White nose syndrome (WNS) is a fungal disease that has killed millions of bats across North America since it was first identified in 2006. The disease is caused by a cold-loving fungus that grows on bats while they hibernate, destroying their wing tissue and disrupting their ability to survive the winter. Mortality rates in affected colonies often reach 90 to 100 percent.
How the Fungus Kills Bats
The fungus behind white nose syndrome thrives in the cold, dark environments where bats spend the winter. It invades the skin of a bat’s nose, ears, and most critically, its wings. Bat wings aren’t just for flying. They regulate body temperature, control water balance, and exchange gases. When the fungus eats through that skin, it triggers a cascade of problems that most bats can’t survive.
The damaged wing tissue allows fluids and sodium to leak out of the body. Deeper tissue damage increases the permeability of blood vessels, accelerating that fluid loss even further. The result is a form of severe dehydration that reduces blood volume, drives up blood viscosity, and drops blood pressure. With less blood reaching tissues, cells start producing lactic acid, tipping the bat into metabolic acidosis.
This dehydration and acid buildup force the bat to wake up from hibernation repeatedly, likely driven by thirst and the need to breathe faster to compensate for its disrupted blood chemistry. Each arousal burns through stored fat. Normally, a hibernating bat wakes only occasionally and has enough fat reserves to last the winter. A bat with white nose syndrome arouses far more frequently and may even leave the cave in midwinter searching for food that doesn’t exist. With no insects available, the bat starves. Most die from a combination of starvation and fluid loss.
What It Looks Like
The syndrome gets its name from the visible white fuzz or powder that appears on the noses, faces, ears, and wings of infected bats. That white growth is the fungus itself. Infected bats also develop lesions and scarring on their wings, which can be visible even after the fungus is no longer actively growing.
Behavioral changes are often the first sign something is wrong in a colony. Bats may cluster near cave entrances instead of deep inside, fly during the daytime in winter, or be found on the ground outside hibernation sites. Healthy bats in winter should be tucked away and still. Any bat active and visible during cold months is a red flag.
How It Spreads
Bats transmit the fungus to each other through direct contact, particularly when they cluster together in caves and mines during hibernation. As populations emerge in spring, disperse, and interact with bats from neighboring colonies, the fungus hitches a ride to new sites. The disease has spread across eastern North America in a pattern consistent with an infectious agent moving through connected populations.
The leading theory is that the fungus originated in Europe, where bat populations appear to tolerate it without mass die-offs. It likely arrived in North America through human activity. The first known case appeared in a popular tourist cave near Albany, New York, raising the possibility that fungal spores were carried on clothing or gear from a European cave. Humans can’t catch the disease, and no illnesses have been reported despite thousands of people visiting affected sites. But cavers and researchers now follow decontamination protocols to avoid carrying spores to new locations on boots and equipment.
Scope of the Devastation
White nose syndrome has been confirmed in at least 33 U.S. states and seven Canadian provinces. Millions of bats have died since 2006, and several species face possible extinction. Little brown bats, once among the most common bats in eastern North America, have experienced population declines of 90 to 100 percent in New York caves. They were the first species affected and have sustained the greatest losses.
The tricolored bat has seen estimated declines of more than 90 percent in affected colonies. In 2022, the U.S. Fish and Wildlife Service proposed listing it as endangered under the Endangered Species Act, citing white nose syndrome as the primary threat. The fungus is now present across 59 percent of the tricolored bat’s range. That same year, the northern long-eared bat was reclassified from threatened to endangered, driven largely by the same disease.
Why Bat Losses Matter Beyond Caves
Insect-eating bats are one of the most effective natural pest controls in North American agriculture. They consume enormous quantities of moths, beetles, and other crop-damaging insects every night. Estimates of the economic value of this pest control range from $3.7 billion to as much as $53 billion per year for U.S. agriculture alone. As bat populations collapse, farmers lose a free, nightly pest suppression service that no amount of pesticide can fully replace.
Vaccines and Conservation Efforts
The most promising intervention so far is an oral vaccine developed by the U.S. Geological Survey’s National Wildlife Health Center and its partners. The vaccine uses a modified raccoon poxvirus as a delivery vehicle and can be given to wild bats by mouth. Researchers are also testing a topical version that bats would consume naturally through grooming.
Since field trials began in 2019, more than 5,000 bats have been treated, including little brown bats and several other species. Results show that vaccinated bats carry lower fungal loads, develop less severe wing damage, and return at higher rates the following year compared to untreated bats. The vaccine appears most effective when given before or immediately after the fungus arrives in a population that hasn’t encountered it yet. Current efforts are focused on species in western states, where the fungus is still advancing, and on endangered species like the northern long-eared bat.
The race is essentially geographic. Vaccination campaigns aim to build resistance in bat populations before the fungal front reaches them. For colonies in the east that have already been decimated, recovery will depend on whether surviving bats, some of which may carry natural resistance, can rebuild their numbers with the help of reduced fungal pressure from vaccination and other interventions.