Fungus Causes High Mortality in Hibernating North American Bats

For over a decade, the white-nose fungus has devastated populations of bats all over eastern Canada and the United States. Millions of bats have died from the disease called the white-nose syndrome or WNS. Scientists have identified the cause as the fungus Pseudogymnoascus destructans, which they think entered the region accidentally from Europe.

Asian and European bat populations are not as affected by the pathogen. One reason for this, scientists think, is because the Eurasian bats have immune systems already familiar with it. Another possible reason is the fact that the North American wintering homes have a high fungal density, and whenever bats return each year to roosts there, they get re-infected.

These points were reported by researchers in an eight-year study. Co-author Virginia Tech disease ecologist Joseph Hoyt said the bats almost immediately become re-infected the moment they return to their contaminated roosting sites. They took into consideration 101 wintering roosts worldwide. In each site, the number of bats was counted, and sample swabs were taken from the animals, the ceilings, and the walls. They found that as winter began, the fungus was considerably more abundant in the roosts in North America compared to those in Asia and Europe. In addition, nearly all of the animals in the former have already been infected shortly after settling down to start their hibernation period. The infection in these animals was more serious. Not surprisingly, mortalities were also much higher.

Since the fungus thrives in colder conditions, the bats are susceptible only during the winter. Hibernating bats also physiologically lower their body temperature in order to save more energy, so that they have enough nutrient stores to last the entire winter. The colder conditions combined with the cool bodies of the animals are the perfect conditions for the fungus to grow.

In WNS, white growth is seen on the wings and muzzles. The affected bats experience restlessness, move more, and wake up more frequently. This causes them to burn more fat reserves that they can afford for the winter season. Aside from the restlessness, the infection itself also causes energy burn up, as shown by a study by scientists from the University of Wisconsin..

The WNS infection shows visible signs at the latter stage of the disease. By this time, the affected bats have already undergone dehydration and acidification and a host of other bodily changes that are life-threatening. If they can somehow manage to survive and their immune system is healthy, then they can survive until spring. For Eurasian bats, they only typically get infected later in the winter season, which gives them a better fighting chance for survival.

When summer comes, a drop in P. destructans colonies also occur in the roosts in Eurasia, which may be due to competition with or consumption by other microorganisms. Unfortunately in North America, the fungus propagates due to the lack of natural enemies. This is the negative impact of introduced species on wildlife populations unfamiliar with them, which can wreak havoc on ecosystems and even cause extinctions.

Hoyt's team hopes to help North American bats recover by applying disinfectants to reduce the fungal colonies and simulate Eurasian conditions.