Emerging Fungal Diseases in Amphibians and Reptiles

About the Author

Mark Scherz is a herpetologist specialising in the reptiles and amphibians of Madagascar, and is a member of the IUCN Amphibian Specialist Group for Madagascar. His current research is mostly concerned with biodiversity assessments of remote forests, threats these forests are facing, and the systematics of Malagasy herps. Mark has a BSc in Zoology from the University of Edinburgh, and is currently studying for a Master’s in Ecology, Evolution, and Systematics at the Ludwig-Maximilian’s University in Munich. You can follow Mark on twitter, tumblr, Academia.edu, ResearchGate, or his website.

Emerging Diseases

Emerging diseases in humans, and even in livestock, receive a great deal of media attention, and are also typically able to acquire a great deal of funding and research interest, allowing for rapid development of treatments (though perhaps not as rapid as could be desired). Our infrastructure for dealing with direct threats to our species is, understandably, well developed. The same cannot be said for emerging diseases in other animals, which do not necessarily have direct impacts on humans. I would argue that this is a critical flaw however: today, there are a number of emerging and established pandemics that do not obviously affect humans in the short term, but that might have long term ramifications for ecosystems. What’s more, several of the pandemics to which I am referring are undoubtedly human-caused. It is unsurprising that this has led to a surge in the international attention to these issues.

Chytridiomycosis – A plague for frogs

The past 30 years has seen a massive crash in many species of frog from around the globe (Stuart, et al., 2004). At least 200 species have been driven to or over the brink of extinction. While anthropogenic habitat change is undoubtedly responsible for several of these extinctions and near-extinctions, many species have also disappeared from within protected, undamaged forests (Stuart, et al., 2004). It is clear that deforestation is not causing the disappearance of these species. What then, is the culprit? A fungus called Batrachochytrium dendrobatidis, or “Bd”, which causes a disease called Chytridiomycosis or “Chytrid”.

Bd kills host frogs (and possibly also salamanders) by stopping them from being able to breathe through their skin, altering their osmotic regulation, increasing their skin’s thickness, and causing it to slough off. It is a fast-acting, fast-spreading, generally fatal disease. It is concerning, therefore, that the disease has been positively identified in samples from more than 350 frog species. Perhaps more concerning still, these frogs are not all confined to one region of the planet, but are spread across the globe (see Fig. 1).

Fig. 1 – The distribution of Chytrid around the world. Red = positive samples; Blue = negative sample site (unkown sample size); White = negative sample site. Source

For a disease to spread so far around the world in so short a time it can only have one cause: humans. In 2004, an examination of early specimens of the African Clawed Frog, Xenopus laevis, was carried out by Ché Weldon and his colleagues (Weldon, et al., 2004). They identified chytridiomycosis in a specimen from 1938, and concluded that, prior to human dissemination of these frogs around the world – first for pregnancy testing, and later for pharmaceutical testing – the disease had been Africa-endemic (Weldon, et al., 2004). After chytrid had established in wild populations in these new locations, transmission was made possible by the natural movements of amphibians, potentially reptilian incubating hosts, water flow, and people failing to clean their boots when they had chytrid spores on them, and traipsing these across the world. By way of example, Fig. 2 shows the rapid spread of confirmed cases of Chytrid through central America.

Fig. 2 – The spread of Bd through central America. Source

The prognosis now is bleak. Chytrid continues to spread, and species in its path continue to dwindle and vanish. There is increasing concern for the planet’s amphibian hotspots; Madagascar is amongst the last hotspots where Chytrid has not yet been found. Hope, however, is not lost. Emergency breeding programmes have been put into action in many parts of the world, including Central America and, prophylactically, Madagascar. Additionally, some treatments are slowly coming to light, although everything so far seems to require either individualistic treatment of all frogs, or replacing all water with rooibos tea. These are impractical solutions, but may be the saving graces for those species for which conservation intervention will arrive too late.

