Field of Science

Update on Bats and Fungi

I previously posted on "White Nose Syndrome," a disease that has been devastating bat colonies in the NE US recently. Well the newest issue of Science has a new Brevia by the same group. (BTW Science, loving the cover this month.) There is not really anything too new here, basically a variety of bats from VT, MA, CT, and NY were autopsied and again fungi were found and identified as belonging to the Geomyces family. Remember these are psychrophilic (cold-loving) fungi, which makes sense based on where they are growing. Anyway, this was a nice little addition to the issue of WNS and a decline in the bat populations in these caves.

However, I do not believe there is sufficient reason for anyone (and I am NOT suggesting the authors are doing this) to conclude this fungus causes WNS. The fungus could in fact be killing all these bats analogous to how Salmonella causes food poison. Conversely, the fungal infection could simply be an additional symptom of an unidentified problem. A think a perfect analogy is the onset of oral thrush (caused by the fungus Candida albicans) and fungal meningitis (caused by Cryptococcus neoformans) in young men during the 80s. These infections, the latter being extremely lethal, were caused by essentially run-of-the-mill fungi, one of which (C. albicans) lives in essentially every person on the planet. Death was often the endpoint, however the reason these fungi were causing diseases in the first place was as a complication of an immunodeficiency caused by the HIV virus. So, in this human example a fungal disease was due to a problem caused by the viral disease. If we take care of the C. albicans infection, the patient would still be sick with pneumonias, acquiring rare cancers, etc. The problem was HIV.

So, what do I think is killing the bats? In reality, I don't know. Here are some thoughts though. I think three possibilities are most likely: 1. its a problem caused by the fungus; 2. its a disease analogous to the HIV example above; 3. same as #2 except the underlying problem is not due to a biological entity, but an environmental entity like a toxic chemical.

I am not a fan of #1 for the following reason. Not all bats that die have overt fungal growth (not the best evidence I agree). Also the fungus seems to be restricted to the surface of the bat, infecting the epithelial (skin), hair follicles, and sweat glands. There is no internal organ involvement or other major dissemination that would clearly lead to death. Also, and most interestingly, these bats lack brown fat reserves. These are the reserves that sustain the bats during hibernation. The lack of brown fat explains why some bats wake up too early (starving) or lack the strength to forage when they wake up on time. But why would they be depleted? Maybe a virus is causing a metabolic problem that leads to the depletion. BTW fighting off microbes takes a lot of energy, so maybe as these reserves are depleted the bats immune systems go south. Similarly, an environmental toxin could cause a similar problem. To confound things further, I do not believe it has been determined when the brown fat depletion occurs. Maybe the problem is that the fat reserves never build up in the first place or that they never build up sufficiently or that they build up normally but are not maintained or used correctly. These are, I think, important and interesting questions and they do matter. If we decide to treat the fungus, but it isnt really the underlying problem, then we probably haven't dealt with the problem. So how can these kinds of questions be answered? Not easy. One possibility is to capture a fair number of bats from a cave suffering from WNS during the growing season, some early, some in the middle, some late, and then those after hibernation. We can autopsy the animals and see what's going on physiologically. Since not every bat suffers from WNS, we would need a collection of bats to analyze. Another possibility, if we can determine brown fat without euthanizing the bats, we could tag 20, 50, 100 bats and see what's happening to them over the course of a winter without having to euthanize them. In reality there are lots of ways to address these and other questions. None of this means it will be easy, but I do think people should be asking these questions.

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