Snake Fungal Disease

Cause

Ophidiomycosis, formerly referred to as Snake Fungal Disease (SFD), is caused by the fungus Ophidiomyces ophiodiicola. This fungus is thought to be an emergent pathogen on the North American landscape and poses a significant threat to snake health and population sustainability. To date, O. ophiodiicola has been documented in over 15 genera of wild and captive snakes.

Significance

Ophidiomycosis is an emerging disease that affects wild and captive snakes across North America. The incidence of Ophidiomycosis has steadily increased over the last few years and has recently (November 2019) been documented in California, Oklahoma, Puerto Rico, and Idaho (Allender et al., 2019).

Species Affected

Ophidiomycosis can infect wild or captive snakes and is the predominant cause of skin infections in wild snakes. It has been documented in over 15 Genera of wild and captive snake. Species confirmed with diagnosis of Ophidiomycosis include: the eastern indigo snake, the northern water snake, the eastern racer, the rat snake, the timber rattlesnake, the massasauga, the pygmy rattlesnake, garter snakes, cottonmouth snakes, the milk snake, queensnake, and eastern fox snake. The fungus causing Ophidiomycosis is not known to affect humans. 

Distribution

As of November 2019, the fungus O. ophiodiicola has been found in at least 25 states, including recent findings in California, Idaho, Oklahoma (Allender et al., 2019). It is also present in one U.S. territory (Puerto Rico) and one Canadian Providence (Ontario). In some species (i.e., Eastern Massasauga) the mortality rate may be over 90% in the Eastern US (unpublished data, reviewed in Allender et al., 2018).

Transmission

At this time, it is not well understood how the fungus causing Ophidiomycosis is spread. It is likely that the pathogen shed into the environment by infected snakes and spread from the environment to other snakes (i.e., snakes that share dens). A recent report indicates O. ophiodiicola may be spread via vertical transmission (i.e., from dam to offspring) (Stengle et al., 2019). There is no evidence of horizontal transmission (i.e., snake-to-snake transmission) of O. ophiodiicola. Host and environmental factors that lead to disease remain largely unknown (Allender et al., 2018). For example, it is thought that Timber rattlesnake populations are at higher risk of Ophidiomycosis in years with higher rainfall (Clark et al., 2011).   

Clinical Signs

Clinical signs of SFD and the severity of disease may vary by species. The most common clinical signs include accelerated ecdysis cycles, flaking and crusting of the scales/epidermis, displaced or discolored scales, granulomas, nodules, swelling or disfiguration of infected tissues (Baker et al., 2019). The disease may progress internally (via the eyes, throat and/or lungs) and cause eye infections or pneumonia.

Diagnosis

Ophidiomycosis is diagnosed by identification of the classic clinical signs in combination with either molecular detection (PCR or qPCR) of the fungus, histopathology, and/or a positive fungal culture (Allender et al., 2019). Ophidiomycosis status should be characterized as defined in the case definition (Baker et al., 2019):

1) negative (no clinical lesions or qPCR detection of O. ophiodiicola)

2) Ophidiomyces present (qPCR detection in absence of clinical signs)

3) possible ophidiomycosis (presence of clinical signs in the absence of qPCR detection)

4) apparent ophidiomycosis (clinical signs present and qPCR detection) (Allender et al., 2019).

 

It must be noted that there may be difficulty in sampling for and detecting (by PCR/qPCR) O. ophiodiicola by skin/epidermal swabs (i.e., low DNA quantity on the skin or the fungi are deep within the epidermis). Therefore, hesitation should be taken in assigning causation to O. ophiodiicola to animals with skin lesions that are qPCR negative in the presence of skin lesions. There are numerous causes of skin lesions in wild snakes (i.e., trauma, another pathogen causing skin lesions, etc). Conversely, the absence of clinical signs of ophidiomycosis is more reliable to eliminate a diagnosis of apparent ophidiomycosis (Allender et al., 2019). 

