By Danny L. Bryan, Ph.D.
Associate Professor of Biology
The timber rattlesnake (Crotalus horridus) is facing serious threats throughout the United States, with population studies indicating a decreasing trend throughout its range. Once considered a vile and deadly reptile, sentiment toward the snake is changing as further details of its life history are determined. Timber rattlesnakes are not aggressive and rely on camouflage as their primary defense. Timber rattlesnakes are found in the eastern United States from New England to northern Florida and eastern Texas to Minnesota. Originally found in 34 states, the species has been extirpated from Delaware, Maine, and Rhode Island. It is listed as endangered in Connecticut, Indiana, Massachusetts, New Hampshire, New Jersey, Ohio, Vermont, and Virginia. The species is considered threatened in Illinois, Minnesota, New York, and Texas. Timber rattlesnakes are protected in Arkansas, Kansas, Maryland, Tennessee, and Wisconsin and are partially protected in Mississippi, Missouri, Nebraska, and Pennsylvania. In Tennessee, it is listed as a species of Greatest Conservation Need and protected from take.
Cumulative effects of land clearing for timber and agriculture, habitat alteration for road construction, residential and commercial development, fragmentation of foraging habitat, destruction of hibernacula, vehicular mortality, natural predation, and collection from the wild have all contributed to decline of the species, with future extinction not outside of the realm of possibility. Roads and development fragment habitat, creating barriers to movement and decreasing successful migration through habitat patches. Habitat fragmentation also isolates populations, thereby reducing gene flow and possibly leading to inbreeding depression. Roads serve as ecological traps for rattlesnakes because the animals are more conspicuous when crossing roads and thus are more easily exposed to depredation or killing by humans.
Because of the fate of timber rattlesnakes in other states, the species became my subject of study in Tennessee as early as 1990. In 2000, I became the Tennessee representative for a group of scientists involved in developing the Timber Rattlesnake Conservation Action Plan for the United States Fish and Wildlife Service. Little was known about timber rattlesnakes in Tennessee during that time, so the natural history and survivability of the species became my dissertation topic.
I was working on a translocation project when a wildlife officer from Bear Hollow Wildlife Management Area brought me a nuisance timber rattlesnake on August 27th, 2005. The snake was using a resident’s driveway culvert for cover and the resident wanted the snake relocated for the safety of his family. After inspecting the snake, I noticed several large masses along the body of that snake and decided it was not a good candidate for translocation. I contacted the University of Tennessee Veterinary College to inquire about the tumors. Initially we suspected it was a mycobacterial infection, caused by a relative of the bacterium causing tuberculosis in humans. The snake died the following day with no definitive diagnosis and no necropsy was performed.
I was tracking a male timber rattlesnake on August 7th, 2007, and during this time I discovered a new female timber rattlesnake. This female was utilizing habitat that would be more appropriate for gravid females than foraging females during this stage of the active season. After I captured the snake, I noticed she was extremely lethargic, anorexic, and in need of a shed. As I was returning to base to process the snake, she died and clear fluid began draining from her nares. I strongly believe these were my first two encounters with Snake Fungal Disease.
On August 13th, 2012, I was invited, along with the Tennessee Wildlife Resources, to assist the United States Army Corps of Engineers in capturing and moving wildlife as the Corps was cleaning debris from a designated work site. A large male timber rattlesnake was discovered at the site and captured. I immediately noticed the snake was lethargic, had discolored scutes, and a mass above the left eye. I had recently read an article by Dr. Matt Allender on a disease infecting massasaugas in Illinois. I observed too many similarities associated with the timber rattlesnake and the snakes in his study. I took samples from the timber rattlesnake and sent them to Dr. Allender to test for Snake Fungal Disease. The tests confirmed presence of the disease and this was the first confirmed case in Tennessee.
Snake Fungal Disease (SFD), caused by Ophidiomyces ophiodiicola, is an emerging threat to rattlesnake populations. This fungus was first confirmed in an infected eastern rat snake (Pantherophis spiloides) in Sparta, Georgia in 2009 and has been linked to declines in the eastern massasauga (Sistrurus catenatus catenatus). The infection has a high mortality rate and has been linked to a 50% decline in the timber rattlesnake population in New Hampshire. The first confirmed case of SFD in Tennessee was from the timber rattlesnake I collected at Center Hill Lake on August 13th, 2012.
