White Nose Syndrome: The Deadliest Disease You’ve Never Heard Of

All across the northeastern United States, bats are waking up early from their winter hibernation. Sensing the cold, most go back to sleep. Come springtime, some will never wake up. Others get up to find themselves unusually emaciated and with lesions on their wings that make it too difficult to fly to find food. A few make it through the season, but many of the afflicted will carry the deadly infection that decimated their population to other caves with other bats.

Scientists who keep a close eye on bat populations recently began to notice that all of these bats were afflicted with a powdery white fungus growing on and in their bodies. Dubbed Geomyces destructans, the causative agent of white nose syndrome (WNS) infects thousands of bats per year, having killed about 5 million of them already (2). Although it was first detected in New York state, G. destructans has since been found as far north as Manitoba, Canada and as far west as Oklahoma (Figure 1, 1). The infection spreads farther every year.

This isn’t an old problem: before 2006, no one had ever seen anything like it (2). It’s thought that the fungus has lived in concert with European bats throughout their evolutionary history without ever causing them harm. However, upon reaching North America, G. destructans was able to infect new species of bats that had not evolved to defend themselves against it. Researchers were able to test this hypothesis by sampling G. destructans from bat populations in both Europe and North America and using them to infect North American bat species. They found that both types of isolates caused pathology and mortality in the North American bats while European bats were observed carrying low levels of G. destructans on their skin without showing any signs of sickness (1).

European bats’ resistance is no small feat; hibernating bats are near-ideal hosts for G. destructans. To conserve energy during the long winter, bats’ body temperature is dramatically lowered, usually matching the chilly ambient temperature of the cave. They also slow their metabolic processes and repress immune system functioning, leaving them vulnerable to infection (4). G. destructans is one of few fungal species that grows best at cold temperatures, making it uniquely suited to growth on hibernating bats, which typically hover around 7 ℃ (3). For these unicellular fungi, hibernating bats are a wealth of nutrients: G. destructans spores land on the bats and begin to colonize areas at the optimal intersection of moisture and nutrients, which are typically the muzzle, nasal cavity, and the delicate membranes of the wings. For Myotis lucifugus, commonly known as the little brown bat, the conservation of moisture is absolutely crucial to survival during hibernation. Because they have such a large surface area-to-mass ratio, the wing membranes lose a great deal of moisture on their own. When G. destructans invades the wing, it either steals moisture or increases moisture dissipation such that the bat becomes so dehydrated it cannot recover (4).

To investigate this further, researchers infected M. lucifugus with G. destructans in the lab and examined the animals’ tissues both during the active infection and after they died. The fungal hyphae, a type of long, branching cell that can invade tissues and soil, were observed to cause significant tissue and blood vessel damage. The destruction of the circulatory system in the bats’ wings lends support to the hypothesis that water and heat loss are the primary mechanisms of pathogenesis in G. destructans infection in little brown bats. However, they also showed that in some cases the fungus actually “eats” holes in the bats’ wings (Figure 2), making it difficult for them to properly fly. The pattern of tissue damage, though, pointed most conclusively to the idea that G. destructans caused enough moisture loss to awaken hibernating bats before winter’s end, driving them to search for water (1). Water’s scarcity during winter is one of the biggest reasons bats hibernate to begin with; their frantic search does not often pan out, leading to death by dehydration or starvation.

To date, the strategies that have been proposed for combating G. destructans range from highly impractical to slightly less impractical. Because the geographic area of infection spans multiple states and countries, a concerted, coordinated implementation effort would be necessary. Catch-treat-release programs have been moderately successful, albeit costly and time-consuming. The same can be said for visiting caves and delivering antifungal treatment to bats individually. Treating bats en masse by delivering antifungals to entire caves is ecologically unsound because it would also kill important members of the natural fungal community in caves. The best options appear to be those that involve delivering supportive care to the caves by only subtly manipulating the environment, such as by adding humidity or food and water sources so that sick bats who awaken can feed and drink and have a better chance of recovering (5). Some additional, more out-of-the-box strategies include adding heated sections to caves so that infected bats can retreat there, killing off the cold-loving G. destructans, or even placing infected bats into a chamber with a strain of bacteria that produces airborne compounds that naturally inhibit growth of G. destructans (6, 7).

All crazy ideas aside, the scourge of G. destructans is a new, evolving problem that must be actively combated with human intervention in order to conserve affected bat populations and prevent the rapid spread of the pathogen across the U.S.