Prevalence Rates of Snake Fungal Disease and Its Population-level Impacts in a Snake Assemblage in Southwest Louisiana

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WARC researchers used visual encounter surveys to determine prevalence rates of snake fungal disease in south-central Louisiana.

The Science Issue and Relevance: In the last 20 years, an extraordinary number of fungal and fungal-like diseases have caused some of the most severe die-offs and extinctions ever observed in wild species. Snake fungal disease (SFD) is a recently documented mycotic disease that has been deemed a global conservation issue and an emerging threat to wild snake. Snake fungal disease became more widely known after two 2011 publications documented high mortality rates caused by SFD in two protected species of pit viper in the Midwestern and Northeastern United States. We confirmed SFD in Louisiana in 2015, but virtually nothing is known about prevalence rates or populations impacts of SFD in the Southeastern United States.

snake fungal disease

A young broad-banded watersnake from south-central Louisiana confirmed to have snake fungal disease

(Credit: Brad Glorioso. Public domain.)

Methodology for Addressing the Issue: We use visual encounter surveys to sample snakes in a capture-mark-recapture (CMR) approach in a small area (~ 3 ha) of semi-permanent and ephemeral wetlands at Palmetto Island State Park in Louisiana. This site was chosen due to the presence of snakes with clinical signs of SFD and the thought that snakes, notoriously hard to recapture, may be recaptured here with some success. After sexing and taking standard measurements of captured snakes, each snake is marked by injection of a passive integrated transponder (PIT) tag and then marked for future identification. After diagramming all areas of possible dermatitis, we take 1-4 swabs of each snake dependent upon the severity of clinical signs of SFD. All areas swabbed are also photographed. We use a real-time polymerase chain reaction (qPCR) assay to detect O. ophiodiicola, the causative agent of SFD, on our swab samples. Marking, swabbing, and photographing each capture allows us to determine prevalence rates by species, and investigate disease progression and its possible effects on fitness and survival. Multi-state CMR models are used to estimate survival of infected and uninfected snakes as well as the probability of an individual transitioning from one state to the other.

 

Future Steps: We suspended sampling at Palmetto Island State Park in early 2019. We are working to put together the results in anticipation of one or more publications.