Over the past four decades, emergent fungal diseases have been the most devastating relative to species declines and extinctions. While most research has focused on fungal diseases affecting amphibians and bats, less has focused on diseases like snake fungal disease (SFD), caused by Ophidiomyces ophiodiicola (Oo). SFD was first described in 2006 in North America within a Timber rattlesnake (Crotalus horridus) population in New Hampshire. Since then SFD has been documented in 19 US states, one US territory (Puerto Rico), and Europe. SFD causes high mortality in some species, including the Eastern Massasauga rattlesnakes (Sistrurus catenatus), which is an endangered species. Given the ecosystem services snakes provide (e.g., control of rodent vectors) and known linkages between disease and stress, it is important to understand the relationships between SFD and stress in snakes as well as the distribution of the disease.
One indicator of stress is circulating corticosterone (CORT) concentrations. Along with epinephrine, CORT helps mediate the fight-or-flight response by mobilizing energy stores and modulating the immune system. Previous research has evaluated relationships between baseline corticosterone (CORT) levels and SFD status; however, to my knowledge no research has investigated relationships between SFD and elevated CORT, CORT reactivity (percent increase from baseline to elevated CORT), or CORT variability. My first objective was to examine Timber rattlesnakes for the presence or absence of SFD and measure their circulating CORT levels in one-month intervals. I conducted a three-year study evaluating relationships between SFD disease state and CORT, including baseline and elevated CORT and CORT reactivity along with individual variability of each of these metrics. From May through September 2018 and 2019, I captured and surgically implanted radio-transmitters into 20 snakes. Following implantation, I tracked snake movements at least once per week and obtained blood and swab samples once per month during the active season (May - September) from 2018 - 2020. I found no difference in baseline and elevated CORT between SFD positive and negative individuals except for August, when elevated CORT was 2 times greater in SFD positive snakes. I also found a positive correlation between the proportion of times a snake tested positive for SFD and variability in CORT reactivity. Greater CORT levels in August could have implications for SFD positive snakes as they prepare for the winter inactive period by way of suppressed immune function. Moreover, snakes that test positive for SFD more often show highly variable, inconsistent CORT measures.
To target conservation efforts, we must also delineate the geographic scope of SFD and obtain a better understanding of host taxa. For objective 2, I conducted a descriptive survey to monitor the presence of SFD in snakes in Western Kentucky. I focused my survey efforts in the Jackson Purchase region, including the Land Between the Lakes National Recreation Area, where data on SFD is largely lacking. I collected 124 individual snakes across 18 species. Sixteen percent of the snakes sampled tested positive for SFD with the majority being terrestrial species, particularly the Crotalines. This may indicate taxa specific differences in infection risk. My data on the linkages between SFD and CORT and my monitoring efforts may help managers implement and better strategize conservation and management practices for snakes in an increasingly human dominated landscape.
Year manuscript completed
Year degree awarded
neuroendocrinology, ecology, wildlife disease, herpetology, crotalus, snake fungal disease
Andrea K. Darracq
Hewlett, John Bromley, "MONITORING RELATIONSHIPS BETWEEN CORTICOSTERONE AND SNAKE FUNGAL DISEASE IN TIMBER RATTLESNAKES (CROTALUS HORRIDUS) IN WESTERN KENTUCKY" (2021). Murray State Theses and Dissertations. 218.