Why Are Bats Dying in North America? White-Nose Syndrome

What is white-nose syndrome?

White-nose syndrome is named for the fuzzy white fungus that often appears on the muzzle, ears and wings of affected bats. The cause is Pseudogymnoascus destructans, a cold-loving fungus that grows best at about 4–15 °C (39–59 °F) and cannot grow above about 20 °C (68 °F).

Those are the temperatures of caves and mines in winter, and also the body temperature of a bat in deep hibernation. While a bat is active in summer, its warm body keeps the fungus in check. During hibernation, when the body cools and the immune system slows down, the fungus can invade the skin. It is the first known disease that kills mammals mainly while they are in torpor.

How it kills bats

The fungus eats into the thin skin of the wings, which bats use not only to fly but also to regulate water, breathing gases and body temperature. The damage starts a chain of problems:

• Too many wake-ups: infected bats rouse from hibernation far more often than healthy ones.
• Burning fat: each arousal is costly, and infected bats can use energy about twice as fast as healthy bats.
• Dehydration and blood chemistry: water loss through damaged wings and changes in blood acidity and potassium add to the stress.
• Starvation: many bats run out of fat before spring. Some fly out of caves in midwinter, searching for insects that are not there, and die from cold.

Deaths usually rise a few months into hibernation, peaking in late winter. Learn more about the energy balance of winter bats in Do bats hibernate?.

When and where it started

The first evidence of white-nose syndrome is a photograph taken in February 2006 in a cave in Schoharie County, New York, west of Albany. Biologists reported dying bats the following winter, and the disease spread quickly to Vermont, Massachusetts and Connecticut, then across the eastern United States and Canada.

The fungus is native to Europe and Asia, where bats carry it without mass deaths. Because bats do not migrate between Europe and North America, scientists think it was most likely brought in accidentally by people, for example on caving gear or clothing. In 2016 the disease turned up in a little brown bat in King County, Washington, a jump of about 2,100 km (1,300 mi) from the nearest known case. By the 2020s, the fungus or disease had been confirmed in dozens of US states and several Canadian provinces, from the Atlantic coast to the Pacific Northwest and Texas.

Millions of bats lost

The losses have been among the fastest declines of wild mammals ever recorded. By 2012, the US Fish and Wildlife Service estimated that 5.7–6.7 million bats had died. At many hibernation sites in the northeastern US, counts of the hardest-hit species dropped by more than 90%, and some caves that once held tens of thousands of bats now hold only a few.

The little brown bat is the best-known victim. Once one of the most common bats in North America, it was estimated at about 6.5 million before the disease reached its range. In 2010, a study in Science warned that it faced a high risk of disappearing from the northeastern US within about two decades.

Why it matters

Bats that hibernate in North America are insect-eaters. Each one eats large amounts of moths, beetles, mosquitoes and other insects, including crop pests; see how many insects bats eat. Fewer bats may mean more insects and higher costs for farmers, and scientists are still measuring these knock-on effects.

Signs of hope and research

There is some good news. In parts of the northeast, small colonies of little brown bats have survived, and some populations appear to have stabilized at low numbers. Survivors tend to hibernate at lower body temperatures, wake less often and carry more fat, and some may have genes that help them resist the disease.

Researchers are testing ways to help, including:

• Probiotic bacteria applied to bats or roosts, which improved survival in some trials.
• Ultraviolet light, because the fungus is unusually sensitive to UV.
• Vaccines, given to bats in experimental studies.
• Cave and mine management, such as protecting sites from disturbance and changing conditions to make them less suitable for the fungus.

How people can help

Disturbance is especially dangerous for infected bats, because every extra arousal burns fat. Many caves and mines are closed to visitors in winter, and cavers follow national decontamination protocols to avoid spreading spores between sites.

Anyone can help by obeying cave closures, never entering a hibernation site without permission, cleaning gear and clothing between caves and supporting local bat groups and surveys. Bat roosts in buildings and trees are also important for summer survival and breeding, so protecting them helps surviving populations recover.