophidiicola were not observed or detected. The earliest detection (1955) is indicated by the arrow.

FIGURE 2

Preserved Texas Nerodia species collected from 1905 through 2019 grouped by time intervals and surveyed for signs of Ophidiomyces ophidiicola infection (SOI). (a) The proportion of Nerodia that exhibited SOI; and (b) the prevalence of O. ophidiicola among those snakes exhibiting SOI. The prevalence of SOI and O. ophidiicola prevalence for all Nerodia surveyed are indicated by the dotted line.

FIGURE 3

Mapped clusters (red = statistically significant; gray = not significant) derived from SaTScan spatial analysis of Ophidiomyces ophidiicola detections among preserved Texas Nerodia. Hatching demarcates the counties encompassing the range of N. harteri.