Field of Science

Its Official: Fungi Kick Mammalian Butt
Bats with WNS
Since 2006 certain species of hibernating bats have been dying off in dramatic fashion by what has been called White Nose Syndrome (WNS). It is named as such because of some fungal growth around the nose of many affected bats. In 2008, Blehert and colleagues identified the fungus as Geomyces destructans and showed in early 2009 that the fungus was widespread throughout affected populations. In previous posts on these issues, I raised concerns because there was no data demonstrating causation. In fact, while noting that G. destructans could indeed be the etiologic agent of WHS, I also noted that it could be an indirect effect of some underlying problem. For example, the bat immune system could be impaired by a biological or chemical agent that allows G. destructans to infect and ultimately kill the bats (akin to HIV in people).

Well Lorch et al report in Nature that G. destructans is directly causing WNS in bats. Lorch et al essentially test the third of Koch's postulates, which are:
1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
2. The microorganism must be isolated from a diseased organism and grown in pure culture.3. The cultured microorganism should cause disease when introduced into a healthy organism.
4. The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
Now even Koch realized his postulates are not universal laws. For example, asymptomatic carriers kind of screw over postulate #1 and non-culturable organisms make #2 an impossibility. However, when fulfilled even partially, these postulates provide powerful information in the etiology of disease.

In the case of G. destructans, postulate #2 was fulfilled previously. In the study by Lorch et al, postulate #3 is shown to be true. If you grow G. destructans in culture and then expose healthy (but susceptible) bats to the fungus, they get l00% infection (Treated in table below), but similarly treated, but without the fungus, control animals should absolutely no development of WNS. More than 95% of infected bats succumbed to WNS within 3 months on infection! (Although it sucks for the bats, this provides definitive evidence that the fungus is the causative agent of WNS!!!1111!

Nature Table 1 (partial)
Furthermore, the authors found the fungi in lesions on the wings where most of the disease damage is thought to occur (despite the 'nose' being part of the name). This helps fulfill postulate #4.

This work is important because it affixes a firm target on the culprit. We can rule out other biological or chemical agents causing susceptibility to WNS. This also helps deal with postulate #1. Postulate #1 has been a complete dick in the case of WNS. This is due to the fact that G. destructans is found associated with European bats that are healthy. What Lorch et al's work tells us is that the situation is more complex than initially realized (but the truth of the matter is that life is always more complex). Maybe European bat species have immune mechanisms that prevent WNS. Maybe the G. destructans strain in the US is more pathogenic than the European isolates. Regardless, these are testable hypotheses. We can also definitively add mammals to the animals fungi feed on. Happy Halloween!

Lorch, J., Meteyer, C., Behr, M., Boyles, J., Cryan, P., Hicks, A., Ballmann, A., Coleman, J., Redell, D., Reeder, D., & Blehert, D. (2011). Experimental infection of bats with Geomyces destructans causes white-nose syndrome Nature DOI: 10.1038/nature10590

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