In a new study that recalls the white-nose syndrome plaguing American bats, and draws heavily on recent observations in Minnesota and Wisconsin, scientists report this week that a new fungal disease is afflicting American snakes far more widely than previously believed.
The exact pathogen in what’s being called “snake fungal disease” or SFD is not yet completely clear, and its ultimate threat to snake populations is a matter of debate.
But high mortality has been seen in some groups of infected snakes, and extinctions are thought to be a real possibility — with serious consequences for ecosystems that might lose these important if little-loved reptiles.
The first cases to be described as SFD turned up in the northeastern U.S. in 2006, the same region and year as white-nose. Over the next decade it has come to be detected in nine states, including Minnesota, and about 16 species.
The new paper raises those measures to 20 states — stretching westward to a tier of states just west of the Mississippi River, from Minnesota down to Louisiana — and 30 species, including the eastern massasauga rattlesnake. The massasauga is one of two rattlers resident in Minnesota, and is already listed as threatened under the Endangered Species Act.
Until this study, most cases of SFD were diagnosed in snakes that various wildlife agencies had sent to the U.S. Geological Service’s National Wildlife Health Center in Madison, Wisconsin. Rattlesnakes have appeared to be especially vulnerable, although the paper says that may simply reflect “more intensive monitoring and sampling of rattlesnake populations compared with other snake species.”
The new research combined a review of all previous findings with a special effort to assess SFD prevalence in more than 200 live snakes gathered for the purpose in Minnesota and Wisconsin.
Mild cases in Minnesota
The bad-news discovery was that 41 percent of these snakes displayed the skin lesions and other damage that define SFD; the good news was that most cases were mild enough that they appeared not to be life-threatening.
Still, as the center’s Jeffrey Lorch, lead author of the paper, observed:
Some snake populations in the eastern and midwestern U.S. could eventually face extinction as a result of SFD. Our new findings increase our understanding of the geographic extent, species susceptibility and manner of development of this disease. These results will offer important clues regarding how to manage SFD.
Management will be a challenge, as detecting and assessing the prevalence of SFD has been. Snakes are well camouflaged against easy detection, and they are solitary — unlike bats, for example, which often roost and hibernate in large groups.
Unlike white-nose-afflicted bats, for example, which roost and hibernate in large groups, snakes are solitary and well camouflaged against easy detection in their environments.
But the stakes for saving them could potentially be quite high, because snakes are not only predators on agricultural pests and disease-carrying rodents, but in turn become important food sources for a range of birds and other animals.
Snake populations are declining worldwide, and although the declines are multifaceted, the role of disease may be overlooked in species where available data on health or long-term population trends are lacking. … Future studies to quantify the benefits of snakes, as has been done for other previously maligned wildlife such as bats, will be essential in gauging the impacts that the loss of snakes could have and will also help the public to understand the importance of protecting these reptiles from emerging threats such as SFD.
Indeed, the Lorch paper is published in a journal of the UK’s Royal Society that has devoted its entire current issue to the rise of fungal-type diseases like SFD and white-nose syndrome — pathogens that, for the most part, used to be self-limiting but appear to be expanding in geographic scope and/or virulence for reasons that are not well understood.
This “alarming increase,” as the Lorch paper assesses it, is causing “some of the most important conservation crises of modern times. Of particular note are global loss of amphibian diversity due to chytridiomhycosis and massive population declines of some bat species due to white-nose syndrome.”
The primary pathogen involved in SFD appears to be Ophidiomyces ophiodiicola, or OO. In addition to attacking snakes’ skin, including eye coverings, and causing ulcer-like lesions, the fungus appears capable of damaging bones, lungs and liver.
If this damage does not kill the snakes outright, it may weaken their ability to respond to other injuries or infections. It may cause them to molt more frequently, which consumes a lot of energy and creates other vulnerabilities:
A snake with SFD may need to molt several times in rapid succession to completely rid itself of the infection. Portions of the old infected epidermis sometimes adhere to the new skin, potentially facilitating reinfection. …
Infections of the head that affect vision, olfaction and infrared sensing (in the case of pit vipers) probably impact the ability to procure food. Indeed, anorexia has been observed experimentally infected captive snakes and emaciation is a common finding in wild snakes with SFD.
OO was present in more than three-quarters of the diseased live snakes captured in Minnesota and Wisconsin for the study, raising confidence that it is the primary if not singular cause of SFD.
But mysteries remain about why SFD seems to afflict primarily snakes in the U.S. — although cases have been documented in Canada, Germany, Australia and the U.K. — and why it appears to be rising rapidly in prevalence just now. The leading speculation lies in familiar territory for those attuned to the many forces, including our changing climate, that threaten wildlife.
As an alternative to the notion that OO is in some way a new or newly virulent infection agent, the Lorch paper says it may be that the fungus
has been present in North America for a long time, and recent environmental changes are driving SFD emergence. The 2006 outbreak of severe dermatitis (now thought to have been SFD) in a timber rattlesnake population in the northeastern USA was associated with extremely wet conditions.
Ophidiomyces ophiodiicola is thought to be able to survive in the environment without a host, and moist conditions could play an important role in disease by promoting fungal growth and persistence in the environment. In addition, precipitation and cloud cover could negatively impact host thermoregulation mechanisms for fighting infection.
Hibernation appears to be important in SFD, and slight temperature increases during the hibernation season (resulting from climate change) may allow O. ophiodiicola to grow at a faster rate and establish more severe infections.
Although it urges more research to clarify what’s going on with SFD, the paper notes that there are few options for managing the disease or protecting snake populations beyond rehabilitating individuals that fall ill — especially if they happen to be, like the massasauga, already endangered for other reasons.
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The full paper, “Snake fungal disease: an emerging threat to wild snakes,” was published in the Royal Society’s journal Philosophical Transactions B, and can be read without charge here [PDF], as can the rest of the special issue on fungal threats to ecosystems, as far as I can tell.