Dr. David Blehert, a microbiologist with the US Geological Survey’s National Wildlife Health Center, presents on white-nose syndrome, an infectious disease among bats and its impact on the ecosystem in the 21st century… this presentation took place in March, 2012.
Dr. David Blehert – Microbiologist, USGS, National Wildlife Health Center
Thank you, Bill, for the very nice introduction and thanks to Hannah Hamilton for inviting me and the USGS communications group for arranging my visit.
So today, I’m going to talk about bats and new bat disease. Let’s see if I can get my slides to advance. Can everybody hear me okay?
So let me just begin by saying a little bit about bats to get us all on the same page.
Introduction – Bats
Bats are the only mammals that are capable of self-powered flights. Most are nocturnal. Perhaps these adaptations of being one of the few animals that’s out flying in the night sky when there’s otherwise lack of aerial predators has afforded this group of animals with the evolutionary opportunity to diverge into a huge number of species.
Bats are the second most species diverse group of mammals on the planet. Only behind rodents. There are about 1,100 species of bats out of a total 5,500 or so species of mammals. And just an amazing amount of adaptations, if you look at this giant fruit bat on the screen here, this animal consumes ripe fruit. This is nectar-feeding bat that can hover in front of cactus flowers and lap the nectar.
Here’s a bat that’s got both white and black fur. It’s the spotted bat in the American Southwest. With these huge ears, I think this animal is known to forge for insects on the ground and they can say that it can actually hear their footsteps.
And here we have a bulldog bat — which actually eats fish and it catches them right out of the water.
Fungal disease and fungal pathogens
So, to transition the topic of today’s talk, a disease of bats — white-nose syndrome is a fungal disease. Of all the various pathogens or disease agents that we know of, and I’m a microbiologist so I know of a lot of these — even for microbiologist, fungi are not often the first that come to mind when talking about disease.
When it comes to disease of humans, there are really only six major recognized groups or genre of fungi that cause disease and to name them, maybe you’ve heard of some of them, includes the yeast Candida and other fungi Aspergillus, Histoplasma, Blastomyces, Coccidioides and Cryptococcus. But, that’s six out of literally hundreds of thousands of species.
Despite fungal pathogens not necessarily being the first thing that people think of when talking about disease, it’s indisputable as I’ve outlined on this slide here, that fungi have had major impacts on the world. So for example, fungal diseases transformed landscapes by ravaging both American chestnut trees and elm trees, all within the 20th century. The Irish potato famine caused by a fungal-like organism that’s now referred to as called Oomycete, caused the death and immigration of over 2 million people.
Fungi remain potent pathogens of plants, humans and wildlife today, yet fewer than 10% of all fungal species are known to science. There are limited antifungal therapeutic drugs available and they often have associated toxicity because in terms of classes of animals, fungi are actually very closely related to people. So when we target bacterial pathogens with antibiotics, we’re targeting mechanisms of replication for that microorganism that are distinctly different than those in our own bodies.
When it comes to treating a fungal pathogen, there are also what are known as eukaryotes just like us. And so it becomes challenging to kill the fungus without harming the host.
There’s no antifungal vaccines. And then when it comes in particular to plants or animals, like wildlife, that don’t have intrinsic or obvious economic value, disease management can be very challenging.
Another point of interest is that fungal diseases in humans and other mammals are most commonly associated with hosts that have compromised immune systems. So there’s something else going on. Or, in cases where that host was exposed to a huge dose of the organism, say, through – fungal diseases sometimes arise in military personnel doing training exercises where they’re crawling through soil with their faces close to the soil that’s enriched with fungal spores of certain disease agents.
This graphic shows that with respect to human health — and likely driven changes in host’s immune status — caused by the emergence of HIV, increased use of immunosuppressive medications like steroid treatments, other intensive care-based therapies that have people in hospitals with in-blowing catheters that are providing a conduit between the outside world and the inside of the body, incidents of systemic fungal disease and people has been exponentially on the rise since about the 1950s.
As I move along through this presentation, and along these same lines, I hope to convey to you the unique world that hibernation plays in white-nose syndrome as one of these predisposing factors to this novel of disease.
So what is White-nose Syndrome?
White-nose Syndrome is an emerging fungal disease of bats and it’s a very interesting — or maybe insidious would be a better word — disease because bats are specifically infected with this fungus when they hibernate – which, based on research done in other hibernating mammals, is believed to be — it’s known to be in these other mammals, and we believed it to be in bats, to be a time of natural immunosuppression. So, there is one of the potential predisposing factors.
Furthermore, a large scale disease epidemic, or we’d say epizootic when we’re talking about mammals, a large scale epizootic among bats like White-nose Syndrome is not only unprecedented among the bats that live in the United States, but among all of the 1,100 plus species of bats and perhaps even among all mammalian species around the world.
A paper came out in the journal Science two summers ago now in which the authors predicted a 99% chance for regional extinction of little brown bats — once the most numerous hibernating insect killer bats species in the Northeastern United States — within about the next 16 years as a result of this disease.
So do I think this is a feasible hypothesis?
I would say that it actually is because unlike other pathogens, say like a virus, that is not as autonomously replicating organism, or something that’s capable of survival on its own, most fungi are. Almost all fungi, including those that are pathogenic, can also live a second type of life phase, where they’re just existing as saprobes or decomposers of organic matter in soil.