16.9 Snake fungal disease

16.9 Snake fungal disease

Beginning in 2006, severe skin infections were reported along with a rapid decline in a timber (Crotalus horridus) population in the northeastern USA. In 2008, similar infections with a possible fungal cause emerged in Illinois, USA in an endangered population of Massasauga rattlesnakes (Sistrurus catenatus). This infectious disease became known as Snake Fungal Disease (SFD), and by 2015 SFD had been documented in most of the eastern USA. SFD is a major conservation concern in North America because it has the potential to cause lethal infections and contribute to extinction of localised snake populations.

SFD is caused by Ophidiomyces ophiodiicola, a keratinophilic fungus that is widely distributed in eastern North America. It has a broad host range and is the predominant cause of fungal skin infections in wild snakes. It often causes mild infections in snakes coming out of hibernation and is an emerging pathogen of captive snakes in North America. SFD has been documented in 23 species of snakes, although this is likely to be an underestimate of the number of susceptible species. SFD hosts are both phylogenetically and ecologically very different, which indicates that other species of snakes in the US might now be infected or susceptible to SFD (Lorch et al., 2016; Burbrink et al., 2017).

Ophidiomyces ophiodiicola has been isolated from captive snakes outside North America, but the pathogen had not been reported from wild snakes elsewhere until carcasses and moulted skins from wild snakes collected during 2010-2016 in Great Britain and the Czech Republic were screened for the presence of skin lesions, and for O. ophiodiicola using PCR DNA detection. The fungus was detected in 8.6% of the specimens and further analysis confirmed that Ophidiomyces-SFD occurs in wild European snakes. Phylogenetic analyses indicated that Ophidiomyces isolated from European wild snakes belonged to a clade distinct from the North American isolates; so, the European and North American diseases involve different strains of Ophidiomyces ophiodiicola (Franklinos et al., 2017).

Potentially, therefore, Ophidiomyces ophiodiicola is a global emerging fungal pathogen of reptiles in the wild. We’re talking about reptiles and potential extinctions here, so this is about the right place to remind you about dinosaurs. They are mostly dead. Mostly, because all those birds that are flapping around out there are the remnants of a group of reptiles that once ruled the Earth for several hundred million years; until they went extinct. Taking advantage of the wide-scale extinction events of other organisms is an often-repeated feature of fungal evolution. The period 800 to 600 million years ago featured successive virtually global glaciations (snowball Earth episodes). The Permian-Triassic (P-Tr) extinction event that occurred approximately 251 million years ago (known as the Great Dying and the Earth’s most severe extinction event so far) was studied by Visscher et al. (1996) who summed it up with the quotation:

‘…sedimentary organic matter preserved in latest Permian deposits is characterised by unparalleled abundances of fungal remains, irrespective of depositional environment (marine, lacustrine [= lake sediments], fluviatile [= river/stream deposits]), floral provinciality, and climatic zonation.’

Much the same is true for the Cretaceous-Tertiary (K-T) extinction of 65 million years ago, the result of a meteor collision that caused the Chicxulub crater in Mexico, which is blamed for the extinction of the dinosaurs. There was also widespread deforestation right at the end of the Cretaceous, which is assumed to be due to post-impact conditions. However, coincident with all this death and destruction of animal and plant life at the K-T boundary there is a massive proliferation of fungal fossils:

‘…This fungi-rich interval implies wholesale dieback of photosynthetic vegetation at the K-T boundary in this region. The fungal peak is interpreted to represent a dramatic increase in the available substrates for [saprotrophic] organisms (which are not dependent on photosynthesis) provided by global forest dieback after the Chicxulub impact...’ (Vajda & McLoughlin, 2004).

So, it is the same story as at the other extinction boundaries: while the rest of the world was dying, the fungi were having a party!

But that Chicxulub meteor might not have had the last word on dinosaur extinction, because the massive increase in the number of fungal spores in the atmosphere of the time may have caused fungal diseases that

‘…could have contributed to the demise of dinosaurs and the flourishing of mammalian species…’ (Casadevall, 2005).

A reminder, perhaps, that the fungi started the eukaryote journey by spring-cleaning the early Earth, and they’ve been cleaning up and modifying the planet and its biosphere ever since (Moore, 2013).

Updated July, 2018