G. destructans – Little Brown Bats Never Heard It Coming.

by DH

As scientists traversed several abandoned, echoing caves in upstate New York they were mortified, astonished, and mystified. As they surveyed the cave floor—not the ceiling as expected—the carnage suggested one thing: the American bat population was dying off at an alarming rate. In that year alone, 2006, an estimated one million little brown bats (Myotis lucifugus) were found dead in their hibernacula. Bats have a tight ecologic relationship with humans; they prey upon many of the insect species which target agricultural products (1).  Therefore, much speculation and consideration was given to this phenomenon. Immediately after its discovery, nearby research facilities like the New York Department of Health’s Wadsworth Center and the United States Geological Survey (USGS) began receiving numerous requests for rabies testing on deceased bats. Luckily, none of these bats tested positive for the zoonotic virus and a humans shared a collective sigh of relief. But, all bats did present with a peculiar, fuzzy white growth on their snout. In 2009, the research team of Dr. Robert Blehert isolated a unicellular fungus responsible for the White Nose Syndrome (WNS) and tentatively deemed it Geomyces destructans. But, the question remained how a fungus from a traditionally non-pathogenic family could be causal agent of such a massacre.

For Dr. Blehert and his team at the USGS, isolation of G. destructans proved difficult. Though stained tissue cultures clearly revealed the presence of fungal growth, no species with similar morphology was obtained by initial culturing methods. After several weeks of incubation at a lowered temperature, approximately 15°C, they were able to obtain and isolate G. destructans (1).  Because of its ability to flourish in cool temperatures, G. destructans is considered a psychrophile. In fact, the fungus is incapable of growth at room temperature. With additional research of Dr. Vishnu Chaturvedi at the Wadsworth Center, the psychrophilic nature of the fungus proved to be a key factor in its ability to infect the little brown bats. Their chilled skin during winter hibernation is perfect for G. destructans growth (2).  But, this confirmation of bat/fungal colonization seemed to raise more questions than it answered. There was not a direct pathogenic factor determined which would kill the bats, such as a toxin, in any related species of fungus. Because of this many believed that G. destructans was a secondary infection to some other pathology in the bat. This is akin to the AIDs emergence in the 1980s wherein males presented with fungal infections due to lack of immune function. There existed a great debate within the scientific community, and the race was on to determine which model was correct (4).

The opposition was silenced in 2011 when Dr.  Jeffery Lorch at the University of Wisconsin-Madison published an article clearly identifying G. destructans as the causative agent of WNS (3). Basically, they placed healthy bats in a cool environment with bats isolated from New York hibernacula. The wild bats were symptomatic of WNS: white fuzzy growth and G. desctructans infection.  After several weeks all the healthy bats exhibited the white fuzzy growth on their snouts and G. destructans was cultured. Interestingly, healthy bats which were physically separated from infected bats by distance of no more than 3 inches were not infected over the entire study. This showed that G. destructans is only transmittable via direct contact. But, in the two month study no bats died. Why was this data sufficient to support the G. destructans model for bat mortality? First, Dr. Lorch’s study was limited by cost—a detail common to all research. Bats are very expensive to house, feed, support and evaluate. Second, they examined previously understood characteristics of the little brown bats’ hibernation. Before hibernating, bats participate in “swarming” behavior, a spectacle in which thousands of bats fly in tight circles to mate; a perfect time for G. destructans to spread.  Also, these bats are known to hibernate in early to mid-September (3). Each year, the mass die offs were observed in late March, nearly six months after entering hibernation. It is proposed that the six month metabolic slowing of the bats lowers immune response; this, coupled with the prolonged infection with G. destructans results in bat death.

Geomyces destructans offers insight into modern fungal pathogenesis. It is a cold reminder that an apparent healthy population needs only a miniscule ecological hiccup to be devastated.  Due to developed global infrastructure, and especially easy international travel, non-pathogenic microbes have opportunities to reach parts of the globe like never before.  A fungus which is harmless in one part of the world, like European Geomyces—a proposed ancestor of G. destructans, is now able to spread to a genetically similar host that is unable to curtail invasion by the fungus.  Massive die off would ensue, and have in the case of G. destructans. The implication of such an event in mammals so closely related to humans is disturbing. Isolated populations of human beings exist worldwide. Evolution has shown that it takes only one event, no matter how improbable, to change the course of ecology. Further study is needed to see how G. destructans is such a lucky pathogen, and how a fungus with such specific environmental requirements seemingly appeared to expunge the  little brown bats of America.

References

1. Blehert DS, Hicks AC, Behr M, Meteyer CU, Berlowski-Zier BM, Buckles EL, Coleman JT, Darling SR, Gargas A, Niver R, Okoniewski JC, Rudd RJ, Stone WB. Bat white-nose syndrome: an emerging fungal pathogen? Science. 2009 Jan 9;323(5911):227

2. Chaturvedi V, Springer DJ, Behr MJ, Ramani R, Li X, et al. (2010) Morphological and molecular characterizations of phychrophilic fungus Geomyces desctructans from New York bats with white nose syndrome (WNS). PLoS One 5: e10783

3. Lorch, JM, Meteyer CU, Behr MJ,Boyles JG ,Cryan PM,Hicks AC, Ballmann AE, Coleman JTH,Redell DN, Reeder DM and Blehert DS. Experimental infection of bats with Geomyces destructans causes white-nose syndrome. 2011. Nature 480(7377): 376–378.

4. Verant ML, Boyles JG, Waldrep W Jr, Wibbelt G, Blehert DS. Temperature-Dependent Growth of Geomyces destructans, the Fungus That Causes Bat White-Nose Syndrome.PLoS One. 2012;7(9):e46280. doi: 10.1371/journal.pone.0046280. Epub 2012 Sep 28.