Houston ChandlerViews:
Wildlife diseases have become increasingly prevalent over the last several decades. Chytrid fungus has been linked to dramatic population declines and extinctions in numerous amphibian species, and white nose syndrome has decimated many bat populations in the eastern U.S. Since originally implicated in the decline of a Timber Rattlesnake population (Clark et al. 2011), there has been increased concern about ophidiomycosis (Snake Fungal Disease) and its potential negative effects on snake populations. Ophidiomycosis is caused by the fungal pathogen Ophidiomyces ophiodiicola, which typically infects the dermal layer of snakes. Infected individuals exhibit a range of symptoms, including scabs, lesions, eroded and discolored scales, swelling, and ulcers. Severe infections can be fatal, and there is evidence that infected individuals alter their behavior in an attempt to clear infections (Tetlaff et al. 2017). Ophidiomycosis has now been observed in snakes across the eastern U.S. and from overseas, increasing concern that it could cause declines in snake populations similar to those caused by fungal pathogens in other taxa.
From 2016–2018, we sampled for ophidiomycosis in Georgia’s native snake fauna, collecting 962 swab samples from 786 individual snakes (representing 34 species). Swab samples were then tested for O. ophiodiicola DNA and, combined with a description of any skin lesions present, were used to determine whether or not the snake had ophidiomycosis. The results of this study were recently published in the journal Scientific Reports and can be read in full here. Briefly, 137 individuals (17.4%) tested positive for O. ophiodiicola, and a majority of those snakes (85%) had skin lesions, indicating that they apparently had ophidiomycosis. Another set of individuals (159, 20.3%) had skin lesions but tested negative for fungal DNA, indicating that they could possibly have ophidiomycosis. The majority of individuals (490, 62.3%) included in the study tested negative for fungal DNA and had no lesions.
One of the interesting results from this 2-year project was that we observed significant variability between species in the four ophidiomycosis categories from negative to apparent (see below network figure). This variability even occurred within genera and among snakes that live in similar habitats. For example, both Brown and Plain-bellied Watersnakes (Nerodia taxispilota, N. erythrogaster) had relatively high rates of apparent ophidiomycosis. However, Banded Watersnakes (N. fasciata) had low rates. Furthermore, other aquatic snakes in the genera Liodytes and Haldea also had low rates of infection (although sample sizes were lower for these difficult to encounter species). These results strongly indicate that there are differences in susceptibility across species and even within the same genus. It is currently unclear what causes these differences, and it could be linked to several factors, including skin structure, life history traits, or even sampling bias when attempting to sample smaller species.
One of the most concerning findings from our research was the high prevalence of skin lesions and O. ophiodiicola DNA observed in Eastern Indigo Snakes (Drymarchon couperi) (read the full paper here). The presence of skin lesions in Georgia’s indigo snake populations is not especially surprising as other authors have often described similar skin ailments that occur during the winter months. However, the high prevalence of fungal DNA combined with multiple individuals clearly exhibiting severe infections that were decreasing overall body condition is cause for concern. We have continued to sample indigo snakes for ophidiomycosis after the conclusion of the initial study. Of the 42 indigo snakes sampled from November 2018 – February 2019, 90% tested positive for fungal DNA. We have now observed apparent ophidiomycosis in indigo snake populations from across their Georgia range, and there appear to be few if any populations that aren’t exposed to this fungal pathogen. It remains unclear whether or not Florida populations experience similarly high infection rates.
The survey results for indigo snakes mirror what has been observed across most studies. Ophidiomycosis appears to be most places, and snakes from a large group of species are susceptible. I see likely cases on social media frequently. Even with the prevalence of this fungal pathogen, we still know relatively little about how it effects snakes at the population level. Does it cause frequent mortality? Does it lead to reduced reproductive output or behavior changes that could impact population viability? A lack of clear answers to these questions makes it incredibly difficult to say what effect ophidiomycosis is actually having on most snake species. Long-term studies with marked snakes are one of the best methods for assessing population level effects, and we plan to continue monitoring indigo snake populations in Georgia to answer some of these questions. Other studies involving telemetry and additional DNA work will also need to be completed to better understand the behavior and reproductive output of infected snakes and how snakes come into contact with O. ophiodiicola in the environment. Overall, given the severe consequences that fungal pathogens have had on other groups, we clearly have insufficient data to feel confident that ophidiomycosis will not join a long list of factors threatening native snake populations.
Literature Cited
Chandler, H. C., M. C. Allender, B. S. Stegenga, E. Haynes, E. Ospina, and D. J. Stevenson. 2019. Ophidiomycosis prevalence in Georgia’s Eastern Indigo Snake (Drymarchon couperi) populations. PLoS ONE 14:e0218351.
Clark, R. W., M. N. Marchand, B. J. Clifford, R. Stechert, and S. Stephens. 2011. Decline of an isolated timber rattlesnake (Crotalus horridus) population: Interactions between climate change, disease, and loss of genetic diversity. Biological Conservation 144:886–891.
Haynes, E., H. C. Chandler., B. S. Stegenga, L. Adamovicz, E. Ospina, D. Zerpa-Catanho, D. J. Stevenson and M. C. Allender. Ophidiomycosis surveillance of snakes in Georgia, USA reveals new host species and taxonomic associations with disease. Scientific Reports 10:10870.
Tetzlaff, S. J., M. J. Ravesi, B. A. Degregorio, M. C. Allender, J. M. Josimovich, E. T. Carters, and B. A. Kingsbury. 2017. Snake fungal disease affects behavior of free-ranging Massasauga Rattlesnakes (Sistrurus catenatus). Herpetological Conservation and Biology 12:624–634.
