The fungus among us: California's bats under siege

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A sinister fungus is spreading through California's bat populations, threatening their survival just as the spooky season takes flight.

First detected on the East Coast around 2006, the fungus has been spreading west ever since. It made headlines this week, as it's now been definitively spotted in the Golden State and may already be as far south as San Bernardino County.

How does this killer operate, how did it get here, and what might it mean for humans? Here to answer these questions are two mycologists from UC Riverside's Microbiology and Plant Pathology Department, Professor Jason Stajich and Associate Professor Sydney Glassman.

What is this fungus, and where did it come from?

Stajich: Pesudogymnoascus destructans or Pd likely originated in Europe but may also have origins in Asia. It was likely introduced into North America as a pathogen around 2006.

Samples of it all across the U.S. generally have the exact same genetic makeup, suggesting it is spreading rapidly. The expectation is that humans visiting caves are moving soil and spores, helping it spread.

This map from the U.S. Geological Survey gives a sense of how it is spreading each year. Sadly, we see this east to west movement and now a north to south change. Because it is a cold adapted fungus, I am curious how it is being spread within this state, or if there are local adaptations yet to the temperature differences.

Glassman: It causes something called white nose syndrome, which makes bats' muzzles and wings turn white and fuzzy. This damages the wings, but also makes bats more active. The additional movement uses up their fat reserves and causes them to starve to death. It can travel through bat dung (guano) and gets to the bats while they're in caves.

Does this fungus pose a danger to humans?

Glassman: It's not likely to affect humans, unless you consider the cascading impacts of killing off bats, which eat a lot of mosquitoes, which are vectors of diseases that affect humans. So, if the bats aren't helping to control mosquitoes, then humans can get more diseases indirectly.

Stajich: Over time with climate change, some fungi are becoming more adapted to warmer temperatures. As they do so, they may gain properties to allow more effective colonization in other mammals.

The fungus itself is sensitive to UV light, as well as warm temperatures, so the leap to being a competent pathogen of humans would require more adaptations. Generally, it is a skin disease which doesn't require survival at human body temperature (37C), so the transition to being a true human pathogen would require several changes in its biology.

How does the fungus operate?

Stajich: It secretes enzymes called peptidases which damage connective tissue in the bats. It can appear as a white fuzzy mold on the bat's skin and tissue. It will invade tissue and cause damage; the worst is in the wing connective tissue which will impair ability to fly and hunt further reducing food intake and limiting ability to recover from the disease. Some bats are acquiring immunity and populations are recovering in some areas, but I think the means for this are still under study to see what the surviving genotypes in the bat populations are.

Everything has a role in nature, even fungal predators. What benefit does this fungal predator have for the environment?

Stajich: More generally, fungi's role in the environment is nutrient cycling. They provide the means to degrade complex molecules like lignin and cellulose in plants, and proteins in deceased animals, which releases the carbon and nitrogen in these organisms.

In caves, the only food sources are things introduced by the environment, and the animals that transit the caves. So, the fungi that live there have to be efficient with the material present.

I generally do not think of these in terms of predator and prey. This is an opportunistic pathogen that probably initially had some foothold in weak or young bats. Over time, it evolved and became more successful. But European bats evolved with it and aren't harmed by it in the same way. In Europe, Pd might not be called a pathogen at all.

Glassman: I agree that most things have a role in the environment, but since this pathogen was introduced here beyond its native range, and North American bats don't have any resistance to it, I'd say this is an instance where I can't think of any benefit from this particular fungus.

Usually, natural pathogens serve to control populations from getting too out of control, but if it's introduced somewhere there is no known resistance to it via globalization or some other force like cutting down rainforests, often that can have catastrophic results for the prey.

What have we not asked about this fungus that would be interesting to note?

Stajich: There is ongoing work from Bruce Klein at the University of Wisconsin and David Blehert at the USGS in Madison, Wisconsin, to develop a vaccine strategy for Pd.

There are also interesting, parallel stories about other recently emerged wildlife fungal diseases. In addition to this one affecting bats, there are ones affecting snakes and amphibians. They've all evolved the means to be effective degraders of vertebrate tissues.

Evolutionarily, they make up very distinct and different lineages in fungi, indicating there are multiple or repeated transitions they've made to be able to colonize and overtake these animals. All of these fungal diseases have only been recognized as they emerged in the last 20 or 30 years, so there are clearly changes in how they are being dispersed in the environment, which seems linked to humans at least in some part.

Witches, take note. Thanks to the emergence of this relatively new fungal disease, a potion that requires eye of newt, fang of snake and wing of bat might soon be harder to make.

Provided by University of California - Riverside