Treatment

Treatment with an antifungal, Terbinafine, either by a slow-release, long-lasting implant or nebulization has been found to reach therapeutic levels in cottonmouth snakes; methods and doses for treatment are thus based on these findings (Kane et al., 2017). Other treatment options include thermal and nutritional supportive therapy.

Management

Ophidiomycosis appears to be negatively impacting several species and populations of snakes across North America. However, its long-term impact remains unknown. It is also very difficult to assess changing snake populations because of the solitary nature of many snakes and, importantly, the absence of long-term monitoring data for most species of snake. Any sightings of snakes with suspected SFD infection should be reported to the Northeast Wildlife Disease Cooperative for further investigation. Wildlife specialists, veterinarians, and any other individuals who routinely come in contact with wild snakes should keep gear, snake holding containers, and clothes clean and disinfected to prevent spreading the fungus to other snakes. For information on field gear disinfection and biosecurity, please review Gray et al., 2017. This paper provides a fantastic review of disinfectants proven effective against O. ophiodiicola (Appendix I, Gray et al., 2017).

Suggested Reading

  • Allender MC, Ravesi MJ, Haynes E, Ospina E, Peterson C, Phillips CA, Lovich R. 2019. DOD Snake Fungal Disease Survey: Natural Resource Manager Training and Data Collection. Project Number: 17-838. DOD Legacy Resource Management Program.
  • Allender MC, Baker S, Britton M, Kent AD. 2018. Snake Fungal Disease alters skin bacterial and fungal diversity in an endangered rattlesnake. Scientific Reports, 8:12147. DOI:10.1038/s41598-018-30709-x
  • Baker SJ, Haynes E, Gramhofer M, Stanford K, Bailey S, Christman M, Conley K, Frasca Jr S, Ossiboff RJ, Labato D, Allender MC. 2019. Case Definition and Diagnostic Testing for Snake Fungal Disease. Herpetological Review, in press
  • Clark RW, Marchand MN, Clifford BJ, Stechert R, Stephens S. 2011. Decline of an isolated timber rattlesnake (Crotalus horridus) population: interactions between climate change, disease, and loss of genetic diversity. Biological Conservation, 144: 886-89
  • Sleeman, J. 2013. Snake Fungal Disease in the United States. USGS National Wildlife Health Center, Wildlife Health Bulletin 2013-02.
  • Stengle AG, Farrell TM, Freitas KS, Lind CM, Price SJ, Butler BO, Tadevosyan T, Isidoro-Ayza M, Taylor DR, Winzeler M, Lorch JM. 2019. Evidence of vertical transmission of the snake fungal pathogen Ophidiomyces ophiodiicola. Journal of Wildlife Diseases
  • Allender, M.C., M. Dreslik, S. Wylie, C. Phillips, D.B. Wylie, C. Maddox, M.A. Delaney, and M.J. Kinsel. 2011. Chrysosporium sp. infection in eastern massasauga rattlesnakes. Emerging Infectious Diseases 17:2383-2384.
  • Rajeev, S., D.A. Sutton, B.L. Wickes, D.L. Miller, D. Giri, M. Van Meter, E.H. Thompson, M.G. Rinaldi, A.M. Romanelli, J.F. Cano, and J. Guarro. 2009. Isolation and characterization of a new fungal species, Chrysosporium ophiodiicola, from a mycotic gra
  • http://www.cwhc-rcsf.ca/docs/technical_reports/CWHC_Snake_Fungal_Disease_Threat_...
  • http://northeastparc.org/wp-content/uploads/2015/11/NEPARC_SFD_FAQver3.pdf
  • https://vetmed.illinois.edu/wel/what-is-snake-fungal-disease/
  • https://cdfgnews.wordpress.com/2019/11/05/snake-fungal-disease-detected-in-calif...

Images

Eastern racer showing signs of fungal skin infection.Eastern racer showing signs of fungal skin infection. Image credit: USGS NWHC
Northern water snake with crusty and thickened scales.Northern water snake with crusty and thickened scales. Image credit: USGS NWHC