Snake Fungal Disease in North America: What USGS Scientists Have Learned

From a U.S. Geological Survey fact sheet by Noelle E. Thompson, Emily W. Lankau and Gail Moede Rogall, January 2018.

Snake fungal disease (SFD) is a skin infection that has been documented only in snakes. It is caused by the fungus Ophidiomyces ophiodiicola and has been confirmed in many species. Reports of snakes with skin infections of unknown cause used to be occasional; recently they have risen sharply.

As of August 2017 the fungus had been found in at least 23 States and one Canadian province. Researchers suspect SFD is more widespread than that, because few snake populations are monitored, and because the disease may go unreported where it strikes rarely or in species that get only mildly ill.

Why snakes matter

Snakes are both predators and prey in food webs. Many mammals and birds eat them, and they eat rodents that damage crops or carry diseases affecting people and other animals. They can even lower the local risk of Lyme disease by eating rodents and other small mammals carrying the ticks that spread it: mathematical models suggest a single timber rattlesnake (Crotalus horridus) removes 2,500–4,500 ticks a year from its foraging range, and other vipers likely do the same.

Many snake populations are already declining from habitat loss and shrinking prey. SFD may speed that decline and wipe out some species in some places.

From "hibernation sores" to a known cause

For decades, field biologists occasionally found skin sores on wild North American snakes, most often after hibernation, and called them "hibernation sores". Their cause was unknown until 2008, when an investigation of deadly outbreaks of skin lesions in eastern massasauga rattlesnakes (Sistrurus catenatus) pointed to a fungus.

• The fungus, then called Chrysosporium ophiodiicola, was also isolated from a skin nodule on a captive eastern rat snake (Pantherophis alleghaniensis).
• It turned up again in a study of archived tissue samples from skin lesions in captive snakes.
• After 2008, more and more infected snakes were reported, including in threatened populations of eastern massasaugas in central Illinois and timber rattlesnakes in New England — raising fears of local extinction for both.

USGS scientists at the National Wildlife Health Center and the Wetland and Aquatic Research Center have worked with government, nonprofit and academic partners to understand SFD. Together they linked the physical signs to a diagnosis, showed by experiment that O. ophiodiicola causes the disease, and set up surveillance to learn which species are infected where.

Signs

The signs and their severity vary between species and between individual snakes, for reasons that are poorly understood. They can include:

• scabs, skin ulcers or nodules (abnormal bumps) under the skin;
• crusted or discoloured scales;
• cloudy eyes;
• a swollen or disfigured face, sometimes severely.

A western foxsnake captured in Wisconsin with signs of snake fungal disease: ulcers on the nose and near the eyes (top), and discoloured, crusted scales on the body (bottom). Stephanie Steinfeldt, USGS.

Infected snakes moult abnormally, and may shed the infected top layer of skin again and again as they try to get rid of the fungus. They are often emaciated, perhaps because they cannot catch prey as well, and they often rest out in the open, exposed to bad weather and predators. The disease can end in starvation and death.

How it spreads

• Temperature: the fungus survives a fairly wide range of pH and temperature, but its growth is held back above 100 degrees Fahrenheit and below 50 degrees Fahrenheit.
• Contact: it passes from snake to snake, and may survive in the environment without a snake.
• Skin: broken skin makes infection easier, but a snake can be infected without any scrape.
• Carriers: snakes can carry the fungus on their skin without being sick, so finding the fungus alone does not show that a snake has SFD.

Diagnosis

Lab tests can detect the fungus, but confirming SFD requires a biopsy — a piece of skin and the tissue under it — examined under a microscope for inflammation and other signs of an invasive infection. That tells a true infection apart from fungus simply sitting on a healthy snake's skin.

National Wildlife Health Center pathologist Susan Knowles takes a skin biopsy from a North American racer submitted for testing. Jennifer Swan, USGS.

Why now?

It is unclear when or why the fungus began harming snakes. It has been found throughout the eastern United States and may be native there, yet severe outbreaks have been recorded in only a few places. Scientists suspect that environmental and climate changes — habitat destruction and fragmentation, rising average temperatures, and cooler, wetter springs — may be helping the disease emerge. More monitoring may also make the disease look more frequent or severe than before, simply because more people are looking.

Which snakes (as of January 2018)

In North America the fungus has been found on captive and wild snakes of 12 genera in 2 families, including two threatened species, the Atlantic saltmarsh watersnake (Nerodia clarkii taeniata) and the eastern massasauga, and one endangered species, the Louisiana pinesnake (Pituophis ruthveni). Not every snake it was found on has been studied enough to know how susceptible it is or how severe the disease would be. The fungus, together with signs of SFD, has been found in:

How you can help

• Report snakes with skin sores or strange behaviour to a local or state wildlife agency — but do not disturb wild snakes.
• Professionals handling wild snakes should clean and disinfect clothing and equipment, and release rehabilitated snakes as close as possible to where they were caught.

What the USGS is doing

• establishing that O. ophiodiicola causes SFD, and developing lab methods to detect it;
• studying how the disease develops in captive snakes;
• diagnosing SFD in snakes sent in by partner agencies to the National Wildlife Health Center;
• coordinating field surveillance for SFD in new populations and species;
• working out where SFD has been in the past and projecting where it will go;
• identifying other fungi, bacteria and viruses that may cause skin infections in snakes or contribute to SFD;
• studying how SFD changes infected snakes' behaviour and survival, to understand its effect on populations.

Sources

• Noelle E. Thompson, Emily W. Lankau and Gail Moede Rogall, Snake Fungal Disease in North America: U.S. Geological Survey Updates, U.S. Geological Survey Fact Sheet 2017–3064, January 2018. https://doi.org/10.3133/fs20173064 — the three USGS photographs recovered from the PDF.
• The fact sheet's photograph of an eastern massasauga, credited to the Wisconsin Department of Natural Resources, is not reproduced.
• Corrections: the source's "Sistrusrus catenatus" is Sistrurus catenatus, and its "Nerodia clarkia taeniata" is Nerodia clarkii taeniata.
• Rewritten in hubnx's own words.