White-Nose Syndrome Proves Resilient as Little Brown Bats Face Further Declines

By Alexandra DeCandia

The situation may be worse than we anticipated for the little brown bat (Myotis lucifugus). In a harrowing new study published by University of Illinois researchers earlier this week, it appears that the fungus Pseudogymnoascus (Geomyces) destructans (the cause of White-Nose Syndrome or WNS in bats) is even more resilient than previously thought. Able to colonize any complex carbon source found within the confines of a cave environment, the fungus can persist on a plethora of organisms and at a variety of pH levels. For the little brown bat, this implies that any attempt at the fungus’ eradication from known hibernacula proves futile. The fungus will merely lay in wait on another organism until its preferred host reappears en masse each fall.

The North American strain of Pseudogymnoascus destructans(Gd) examined in this study first appeared in 2006. Infecting only a few hibernacula in upstate New York, the fungus has since spread to over two-dozen states and migrated as far northward as Canada. Highly transmissible, highly persistent, and incredibly lethal, Gd has already claimed the lives of over 5.7 million North American bats with no perceivable end to its destructive reign yet in sight.

Gd infects bats while they hibernate, passing from one individual to the next in the cramped conditions of a M. lucifugus colony. The fungus grows on the cold cutaneous tissues of their muzzles and wings and specifically degrades their epidermal keratin. Resultant lesions form and increase the bat’s vulnerability to other pathogens and parasites lurking within the caves.
Of even greater concern, though, is the fungus’ effect on the patterning of torpor and consciousness during hibernation. As an order, chiropterans possess incredibly efficient metabolisms. Flying or even heating their bodies above ambient temperature can deplete their energy stores to the point of emaciation within days. Therefore, remaining in a state of torpor (i.e. decreased body temperature, lowered metabolic rate, etc.) proves crucial when ambient temperature and food availability decrease in winter. Bats infected with Gd cannot remain in hibernation undisturbed, due either to fungal itch or rapid dehydration. With increasing frequency, they arouse until ultimately perishing from starvation.

Intrinsic value of the species aside, the loss of so many little brown bats at the hands of Gd-induced starvation poses a serious economic risk to North Americans. Through insect predation, consequential reduction in pesticide utilization, and natural agricultural pollination, bats provide ecosystem services worth an estimated $3.7 to $53 billion USD per annum (Boyles et al., 2011). Should WNS eradicate certain chiropterans from the continent as it seems poised to do (at least as far as M. lucifugus is concerned), thousands of metric tons of insects will pour into our fields and lead to a cascade of negative ecological, economic, and human health implications.
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Combatting Gd and WNS has proven difficult thus far, to say the least. Indeed, studies have concluded that even if little brown bats manage to evolve means of surviving infection (as their European cousins have done), their populations will still decrease to fewer than 1% of their initial numbers within 20 years (Frick et al., 2010). Such estimates combined with the newfound resilience of Gd paint a grim depiction of the future for M. lucifugus, but they by no means necessitate surrender. Scientists continue to seek physical, chemical, and biological means of impeding the fungus, and some have even developed artificial hibernacula devoid of spores for bats to roost in unaffected. While neither management strategy has yet proven to drastically mitigate the spread of WNS, they represent steps in the right direction towards preserving an often overlooked but economically and intrinsically significant species, North America’s little brown bat.