To date, the disease has been confirmed in at least 18 states: Illinois, Florida, Massachusetts, Minnesota, New Jersey, New York, Ohio, Tennessee, Wisconsin, South Carolina, North Carolina, Georgia, Connecticut, Arkansas, New Hampshire, Virginia, and Michigan. It has also been detected in southern Canada. The spread of the disease appears to be following a path very similar to that of White Nose Syndrome in our bat populations. Snake species that have been confirmed with the disease so far include: northern watersnakes (Nerodia sipedon), eastern racers (Coluber constrictor), rat snakes (Pantherophis spiloides), timber rattlesnakes (Crotalus horridus), mud snakes (Farancia abacura), pigmy rattlesnakes (Sistrurus miliarius), massasaugas (Sistrurus catenatus), copperheads (Agkistrodon contortix), milksnakes (Lampropeltis Triangulum), cottonmouths (Agkistrodon piscivorous), ribbon snakes (Thamnophis sauritus), corn snakes (Pantherophis guttatus), indigo snakes (Drymarchon couperi), kingsnakes (Lampropeltis getula), and ringneck snakes (Diadophis punctatus).
Clinical signs of the disease include cellulitis, oral swelling, cutaneous abscessation, ocular swelling, respiratory distress, scabs, crusty scales, subcutaneous nodules, abnormal molting, cloudiness of the eyes, hyperkeratosis, skin ulcers, swelling of the face, anorexia, and nodules in deep tissues and facial regions. The disease can begin at any time, but tends to be observed more frequently at emergence from hibernation. Not all hibernation sores are SFD. Other skin infections can resemble SFD and include fungal species of Trichophyton (ringworm), Epidermophyton, Cladosporium, Fusarium, and bacterial keratitis. The only way to confirm the causative agent of an infection is through clinical diagnosis. Snakes may shed out of these infections without difficulty.
The fungal disease begins as a dermatitis, and in the early stages, some snakes may effectively shed free of the disease. However, if the disease is able to take hold, it penetrates through the dermal layers into muscle and bone and may subsequently become systemic. Mortality rates are very high in this phase of the disease. I have noticed timber rattlesnakes that have tested positive for the disease in Tennessee have abnormal loreal pits. The loreal pit is a deep depression behind the nostril which serves to thermoregulate and also opens to a sensitive infrared detecting organ. These pits may serve as an entry point for infection. The disease can also change the behaviors of the snake, which may increase opportunities for depredation. Snakes may come out of hibernation during mild winter days to bask. Basking raises the temperature of snakes in order to stimulate an immune response. However, in winter months, the temperature increase from basking does not raise temperatures enough to illicit an effective immune response. Conversely, basking-induced temperature increases during winter will actually stimulate additional fungal growth.
Snakes have been treated for SFD with an approximate 50% success rate. The process involves surgically removing fungal masses, treatment with anti-fungal drugs given intramuscularly, and housing in elevated temperatures. Anti-fungal drugs are administered for approximately six months. These drugs can have an adverse effect on the individual’s organ systems, not to mention the added stress placed on confining wild animals. At a population level, this type of treatment is not feasible and the laws of various states may deem it illegal to house venomous snakes for treatment and also to release the snakes once they have been held in captivity.
The timber rattlesnake in the photographs above also experienced neurological disorders. The snake’s side-to-side head movements were not smooth, but rather erratic and jerky – one might say the movements were reminiscent of those associated with Parkinson’s disease. This is a clear indication that the central nervous system was affected by SFD. This 137 cm male timber rattlesnake was found killed by a predator within 24 hours of its release, but was not consumed. It appeared a violent death for the rattle was broken off, the skin stripped from the neck as he was likely shaken, and sharp punctures covered the entire body
Snakes can spread the disease through direct contact; however, as snakes crawl through their habitat prodding and probing rock crevices and rodent holes, they often develop micro-abrasions. These openings in the skin allow entry of fungal spores from ground contact. Due to climate change, the winters in Tennessee are milder and wetter. These conditions can facilitate active fungal growth during hibernation when the snakes are most vulnerable.
There is still much to learn about SFD. The ecology of the fungus, the transmission of the disease, potential variants of Ophidiomyces, the effects on snake populations, and timing of morbidity and death are all questions yet to be answered about this emerging disease.
About the Author
Dr. Danny L. Bryan is Program Director and Assistant Professor of Biology at Cumberland University in Lebanon, TN and serves as a scientific and species review expert for the Tennessee State Wildlife Action Plan. Dr. Bryan is currently conducting research on the distribution, natural history and conservation of the timber rattlesnake in Tennessee and has spent the past several years studying the impact of development in Middle Tennessee on the Timber Rattlesnake population. Prior to Professor Bryan's study, which is in conjunction with the Tennessee Wildlife Resource Agency and the U.S. Fish & Wildlife Service, very little was known about the Timber Rattlesnake population in the state. The species is currently listed as a "Species of Greatest Conservation Need " in Tennessee; however, Bryan's study - which focuses on the Timber Rattlesnakes' population movements and repopulation - suggests the species may be endangered due to encroaching development and habitat loss. Bryan was also the first to identify the fungal disease, Ophidiomyces ophiodiicola, in Tennessee which may add an additional threat to the species.