The issue is receiving a lot of media attention as frogs continue to vanish at unprecedented rates, and several initiatives have been set up to try to bring critically endangered species into captive breeding programmes, to monitor the spread of chytrid in the wild, and to try to find vaccines, pesticides, or other chemical solutions to hopefully eradicate this threat before it eradicates the planet’s frogs. Indeed, on the 23rd of July 2013, the Amphibian Specialist Group will meet for a symposium titled “The Global Amphibian Conservation Action Plan: Connecting systems, disciplines and stakeholders to save amphibians worldwide” in Boston. The agenda is concerned almost exclusively with Bd and saving the species which have been driven to the brink of extinction by it. Fortunately, it looks as though the news is not all bad: at least one talk concerns the reintroduction of a frog species that was previously extinct in the wild.

If you want to learn more about Chytrid, I highly recommend this phenomenal resource as well as amphibianark.org.

Snake Fungal Disease

While chytridiomycosis is well established and much work has already been done on it, the same cannot be said of SFD, a disease that has emerged in the last few years in snakes in the United States. The new SFD epidemic is associated with, but not conclusively caused by, the fungus Ophidiomyces (=Chrysosporium) ophiodiicola. It causes lesions on the head and body of snakes, and potentially eventual mortality (see fig. 3).

Fig. 3 – A northern water snake (Nerodia sipedon) showing typical signs of SFD – crusty scales and lesions. Source

The National Wildlife Health Centre (NWHC) in America has found SFD in snakes from nine states and numerous species (NWHC, 2013). It is unclear, however, whether this epidemic is causing significant changes in snake population sizes as monitoring efforts have been lacking. The concern is that SFD may act like white-nose disease, a condition which has caused the death of millions of bats across the US. This is especially concerning because many American snakes hibernate in communal and often mixed-species hibernacula in a manner akin to bats – a behaviour that facilitates disease spread.

The course of action for SFD is unclear. The disease is not so distinguished, nor so destructive, as to elicit enormous concern. Nonetheless, the NWHC has begun collaborating with key stakeholders to assess the threat posed to America’s snakes by SFD. This assessment will then inform conservation agencies as to how to proceed.

Comparisons may now be drawn between SFD and Chytrid. While SFD is not currently known to be causing mass fatalities on the scale that Chytrid tends to cause them, the paucity of research on it means that a conclusion that it is not a major threat would be premature. The major advantage of SFD over Chytrid is that snakes are not commonly transported around the world except on the pet trade, an industry which scarcely bares comparison to the medical industry that was responsible for the dispersal of Chytrid via African Clawed Frogs.

It is hoped that timely action and an increasingly aware international community will be able to stop SFD in its tracks, and mitigate the damage done by Chytrid. It will require massive scientific breakthroughs and international cooperation to eradicate Chytrid altogether, and this may never be achieved. However, captive breeding programmes and careful management may minimize damage. It also falls to us to take appropriate precautions to avoid making the situation any worse than it already is: when traveling, always be sure to wash your boots thoroughly. Try to avoid handling frogs without good reason, and certainly avoid transporting them from one place to another. If you receive a wild-caught snake or frog as a pet, be sure to keep it in quarantine for a while to make sure that no diseases can piggy-back onto other animals. And if you are out hiking and you come across what looks like a frog massacre, report it to the Amphibian Specialist Group or Amphibian Ark.

Featured image: Boophis madagascariensis by Mark Scherz.

Works Cited:

NWHC, 2013. Snake Fungal Disease. [Online] Available at: http://goo.gl/i3BZP [Accessed 12 July 2013].

Stuart, S. N. et al., 2004. Status and Trends of Amphibian Declines and Extinctions Worldwide. Science, 3 December, Volume 306, pp. 1783-1786.

Weldon, C. et al., 2004. Origin of the Amphibian Chytrid Fungus. Emerging Infectious Diseases, 10(12).

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