Usually when you see the words "snake" and "deadly" in the same sentence, you might assume the serpents are the ones doing the killing. But for the past several years, there has been a snake-slayer on the loose – a mysterious fungus that has left victims all across North America. And now, in a worrying new development, the disease's genetic fingerprints have been found on dead wild snakes in Europe for the first time.

We covered Snake Fungal Disease (SFD) back in December last year. At the time, the big story was that the disease-causing fungus (a species with the tongue-twisting name Ophidiomyces ophidiicola) had been spotted all over eastern North America, from Ontario down to Louisiana, and had been reported infecting more than 30 different species of snakes. This was – and still is – troubling news.

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A northern water snake (Nerodia sipedon) with crusty and thickened scales overlaying raised blisters as a result of a fungal skin infection, captured in western Lake Erie, Ohio. Image: D.E. Green, USGS National Wildlife Health Center.

Snake Fungal Disease isn't a new problem – herpetologists have documented it in captive snakes for decades. But recently, the fungus has caused a number of severe outbreaks in wild snake populations, starting with one such event in New Hampshire in 2006, which cut the local timber rattlesnake numbers in half. In the years since, the situation has not improved, and some of the disease's victims have been species that are already threatened.

Until now, the infection, though widespread and concerning, was limited to North America. But a new study led by researchers from the Zoological Society of London (ZSL) and the US Geological Survey (USGS) has revealed the presence of the disease in wild snakes on the other side of the Atlantic.

The researchers examined samples of over 300 snakes from around Europe, gathered over the past several years. They found 24 cases of SFD-infected grass snakes from across the UK, as well as one afflicted dice snake from the Czech Republic. It seems the fungus has been present in Europe since 2010, and perhaps much earlier.

The new study found evidence of SFD in grass snakes (Natrix natrix), like the one on the left, and dice snakes (Natrix tessellata), on the right. Image: Taema/Flickr (left) and Alexandre Roux/Flickr (right)
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Snakes showing multiple signs of fungal disease. Image: USGS

In most of these European snakes, the infection appears to have been quite mild, but some cases were so severe the researchers suspect the fungus was related to the snakes' deaths, similar to what has been seen in the North American epidemic.

Intriguingly, the North American and European strains of the fungus are quite distinct, which makes lead author Lydia Franklinos wonder if this troublesome microbe has actually been hanging out on both continents for a while. "[R]ather than being introduced across the Atlantic, or vice versa," she said in a press release, "the disease could have been present below the radar in European snakes for some time."

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Fungal disease lesions (a) on a European grass snake, as seen under magnification (b and c). Image: LH Franklinos et al., 2017

Snake Fungal Disease targets the skin, causing nasty symptoms like blisters, scabs and crusty scales. Much of the time, these effects are simply a nuisance, and researchers are still trying to figure out why the disease has been striking with such deadly ferocity in recent years. It seems something has changed, but what?

"Environmental change, such as habitat degradation and climate change, together with host factors such as inbreeding depression and underlying impaired health, might be implicated in the emergence of SFD," the researchers suggest in the new study, published in Scientific Reports.

Pathogenic (that is, disease-causing) fungi can be particularly effective at spreading through ecosystems when the opportunity arises. Over the past decade, a species named Pseudogymnoascus destructans has been behind the rise of White Nose Syndrome, which has claimed the lives of over six million bats in North America. The even more intimidatingly named Batrachochytrium dendrobatidis has spent the past 30 years causing population crashes in at least 200 species of amphibians around the world.    

These ferocious fungi, including the culprit behind SFD, don't seem to be a direct threat to humans. But snakes are crucial components of their environments – in many parts of the world, a healthy ecosystem depends on a healthy snake population.

One of the goals of the Zoological Society of London is to tackle the problem that diseases pose for conservation, particularly in a world where tiny troublemakers find it easier than ever to hitchhike around the globe on boats and planes, and where rapid environmental changes are leaving wild populations of animals particularly vulnerable to new threats.

But the identification of the SFD fungus in Europe is not all doom and gloom. In fact, it may even be a boon for scientists who are hoping to learn more about the disease.

"Comparing how SFD affects wild snakes on different continents may help us pinpoint the factors causing the disease to emerge and help managers identify mitigation strategies," says Jeffrey Lorch of the USGS.

"Of all vertebrate wildlife, we probably know least about health conditions that affect terrestrial reptiles such as snakes," Franklinos adds, "so this study represents an important milestone and one that will hopefully encourage greater focus in understanding the threats facing these animals."



Top header image: Alexandre Roux/Flickr