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Mysterious Fungal Disease Strikes US Snakes

March 07, 2017

Story at-a-glance

  • In 2015, scientists identified a soil fungus called Ophidiomyces ophiodiicola (O. ophiodiicola) as the culprit behind snake fungal disease
  • Why the fungus, which is widespread in the U.S., is suddenly killing snakes, remains a mystery
  • Across the U.S., at least 30 species of snakes in more than 16 states have been infected with snake fungal disease, as have snakes in Canada

By Dr. Becker

Fungal diseases seem to be an emerging threat to animals worldwide, as we’ve seen with the millions of bats killed due to white-nose syndrome (WNS). A similar fungal disease is now striking wild snakes, and researchers are scrambling to figure out why so many animals are becoming infected — and stop it before it’s too late.

In New Hampshire, for instance, half of the state’s population of rare timber rattlesnakes were wiped out due to the disease, known as snake fungal disease, between 2006 and 2007. Across the U.S., at least 30 species of snakes in more than 16 states have been affected, as have snakes in Canada.1

What Causes Snake Fungal Disease?

In 2015, scientists identified a soil fungus called Ophidiomyces ophiodiicola (O. ophiodiicola) as the culprit behind snake fungal disease.2 Why the fungus, which is widespread in the U.S., is suddenly killing snakes, remains a mystery.

In an analysis of rattlesnake specimens in Illinois from 1880 to the present, skin lesions that appear to have been caused by O. ophiodiicola do not appear until 2000.3

As BBC news reported, “This suggests that an event at the turn of the millennium led a relatively benign fungus to become a potent snake killer. However, nobody knows what that trigger was.”4 Several theories have been suggested, however, including:5

Signs and Symptoms of Snake Fungal Disease

Snake fungal disease often leads to skin lesions, including open wounds, crusty scabs and swelling. There may be also be swelling of the eyes, thickened skin on the snout, abnormal scales, bumps below the skin and cloudy eyes not associated with molting.

While in some cases snake fungal disease may kill up to 90 percent of infected snakes, some do recover and survive (why some snakes seem to be able to fight off the disease while others can’t is another mystery).9

The disease also leads to behavioral changes in the snakes. In one study in which involved captive-bred corn snakes infected with the fungus, in addition to the characteristic skin lesions, the infected snakes also became lethargic and lacked an appetite.

They molted more often than usual and displayed unusual behaviors, such as basking in “conspicuous” areas. The responses may be intended to help the snakes fight the infection, but ultimately could make them more vulnerable to predators in the wild.10

It’s possible the fungi could be spread internationally through the pet trade and may also be transferred on clothing, boots or field equipment used to handle snakes in the wild. The fungi also share similarities with chytrid fungi, which is spreading among frogs and salamanders.

Are Wild Snake Populations at Serious Risk?

The U.S. Geological Survey (USGS) has noted that the number of wild snakes submitted to the USGS National Wildlife Health Center (NWHC) and other diagnostic laboratories has been increasing. They noted that the snakes often show evidence of not only O. ophiodiicola but also other types of fungi. According to USGS:11

“[I]t is suspected that SFD [snake fungal disease] is more widespread in the United States than is currently documented.

Multiple species of snakes have been diagnosed with SFD at the NWHC … including northern water snake (Nerodia sipedon), eastern racer (Coluber constrictor), rat snake (Pantherophis obsoletus species complex), timber rattlesnake (Crotalus horridus), massasauga (Sistrurus catenatus), pygmy rattlesnake (Sistrurus miliarius) and milk snake (Lampropeltis triangulum).”

It’s been difficult for researchers to determine whether snake fungal disease is threatening the viability of different snake species, in part because of the solitary nature of snakes. However, those species that occur in small, isolated populations are thought to be most at risk.

That being said, separate research published in Biology Letters revealed that, of 17 snake populations from the U.K., France, Italy, Nigeria and Australia, 11 had sharp population declines over a relatively short period of time, with no signs of recovery in the decade following the population crash.

“Unfortunately, there is no reason to expect a reversal of this trend in the future,” they wrote.12

Snakes’ Loathsome Reputation May Be Hindering Further Research

Much more research is needed to determine the extent of snake fungal disease, as well as its future implications to snake species. Also of interest is whether other factors, like habitat loss or pollution, could be complicating matters and making snakes less able to fight off what should be a mild infection.

Matt Allender, Ph.D., snake researcher at the University of Illinois in Urbana, further told BBC News that the public’s generally negative attitude toward snakes is another hindrance:13

"It's hard to get people excited about snakes. It's not a group of species that many people care much about … And most cases of the disease are in venomous species, which makes it even more difficult to get them excited."

Snakes, however, represent an important part of the ecosystem, feeding on rodents, insects and other potentially disease-carrying “pests.” Snakes are also an important food source for many mammals, birds, fish and other reptiles, while their venom is being used for its medicinal properties.

If you happen to see a potentially infected snake, report the sighting to your state wildlife agency so they can be alerted that snake fungal disease may be spreading in the area. At this time, it’s not thought that snake fungal disease is dangerous to humans.

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Sources and References

  • 1, 4, 5, 13 BBC November 23, 2016
  • 2, 7 Fungal Ecology October 2015
  • 3 Illinois Department of Natural Resources August 2013
  • 6 Philosophical Transactions of the Royal Society B October 24, 2016
  • 8 Journal of Wildlife Diseases: July 2016, Vol. 52, No. 3, pp. 759-762.
  • 9 Northeast Partners in Amphibian and Reptile Conservation, Snake Fungal Disease
  • 10 mBio November 17, 2015
  • 11 USGS Snake Fungal Disease
  • 12 Biology Letters June 9, 2010
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